Archaeology Dating Methods

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Archaeology dating methods http://en.allexperts.com/q/Archaeology-654/Dating-methods.htm Answer There are several others: Potassium-Argon Protien racitimization Magnetic dating Ceramic Thermoluminesence Tree Ring dating Varve Dating Other than Potasium-Argon and Carbon 14 there are other radioactive decay clocks which are being brought into use but these are the two most common. Protien racitimization looks at the rate at which protiens in bone "flip" and this occurs at a reasonably predictive rate. Its usefulness is limited to about 3 to 400 years. Magnetic dating is highly accurate (in 20 year time spans) and looks at the clay materials of a hearth which when heated fixes the magnetic particles in the direction of north at the last firing of the hearth. This along with any charcoal may give a very precise date for the hearth. The samples are carefully encased in plaster with the modern north indicated and then the sample is taken into the lab where it is reoriented to north and the micro magnetic differences are tested and then compared to a chart of how long ago this "north" occurred. Ceramic Thermoluminesence uses a similar principal as the magnetic dating above but looks at the molecular structure under high temp to determine its magnetic seting. Tree ring dating is rather obvious 1

Transcript of Archaeology Dating Methods

Page 1: Archaeology Dating Methods

Archaeology dating methodshttp://en.allexperts.com/q/Archaeology-654/Dating-methods.htm

AnswerThere are several others:

Potassium-ArgonProtien racitimizationMagnetic datingCeramic ThermoluminesenceTree Ring datingVarve Dating

Other than Potasium-Argon and Carbon 14 there are other radioactive decay clocks which are being brought into use but these are the two most common.

Protien racitimization looks at the rate at which protiens in bone "flip" and this occurs at a reasonably predictive rate.  Its usefulness is limited to about 3 to 400 years.

Magnetic dating is highly accurate (in 20 year time spans) and looks at the clay materials of a hearth which when heated fixes the magnetic particles in the direction of north at the last firing of the hearth. This along with any charcoal may give a very precise date for the hearth.  The samples are carefully encased in plaster with the modern north indicated and then the sample is taken into the lab where it is reoriented to north and the micro magnetic differences are tested and then compared to a chart of how long ago this "north" occurred.

Ceramic Thermoluminesence uses a similar principal as the magnetic dating above but looks at the molecular structure under high temp to determine its magnetic seting.

Tree ring dating is rather obvious

Varve dating is similar to tree ring dating in that in sediments in ponds and other water sources, thin layers are deposited.  And like tree rings can be counted and the age of common deposits can be determined.  

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Dendrochronology

By K. Kris Hirst, About.com

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Definition:

Dendrochronology is an archaeological dating technique which uses the growth rings of long-lived trees as a calendar. Tree-ring dating was one of the first absolute dating method, and was invented in the early decades of the 20th century by astronomer Andrew Ellicott Douglass and archaeologist Clark Wissler.

In general, during the lifetimes of trees, each year the tree grows is marked by a growth ring; the tree gains a little bit of girth each year. The width of the ring added to the outside of the tree is in part dependent on the amount of moisture available to the tree--thus trees in the same area add thin rings during dry years and thick rings during wet years. If a research can obtain a string of tree samples that overlap, a precise sequence of tree rings can be derived.

Over the past hundred years or so, tree ring sequences have been built all over the world, with the longest to date consisting of a 10,000 year sequence in central Europe completed on oak trees by the Hohenheim Laboratory.

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Potassium-Argon Dating Methods

Filed In:

1. Fossils, Time & Evolution 2. > Geologic Time and Dating

The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas. Developed in the 1950s, it was important in the development of plate tectonics and calibration of the geologic time scale.

Potassium-Argon Basics

Potassium occurs in two stable isotopes (41K and 39K) and one radioactive isotope (40K). Potassium-40 decays with a half-life of 1250 million years, meaning that half of the 40K atoms are gone after that span of time. Its decay yields argon-40 and calcium-40 in a ratio of 11 to 89. The K-Ar method works by counting the number of these radiogenic 40Ar atoms trapped inside a mineral grain.

Potassium is a reactive metal and argon is an inert gas, which simplifies some things. Potassium is always locked up in minerals whereas argon is not part of any minerals. Argon makes up 1 percent of the atmosphere. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content. That is, a fresh mineral grain has its K-Ar "clock" set at zero.

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The method relies on satisfying some important assumptions:

1. The potassium and argon must stay put in the mineral over geologic time. This is the hardest one to satisfy.

2. We can measure everything accurately. Advanced instruments, rigorous procedures and the use of standard minerals ensure this.

3. We know the precise natural mix of potassium and argon isotopes. Decades of basic research has given us this data.

4. We can correct for any argon from the air that gets into the mineral. This requires an extra step.

Given careful work in the field and in the lab, these assumptions can be met.

The K-Ar Method in Practice

The rock sample to be dated must be chosen very carefully. Any alteration or fracturing means that the potassium or the argon or both have been disturbed. The site must be geologically meaningful, clearly related to fossil-bearing rocks or other features that need a good date to join the big story. Lava flows above and below rock beds holding ancient human fossils are a good—and true—example.

The mineral sanidine, the high-temperature form of potassium feldspar, is the most desirable. But micas, plagioclase, hornblende, clays and other minerals can yield good data, as can whole-rock analyses. Young rocks have low levels of 40Ar, so as much as several kilograms may be needed. Rock samples are recorded, marked, sealed and kept free of contamination and excessive heat on the way to the lab.

The rock samples are crushed, in clean equipment, to a size that preserves whole grains of the mineral to be dated, then sieved to help concentrate these grains of the target mineral. The selected size fraction is cleaned in ultrasound and acid baths, then gently oven-dried. The target mineral is separated using heavy liquids, then hand-picked under the microscope for the purest possible sample. This mineral sample is then baked gently overnight in a vacuum furnace. These steps help remove as much atmospheric 40Ar from the sample as possible before making the measurement.

Next the mineral sample is heated to melting in the vacuum furnace, driving out all the gas. A precise amount of argon-38 is added to the gas as a "spike" to help calibrate the measurement, and the gas sample is collected onto activated charcoal cooled by liquid nitrogen. Then the gas sample is cleaned of all unwanted gases such as H2O, CO2, SO2, nitrogen and so on until all that remains are the inert gases, argon among them.

Finally the argon atoms are counted in a mass spectrometer, a machine with its own complexities. Three argon isotopes are measured: 36Ar, 38Ar, and 40Ar. If the data from this step is clean, the abundance of atmospheric argon can be determined and then subtracted to yield the radiogenic 40Ar content. This "air correction" relies on the level of argon-36, which is not created from any other nuclear decay reaction. It is subtracted, and a proportional amount of the 38Ar and 40Ar are also subtracted. The remaining 38Ar is

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from the spike, and the remaining 40Ar is radiogenic. Because the spike is precisely known, the 40Ar is measured by comparison to it.

Variations in this data may point to errors anywhere in the process, which is why all the steps of preparation are recorded in detail.

K-Ar analyses cost several hundred dollars per sample and take a week or two.

The 40Ar-39Ar Method

A variant of the K-Ar method gives better data by making the overall measurement process simpler. The key is to put the sample in a neutron beam, which converts potassium-39 into argon-39. Because 39Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, thus it makes a clean indicator of the potassium content. The advantage is that all the information needed for dating the sample comes from the same argon measurement. Accuracy is greater and errors are lower. This method is commonly called "argon-argon dating."

The physical procedure for 40Ar-39Ar dating is the same except for three differences:

Before the mineral sample is put in the vacuum oven, it is irradiated along with samples of standard materials by a neutron source.

There is no 38Ar spike needed. Four Ar isotopes are measured: 36Ar, 37Ar, 39Ar and 40Ar.

The analysis of the data is more complex than in the K-Ar method, because the irradiation creates argon atoms from other isotopes beside 40K. These effects must be corrected, and the process is intricate enough to require computers.

Ar-Ar analyses cost around $1000 per sample and take several weeks.

Conclusion

The Ar-Ar method is considered superior, but some of its problems are avoided in the older K-Ar method. Also, the cheaper K-Ar method can be used for screening or reconnaissance purposes, saving Ar-Ar for the most demanding or interesting problems.

These dating methods have been under constant improvement for more than 50 years. The learning curve has been long and is far from over today. With each increment in quality, more subtle sources of error have been found and taken into account. Good materials and skilled hands can yield ages that are certain to within 1 percent, even in rocks only 10,000 years old, in which quantities of 40Ar are vanishingly small.

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About Isotopic Dating: Yardsticks for Geologic Time

The Century in Review

By Andrew Alden, About.com

The work of geologists is to tell the true story of Earth's history—more precisely, a story of Earth's history that is ever more true. A hundred years ago, we had little idea of the story's length—we had no good yardstick for time. Today, with the help of isotopic dating methods, we can determine the ages of rocks nearly as well as we map the rocks themselves. For that we can thank radioactivity, discovered at the turn of the last century.

The Need for a Geologic Clock

A hundred years ago, our ideas about the ages of rocks and the age of the Earth were vague. But obviously rocks are very old things. Judging from the amount of rocks there are, plus the imperceptible rates of the processes forming them—erosion, burial, fossilization, uplift—the geologic record must represent untold millions of years of time. It is that insight, first expressed in 1785, that made James Hutton the father of geology.

So we knew about "deep time," but exploring it was frustrating. For more than a hundred years the best method of arranging its history was the use of fossils or biostratigraphy. That only worked for sedimentary rocks, and only some of those. Rocks of Precambrian age had only the rarest wisps of fossils. No one knew even how much of Earth history was unknown! We needed a more precise tool, some sort of clock, to begin to measure it.

The Rise of Isotopic Dating

In 1896, Henri Becquerel's accidental discovery of radioactivity showed what might be possible. We learned that some elements undergo radioactive decay, spontaneously changing to another type of atom while giving off a burst of energy and particles. This process happens at a uniform rate unaffected by ordinary temperatures or ordinary chemistry.

The principle of using radioactive decay as a dating method is simple. Consider this analogy: a barbecue grill full of burning charcoal. The charcoal burns at a known rate, and if you measure how much charcoal is left and how much ash has formed, you can tell how long ago the grill was lit.

The geologic equivalent of lighting the grill is the time at which a mineral grain solidified, whether that is long ago in an ancient granite or just today in a fresh lava flow. The solid mineral grain traps the radioactive atoms and their decay products, helping to ensure accurate results.

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Within a few years, experimenters published some trial dates of rocks. Realizing that the decay of uranium produces helium, Ernest Rutherford in 1905 determined an age for a piece of uranium ore by measuring the amount of helium trapped in it. Bertram Boltwood in 1907 used lead, the end-product of uranium decay, as a method to assess the age of the mineral uraninite in some ancient rocks.

The results were spectacular but premature. The rocks appeared to be astonishingly old, ranging in age from 400 million to more than 2 billion years. But at the time, no one knew about isotopes. Once isotopes were explicated, during the 1910s, it became clear that radiometric dating methods were not ready for prime time.

With the discovery of isotopes, the dating problem went back to square one. For instance, the uranium-to-lead decay cascade is really two—uranium-235 decays to lead-207 and uranium-238 decays to lead-206, but the second process is nearly seven times slower. Some 200 other isotopes were discovered in the next decades; those that are radioactive then had their decay rates determined in painstaking lab experiments.

By the 1940s, this fundamental knowledge and advances in instruments made it possible to start determining dates that mean something to geologists. But techniques are still advancing today because with every step forward, a host of new scientific questions can be asked and answered.

Methods of Isotopic Dating

There are two main methods of isotopic dating. One detects and counts radioactive atoms through their radiation. The pioneers of radiocarbon dating used this method because carbon-14, the radioactive isotope of carbon, is very active, decaying with a half-life of just 5730 years. The first radiocarbon laboratories were built underground, using antique materials from before the 1940s era of radioactive contamination, with the aim of keeping background radiation low. Even so, it can take weeks of patient counting to get accurate results, especially in old samples in which very few radiocarbon atoms remain. This method is still in use for scarce, highly radioactive isotopes like carbon-14 and tritium (hydrogen-3). (Anne Marie has also prepared this worked-out example of radiocarbon dating.)

Most decay processes of geologic interest are too slow for decay-counting methods. The other method relies on actually counting the atoms of each isotope, not waiting for some of them to decay. This method is harder, but more promising. It involves preparing samples and running them through a mass spectrometer, which sifts them atom by atom according to weight as neatly as one of those coin-sorting machines.

For an example, consider the potassium-argon method. Atoms of potassium come in three isotopes. Potassium-39 and potassium-41 are stable, but potassium-40 undergoes a form of decay that turns it to argon-40 with a half-life of 1,277 million years. Thus the older a sample gets, the smaller the percentage of potassium-40, and conversely the greater the percentage of argon-40 relative to argon-36 and argon-38. Counting a few million atoms (easy with just micrograms of rock) yields dates that are quite good.

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Isotopic dating has underlain the whole century of progress we have made on Earth's true history. And what happened in those billions of years? That's enough time to fit all the geologic events we ever heard of, with billions left over. But with these dating tools we've been busy mapping deep time, and the story is getting more accurate every year.

Isotope Help from About Chemistry

About Atoms Radiocarbon Dating Example

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Uranium-Lead Dating

By Andrew Alden

Concordia diagram, with ages along the curve measured in million years.

Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable. Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence.

Basics of Uranium-Lead

Uranium comes in two common isotopes with atomic weights of 235 and 238 (we'll call them 235U and 238U). Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't stop until they become lead (Pb). The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb. What makes this fact useful is that they occur at different rates, as expressed in their half-lives (the time it takes for half the atoms to decay). The 235U–207Pb cascade has a half-life of 704 million years and the 238U–206Pb cascade is considerably slower, with a half-life of 4.47 billion years.

So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero. Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time. If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept. In a 704-million-year-old rock, 235U is at its half-life and there will be an equal number of 235U and 207Pb atoms (the Pb/U ratio is 1). In a rock twice as old there will be one 235U atom left for every three 207Pb atoms (Pb/U = 3), and so forth. With 238U the

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Pb/U ratio grows much more slowly with age, but the idea is the same. If you took rocks of all ages and plotted their two Pb/U ratios from their two isotope pairs against each other on a graph, the points would form a beautiful line called a concordia (see the example in the right column).

Zircon in Uranium-Lead Dating

The favorite mineral among U-Pb daters is zircon (ZrSiO4), for several good reasons.

First, its chemical structure likes uranium and hates lead. Uranium easily substitutes for zirconium while lead is strongly excluded. This means the clock is truly set at zero when zircon forms.

Second, zircon has a high trapping temperature of 900°C. Its clock is not easily disturbed by geologic events—not erosion or consolidation into sedimentary rocks, not even moderate metamorphism.

Third, zircon is widespread in igneous rocks as a primary mineral. This makes it especially valuable for dating these rocks, which have no fossils to indicate their age.

Fourth, zircon is physically tough and easily separated from crushed rock samples because of its high density.

Other minerals sometimes used for uranium-lead dating include monazite, titanite and two other zirconium minerals, baddeleyite and zirconolite. However, zircon is so overwhelming a favorite that geologists often just refer to "zircon dating."

But even the best geologic methods are imperfect. Dating a rock involves uranium-lead measurements on many zircons, then assessing the quality of the data. Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge. In these cases, the concordia diagram is a valuable tool.

Concordia and Discordia

Consider the concordia: as zircons age, they move outward along the curve. But now imagine that some geologic event disturbs things to make the lead escape. That would take the zircons on a straight line back to zero on the concordia diagram. The straight line takes the zircons off the concordia.

This is where data from many zircons is important. The disturbing event affects the zircons unequally, stripping all the lead from some, only part of it from others and leaving some untouched. The results from these zircons therefore plot along that straight line, establishing what is called a discordia.

Now consider the discordia. If a 1500-million-year-old rock is disturbed to create a discordia, then is undisturbed for another billion years, the whole discordia line will migrate along the curve of the

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concordia, always pointing to the age of the disturbance. This means that zircon data can tell us not only when a rock formed, but also when significant events occurred during its life.

The oldest zircon yet found dates from 4.4 billion years ago. With this background in the uranium-lead method, you may have a deeper appreciation of the research presented on the University of Wisconsin's "Earliest Piece of the Earth" page, including the 2001 paper in Nature that announced the record-setting date.

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Geologic Time Scale: Eons and Eras

Eon Era Dates (m.y.)

Phanerozoic Cenozoic 66-0

Mesozoic 251-66

Paleozoic 542-251

Proterozoic Neoproterozoic 1000-542

Mesoproterozoic 1600-1000

Paleoproterozoic 2500-1600

Archean Neoarchean 2800-2500

Mesoarchean 3200-2800

Paleoarchean 3600-3200

Eoarchean 4600-3600

(c) 2005 Andrew Alden, licensed to About.com, Inc. (fair use policy). Data from Geologic Time Scale of 2004)

All of geologic time, from the Earth's origin about 4600 million years ago to today, is divided into three eons. The first two eons, Archean and Proterozoic, and their seven eras are together informally referred to as Precambrian time. Read summaries of the Archean Eon and the Proterozoic Eon.

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The eras of the Proterozoic and Phanerozoic eons are each further divided into periods, shown in this geologic time scale.

The periods of the three Phanerozoic eras are divided in turn into epochs. (See the Phanerozoic epochs listed together.) Epochs are subdivided into ages. Because there are so many ages, they are presented separately for the Paleozoic Era, the Mesozoic Era and the Cenozoic Era.

The dates and colors shown on this table were specified by the International Commission on Stratigraphy in 2004. Colors are used to indicate the age of rocks on geologic maps. There are two major color standards, the international standard and the U.S. Geological Survey standard. (All of the geologic time scales here are made using the USGS standard.)

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QuestionI asked a creationists about current dating methods and this is what he said, is what he's saying valid?

You are  repeating some claims by the evolutionists that are simply NOT TRUE.   "scientists use several dating methods which all coincide with the same age"  They don't tell you they select those that agree and throw away MANY others that don't agree.  "dating methods on the fossil records on the continents seems conclusive"  NOT TRUE!!!

Dr. John Morris, a geologist, explains in easy-to-understand terms how true science supports a young Earth. Includes a critique of major dating methods. Filled with facts that will equip layman and scientist alike. Transparency masters are provided in the second half of this book. Use them in your Sunday school, church or youth group to challenge and teach.  

Radioactive dating in general depends on three major assumptions:

1. When the rock forms (hardens) there should only be parent radioactive atoms in the rock and no daughter radiogenic (derived by radioactive decay of another element) atoms;

2. After hardening, the rock must remain a closed system, that is, no parent or daughter atoms should be added to or removed from the rock by external influences such as percolating ground water; and

3.   The radioactive decay rate must remain constant.

Radiometric dating methods make assumptions that have been proven to be inconsistent.  If any of these assumptions are violated, then the technique fails and any ‘dates' are false.  Leaching, varying isotope ratios, etc. indicate the methods at best are unreliable.  Dr. Steve Austin at ICR tested rocks from the bottom and top of Grand Canyon.  Three of the four methods showed that the bottom rocks were younger than the top rock an IMPOSSIBLE conclusion.  The radiological methods date rocks that were liquid and are now solid.  These rocks do not contain fossils.  The fossil bearing rock is sedimentary (laid down under water in Noah's flood).  Evolutionists consistently disregard radiometric dates that conflict with their time scheme.  If a date doesn't support evolution they throw it away.  See John Morris' "Young Earth" and the book “Bone of Contention” for a detailed study of “missing links of apes to men.

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Carbon Dating is a method that is often cited to “prove” evolution.  It does not because:

1) as a method it can only extend to 50,000 years not millions of years,

2) C14 can only date tissue not fossils (stone impressions of bones).  When you find (some have been found) real dinosaur bones it is a testimony to creation because biological tissue won't last millions of years.  

3) Carbon 14 has not come to balance in the atmosphere indicating a young not old atmosphere.

Answer

Dear Eric

There are so many things wrong with this, I scarcely know where to begin.  The cardinal assumptions (1-3) about radiometric dating that your creationist correspondent cites are correct, and they hold true for most applications of the major dating methods we use to reconstruct earth history (i.e., potassium-argon, uranium-series, radiocarbon).  

It is not true that geologists discard MANY dates that disagree with previous ones.  It is the case that occasionally contamination, bad preparation, or mechanical errors result in one or two dates in a much larger series of dates for a particular rock sample being discordantly young or old.  What the creationist fails to point out is that if their young chronology was true, then ALL the dates should be young, not just one or two that are explicable in terms of contamination or processing errors.  

When the results of radiometric dating studies are published in the scientific literature, they typically include ALL the dates, regardless of how well they fit with prior hypotheses about the age of the samples. Then in the discussion section of the paper the authors explain why outlying dates might be rejected. Other scientists who disagree with these arguments can then examine the evidence and make their own case for accepting the discordant dates.

Applying more than one dating method to the same rock formation is not the exception, it is the rule in geochronology. Agreements among different dating systems are what we scientists call "independent verification".  The more lines of evidence that point to the same conclusion, the surer you can be that your conclusion is valid.  (Like having both fingerprints and DNA in a crime scene analysis.)  It is true that popular summaries of geological dating (like high school or college textbooks) might list a single value for a geological event, for the purpose of simplicity or presentation, but if you dig into the mainstream scientific literature, you will easily enough find the actual dates accompanied by their lab numbers, the proportions of radioisotopes upon which they are based, the assumptions of the dating model, and the debate about the validity of the dates.

Grand Canyon Study by the ICR's Steve Austin?  Never heard of it.  Were this finding of reversed chronology in the "Grand Canyon" valid, and more to the point, replicated by other scientists it would have been a headline paper in Science or Nature. That it isn't speaks volumes about its validity.  ICR publishes all kinds of silly pseudo-scientific sounding stuff that takes issue with some of the minor problems with geology, paleontology, and anthropology, magnifying them through rhetorical tricks into allegedly fatal  flaws.  But what they consistently fail to point out that when real scientists discover these mistakes they work hard to fix them, or, failing that to modify their hypotheses accordingly.  

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Creationists, in contrast, are still pushing the same 3000-year old Genesis-based cosmogenic myth, in spite of the fact that nearly every major discovery in biology, chemistry, geology, history, and anthropology has proven their account of human origins wrong over and over.  This is why, when you step back from the debate and look at the behavior of the participants with an anthropologist's eye, you instantly recognize that "creation science" is a faith-based argument, not science.  It is just as immune from being proven wrong to its adherents as are the tenets of any of the world's religions.

Their comments on radiocarbon dating are gibberish.  Inasmuch as Willard Libby won the Nobel Prize for Physics for discovering the principle of radiocarbon dating, you'd think that such flaws as are claimed here would have been discovered in the review process for that award.

For a good take on these issues, see Niles Eldredges' book, The Triumph of Evolution and the Failure of Creationism.  The title pretty much says it all.

Cheers,

John Shea

A Short Course in Scientific Dating Methods

This course on archaeological dating methods is provided here as an introduction to the primary methods of dating in archaeology.

Stratigraphy and SeriationAn introduction to relative dating--the first ways used by archaeologists to tell how old one site, one part of a site, or one object is, relative to another.

Chronological Markers and DendrochronologyThis lesson includes an introduction to absolute dating, as well as descriptions of the techniques using chronological markers and dendrochronology.

The Radiocarbon RevolutionAn introduction to the first method developed using the measurement of biologrical processes.

New Fangled MethodsDating techniques such as obsidian hydration, potassium-argon and fission track are not really new, except to an archaeologist, who believes anything under a century is new.

A Few Cautionary NotesAll dating techniques work to one degree or another; but all need to be used with some caution. Here's why and how you can overcome the obstacles.

http://archaeology.about.com/cs/datingtechniques/a/timing.htm

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Stratigraphy and Seriation

Archaeologists use lots of different techniques to determine the age of a particular artifact, site, or part of a site. The two categories of chronometric techniques that archaeologists use are called relative and absolute dating. Relative dating determines the relative age of artifacts or site, as compared to others, but does not produce precise dates. Absolute dating, methods that produce specific chronological dates for objects and occupations, was not available to archaeology until well into the 20th century.

Stratigraphy and the Law of Superposition

Stratigraphy is the oldest of the methods that archaeologists use to date things. Stratigraphy is based on the law of superposition--like a layer cake, the lowest layers must have been formed first.

That is, artifacts found in the upper layers of a site will have been deposited more recently than those found in the lower layers. Cross-dating of sites, when one compares geologic strata at one site with another location, and extrapolates relative ages in that manner is still used today, primarily when sites are far too old for absolute dates to have much meaning.

The scholar most associated with the rules of stratigraphy (or law of superposition) is probably the geologist Charles Lyell. The basis for stratigraphy is quite intuitive, but its applications were no less than earth-shattering to archaeological theory. For example, Worsaae used this law to prove the Three Age system.

Seriation

Seriation, on the other hand, was a stroke of genius. First used, and probably invented by the archaeologist Sir William Flinders-Petrie in 1899, seriation (or sequence dating) is based on the idea that artifacts change over time. Like fins on the back end of a Cadillac, artifact styles and characteristics change over time, coming into fashion, then fading in popularity.

Generally, seriation is manipulated graphically. The standard graphical result of seriation is a series of "battleship curves," which are horizontal bars representing percentages plotted on a vertical axis. Plotting several curves can allow the archaeologist to develop a relative chronology for an entire site or group of sites.

For detailed information about how seriation works, see Seriation: A Step by Step Description. Seriation is thought to be the first application of statistics in archaeology. It certainly wasn't the last.

The most famous seriation study was probably Deetz and Dethlefson's study on changing styles on gravestones in New England cemeteries. The method is still a standard for cemetery studies.

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Chronological Markers and Dendrochronology

Absolute dating, the ability to attach a specific chronological date to an object or collection of objects, was a breakthrough for archaeologists. Until the 20th century, with its multiple developments, only relative dates could be determined with any confidence. Since the turn of the century, several methods to measure elapsed time have been discovered.

Chronological Markers

The first and simplest method of absolute dating is using objects with dates inscribed on them, such as coins, or objects associated with historical events or documents. For example, since each Roman emperor had his own face stamped on coins during his realm, and dates for emporer's realms are known from historical records, the date a coin was minted may be discerned by identifying the emperor depicted. Many of the first efforts of archaeology grew out of historical documents--for example, Schliemann looked for Homer's Troy, and Layard went after the Biblical Ninevah--and within the context of a particular site, an object clearly associated with the site and stamped with a date or other identifying clue was perfectly useful.

But there are certainly drawbacks. Outside of the context of a single site or society, a coin's date is useless. And, outside of certain periods in our past, there simply were no chronologically dated objects, or the necessary depth and detail of history that would assist in chronologically dating civilizations. Without those, the archaeologists were in the dark as to the age of various societies. Until the invention of dendrochronology.

Dendrochronology

The use of tree ring data to determine chronological dates, dendrochronology, was first developed in the American southwest by astronomer Andrew Ellicott Douglass. In 1901, Douglass began investigating tree ring growth as an indicator of solar cycles. Douglass believed that solar flares affected climate, and hence the amount of growth a tree might gain in a given year. His research culminated in proving that tree ring width varies with annual rainfall. Not only that, it varies regionally, such that all trees within a specific species and region will show the same relative growth during wet years and dry years. Each tree then, contains a record of rainfall for the length of its life, expressed in density, trace element content, stable isotope composition, and intra-annual growth ring width.

Using local pine trees, Douglass built a 450 year record of the tree ring variability. Clark Wissler, an anthropologist researching Native American groups in the Southwest, recognized the potential for such dating, and brought Douglass subfossil wood from puebloan ruins.

Unfortunately, the wood from the pueblos did not fit into Douglass's record, and over the next 12 years, they searched in vain for a connecting ring pattern, building a second prehistoric sequence of 585 years.

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In 1929, they found a charred log near Show Low, Arizona, that connected the two patterns. It was now possible to assign a calendar date to archaeological sites in the American southwest for over 1000 years.

Determining calendar rates using dendrochronology is a matter of matching known patterns of light and dark rings to those recorded by Douglass and his successors. Dendrochronology has been extended in the American southwest to 322 BC, by adding increasingly older archaeological samples to the record. There are dendrochronological records for Europe and the Aegean, and the International Tree Ring Database has contributions from 21 different countries.

The main drawback to dendrochronology is its reliance on the existence of relatively long-lived vegetation with annual growth rings. Secondly, annual rainfall is a regional climatic event, and so tree ring dates for the southwest are of no use in other regions of the world.

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| Part 3: The Radiocarbon Revolution | Part 4: New Fangled Methods

It is certainly no exaggeration to call the invention of radiocarbon dating a revolution. It finally provided the first common chronometric scale which could be applied across the world. Invented in the latter years of the 1940s by Willard Libby and his students and colleagues James R. Arnold and Ernest C. Anderson, radiocarbon dating was an outgrowth of the Manhattan Project, and was developed at the University of Chicago Metallurgical Laboratory.

Although I am hardly a chemist or a physicist, and so will leave the detailed explanations to those who are better at it than I (for example, Anne Marie Helmenstine's page in About Chemistry), essentially radiocarbon dating uses the amount of carbon 14 available in living creatures as a measuring stick. All living things maintain a content of carbon 14 in equilibrium with that available in the atmosphere, right up to the moment of death. When an organism dies, the amount of C14 available within it begins to decay at a half life rate of 5730 years; i.e., it takes 5730 years for 1/2 of the C14 available in the organism to decay. Comparing the amount of C14 in a dead organism to available levels in the atmosphere, produces an estimate of when that organism died. So, for example, if a tree was used as a support for a structure, the date that tree stopped living (i.e., when it was cut down) can be used to date the building's construction date.

The organisms which can be used in radiocarbon dating include charcoal, wood, marine shell, human or animal bone, antler, peat; in fact, most of what contains carbon during its life cycle can be used, assuming it's preserved in the archaeological record. The farthest back C14 can be used is about 10 half lives, or 57,000 years; the most recent, relatively reliable dates end at the Industrial Revolution, when humankind busied itself messing up the natural quantities of carbon in the atmosphere. Further limitations, such as the prevalence of modern environmental contamination, require that several dates (called a suite) be taken on different associated samples to permit a range of estimated dates.

Calibration

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In the 50 or so years since Libby and his associates created the radiocarbon dating technique, refinements and calibrations have both improved the technique and revealed its weaknesses. Calibration of the dates may be completed by looking through tree ring data for a ring exhibiting the same amount of C14 as in a particular sample--thus providing a known date for the sample. Such investigations have identified wiggles in the data curve, such as at the end of the Archaic period in the United States, when atmospheric C14 fluctuated, adding further complexity to calibration.

One of the first modifications to C14 dating came about in the first decade after the Libby-Arnold-Anderson work at Chicago. One limitation of the original C14 dating method is that it measures the current radioactive emissions; Accelerator Mass Spectrometry dating counts the atoms themselves, allowing for sample sizes up to 1000 times smaller than conventional C14 samples.

While neither the first nor the last absolute dating methodology, C14 dating practices were clearly the most revolutionary, and some say helped to usher in a new scientific period to the field of archaeology.

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New Fangled Methods

Since the discovery of radiocarbon dating in 1949, science has leapt onto the concept of using atomic behavior to date objects, and a plethora of new methods was created. Here are descriptions of a few of the many new methods.

Potassium-Argon

The potassium-argon dating method, like radiocarbon dating, relies on measuring radioactive emissions. The Potassium-Argon method dates volcanic materials and is useful for sites dated between 50,000 and 2 billion years ago. It was first used at Olduvai Gorge. A recent modification is Argon-Argon dating, used recently at Pompeii.

Fission Track

Fission track dating was developed in the mid 1960s by three American physicists, who noticed that micrometer-sized damage tracks are created in minerals and glasses that have minimal amounts of uranium. These tracks accumulate at a fixed rate, and are good for dates between 20,000 and a couple of billion years ago. (This description is from the Geochronology unit at Rice University.) Fission-track dating was used at Zhoukoudian. A more sensitive type of fission track dating is called alpha-recoil. There is a newsletter for the fission track community called OnTrack.

Obsidian HydrationObsidian hydration uses the rate of rind growth on volcanic glass to determine dates; after a new fracture, a rind covering the new break grows at a constant rate. Dating limitations are physical ones; it takes

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several centuries for a detectable rind to be created, and rinds over 50 microns tend to crumble. The Obsidian Hydration Laboratory at the University of Auckland, New Zealand describes the method in some detail. Obsidian hydration is regularly used in Mesoamerican sites, such as Copan.

Thermoluminescence dating

Thermoluminescence (called TL) dating was invented around 1960 by physicists, and is based on the fact that electrons in all minerals emit light (luminesce) after being heated. It is good for between about 300 to about 100,000 years ago, and is a natural for dating ceramic vessels. TL dates have recently been the center of the controversy over dating the first human colonization of Australia. There are several other forms of luminescence dating as well, but they are not as frequently used to date as TL.

Archaeo- and Paleo-magnetism

Archaeomagnetic and paleomagnetic dating techniques rely on the fact that the earth's magnetic field varies over time. The original databanks were created by geologists interested in the movement of the planetary poles, and they were first used by archaeologists during the 1960s. Jeffrey Eighmy's Archaeometrics Laboratory at Colorado State provides details of the method and its specific use in the American southwest.

Oxidized Carbon Ratios

This newly developed method is a chemical procedure that uses a dynamical systems formula to establish the effects of the environmental context (systems theory), and was developed by Douglas Frink and the Archaeological Consulting Team. OCR has been used recently to date the construction of Watson Brake.

Racemization Dating

Racemization dating is a process which uses the measurement of the decay rate of carbon protein amino acids to date once-living organic tissue. All living organisms have protein; protein is made up of amino acids. All but one of these amino acids (glycine) has two different chiral forms (mirror images of each other). While an organism lives, their proteins are composed of only 'left-handed' (laevo, or L) amino acids, but once the organism dies the left-handed amino acids slowly turn into right-handed (dextro or D) amino acids. Once formed, the D amino acids themselves slowly turn back to L forms at the same rate. In brief, racemization dating uses the pace of this chemical reaction to estimate the length of time that has elapsed since an organism's death. For more details, see racemization dating

Racemization can be used to date objects between 5,000 and 1,000,000 years old, and was used recently to date the age of sediments at Pakefield, the earliest record of human occupation in northwest Europe. Xxx

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A Few Cautionary Notes about Dating

In this series, we've talked about the various methods archaeologists use to determine the dates of occupation of their sites. As you've read, there are several different methods of determining site chronology, and they each have their uses. One thing they all have in common, though, is they cannot stand alone.

Each method that we've discussed, and each of the methods we haven't discussed, may provide a faulty date for one reason or another.

Radiocarbon samples are easily contaminated by rodent burrowing or during collection.

Thermoluminescence dates may be thrown off by incidental heating long after the occupation has ended.

Site stratigraphies may be disturbed by earthquakes, or when human or animal excavation unrelated to the occupation disturbs the sediment.

Seriation, too, may be skewed for one reason or another. For example, in our sample we used the preponderance of 78 rpm records as an indicator of relative age of a junkyard. Say a Californian lost her entire 1930s jazz collection in the 1993 earthquake, and the broken pieces ended up in a landfill which opened in 1985. Heartbreak, yes; accurate dating of the landfill, no.

Dates derived from dendrochronology may be misleading if the occupants used relict wood to burn in their fires or construct their houses.

Obsidian hydration counts begin after a fresh break; the obtained dates may be incorrect if the artifact was broken after the occupation.

Even chronological markers may be deceptive. Collecting is a human trait; and finding a Roman coin a ranch style house which burned to the ground in Peoria, Illinois probably doesn't indicate the house was built during the rule of Caesar Augustus.

Resolving the Conflict with Context

So how do archaeologists resolve these issues? There are four ways: Context, context, context, and cross-dating. Since Michael Schiffer's work in the early 1970s, archaeologists have come to realize the critical significance of understanding site context. The study of site formation processes, understanding the processes that created the site as you see it today, has taught us some amazing things. As you can tell from the above chart, it is an extremely crucial aspect to our studies. But that's another feature.

Secondly, never rely on one dating methodology. If at all possible, the archaeologist will have several dates taken, and cross check them by using another form of dating. This may be simply comparing a suite

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of radiocarbon dates to the dates derived from collected artifacts, or using TL dates to confirm Potassium Argon readings.

I believe it is safe to say that the advent of absolute dating methods completely changed our profession, directing it away from the romantic contemplation of the classical past, and toward the scientific study of human behaviors. Not everybody is happy with this change, but it is a change towards science nonetheless.

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Carbon 14 Dating of Organic Material

Worked Chemistry Problems

By Anne Marie Helmenstine, Ph.D., About.com

See More About:

carbon dating radioactive decay isotopes worked chemistry problems

Introduction

In the 1950s W.F. Libby and others (University of Chicago) devised a method of estimating the age of organic material based on the decay rate of carbon-14. Carbon-14 dating can be used on objects ranging from a few hundred years old to 50,000 years old.

Carbon-14 is produced in the atmosphere when neutrons from cosmic radiation react with nitrogen atoms:

147N + 1

0n --> 146C + 1

1H

Free carbon, including the carbon-14 produced in this reaction, can react to form carbon dioxide, a component of air. Atmospheric carbon dioxide, CO2, has a steady-state concentration of about one atom of carbon-14 per every 1012 atoms of carbon-12. Living plants and animals that eat plants (like people) take in carbon dioxide and have the same 14C/12C ratio as the atmosphere.

However, when a plant or animal dies, it stops intaking carbon as food or air. The radioactive decay of the carbon that is already present starts to change the ratio of 14C/12C. By measuring how much the ratio is lowered, it is possible to make an estimate of how much time has passed since the plant or animal lived. The decay of carbon-14 is:

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146C --> 14

7N + 0-1e (half-life is 5720 years)

Example Problem

A scrap of paper taken from the Dead Sea Scrolls was found to have a 14C/12C ratio of 0.795 times that found in plants living today. Estimate the age of the scroll.

Solution

The half-life of carbon-14 is known to be 5720 years. Radioactive decay is a first order rate process, which means the reaction proceeds according to the following equation:

log10 X0/X = kt / 2.30

where X0 is the quantity of radioactive material at time zero, X is the amount remaining after time t, and k is the first order rate constant, which is a characteristic of the isotope undergoing decay. Decay rates are usually expressed in terms of their half-life instead of the first order rate constant, where

k = 0.693 / t1/2

so for this problem:

k = 0.693 / 5720 years = 1.21 x 10-4/year

log X0 / X = [(1.21 x 10-4/year] x t] / 2.30

X = 0.795 X0, so log X0 / X = log 1.000/0.795 = log 1.26 = 0.100

therefore, 0.100 = [(1.21 x 10-4/year) x t] / 2.30

t = 1900 years

Recent Chemistry Features

More Worked Problems

Worked Chemistry Problem Index Atomic Mass & Isotope Abundance Isotopes & Nuclear Symbols

New posts to the Chemistry forums:

Salt Water ? Electronic configuration of nickel.

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Suggested Reading

Carbon Facts Half Life Definition Isotope Definition

Related Articles

Rate of Radioactive Decay - Worked Chemistry Problems Radiocarbon Dating Method

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http://www.c14dating.com/index.html

Radio Carbon DatingINDEX | Introduction | Measurement | Applications | WWW Links | k12 | Publication | Corrections | Age calculation | Calibration | Pretreatment | References |

INTRODUCTION

http://www.c14dating.com/int.html

BY: THOMAS HIGHAM"Everything which has come down to us from heathendom is wrapped in a thick fog; it belongs to a space of time we cannot measure. We know that it is older than Christendom, but whether by a couple of years or a couple of centuries, or even by more than a millenium, we can do no more than guess." [Rasmus Nyerup, (Danish antiquarian), 1802 (in Trigger, 1989:71)].

Nyerup's words illustrate poignantly the critical power and importance of dating; to order time. Radiocarbon dating has been one of the most significant discoveries in 20th century science. Renfrew (1973) called it 'the radiocarbon revolution' in describing its impact upon the human sciences. Oakley (1979) suggested its development meant an almost complete re-writing of the evolution and cultural emergence of the human species. Desmond Clark (1979) wrote that were it not for radiocarbon dating, "we would still be foundering in a sea of imprecisions sometime bred of inspired guesswork but more often of imaginative speculation" (Clark, 1979:7). Writing of the European Upper Palaeolithic, Movius (1960) concluded that "time alone is the lens that can throw it into focus".

The radiocarbon method was developed by a team of scientists led by the late Professor Willard F. Libby of the University of Chicago in immediate post-WW2 years. Libby later received the Nobel Prize in Chemistry in 1960:

"for his method to use Carbon-14 for age determinations in archaeology, geology, geophysics, and other branches of science."

According to one of the scientists who nominated Libby as a candidate for this honour;

"Seldom has a single discovery in chemistry had such an impact on the thinking of so many fields of human endeavour. Seldom has a single discovery generated such wide public interest."

Today, there are over 130 radiocarbon dating laboratories around the world producing radiocarbon assays for the scientific community.

The C14 technique has been and continues to be applied and used in many, many different fields including hydrology, atmospheric science, oceanography, geology, palaeoclimatology, archaeology and biomedicine.

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The 14C Method

There are three principal isotopes of carbon which occur naturally - C12, C13 (both stable) and C14 (unstable or radioactive). These isotopes are present in the following amounts C12 - 98.89%, C13 - 1.11% and C14 - 0.00000000010%. Thus, one carbon 14 atom exists in nature for every 1,000,000,000,000 C12 atoms in living material. The radiocarbon method is based on the rate of decay of the radioactive or unstable carbon isotope 14 (14C), which is formed in the upper atmosphere through the effect of cosmic ray neutrons upon nitrogen 14. The reaction is:

14N + n => 14C + p

(Where n is a neutron and p is a proton). The 14C formed is rapidly oxidised to 14CO2 and enters the earth's plant and animal lifeways through photosynthesis and the food chain. The rapidity of the dispersal of C14 into the atmosphere has been demonstrated by measurements of radioactive carbon produced from thermonuclear bomb testing. 14C also enters the Earth's oceans in an atmospheric exchange and as dissolved carbonate (the entire 14C inventory is termed the carbon exchange reservoir (Aitken, 1990)).

Plants and animals which utilise carbon in biological foodchains take up 14C during their lifetimes. They exist in equilibrium with the C14 concentration of the atmosphere, that is, the numbers of C14 atoms and non-radioactive carbon atoms stays approximately the same over time. As soon as a plant or animal dies, they cease the metabolic function of carbon uptake; there is no replenishment of radioactive carbon, only decay. There is a useful diagrammatic representation of this process given here

Libby, Anderson and Arnold (1949) were the first to measure the rate of this decay. They found that after 5568 years, half the C14 in the original sample will have decayed and after another 5568 years, half of that remaining material will have decayed, and so on (see figure 1 below). The half-life (t 1/2) is the name given to this value which Libby measured at 5568±30 years. This became known as the Libby half-life.

After 10 half-lives, there is a very small amount of radioactive carbon present in a sample. At about 50 - 60 000 years, then, the limit of the technique is reached (beyond this time, other radiometric techniques must be used for dating). By measuring the C14 concentration or residual radioactivity of a sample whose age is not known, it is possible to obtain the countrate or number of decay events per gram of Carbon. By comparing this with modern levels of activity (1890 wood corrected for decay to 1950 AD) and using the measured half-life it becomes possible to calculate a date for the death of the sample.

As 14C decays it emits a weak beta particle (b ), or electron, which possesses an average energy of 160keV. The decay can be shown:

14C => 14N + b

Thus, the 14C decays back to 14N. There is a quantitative relationship between the decay of 14C and the production of a beta particle. The decay is constant but spontaneous. That is, the probability of decay for an atom of 14C in a discrete sample is constant, thereby requiring the application of statistical methods for the analysis of counting data.

It follows from this that any material which is composed of carbon may be dated.Herein lies the true advantage of the radiocarbon method, it is able to be uniformly applied throughout the world. Included

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below is an impressive list of some of the types of carbonaceous samples that have been commonly radiocarbon dated in the years since the inception of the method:

Charcoal, wood, twigs and seeds. Bone. Marine, estuarine and riverine shell. Leather. Peat Coprolites. Lake muds (gyttja) and sediments. Soil. Ice cores. Pollen. Hair. Pottery. Metal casting ores. Wall paintings and rock art works. Iron and meteorites. Avian eggshell. Corals and foraminifera. Speleothems. Tufa. Blood residues. Textiles and fabrics. Paper and parchment. Fish remains. Insect remains. Resins and glues. Antler and horn. Water.

The historical perspective on the development of radiocarbon dating is well outlined in Taylor's (1987) book "Radiocarbon Dating: An archaeological perspective". Libby and his team intially tested the radiocarbon method on samples from prehistoric Egypt. They chose samples whose age could be independently determined. A sample of acacia wood from the tomb of the pharoah Zoser (or Djoser; 3rd Dynasty, ca. 2700-2600 BC) was obtained and dated. Libby reasoned that since the half-life of C14 was 5568 years, they should obtain a C14 concentration of about 50% that which was found in living wood (see Libby, 1949 for further details).

The results they obtained indicated this was the case. Other analyses were conducted on samples of known age wood (dendrochronologically aged). Again, the fit was within the value predicted at ±10%. The tests suggested that the half-life they had measured was accurate, and, quite reasonably, suggested further that atmospheric radiocarbon concentration had remained constant throughout the recent past. In 1949, Arnold and Libby (1949) published their paper "Age determinations by radiocarbon content: Checks with samples of known age" in the journal Science. In this paper they presented the first results of the C14 method, including the "Curve of Knowns" in which radiocarbon dates were compared with the known age historical dates (see figure 1). All of the points fitted within statistical range. Within a few

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years, other laboratories had been built. By the early 1950's there were 8, and by the end of the decade there were more than 20.

Figure 1: The "Curve of Knowns" after Libby and Arnold (1949). The first acid test of the new method was based upon radiocarbon dating of known age samples primarily from Egypt (the dates are shown in the diagram by the red lines, each with a ±1 standard deviation included). The Egyptian King's name is given next to the date obtained. The theoretical curve was constructed using the half-life of 5568 years. The activity ratio relates to the carbon 14 activity ratio between the ancient samples and the modern activity. Each result was within the statistical range of the true historic date of each sample.

In the 1950s, further measurements on Mediterranean samples, in particular those from Egypt whose age was known through other means, pointed to radiocarbon dates which were younger than expected. The debate regarding this is outlined extensively in Renfrew (1972). Briefly, opinion was divided between those who thought the radiocarbon dates were correct (ie, that radiocarbon years equated more or less to solar or calendar years) and those who felt they were flawed and the historical data was more accurate. In the late 1950's and early 1960's, researchers measuring the radioactivity of known age tree rings found fluctuations in C14 concentration up to a maximum of ±5% over the last 1500 years.

In addition to long term fluctuations, smaller 'wiggles' were identified by the Dutch scholar Hessel de Vries (1958). This suggested there were temporal fluctuations in C14 concentration which would neccessitate the calibration of radiocarbon dates to other historically aged material. Radiocarbon dates of sequential dendrochronologically aged trees primarily of US bristlecone pine and German and Irish oak have been measured over the past 10 years to produce a calendrical / radiocarbon calibration curve which now extends back over 10 000 years (more on Calibration). This enables radiocarbon dates to be calibrated to solar or calendar dates.

Later measurements of the Libby half-life indicated the figure was ca. 3% too low and a more accurate half-life was 5730±40 years. This is known as the Cambridge half-life. (To convert a "Libby" age to an age using the Cambridge half-life, one must multiply by 1.03).

The major developments in the radiocarbon method up to the present day involve improvements in measurement techniques and research into the dating of different materials. Briefly, the initial solid carbon method developed by Libby and his collaborators was replaced with the Gas counting method in the 1950's. Liquid scintillation counting, utilising benzene, acetylene, ethanol, methanol etc, was developed at about the same time. Today the vast majority of radiocarbon laboratories utilise these two methods of radiocarbon dating. Of major recent interest is the development of the Accelerator Mass Spectrometry method of direct C14 isotope counting. In 1977, the first AMS measurements were

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conducted by teams at Rochester/Toronto and the General Ionex Corporation and soon after at the Universities of Simon Fraser and McMaster (Gove, 1994). The crucial advantage of the AMS method is that milligram sized samples are required for dating. Of great public interest has been the AMS dating of carbonacous material from prehistoric rock art sites, the Shroud of Turin and the Dead Sea Scrolls in the last few years.

The development of high-precision dating (up to ±2.0 per mille or ±16 yr) in a number of gas and liquid scintillation facilities has been of similar importance (laboratories at Belfast (N.Ireland), Seattle (US), Heidelberg (Ger), Pretoria (S.Africa), Groningen (Netherlands), La Jolla (US), Waikato (NZ) and Arizona (US) are generally accepted to have demonstrated radiocarbon measurements at high levels of precision).

The calibration research undertaken primarily at the Belfast and Seattle labs required that high levels of precision be obtained which has now resulted in the extensive calibration data now available. The development of small sample capabilities for LSC and Gas labs has likewise been an important development - samples as small as 100 mg are able to be dated to moderate precision on minigas counters (Kromer, 1994) with similar sample sizes needed using minivial technology in Liquid Scintillation Counting. The radiocarbon dating method remains arguably the most dependable and widely applied dating technique for the late Pleistocene and Holocene periods.

XXX

MEASUREMENT

There are three principal methods of measuring residual C14 activity.

Gas Proportional Counting or GPC [This site is still under construction.] Liquid Scintillation Counting or LSC. Accelerator Mass Spectrometry or AMS.

Liquid Scintillation Counting (LSC)

Introduction

The method of 14C counting used by Libby and his co-workers involved measuring radioactivity using modified Geiger counters. The next development in counting technology was the conversion of sample carbon to CO2 gas for measurement in Gas Proportional counters. In the early 1950's, the first attempts were made to detect 14C by the Liquid Scintillation (LS) counting method. In the 1940's, Broser and Kallman (1947) discovered that certain organic compounds (scintillators) fluoresced when exposed to ionising radiation. Each fluorescence event is proportional to a radioactive decay event, and the frequency of these events is directly proportional to the number of 14C atoms present in the sample.

A brief summary of the LS method and some of the problems associated with the technique are given here. A more detailed report can be found in Polach (1987). Discussion on the LS method can be broken down into a number of topics : counting liquid, counting vials, and the LSC method, including counting instrumentation and potential problems associated with measurement of beta (ß)-decay by LS counting.

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Counting Liquid

In the majority of LSC facilities, the scintillation solvent is benzene (C6H6) or a mixture of benzene and toluene (C6H6CH3). Benzene has been chosen because of its excellent light transmission properties and the high chemical conversion yield of sample C to benzene. The sample is first converted to CO2, then reacted with molten lithium to form lithium carbide (Li2C2), before being catalytically trimerised to benzene. Benzene synthesis procedures vary considerably in different laboratories. The details outlined below describe the procedures used in the Waikato laboratory to give the reader some idea of how the chemical conversions may be carried out.

The carbon is first oxidised to CO2, either by acid hydrolysis (for carbonates), or combustion in an oxygen stream or combustion bomb (for organic materials). The combustion gases are passed over heated CuO to complete the oxidation of CO, NO, N2O and NO2 and also of tar substances. The CO2 may then be then purified using a chain of wet chemical reagents; for example, AgNO3 and Hg(NO3)2 to precipitate halogens, and the oxidising agents KI/I2 and K2Cr2O7 to remove nitrogen and sulphur compounds. Silica gel and dry ice traps (-80°C) remove any water remaining in the gas (see figure 1).

The purified CO2 is then reacted with molten lithium in a stainless steel or inconel reaction vessel in vacuo:

2CO2 + 10Li => Li2C2 + 4Li2O

This reaction was first described by Barker (1953), who developed the reaction using lithium instead of barium, and later improved by Polach and Stipp (1967). The CO2 is bled slowly onto the molten Li where it is converted to Li2C2. The carbide is heated to ca. 800°C (furnace temperature) and placed under active vacuum for 30 minutes to remove any unreacted gases and complete the carbide synthesis (Gupta and Polach, 1985). The lithium carbide is cooled and then hydrolysed to acetylene gas (Li2C2):

Li2C2 + 2H2O => C2H2 + 2LiOH

The acetylene is purified by passing through a phosphoric acid trap to remove ammonia compounds, and again, dry ice traps to remove water vapour. Finally the acetylene is trimerised to benzene using a suitable catalyst. There are a variety of vanadium or chromium activated catalysts available, including a silica-alumina vanadium activated catalyst developed by John Noakes (CAIS, Univ. of Georgia), a vanadium-alumina-silica catalyst produced at the Institute of Geography, Univ. of Petersburg by Dr Kh. A. Arslanov and another at the British Museum by Dr J. Ambers. The catalytic trimerisation for some catalysts may be more efficient at reduced temperatures (e.g. 5°C).

3C2H2 => C6H6

Benzene is then driven off the catalyst at ca. 100°C and collected under vacuum at ca. -65°C. The benzene is then stored in a vial under refrigeration to await counting.

This sequence of reactions requires a high degree of operator skill because of the complexity of the equipment and the nature of the reactions. It is important that a standardised routine is followed carefully and consistently, so that yields remain high and there is little cross-contamination between samples.

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Counting vials

The synthesised benzene is transferred into counting vials of a variety of types. The counting vials contain the sample solvent and the scintillator. Commercially available LS vials used for benzene counting are commonly composed of either Teflon, quartz or low-K glass (e.g. Polach et al.1983; Haas 1979; Devine & Haas 1987; Noakes & Valenta 1989; Hogg & Noakes, 1992; Hogg 1993). Polyethylene and polypropylene vials are very useful for non-aromatic solvents, but their permeability makes them unsuitable for repeatable long-term benzene use.

In a comparison of different vial types, Hogg et al . (1991) concluded that, for spectrometers using true anticoincidence detection and extensive passive shielding (as in the Perkin Elmer 1220 Quantulus), Teflon or synthetic silica vials produced the highest performance in terms of 14C detection efficiency and background. The authors also noted that synthetic silica had superior physical properties to Teflon. The silica vials vary in size according to the weight of the sample benzene being counted, including 0.3 ml and 1 ml minivials, 3 ml standard vials and 12 ml vials, suitable for high precision applications. Low-K glass vials are supplied in either 7 ml or 20 ml sizes.

The most commonly used scintillator in 14C dating is PPO + POPOP, either dissolved in toluene or directly in sample benzene. Polach et al (1983) made a comparative study of various scintillators and concluded that dry powder butyl-PBD dissolved in sample benzene (15g/l) gave superior and stable performance even under extreme quench conditions. Some Packard spectrometers utilise an active plastic holder (Pico adaptor) which helps to reduce background radiation. These instruments require a secondary wave shifter (usually bis-MSB, see Cook, Harkness & Anderson 1989) to be added to the butyl-PBD. In the Waikato laboratory, the scintillator (butyl-PBD) is dissolved directly into the sample benzene at a concentration of 15g/l. The vial is then transferred into a Quantulus spectrometer and allowed to cool and dark adapt for a minimum of 8 hours prior to the commencement of counting.

Liquid Scintillation Spectrometry

The essential electronic components of the Liquid Scintillation (LS) Spectrometer according to Gupta and Polach (1985), are photo multiplier tubes (PMTs), high voltage supply, signal preamplifiers, pulse and summing amplifiers, coincidence logic, timer and scaler (Gupta and Polach, 1985:50). The LS spectrometer measures electronically the pulses of light generated from photon emissions emitted by a scintillator in response to a radioactive decay event. The PMTs register an electronic pulse proportional to the energy of a particular beta decay when a photon or light particle is emitted within the benzene cocktail. The anode current generated from a PMT, then, is a function of the level of radioactivity (Horrocks, 1974).

External factors interfering with ß-decay detection by LSC

Optical cross talk

Optical cross-talk occurs between opposed PMTs. An event occurring in one tube initiates a pulse in the other (Noakes, 1977). Ionising events may occur in either PMT due to interactions between the molecules of the tube material and surrounding radiation. Butterfield and Polach (1983) and Gupta and Polach (1985) have described methods used to reduce optical cross talk in LS counters, both by optimising and refining electronic circuitry to enable differentiation between sample and non-sample events and by masking areas in the counting chamber to reduce reflection. Carefully designed vials and vial holders can also help to reduce cross-talk, by minimising the view of opposing PMTs to each other.

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Line transmission noise

Line transmitted switching noises and variation in the high voltage supply can influence reproducibility in LS counting. Power supplies are therefore frequently filtered and regulated, either with line noise suppression electronic transformers or a suitable uninterruptable power supply (UPS) unit. A clean and stable voltage supply is crucial because of the relationship between electronic particle detection and the energy of the decay event.

Radiofrequency interference

Radiofrequency interference is generated by lights, motors and switches in the vicinity of the counter. RF detection circuitry can be fitted which will identify RF signals and ensure they are removed from the sample 14C spectrum.

Static induced noise

Static induced noise results from the build up of static electricity through friction generated from the movement of vials throughout the counting system. Earthing of vials and holders and an ionising unit which the vials move past, can be used to remove static electricity prior to counting.

Radon contamination

222Rn emits alpha and beta particles, the latter with a decay energy of 5.587 meV. This decay interferes with the 14C spectrum, resulting in higher count rates and radiocarbon ages that are too young. Radon's short half-life of 3.82 days, however, means that a delay in counting eliminates the problem. Sample benzene is therefore often left for 3 - 4 weeks to allow any radon that may be present to decay. Potential 222Rn contamination is more significant in older samples because of their lower count rates.

Natural radioactivity and cosmic radiation

Natural atmospheric radiation influences sample count rates. This is reduced through shielding of the counting environment. This may be passive or active shielding. The passive shield consists of lead which shields the counting chamber from external radiation. The term 'active shielding' describes electronic means of recognising and eliminating background causing events from the sample beta spectrum. Active shielding occurs in two forms in modern LS counters.

The Perkin Elmer Quantulus, utilises an electronic anti-coincidence guard, comprising a liquid scintillator guard containing it's own phototubes, surrounding the counting chamber and sample PMTs. Ionising radiation from non-sample external sources passing through the guard scintillator leaves excited atoms and molecules in it which are seen by the guard PMTs. If an ionising event is recorded simultaneously in the guard and sample tubes, it is rejected as a background count. Some Packard counters utilise 'time resolved counting' to differentiate between, sample events with a low pulse index (see Roessler et al, 1991 for details), and background events with a higher pulse index.

Most higher performance counters use multi-channel analysers (MCAs) to store and evaluate counting data. The MCAs are software-controlled to provide complete flexibility in data acquisition.

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LSC Dating by the Perkin Elmer "Quantulus"

SummaryOne of the major advantages of LSC is that sample, background and modern vials are able to be rotated into the counting chamber by the operator. This is termed "quasi-simultaneous counting" and was first implemented by Noakes, Kim and Stipp (1965). Major developments in LSC in the last decade include the optimisation of spectrometers, incorporating sophisticated computer and electronic hardware which enables the user to validate counting data with ease.

In addition, background count rates have been significantly reduced through the addition of active and passive forms of shielding, including lead and through the placement of LS spectrometers underground. A number of researchers have participated in the development of 'minivial' technology which has enabled samples of <1 g carbon to be dated through the LSC method.

One of the most widely known applications of LSC of 14C is in high precision dating of the radiocarbon timescale for the purposes of calibration. This was conducted primarily at the Belfast Radiocarbon Dating Laboratory (an LSC facility) and the Quaternary Dating Centre at the University of Washington (Gas Proportional Counting). The following LSC laboratories present information on the technique and its application:

University of Waikato Radiocarbon Dating Laboratory (New Zealand). Beta Analytic Radiocarbon Dating Laboratory (USA) Queen's University of Belfast - Radiocarbon Laboratory (UK)

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http://www.rlaha.ox.ac.uk/

Dating o Radiocarbon dating o Luminescence dating o Tephrochronology and Quaternary Geochronology

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Radiocarbon dating

From Wikipedia, the free encyclopedia

Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 (14C) to determine the age of carbonaceous materials up to about 60,000 years.[1] Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" (BP), "Present" being defined as AD 1950. Such raw ages can be calibrated to give calendar dates.

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One of the most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites. When plants fix atmospheric carbon dioxide (CO2) into organic material during photosynthesis they incorporate a quantity of 14C that approximately matches the level of this isotope in the atmosphere (a small difference occurs because of isotope fractionation, but this is corrected after laboratory analysis). After plants die or they are consumed by other organisms (for example, by humans or other animals) the 14C fraction of this organic material declines at a fixed exponential rate due to the radioactive decay of 14C. Comparing the remaining 14C fraction of a sample to that expected from atmospheric 14C allows the age of the sample to be estimated.

The technique of radiocarbon dating was developed by Willard Libby and his colleagues at the University of Chicago in 1949.[2] Libby estimated that the steady state radioactivity concentration of exchangeable carbon-14 would be about 14 disintegrations per minute (dpm) per gram. In 1960, he was awarded the Nobel Prize in chemistry for this work. He first demonstrated the accuracy of radiocarbon dating by accurately measuring the age of wood from an ancient Egyptian royal barge whose age was known from historical documents.[2]

Contents

1 Basic physics 2 Computation of ages and dates 3 Measurements and scales

o 3.1 Calibration 3.1.1 The need for calibration 3.1.2 Calibration methods

4 Radiocarbon half-life o 4.1 Libby vs Cambridge values

5 Carbon exchange reservoir 6 Speleothem studies extend 14 C calibration 7 Examples 8 See also 9 Notes 10 References 11 External links

Basic physicsAtmospheric 14C, New Zealand [3] and Austria.[4] The New Zealand curve is representative for the Southern Hemisphere, the Austrian curve is representative for the Northern Hemisphere. Atmospheric nuclear weapon tests almost doubled the concentration of 14C in the Northern Hemisphere.[5]

Carbon has two stable, nonradioactive isotopes: carbon-12 (12C), and carbon-13 (13C). In addition, there are trace amounts of the unstable isotope carbon-14 (14C) on Earth. Carbon-14 has a half-life of 5730 years and would have long ago vanished from Earth were it not for the unremitting cosmic ray impacts on nitrogen in the Earth's atmosphere, which create more of the

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isotope. The neutrons resulting from the cosmic ray interactions participate in the following nuclear reaction on the atoms of nitrogen molecules (N2) in the atmospheric air:

The highest rate of carbon-14 production takes place at altitudes of 9 to 15 km (30,000 to 50,000 ft), and at high geomagnetic latitudes, but the carbon-14 spreads evenly throughout the atmosphere and reacts with oxygen to form carbon dioxide. Carbon dioxide also permeates the oceans, dissolving in the water. For approximate analysis it is assumed that the cosmic ray flux is constant over long periods of time; thus carbon-14 is produced at a constant rate and the proportion of radioactive to non-radioactive carbon is constant: ca. 1 part per trillion (600 billion atoms/mole). In 1958 Hessel de Vries showed that the concentration of carbon-14 in the atmosphere varies with time and locality. For the most accurate work, these variations are compensated by means of calibration curves. When these curves are used, their accuracy and shape are the factors that determine the accuracy and age obtained for a given sample.

Plants take up atmospheric carbon dioxide by photosynthesis, and are ingested by animals, so every living thing is constantly exchanging carbon-14 with its environment as long as it lives. Once it dies, however, this exchange stops, and the amount of carbon-14 gradually decreases through radioactive beta decay.

Computation of ages and dates

The radioactive decay of carbon-14 follows an exponential decay. A quantity is said to be subject to exponential decay if it decreases at a rate proportional to its value. Symbolically, this can be expressed as the following differential equation, where N is the quantity and λ is a positive number called the decay constant:

The solution to this equation is:

,

where, for a given sample of carbonaceous matter:

N0 = number of radiocarbon atoms at t = 0, i.e. the origin of the disintegration time,

N = number of radiocarbon atoms remaining after radioactive decay during the time t,

λ = radiocarbon decay or disintegration constant.

Two related times can be defined:

mean- or average-life: mean or average time each radiocarbon atom spends in a given sample until it decays.

half-life: time lapsed for half the number of radiocarbon atoms in a given sample, to decay,

It can be shown that:

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= = radiocarbon mean- or average-life = 8033 years (Libby value)

= = radiocarbon half-life = 5568 years (Libby value)

Notice that dates are customarily given in years BP which implies t(BP) = -t because the time arrow for dates runs in reverse direction from the time arrow for the corresponding ages. From these considerations and the above equation, it results:

For a raw radiocarbon date:

and for a raw radiocarbon age:

After replacing values, the raw radiocarbon age becomes any of the following equivalent formulae:

using logs base e and the average life:

and

using logs base 2 and the half-life:

Measurements and scales

Measurements are traditionally made by counting the radioactive decay of individual carbon atoms by gas proportional counting or by liquid scintillation counting. For samples of sufficient size (several grams of carbon) this method is still widely used in the 2000s. Among others, all the tree ring samples used for the calibration curves (see below) were determined by these counting techniques. Such decay counting, however, is relatively insensitive and subject to large statistical uncertainties for small samples. When there is little carbon-14 to begin with, the long radiocarbon half-life means that very few of the carbon-14 atoms will decay during the time allotted for their detection, resulting in few disintegrations per minute.

The sensitivity of the method has been greatly increased by the use of Accelerator Mass Spectrometry (AMS). With this technique 14C atoms can be detected and counted directly vs only detecting those atoms that decay during the time interval allotted for an analysis. AMS allows dating samples containing only a few milligrams of carbon.

Raw radiocarbon ages (i.e., those not calibrated) are usually reported in "years Before Present" (BP). This is the number of radiocarbon years before 1950, based on a nominal (and assumed constant - see "calibration" below) level of carbon-14 in the atmosphere equal to the 1950 level. These raw dates are also based on a slightly-off historic value for the radiocarbon half-life. Such value is used for consistency with earlier published dates (see "Radiocarbon half-life" below). See the section on computation for the basis of the calculations.

Radiocarbon dating laboratories generally report an uncertainty for each date. For example, 3000±30BP indicates a standard deviation of 30 radiocarbon years. Traditionally this included

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only the statistical counting uncertainty. However, some laboratories supplied an "error multiplier" that could be multiplied by the uncertainty to account for other sources of error in the measuring process. More recently, the laboratories try to quote the overall uncertainty, which is determined from control samples of known age and verified by international intercomparison exercises [6]. In 2008, a typical uncertainty better than ±40 radiocarbon years can be expected for samples younger than 10,000 years. This, however, is only a small part of the uncertainty of the final age determination (see section Calibration below).

As of 2007, the limiting age for a 1 milligram sample of graphite is about ten half-lives, approximately 60,000 years[7]. This age is derived from that of the calibration blanks used in an analysis, whose 14C content is assumed to be the result of contamination during processing (as a result of this, some facilities[7] will not report an age greater than 60,000 years for any sample).

A variety of sample processing and instrument-based constraints have been postulated to explain the upper age-limit. To examine instrument-based background activities in the AMS instrument of the W. M. Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory of the University of California, a set of natural diamonds were dated. Natural diamond samples from different sources within rock formations with standard geological ages in excess of 100 my yielded 14C apparent ages 64,920±430 BP to 80,000±1100 BP as reported in 2007[8].

Calibration

The need for calibrationCalibration curve for the radiocarbon dating scale. Data sources: Stuiver et al. (1998)[9]. Samples with a real date more recent than AD 1950 are dated and/or tracked using the N- & S-Hemisphere graphs. See preceding figure.

A raw BP date cannot be used directly as a calendar date, because the level of atmospheric 14C has not been strictly constant during the span of time that can be radiocarbon dated. The level is affected by variations in the cosmic ray intensity which is in turn affected by variations in the earth's magnetosphere. In addition, there are substantial reservoirs of carbon in organic matter, the ocean, ocean sediments (see methane hydrate), and sedimentary rocks. Changes in the Earth's climate can affect the carbon flows between these reservoirs and the atmosphere, leading to changes in the atmosphere's 14C fraction.

Aside from these changes due to natural processes, the level has also been affected by human activities. From the beginning of the industrial revolution in the 18th century to the 1950s, the fractional level of 14C decreased because of the admixture of large quantities of CO2 into the atmosphere, the combustion production of fossil fuel. This decline is known as the Suess effect, and also affects the 13C isotope. However, atmospheric 14C was almost doubled for a short period during the 1950s and 1960s due to atomic bomb tests.

Calibration methods

The raw radiocarbon dates, in BP years, are calibrated to give calendar dates. Standard calibration curves are available, based on comparison of radiocarbon dates of samples that can be

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dated independently by other methods such as examination of tree growth rings (dendrochronology), deep ocean sediment cores, lake sediment varves, coral samples, and speleothems (cave deposits).

The calibration curves can vary significantly from a straight line, so comparison of uncalibrated radiocarbon dates (e.g., plotting them on a graph or subtracting dates to give elapsed time) is likely to give misleading results. There are also significant plateaus in the curves, such as the one from 11,000 to 10,000 radiocarbon years BP, which is believed to be associated with changing ocean circulation during the Younger Dryas period. Over the historical period from 0 to 10,000 years BP, the average width of the uncertainty of calibrated dates was found to be 335 years, although in well-behaved regions of the calibration curve the width decreased to about 113 years while in ill-behaved regions it increased to a maximum of 801 years. Significantly, in the ill-behaved regions of the calibration curve, increasing the precision of the measurements does not have a significant effect on increasing the accuracy of the dates.[10]

The 2004 version of the calibration curve extends back quite accurately to 26,000 years BP. Any errors in the calibration curve do not contribute more than ±16 years to the measurement error during the historic and late prehistoric periods (0 - 6,000 yrs BP) and no more than ±163 years over the entire 26,000 years of the curve, although its shape can reduce the accuracy as mentioned above.[11]

Radiocarbon half-life

Libby vs Cambridge values

Carbon dating was developed by a team led by Willard Libby. He worked out a carbon-14 half-life of 5568±30 years, the Libby half-life. Later a more accurate figure of 5730±40 years was determined, which is known as the Cambridge half-life. This is, however, not relevant for radiocarbon dating. If calibration is applied, the half-life cancels out, as long as the same value is used throughout the calculations. Laboratories continue to use the Libby figure to avoid inconsistencies with previous publications.

Carbon exchange reservoir

Libby's original exchange reservoir hypothesis assumes that the exchange reservoir is constant all over the world. The calibration method also assumes that the temporal variation in 14C level is global, such that a small number of samples from a specific year are sufficient for calibration.[12] However, since Libby's early work was published (1950 to 1958), latitudinal and continental variations in the carbon exchange reservoir have been observed by Hessel de Vries (1958; as reviewed by Lerman et al., 1959, 1960). Subsequently, methods have been developed that allow the correction of these so-called reservoir effects, including:

When CO2 is transferred from the atmosphere to the oceans, it initially shares the 14C concentration of the atmosphere. However, turnaround times of CO2 in the ocean are similar to the half-life of 14C (making 14C also a dating tool for ocean water[13]. Marine organisms feed on this "old" carbon, and thus their radiocarbon age reflects the time of CO2 uptake by the ocean

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rather than the time of death of the organism. This marine reservoir effect is partly handled by a special marine calibration curve [14], but local deviation of several 100 years exist.

Erosion and immersion of carbonate rocks (which are generally older than 80,000 years and so shouldn't contain measurable 14C) causes an increase in 12C and 13C in the exchange reservoir, which depends on local weather conditions and can vary the ratio of carbon that living organisms incorporate. This is believed negligible for the atmosphere and atmosphere-derived carbon since most erosion will flow into the sea.[15] The atmospheric 14C concentration may differ substantially from the concentration in local water reservoirs. Eroded from CaCO3 or organic deposits, old carbon may be assimilated easily and provide diluted 14C carbon into trophic chains. So the method is less reliable for such materials as well as for samples derived from animals with such plants in their food chain.

Volcanic eruptions eject large amount of carbonate into the air, causing an increase in 12C and 13C in the exchange reservoir and can vary the exchange ratio locally. This explains the often irregular dating achieved in volcanic areas.[15]

The earth is not affected evenly by cosmic radiation, the magnitude of the radiation depends on land altitude and earth's magnetic field strength at any given location, causing minor variation in the local 14C production. This is accounted for by having calibration curves for different locations of the globe. However this could not always be performed, as tree rings for calibration were only recoverable from certain locations in 1958.[16] The rebuttals by Münnich et al.[17] and by Barker[18] both maintain that while variations of carbon-14 exist, they are about an order of magnitude smaller than those implied by Crowe's calculations.

These effects were first confirmed when samples of wood from around the world, which all had the same age (based on tree ring analysis), showed deviations from the dendrochronological age. Calibration techniques based on tree-ring samples have contributed to increase the accuracy since 1962, when they were accurate to 700 years at worst.[19]

Speleothem studies extend 14C calibration

Relatively recent (2001) evidence has allowed scientists to refine the knowledge of one of the underlying assumptions. A peak in the amount of carbon-14 was discovered by scientists studying speleothems in caves in the Bahamas. Stalagmites are calcium carbonate deposits left behind when seepage water, containing dissolved carbon dioxide, evaporates. Carbon-14 levels were found to be twice as high as modern levels.[20] These discoveries improved the calibration for the radiocarbon technique and extended its usefulness to 45,000 years into the past.[21]

Examples

Ancient footprints of Acahualinca Chauvet Cave Dolaucothi Haraldskær Woman Kennewick Man Skeleton Lake Shroud of Turin

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Thera eruption Vinland map

See also

Age of the Earth Cosmogenic isotopes Environmental isotopes Discussion of half-life and average-life or mean-lifetime

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Radiometric dating

From Wikipedia, the free encyclopedia

Radiometric dating (often called radioactive dating) is a technique used to date materials, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.[1] It is the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and can be used to date a wide range of natural and man-made materials. Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead dating. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.

Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.

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Contents

1 Fundamentals of radiometric dating o 1.1 Blocking temperature o 1.2 The age equation o 1.3 Limitation of techniques

2 Modern dating techniques o 2.1 U-Pb long time scale method o 2.2 Other long time scale methods

3 Short-range dating techniques o 3.1 Carbon-14 method o 3.2 Other short time scale methods

4 Dating with shortlived extinct radionuclides 5 Types of radiometric dating 6 See also 7 References 8 External links

Fundamentals of radiometric dating

All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including radioactive decay, either by emission of particles (usually electrons (beta decay), positrons or alpha particles) or by spontaneous fission, and electron capture.

While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or decay product. In many cases, the daughter nuclide itself is radioactive, resulting in a decay chain, eventually ending with the formation of a stable (nonradioactive) daughter nuclide; each step in such a chain is characterized by a distinct half-life. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years (e.g., tritium) to over 100 billion years (e.g., Samarium-147).

In general, the half-life of a nuclide depends solely on its nuclear properties; it is not affected[2] by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field. (For some nuclides which decay by the process of electron capture, such as Beryllium-7, Strontium-85, and Zirconium-89, the decay rate may be slightly affected by

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local electron density, therefore these isotopes may not be as suitable for radiometric dating.) But in general, the half-life of any nuclide is essentially a constant. Therefore, in any material containing a radioactive nuclide, the proportion of the original nuclide to its decay product(s) changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock that measures the time from the incorporation of the original nuclide(s) into a material to the present.

The processes that form specific materials are often conveniently selective as to what elements they incorporate during their formation. In the simplest case, the material will incorporate a parent nuclide and reject the daughter nuclide. In this case, the only atoms of the daughter nuclide present in a sample must have been deposited by radioactive decay since the sample formed. When a material incorporates both the parent and daughter nuclides at the time of formation, a correction must be made for the initial proportion of the radioactive substance and its daughter; generally this is done by construction of an isochron, e.g. in Rubidium-strontium dating.

Accurate radiometric dating generally requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation, that the parent has a long enough half-life that it will still be present in significant amounts at the time of measurement (except as described below under "Dating with shortlived extinct radionuclides"), the half-life of the parent is accurately known, and enough of the daughter product is produced to be accurately measured and distinguished from the initial amount of the daughter present in the material. The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate.[citation

needed]

[edit] Blocking temperature

If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusion, setting the isotopic "clock" to zero. The temperature at which this happens is known as the blocking temperature or closure temperature and is specific to a particular material and isotopic system. These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace.

The age equation

Considering that radioactive parent elements decay to stable daughter elements [3], the mathematical expression that relates radioactive decay to geologic time, called the age equation, is [4]:

where

t = age of the sample

D = number of atoms of the daughter isotope in the sample

P = number of atoms of the parent isotope in the sample

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λ = decay constant of the parent isotope

ln = natural logarithm

The decay constant (or rate of decay[5]) is the fraction of a number of atoms of a radioactive nuclide that disintegrates in a unit of time. The decay constant is inversely proportional to the radioactive half-life of the parent isotope, which can be obtained from tables such as the one on this page.

Limitation of techniques

Although radiometric dating is accurate in principle, the precision is very dependent on the care with which the procedure is performed. The possible confounding effects of initial contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created.

Precision is enhanced if measurements are taken on different samples from the same rock body but at different locations. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample.

The precision of a dating method depends in part on the half-life of the radioactive isotope involved. For instance, carbon-14 has a half-life of about 6000 years. After an organism has been dead for 60,000 years, so little carbon-14 is left in it that accurate dating becomes impossible. On the other hand, the concentration of carbon-14 falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades. The isotope used in uranium-thorium dating has a longer half-life, but other factors make it more accurate than radiocarbon dating.[citation needed]

Modern dating techniques

Radiometric dating can be performed on samples as small as a billionth of a gram using a mass spectrometer. The mass spectrometer was invented in the 1940s and began to be used in radiometric dating in the 1950s. The mass spectrometer operates by generating a beam of ionized atoms from the sample under test. The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as "Faraday cups", depending on their mass and level of ionization. On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams.

U-Pb long time scale method

The uranium-lead radiometric dating scheme is one of the oldest available, as well as one of the most highly respected. It has been refined to the point that the error in dates of rocks about three billion years old is no more than two million years.[citation needed]

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Uranium-lead dating is often performed on the mineral "zircon" (ZrSiO4), though it can be used on other materials. Zircon incorporates uranium atoms into its crystalline structure as substitutes for zirconium, but strongly rejects lead. It has a very high blocking temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. In situ micro-beam analysis can be achieved via laser ICP-MS or SIMS techniques [6] .

One of its great advantages is that any sample provides two clocks, one based on uranium-235's decay to lead-207 with a half-life of about 700 million years, and one based on uranium-238's decay to lead-206 with a half-life of about 4.5 billion years, providing a built-in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost.

Other long time scale methods

Two other radiometric techniques are used for long-term dating. Potassium-argon dating involves electron capture or positron decay of potassium-40 to argon-40. Potassium-40 has a half-life of 1.3 billion years, and so this method is applicable to the oldest rocks. Radioactive potassium-40 is common in micas, feldspars, and hornblendes, though the blocking temperature is fairly low in these materials, about 125°C (mica) to 450°C (hornblende).

Rubidium-strontium dating is based on the beta decay of rubidium-87 to strontium-87, with a half-life of 50 billion years. This scheme is used to date old igneous and metamorphic rocks, and has also been used to date lunar samples. Blocking temperatures are so high that they are not a concern. Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample.

Short-range dating techniques

There are a number of dating techniques that have short ranges and are so used for historical or archaeological studies. One of the best-known is the carbon-14 (C14) radiometric technique.

Carbon-14 methodMain article: Carbon-14 dating

Carbon-14 is a radioactive isotope of carbon, with a half-life of 5,730 years (very short compared with the above). In other radiometric dating methods, the heavy parent isotopes were synthesized in the explosions of massive stars that scattered materials through the Galaxy, to be formed into planets and other stars. The parent isotopes have been decaying since that time, and so any parent isotope with a short half-life should be extinct by now.

Carbon-14 is an exception. It is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere. The carbon-14 ends up as a trace component in atmospheric carbon dioxide (CO2).

An organism acquires carbon from carbon dioxide during its lifetime. Plants acquire it through photosynthesis, and animals acquire it from consumption of plants and other animals. When an

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organism dies, it ceases to intake new carbon-14 and the existing isotope decays with a characteristic half-life (5730 years). The proportion of carbon-14 left when the remains of the organism are examined provides an indication of the time lapsed since its death. The carbon-14 dating limit lies around 58,000 to 62,000 years.[7]

The rate of creation of carbon-14 appears to be roughly constant, as cross-checks of carbon-14 dating with other dating methods show it gives consistent results. However, local eruptions of volcanoes or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon-14 and give inaccurate dates. The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon-14 by a few percent; conversely, the amount of carbon-14 was increased by above-ground nuclear bomb tests that were conducted into the early 1960s. Also, an increase in the solar wind or the earth's magnetic field above the current value would depress the amount of carbon-14 created in the atmosphere. These effects are corrected for by the calibration of the radiocarbon dating scale. See the article on radiocarbon dating.

Other short time scale methods

Another relatively short-range dating technique is based on the decay of uranium-238 into thorium-230, a substance with a half-life of about 80,000 years. It is accompanied by a sister process, in which uranium-235 decays into protactinium-231, which has a half-life of 34,300 years.

While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sediments, from which their ratios are measured. The scheme has a range of several hundred thousand years.

Natural sources of radiation in the environment knock loose electrons in, say, a piece of pottery, and these electrons accumulate in defects in the material's crystal lattice structure. Heating the object will release the captured electrons, producing a luminescence. When the sample is heated, at a certain temperature it will glow from the emission of electrons released from the defects, and this glow can be used to estimate the age of the sample to a threshold of approximately 15 percent of its true age. The date of a rock is reset when volcanic activity remelts it. The date of a piece of pottery is reset by the heat of the kiln. Typically temperatures greater than 400 degrees Celsius will reset the "clock". This is termed thermoluminescence.

Finally, fission track dating involves inspection of a polished slice of a material to determine the density of "track" markings left in it by the spontaneous fission of uranium-238 impurities.

The uranium content of the sample has to be known, but that can be determined by placing a plastic film over the polished slice of the material, and bombarding it with slow neutrons. This causes induced fission of 235U, as opposed to the spontaneous fission of 238U. The fission tracks produced by this process are recorded in the plastic film. The uranium content of the material can then be calculated from the number of tracks and the neutron flux.

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This scheme has application over a wide range of geologic dates. For dates up to a few million years micas, tektites (glass fragments from volcanic eruptions), and meteorites are best used. Older materials can be dated using zircon, apatite, titanite, epidote and garnet which have a variable amount of uranium content. Because the fission tracks are healed by temperatures over about 200°C the technique has limitations as well as benefits. The technique has potential applications for detailing the thermal history of a deposit.

Large amounts of otherwise rare 36 Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of 36Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, 36Cl is also useful for dating waters less than 50 years before the present. 36Cl has seen use in other areas of the geological sciences, including dating ice and sediments.

Dating with shortlived extinct radionuclides

At the beginning of the solar system there were several relatively shortlived radionuclides like 26Al, 60Fe, 53Mn, and 129I present within the solar nebula. These radionuclides—possibly produced by the explosion of a supernova—are extinct today but their decay products can be detected in very old material such as meteorites. Measuring the decay products of extinct radionuclides with a mass spectrometer and using isochronplots it is possible to determine relative ages between different events in the early history of the solar system. Dating methods based on extinct radionuclides can also be calibrated with the U-Pb method to give absolute ages.

Types of radiometric dating

argon-argon (Ar-Ar) fission track dating helium (He-He) iodine-xenon (I-Xe) lanthanum-barium (La-Ba) lead-lead (Pb-Pb) lutetium-hafnium (Lu-Hf) neon-neon (Ne-Ne) optically stimulated luminescence dating potassium-argon (K-Ar) radiocarbon dating rhenium-osmium (Re-Os) rubidium-strontium (Rb-Sr) samarium-neodymium (Sm-Nd) uranium-lead (U-Pb) uranium-lead-helium (U-Pb-He) uranium-thorium (U-Th) uranium-uranium (U-U)

See also

Age of the Earth

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Exponential decay Half-life Isochron dating Isotope geochemistry Isotopic signature Radioactive decay Radioactivity sensitive high resolution ion microprobe (SHRIMP)

References

1. ̂ International Union of Pure and Applied Chemistry. "radioactive dating". Compendium of Chemical Terminology Internet edition.

2. ̂ http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/decay_rates.html How to Change Nuclear Decay Rates

3. ̂ Georgia Perimeter College - Radiometric dating 4. ̂ U.S. Geological Survey - Radiometric Time Scale 5. ̂ University of South Carolina - Center for Science Education - Decay rates 6. ̂ SIMS ion micropobes able to achieve zircon analysis are SHRIMP or Cameca IMS 1270–1280.

refer to Trevor Ireland, Isotope Geochemistry: New Tools for Isotopic Analysis, Science, December 1999, Vol. 286. no. 5448, pp. 2289 - 2290

7. ̂ Ingentaconnect Cosmic Background Reduction In The Radiocarbon Measurement By Sci

External links

USGS page on the radiometric time scale Principles of radiometric dating . Simple explanation of relationship between radiometric and biostratigraphic dating

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v • d • e

Chronology

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Retrieved from "http://en.wikipedia.org/wiki/Radiometric_dating"Categories: Radiometric dating

Hidden categories: All articles with unsourced statements | Articles with unsourced statements since June 2007 | Articles with unsourced statements since February 2007 | Articles with unsourced statements since February 2008

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http://id-archserve.ucsb.edu/anth3/courseware/Exercises.html

Assignment 1 - Introduction to Archaeology o Video Introduction to Assignment 1 o Exercise I-1, Introduction to the Software o Exercise 1-1, A Brief History of Archaeology o Exercise 1-2, Chronological Methods

Assignment 2 - Finding the Past o Video Introduction to Assignment 2 o Exercise 2-1, Archeological Survey in the Field - The Basin of Mexico

Assignment 3 - Digging up and Analyzing the Past o Video Introduction to Assignment 3 o No Web Exercises

Assignment 4 - Human Origins o Video Introduction to Assignment 4 o Exercise 4-1, The Ice Age o Exercise 4-2, Introducing the Skeletons in Your Closet o Exercise 4-3, Principles of Lithic Technology o Exercise 4-4, The Formation of Olduvai Gorge o Exercise 4-5, The Archaeology of Olduvai Gorge

Assignment 5 - The Origins and Spread of Modern Humans

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o Video Introduction to Assignment 5 o Exercise 5-1, Homo erectus: A Comparison o Exercise 5-2, Middle Paleolithic Stone Technology o Exercise 5-3, The Technology of Emerging Homo sapiens o Exercise 5-4, Peopling of the Globe

Assignment 6 - The Origins of Agriculture and Animal Domestication

o Video Introduction to Assignment 6 o Exercise 6-1, The End of the Ice Age o Exercise 6-2, The Gwembe Tonga Simulation

Assignment 7 - States: Ancient Egypt o Video Introduction to Assignment 7 o Exercises 7-1, A Monopoly Game: State Formation

Assignment 8 - States: Maya Civilization o Video Introduction to Assignment 8 o Exercise 8-1, Tikal: A Maya City

Assignment 9 - Archaeology and Society o Video Introduction to Assignment 9 o No Web Exercises

Exercise 1-2 Chronological Methods

George H. Michaelsand Brian M. Fagan

Introduction  Superposition

Stratigraphy  Cross Dating

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Artifacts of Known Age   Dendrochronology

Radiocarbon Dating  Potassium Argon Dating

Obsidian Hydration Dating  Paleomagnetic and Archaeomagnetic Dating

Luminescence  Other Isotopic Dating

Conclusion   

Chronological Methods 2 - Introduction

This exercise is designed to introduce you to basic techniques used in determining the age of archaeological materials and sites. There are two general categories of methods for dating in archaeology. One is Relative Dating, and the other is Absolute Dating. In general, archaeologists use both methods in conjunction. Absolute dating to determine the actual age of an object or stratigraphic layer, and relative dating to tie associated artifacts and layers into the sequence provided by the absolute dates.

Chronological Methods 3 - Superposition

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One of the most fundamental principles of archaeology is the Law of Superposition. The law states that strata that are younger will be deposited on top of strata that are older, given normal conditions of deposition. This law is the guiding principle of stratigraphy, or the study of geological or soil layers. Stratigraphy is still the single best method that archaeologists have for determining the relative ages of archaeological materials.

Chronological Methods 4 - Stratigraphy

Stratigraphy is the study of strata, or layers. Specifically, stratigraphy refers to the application of the Law of Superposition to soil and geological strata containing archaeological materials in order to determine the relative ages of layers. In addition, stratigraphy can tell us much about the processes affecting the deposition of soils, and the condition of sites and artifacts. These are called postdepositional processes, and their study is part of Middle Range Theory.

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As this example has shown you, post depositional processes, both natural and human, can result in very complex stratigraphy. Although stratigraphy and the Law of Superposition can help us determine the relative ages of occupations, one must be very alert to alterations in stratigraphy that may throw chronological reckoning off. Various phenomena, such as hole digging or mudslides, can completely reverse stratigraphy. Thus, long profiles or profiles from a number of units are necessary to avoid misinterpretation.

Chronological Methods 5 - Cross-Dating

Cross-dating is a technique used to take advantage of consistencies in stratigraphy between parts of a site or different sites, and objects or strata with a known relative

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chronology. A specialized form of cross-dating, using animal and plant fossils, is known as biostratigraphy. The following animation will provide you with an example of how cross-dating is used. Click on the button below to start the animation.

Chronological Methods 6 - Artifacts of Known Age

Relative dating is an invaluable tool, but does not tell us WHEN an event occurred, just the ORDER in which events occurred. The oldest technique for establishing the actual ages of deposits is to use artifacts of a known age. These can be coins with minting dates stamped on them, writings with dates included, or objects that we know were only manufactured during a certain time.

Chronological Methods 7 - Dendrochronology

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Dendrochronology is another traditional technique for establishing the abolute date of events. This is also called Tree-Ring Dating. Tree-Ring dating is based on the principle that the growth rings on certain species of trees reflect variations in seasonal and annual rainfall. Trees from the same species, growing in the same area or environment will be exposed to the same conditions, and hence their growth rings will match at the point where their lifecycles overlap.   

Limitations of Dendrochronology

There are limitations on dendrochronology. Some of those limitations include:

In some areas of the world, particularly in the tropics, the species available do not have sufficiently distinct seasonal patterns that they can be used.

Where the right species are available, the wood must be well enough preserved that the rings are readable. In addition, there must be at least 30 intact rings on any one sample.

There also must be an existing master strip for that area and species. There is an absolute limit on how far back in the past we can date things with tree rings. Although bristle cone pine trees can live to 9,000 years, this is a very rare phenomenon. As we try to push our matching of archaeological

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specimens beyond the range for which we have good control data, our confidence in the derived dates diminishes.

Finally, the prehistoric people being studied had to have built fairly substantial structures using wood timbers. In most of the world that did not begin to happen until about 4,000 to 5,000 years ago!

Links

The Ultimate Tree-Ring Web PagesThe Malcolm and Carolyn Wiener Laboratory for Aegean and Near Eastern Dendrochronology at Cornell UniversityThe Laboratory of Tree-Ring Research, University of Arizona

References

Chronological Methods 8 - Radiocarbon Dating

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Radiocarbon, or Carbon-14, dating is probably one of the most widely used and best known absolute dating methods. It was developed by J. R. Arnold and W. F. Libby in 1949, and has become an indispensable part of the archaeologist's tool kit since. Its development revolutionized archaeology by providing a means of dating deposits independent of artifacts and local stratigraphic sequences. This allowed for the establishment of world-wide chronologies.

  Where does C -14 Come From?

Radiocarbon dating relies on a simple natural phenomenon. As the Earth's upper atmosphere is bombarded by cosmic radiation, atmospheric nitrogen is broken down into an unstable isotope of carbon - carbon 14 (C-14).

Bombardment Reactions

The unstable isotope is brought to Earth by atmospheric activity, such as storms, and becomes fixed in the biosphere. Because it reacts identically to C-12 and C-13, C-14 becomes attached to complex organic molecules through photosynthesis in plants and becomes part of their molecular makeup. Animals eating those plants in turn absorb Carbon-14 as well as the stable isotopes. This process of ingesting C-14 continues as long as the plant or animal remains alive.

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Diffusion Ingestion

  

C-14 Decay Profile

The C-14 within an organism is continually decaying into stable carbon isotopes, but since the organism is absorbing more C-14 during its life, the ratio of C-14 to C-12 remains about the same as the ratio in the atmosphere. When the organism dies, the ratio of C-14 within its carcass begins to gradually decrease. The rate of decrease is 1/2 the quantity at death every 5,730 years. That is the half-life of C-14. The animation provides an example of how this logarithmic decay occurs. Click on the "Show Movie" button below to view this animation.

C-14 Decay Profile

  

How is a C-14 Sample Processed?

Clicking on the "Show Movie" button below will bring up an animation that illustrates how a C-14 sample is processed and the calculations involved in arriving at a date. This is actually a mini-simulator, in that it processes a different sample each time and generates different dates.

C-14 Processing

The Limitations of Carbon 14 Dating

Using this technique, almost any sample of organic material can be directly dated. There are a number of limitations, however.

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First, the size of the archaeological sample is important. Larger samples are better, because purification and distillation remove some matter. Although new techniques for working with very small samples have been developed, like accelerator dating, these are very expensive and still somewhat experimental.

Second, great care must be taken in collecting and packing samples to avoid contamination by more recent carbon. For each sample, clean trowels should be used, to avoid cross contamination between samples. The samples should be packaged in chemically neutral materials to avoid picking up new C-14 from the packaging. The packaging should also be airtight to avoid contact with atmospheric C-14. Also, the stratigraphy should be carefully examined to determine that a carbon sample location was not contaminated by carbon from a later or an earlier period.

Third, because the decay rate is logarithmic, radiocarbon dating has significant upper and lower limits. It is not very accurate for fairly recent deposits. In recent deposits so little decay has occurred that the error factor (the standard deviation) may be larger than the date obtained. The practical upper limit is about 50,000 years, because so little C-14 remains after almost 9 half-lives that it may be hard to detect and obtain an accurate reading, regardless of the size of the sample.

Fourth, the ratio of C-14 to C-12 in the atmosphere is not constant. Although it was originally thought that there has always been about the same ratio, radiocarbon samples taken and cross dated using other techniques like dendrochronology have shown that the ratio of C-14 to C-12 has varied significantly during the history of the Earth. This variation is due to changes in the intensity of the cosmic radiation bombardment of the Earth, and changes in the effectiveness of the Van Allen belts and the upper atmosphere to deflect that bombardment. For example, because of the recent depletion of the ozone layer in the stratosphere, we can expect there to be more C-14 in the atmosphere today than there was 20-30 years ago. To compensate for this variation, dates obtained from radiocarbon laboratories are now corrected using standard calibration tables developed in the past 15-20 years. When reading archaeological reports, be sure to check if the carbon-14 dates reported have been calibrated or not.

Finally, although radiocarbon dating is the most common and widely used chronometric technique in archaeology today, it is not infallible. In general,

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single dates should not be trusted. Whenever possible multiple samples should be collected and dated from associated strata. The trend of the samples will provide a ball park estimate of the actual date of deposition. The trade-off between radiocarbon dating and other techniques, like dendrochronology, is that we exchange precision for a wider geographical and temporal range. That is the true benefit of radiocarbon dating, that it can be employed anywhere in the world, and does have a 50,000 year range. Using radiocarbon dating, archaeologists during the past 30 years have been able to obtain a much needed global perspective on the timing of major prehistoric events such as the development of agriculture in various parts of the world.

Links

Radiocarbon Journal

References

 

Chronological Methods 9 - Potassium-Argon Datinghttp://id-archserve.ucsb.edu/Anth3/Courseware/Chronology/09_Potassium_Argon_Dating.html

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Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials. Geologists have used this method to date rocks as much as 4 billion years old. It is based on the fact that some of the radioactive isotope of Potassium, Potassium-40 (K-40) ,decays to the gas Argon as Argon-40 (Ar-40). By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.

  

How Does the Reaction Work?

Potassium (K) is one of the most abundant elements in the Earth's crust (2.4% by mass). One out of every 10,000 Potassium atoms is radioactive Potassium-40 (K-40). These each have 19 protons and 21 neutrons in their nucleus. If one of these protons is hit by a beta particle, it can be converted into a neutron. With 18 protons and 22 neutrons, the atom has become Argon-40 (Ar-40), an inert gas. For every 100 K-40 atoms that decay, 11 become Ar-40.

  

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How is the Atomic Clock Set?

When rocks are heated to the melting point, any Ar-40 contained in them is released into the atmosphere. When the rock recrystallizes it becomes impermeable to gasses again. As the K-40 in the rock decays into Ar-40, the gas is trapped in the rock.

  

The Decay Profile

In this simulation, a unit of molten rock cools and crystallizes. The ratio of K-40 to Ar-40 is plotted. Note that time is expressed in millions of years on this graph, as opposed to thousands of years in the C-14 graph. Click on the "Show Movie" button below to view this animation.

K-Ar Decay Profile

  

How are Samples Processed?

Clicking on the "Show Movie" button below will bring up an animation that illustrates how a K-Ar sample is processed and the calculations involved in arriving at a date. This is actually a mini-simulator, in that it processes a different sample each time and generates different dates.

K-Ar Processing

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Limitations on K-Ar Dating

The Potassium-Argon dating method is an invaluable tool for those archaeologists and paleoanthropologists studying the earliest evidence for human evolution. As with any dating technique, there are some significant limitations.

The technique works well for almost any igneous or volcanic rock, provided that the rock gives no evidence of having gone through a heating-recrystallization process after its initial formation. For this reason, only trained geologists should collect the samples in the field.

This technique is most useful to archaeologists and paleoanthropologists when lava flows or volcanic tuffs form strata that overlie strata bearing the evidence of human activity. Dates obtained with this method then indicate that the archaeological materials cannot be younger than the tuff or lava stratum. Because the materials dated using this method are NOT the direct result of human activity, unlike radiocarbon dates for example, it is critical that the association between the igneous/volcanic beds being dated and the strata containing human evidence is very carefully established.

As the simulation of the processing of potassium-argon samples showed, the standard deviations for K-Ar dates are so large that resolution higher than about a million years is almost impossible to achieve. By comparison, radiocarbon dates seem almost as precise as a cesium clock! Potassium-argon dating is accurate from 4.3 billion years (the age of the Earth) to about 100,000 years before the present. At 100,000 years, only 0.0053% of the potassium-40 in a rock would have decayed to argon-40, pushing the limits of present detection devices. Eventually, potassium-argon dating may be able to provide dates as recent as 20,000 years before present.

Links

Geochron Laboratories

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Helpful References

 Chronological Methods 10 - Obsidian Hydration Dating

Obsidian, or volcanic glass, is formed by the rapid cooling of silica-rich lava. Although its precise chemical composition varies from one outcrop to another, it always contains >70% silica by weight. Humans often used obsidian as a raw material when making chipped stone tools.

In 1948, two geologists, Irving Friedman and Robert Smith, began looking into obsidian's potential as a time marker. They introduced the obsidian hydration dating method to the archaeological community in 1960. It may be used in two ways: as a relative dating method to determine if one artifact is older or younger than another, or as an absolute dating method where a calendar date (AD/BC) is produced. The decision to use it as a relative or absolute dating method depends upon whether the environmental conditions (eg. soil temperature and soil relative humidity) of the archaeological site are known.

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How does Obsidian Hydration Dating work?

Obsidian hydration dating is based on the fact that a fresh surface is created on a piece of obsidian in the tool manufacturing, or flintknapping, process. Obsidian contains about 0.2 percent water. When a piece of obsidian is fractured, atmospheric water is attracted to the surface and begins to diffuse into the glass. This results in the formation of a water rich hydration rind that increases in depth with time. The hydration process continues until the fresh obsidian surface contains about 3.5 percent water. This is the saturation point. The thickness of the hydration rind can be identified in petrographic thin sections cut normal to the surface and observed under a microscope. A distinct diffusion front can be recognized by an abrupt change in refractive index at the inner edge of the hydration rind. These fronts or rinds of hydration are more dense than the unhydrated inside, and the unhydrated zone has different optical properties. Friedman and Smith reasoned that the degree of hydration observed on an obsidian artifact could tell archaeologists how long it had been since that surface was created by a flintknapper.

  

Hydration Profile

Hydration begins after any event which exposes a fresh surface (e.g. cracking of a lava flow on cooling, manufacture of an obsidian artifact, or glacial abrasion of an obsidian pebble). Providing one can identify which process created the exposed surface or crack in the rock, it is possible to date when that process took place. Hydration rind thickness is a (non-linear) function of time. The hydration rate is primarily a function of temperature, though chemical composition of the sample is also an important factor. For this reason, it is necessary to calibrate the samples within a limited geographical area against a sample of known age and similar chemical composition.

Hydration forms at different rates on different obsidians. Under the same conditions of temperature and humidity some glasses will hydrate rapidly while others are very slow. What controls the process? There is a very strong relationship

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between the rate of hydration and the quantity of intrinsic water found within the glass. This is the water trapped in the obsidian at the time the lava hardens into a natural glass. The presence of intrinsic water opens up the glass structure and allows the atmospheric water to diffuse inward from the surface to form the hydration rim. The more intrinsic water present within an obsidian artifact, the faster it will hydrate and the faster the hydration rim will form

 

How is an Obsidian Sample Processed?

Three steps are required to determine a calendar date from an obsidian artifact. These are the determination of: 1) the hydration rate, 2) the thickness of the hydration rim, and 3) the soil temperature and soil relative humidity at the archaeological site.

A hydration rate is determined for every artifact through a measurement of the amount of intrinsic water that is present. This is done by either a direct infrared spectroscopic measurement of the volcanic glass, or by a determination of the volcanic glass density made by submersion in a heavy liquid. Once the quantity of water is known a hydration rate is estimated.

A small sample is cut out perpendicular to the edge of the obsidian artifact using a diamond-impregnated saw. A lapidary machine is used to grind down the obsidian sample until it is very thin. It is glued to a clear microscope slide with Canada balsam. The obsidian sample is ground a second time until it less than 50 microns in thickness. A microscope is then used to optically measure the hydration rind on the petrographic thin section. The hydration layer is measured at 800x using a Watson image-splitting instrument. This is the most precise optical instrument that can be used. It has an error factor of about 0.1 microns.

In order to adjust the experimental hydration rate to the conditions at the archaeological site, the soil temperature and soil relative humidity need to be well estimated. On short term projects, ambient conditions can be estimated from weather records. For studies that take longer than a year, thermal cells can be buried at the archaeological site. The small capsules are placed at multiple depths

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that typically span a depth range of 5 cm to 100 cm below ground surface. About 8 cells are required to establish a temperature and relative humidity curve for the site. With this background work done, the environmental conditions can be determined for any context at a site.

The Limitations of Obsidian Hydration Dating

Using this technique, any sample of obsidian can be dated. There are several limitations, however.

The rate of hydration is not uniform throughout the world. Variations exist in temperature over time from site to site. Temperature effects are particularly difficult to evaluate. Variations also exist in sample chemical composition. Samples from different obsidian sources hydrate at different rates. Moisture is another source of variability. The amount of moisture present at a site can affect the hydration rate of an obsidian sample. It is really necessary to produce a calibration curve for each archaeological site or area being studied, and this is not always possible.

Artifact reuse may lead to an erroneous date. For example, one person fashions a tool out of an obsidian nodule and uses it to skin a deer. Once that person finishes using the tool, it is discarded. Several hundred years later, a second person finds the tool, resharpens it, uses it to shave the bark off of a tree branch, and then later discards it as well. Several thousand years later, an archaeologist discovers the tool and takes it to a laboratory to be dated. The archaeologist found the tool at a site that was an arrowshaft workshop. However, instead of dating the surface on the tool that was used to shave bark, the surface that was used to skin the deer several hundred years earlier is dated. The archaeologist, would be lead to beleive by this erroneous date that arrow production started several hundred years earlier than what was expected.

Links

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The Diffusion Laboratory

The Northwest Research Obsidian Studies Laboratory

International Association for Obsidian Studies

The Obsidian Hydration Laboratory at the Centre for Archaeological Research, University of Auckland, New Zealand

References

Freter, AnnCorinne. 1993. Obsidian-Hydration Dating: Its Past, Present, and Future Application in Mesoamerica. Ancient Mesoamerica 4(2):285-303.Friedman, Irving and F. W. Trembour. 1983. Obsidian Hydration Dating Update. American Antiquity 48(3):544-547.Friedman, Irving Fred W. Trembour,Franklin L. Smith, and George I. Smith. 1994. Is Obsidian Hydration Dating Affected by Relative Humidity? Quaternary Research 41(2):185-190.Michels, Joseph W. and Ignatius S. T. Tsong. 1980. Obsidian Hydration Dating: A Coming of Age. In Advances in Archaeological Method and Theory, Volume 3, edited by M. B. Schiffer, pp. 405-444. Academic Press, New York, New York.Michels, Joseph W., Ignatius S. T. Tsong, and Charles M. Nelson. 1983. Obsidian Dating and East African Archeology. Science 219:361-366.Origer, Thomas M. 1989. Hydration Analysis of Obsidian Flakes Produced by Ishi During the Historic Period. In Current Directions in California Obsidian Studies, edited by Richard E. Hughes, pp. 69-77. Contributions of the University of California Archaeological Research Facility No. 48, University of California, Berkeley, California.

Chronological Methods 11 - Paleomagnetic and Archaeomagnetic Dating

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After World War II, geologists developed the paleomagnetic dating technique to measure the movements of the magnetic north pole over geologic time. In the early to mid 1960s, Dr. Robert Dubois introduced this new absolute dating technique to archaeology as archaeomagnetic dating.

   How does Magnetism work?

Magnetism occurs whenever electrically charged particles are in motion. The Earth's molten core has electric currents flowing through it. As the earth rotates, these electric currents produce a magnetic field that extends outward into space. This process, in which the rotation of a planet with an iron core produces a magnetic field, is called a dynamo effect.

The Earth's magnetic core is generally inclined at an 11 degree angle from the Earth's axis of rotation. Therefore, the magnetic north pole is at approximately an 11 degree angle from the geographic north pole. On the earth's surface, when you hold a compass and the needle points to north, it is actually pointing to magnetic north, not geographic (true) north.

The Earth's magnetic north pole can change in orientation (from north to south and south to north), and has many times over the millions of years that this planet has existed. The term that refers to changes in the Earth's magnetic field in the past is paleomagnetism. Any changes that occur in the magnetic field will occur all over the world; they can be used to correlate stratigraphic columns in different locations. This correlation process is called magnetostratigraphy.

Lava, clay, lake and ocean sediments all contain microscopic iron particles. When lava and clay are heated, or lake and ocean sediments settle through the water, they acquire a magnetization parallel to the Earth's magnetic field. After they cool or settle, they maintain this magnetization, unless they are reheated or disturbed. This

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process is called thermoremanent magnetization in the case of lava and clay, and depositional remanent magnetization in the case of lake and ocean sediments.

 

In addition to changing in orientation, the magnetic north pole also wanders around the geographic north pole. Archaeomagnetic dating measures the magnetic polar wander.

For example, in the process of making a fire pit, a person can use clay to create the desired shape of the firepit. In order to harden the clay permanently, one must heat it above a certain temperature (the Curie point) for a specified amount of time. This heating, or firing, process resets the iron particles in the clay. They now point to the location of magnetic north at the time the firepit is being heated. When the firepit cools the iron particles in the hardened clay keep this thermoremanent magnetization. However, each time the firepit is reheated above the Curie point while being used to cook something, or provide heat, the magnetization is reset. Therefore, you would use archaeomagnetic dating to date the last time the firepit was heated above the Curie point temperature.

  

Paleomagnetic and Archaeomagnetic Profile

Paleomagnetism and Archaeomagnetism rely on remnant magnetism,as was explained above. In general, when clay is heated, the microscopic iron particles within it acquire a remnant magnetism parallel to the earth's magnetic field. They also point toward the location around the geographic north pole where the magnetic north pole was at that moment in its wandering. Once the clay cools, the iron particles maintain that magnetism until the clay is reheated. By using another dating method (dendrochonology, radiocarbon dating) to obtain the absolute date of an archaeological feature (such as a hearth), and measuring the direction of magnetism and wander in the clay today, it is possible to determine the location of the magnetic north pole at the time this clay was last fired. This is called the virtual geomagnetic pole or VGP. Archaeologists assemble a large number of these ancient VGPs and construct a composite curve of polar wandering (a VGP curve). The VGP curve can then be used as a master record, against which the VGPs of samples of unknown age can be compared to and assigned a date.

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How are Paleomagnetic and Archaeomagnetic Samples Processed?

Geologists collect paleomagnetic samples by drilling and removing a core from bedrock, a lava flow, or lake and ocean bottom sediments. They make a marking on the top of the core which indicates the location of the magnetic north pole at the time the core was collected. This core is taken back to a laboratory, and a magnetometer is used to measure the orientation of the iron particles in the core. This tells the geologist the orientation of the magnetic pole when the rock was hot.

Archaeologists collect archaeomagnetic samples by carefully removing samples of baked clay from a firepit using a saw. A nonmagnetic, cube-shaped mold (aluminum) is placed over the sample, and it is filled with plaster. The archaeologist then records the location of magnetic north on the cube, after the plaster hardens. The vertical and horizontal placement of the sample is also recorded. Eight to twelve samples are collected and sent to a laboratory for processing. A magnetometer is used to measure the orientation of the iron particles in the samples. The location of the magnetic pole and age are determined for that firepit by looking at the average direction of all samples collected.

The Limitations of Paleomagnetic and Archaeomagnetic Dating

Using this technique, a core or sample can be directly dated. There are a number of limitations, however.

First, it is necessary to know the approximate age of the sample to avoid miscorrelations. The K-Ar method has been used to place the sample in an approximate age range. However, sometimes the error associated with K-Ar date is greater than the time span being studied using Paleomagnetic or Archaeomagmetic Dating techniques.

Second, when studying depositional remanent magnetization, in the case of lake and ocean sediments, disturbance of the sediments by currents, slumping of sediments, or burrowing animals is a problem. Any of these disturbances can churn up sediments and change the orientation of the iron particles in the sediments, or remove parts of the sedimentary record

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altogether. Therefore, paleomagnetism studies of sediments should be used as an average record of long term changes in the Earth's magnetic field to reduce error in the interpretation of the record.

Third, the microscopic iron particles in some sediments undergo chemical changes after they have settled through the water into strata. These chemical changes cause the iron particles to realign themselves with the Earth's magnetic field at the time of the chemical change. This is called chemical remanent magnetization. The identification of the particular iron minerals that are susceptible to this change can be an early warning that errors can be expected.

Fourth, paleomagnetic dating can only date deposits that are hundreds of thousands to millions of years old. This is useful when studying early fossil hominids, but is not useful when studying modern human beings.

Finally, the skill of the archaeologist collecting the sample, and the number of the samples used to calibrate the archaeomagnetic master curve affect the precision with which archaeologists can determine a date for a feature.

Links

Archaeometry Journal Home Page

Paleomagnetic Data at NOAA National Data Center

Centre for Environmental Magnetism and Palaeomagnetism (CEMP)

Fort Hoofddijk Paleomagnetic Laboratory, Utrecht University, Netherlands

Institute for Rock Magnetism, University of Minnesota

Rock-Magnetism & Paleomagnetism Lab, Geological Survey of Japan

Los Hornos: A Case Study in Chronology

Laboratory of Earth's Magnetism, Saint-Petersburg State University, Russia

CSU Archaeometric Laboratory

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References

Eighmy, J.L. 1980. Archaeomagnetic Dating: A Handbook for Archaeologists.

Eighmy, J.L., and R.S. Sternberg, eds. 1990. Archaeomagnetic Dating.

Butler, R.F. 1992. Paleomagnetism: Magnetic Domains to Geologic Terrains.

Chronological Methods 12 - Luminescence Dating

Scientists in North America first developed thermoluminescence dating of rock minerals in the 1950s and 1960s, and the University of Oxford, England first developed the thermoluminescence dating of fired ceramics in the 1960s and 1970s.During the 1970s and 1980s scientists at Simon Frasier University, Canada, developed standard thermoluminescence dating procedures used to date sediments. In 1985, they also developed optically stimulated luminescence dating techniques, which use laser light, to date sediments.

   How does Luminescence work?

The microscopic structure of some minerals and ceramics trap nuclear radioactive energy. This energy is in constant motion within the minerals or sherds. Most of the energy escapes as heat, but sometimes this energy separates electrons from the molecules that make up the minerals or ceramics. Usually the electrons will

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reconnect with the molecules, but some will not. The electrons that dont reconnect eventually encounter imperfections in the microscopic structure of the ceramics or minerals, and they become trapped by these imperfections. Over time energy (in the form of more and more trapped electrons) is stored in these structural imperfections. By heating the ceramic or mineral to above 500 degrees Celcius, these trapped electrons are released, creating a flash of light called thermoluminescence. When a laser light source is used to stimulate the release of electrons, the process is called optically stimulated luminescence.

  

Luminescence Profile

In the process of making a ceramic vessel, the soft clay vessel must be heated in a kiln to harden it. The process of firing the vessel releases the trapped electrons (energy), and resets the thermoluminescence clock to zero. The process of accumulation of electrons (energy) and then release when heated occurs every time the ceramic vessel is reheated. What an archaeologist would be able to measure using this technique is the last time the vessel was heated above 500 degrees Celcius, either at the time the vessel was first fired or the last time it was heated if it was used as a cooking vessel. In the laboratory, the release of electrons can be induced through heating or the use of a laser beam. The intensity of the light emmisions (luminescence) can be measured to determine the amount of time that has passed since the vessel was last heated and the present laboratory heating of the vessel.

  

How is a Luminescence Sample Processed?

A small sample is cut out of the artifact being dated. An equivalent dose (DE) of nuclear radiation is determined for every artifact through the application of artificial doses of nuclear radiation (through heating or exposure to a laser light beam) to subsamples of the artifact to scale the signal. Next the burial dose rate (DR) is determined by measuring the radioactivity in portions of the sample grains and surrounding sediments. Lastly, the age of the sample is determined by dividing the equivalent dose by the burial dose rate (DE/DR = Age).

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This method is applicable to samples that range in age from a few hundred years to several hundred thousand years.

The Limitations of Luminescence Dating

Using this technique, almost any rock mineral or ceramic sample can be directly dated. However, it works best when dating heated grains in ceramics, obsidian, burned flint, and burned sediments.

Links

Journal of Luminescence

The Dalhousie Thermally and Optically Stimulated Luminescence and ESR Laboratory

The Research Laboratory for Archaeology and the History of Art, Oxford University

Aberystwyth Luminescence Dating Laboratory

The Sheffield Centre for International Drylands Research (SCIDR), England

Center for Applied Dosimetry

Archaeometry Research Group Heidelberg, Germany

The University of Washington Luminescence Dating Laboratory

References

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Aitken, Martin J. 1985. Thermoluminescence Dating. Aitken, Martin J. (in press) Optical Dating: A Review for Non-Specialists. Quaternary Science Reviews Ollerhead, Jeff, David J. Huntley, and Glenn W. Burger. 1994. Luminescence Dating of the Buctouche Spit, New Brunswick. Canadian Journal of Earth Sciences 31: 523-531.

Chronological Methods 13 - Other Isotopic Dating Methods

Technique Date

Range

Materials Date

d

Limitations

Links

Argon 40/Argon 39 K-feldspar

      Michigan Memorial Phoenix Project

Uranium/Thorium/Pb

      UCLA Ion Microprobe

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Uranium Series

upto 70,000 years

calcite, bone, tooth, shell

 

Uranium Series Decay Scheme

Archaeometry Research Group

Lead/Uranium 1-400 years

sediments, lead-based paints

   

 

A Paragraph Nitrogen, Chlorine 36,Beryllium 10, Cesium 137, Oxygen 16/Oxygen 18, Rubidium/Strontium

Web Elements - The Periodic Table on the WWW

Chronological Methods 14 - Conclusion

There are several other dating techniques employed in archaeology. Some of these include: fission-track dating, paleomagnetic and archaeomagnetic dating, obsidian hydration dating, and thermoluminescence dating. Many of these techniques are still experimental, or have not found wide acceptence in archaeology yet, hence they have not been discussed here. For more information on dating techniques consult:

Fagan, Brian M. - In the Beginning: An Introduction to Archaeology .

Michels, Joseph - Dating Techniques in Archaeology .

You may now return to the Exercise menu by clicking on the "Main" button below. Please refer to the next section of your Study Guide for the next assignment.

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Archaeological Survey 2 - Introduction

One of the most important developments in archaeological method and theory in the past twenty years has been the recognition of the importance of regional archaeological surface surveys. This technique was pioneered in the Viru River Valley by Gordon Willey in the early 1940's, and has become an indispensable tool in the archaeologist's arsenal of research tools since that time. Of course, the sophistication of survey techniques has improved since then, as well as the sophistication of archaeologists in understanding the compexities of survey strategy.

One luxury that most archaeologists never have is the ability to check the effectiveness of various survey strategies before commiting themselves in the field. There are a variety of reasons for this, the most obvious is that you must already know where all of the sites in a region are in order to determine the effectiveness of your survey strategy. In the following exercise you will be afforded the opportunity to evaluate a number of strategies if you wish, and then choose the one that makes the most sense to you in terms of effectiveness at discovering sites, the representativeness of the sample, and the cost effectiveness.

Archaeological Survey 3 - Letter

Tuesday, January 2, 1998

On behalf of the National Science Foundation, I would like to be the first to congratulate you on the award of your grant for field research in the Basin of Mexico. Your funding is effective immediately, and is good for one calendar year from this date. Your proposal, "A Study of Changing Settlement Patterns Through Time in the Basin of Mexico," received high marks from all of the reviewers on both clarity of thought and the necessity of your proposed research. You have accordingly been granted the full $50,000 requested. As always, a complete report of your methods and findings is due within six (6) months of the completion of the fieldwork. Good Luck and Congratulations again.

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Yours, John Yellen, PhD.

Archaeological Survey 4 - Explanation

Your proposal indicated that you wanted to do a survey of the Basin of Mexico to determine what changes there were, if any, in the settlement pattern from the earliest agricultural period up to the time of Teotihuacan's dominance of the Basin. Your study area includes the very important pre-Aztec metropolis of Teotihuacan. What you are interested in documenting is any detectable changes in the sizes, locations, and numbers of the smaller support communities from the period of incipient agriculture to state formation.

You have a very good team of assistants lined up to work with you, as well as excellent contacts in Mexico who can provide you with additional experienced personel should you need them. Your labor needs will be determined by the size of your sample, and the amount of time you have to spent in the field. In short, you will have to be careful about your survey strategy to make sure that you do not run out of money.

Of course, your other major concern is that the sample that you do collect is representative of the range of site types, sizes, and temporal affiliations in the Basin. A badly biased sample will significantly affect your conclusions about settlement patterns.

Below you will be able to take a look at the topography of the Basin, and look at the locations of prominent geographic features which may have influenced the distribution of human populations in the Basin.

After examining the map, you should click on the "Next" button to download the survey simulation. The first card of the simulation is a breakdown of your bugetary constraints. This is an indication of the cost factors that you must take into consideration in planning your strategy. It is also where you will determine the numbers of vehicles and assistants that will be used to calculate costs as the survey progresses. Think carefully about this.

After that you can enter the control center of your field camp and begin setting up your survey.

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Please note that there is no specific end to the survey simulation. When you are satisfied with your survey strategy, the sample you have collected and the results of your preliminary analysis then you are finished with the software portion of the assignment. You will still need to discuss the results with your research group and develop a report to share with the rest of your discussion section. Note that unless you are doing a judgemental survey, there is no way to reproduce a survey, so make sure to take notes and print out the pertinent tables and bar charts to shore up your argument BEFORE you quit the program.

Start Survey Simulation

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Exercise 4-1The Ice Age George H. Michaelsand Brian M. Fagan

Introduction  Glaciation

Global Views of the Wurm/Weichsel/Wisconsin Fullglacial 

Comparison of Major World Vegetation Zones

A Pleistocene Bestiary  Sea Level Changes

References  Conclusion

The Ice Age 2 - Introduction

This assignment is designed to familiarize you with the amazing and dynamic world of the Earth during the Late Pleistocene. This exercise covers about the last 130,000 years of geologic history. It begins with an animated tour of the world during the last two major glaciations. After the opening sequence, you can view each stage individually by clicking on the right and left arrow buttons on the QuickTime control bar. Below is a menu of other important geographic data that you can study and that pertains to this important time period in human prehistory. As you go through the assignment, remember that we are technically still in the Pleistocene, and the world that we know may be just another interglacial period punctuating Quaternary history.

The Ice Age 3 - Glaciation

The Ice Age 4 - Global Views of the Wurm/Weichsel/Wisconsin Fullglacial ca. 24,000

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Scroll down for detailed view of the continents.

Detail of Wurm/Weichsel Fullglacial in Eurasia

Detail of Wurm/Weichsel Fullglacial in Northern Asia

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Detail of Wisconsin Fullglacial in Berengia

(Shallow Land Connecting Alaska and Siberia through the Bering Sea)

Detail of Wisconsin Fullglacial in North America

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Detail of Wisconsin Fullglacial in South America

Detail of Wurm/Weichsel Fullglacial in Africa

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Detail of Wurm/Weichsel Fullglacial in Wallacea and Sunda

(Southeast Asia, Australia and New Guinea)

The Ice Age 5 - Comparison of Major World Vegetation Zones

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Global Pleistocene Plant Communities

Modern Distribution of Major World Vegetation Zones

The Ice Age 6 - A Pleistocene Bestiary

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Bison bison

Common Name: Buffalo

Scientific Name: Bison bison

Related to: No Other Living Species

Current Status: Previously endangered, but now recovering.

Distribution: Found throughout the Great Plains and Midwest of North America. Common from the Late Pleistocene until the Nineteenth Century. The species fell prey to heavy hunting by Europeans, and would now be extinct except for early conservation and breeding efforts. Still only found in national parks and state and private game preserves.

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Distribution of Buffalo (Bison bison ) in North America During the Wurm/Weichsel/Wisconsin Fullglacial

This is the Sole Distribution of the North American Buffalo.

Bison bonasus

Common Name: European Bison or Wisent

Scientific Name: Bison bonasus

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Related to: No Other Living Species

Current Status: Endangered.

Distribution: Originally found throughout the Northern and Central European steppes. This species is now only found in the Bialowice Forest of the Polish-Soviet Russian borderland.

Distribution of European Wisent (Bison bonasus ) in Eurasia During the Wurm/Weichsel/Wisconsin Fullglacial

This is the Sole Distribution of the European Wisent.

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Bison priscus

Common Name: Steppe Wisent

Scientific Name: Bison priscus

Related to: Both European and American bison.

Current Status: Extinct.

Distribution: Originally found throughout the Northern and Central European steppes. Migration across Berengia and into North America eventually lead to the evolution of Bison bison. Species apparently became extinct during the Late Pleistocene, being replaced in North America by B. bison , and in Europe by B. bonasus.

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Distribution of Steppe Wisent (Bison priscus ) in Asia During the Wurm/Weichsel/Wisconsin Fullglacial

Distribution of Steppe Wisent (Bison priscus ) in North America During the Wurm/Weichsel/Wisconsin Fullglacial

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Cave Bear

Common Name: Cave Bear

Scientific Name: Ursus spelaeus

Related to: Brown Bear (U. arctos )

Current Status: Extinct

Distribution: Common throughout eastern and western Europe during the last major glacial stage. Species experienced a period of rapid decline and eventual extinction during the last phase of the last glacial maximum.

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Distribution of Cave Bears (Ursus spelea ) During the Wurm/Weichsel/Wisconsin Fullglacial

This is the Sole Distribution of Cave Bears.

Cave Lion

Common Name: Cave Lion

Scientific Name: Panthera leo spelea (probably related to P. leo atrox in North America during the Late Pleistocene)

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Related to: Indian Lion (P. leo persica)

Current Status: Extinct

Distribution: Common throughout Europe and northern Asia (and probably parts of North America in the species P. leo atrox) during the last major glacial stage. Species experienced a period of rapid decline and eventual extinction during the last phase of the last glacial maximum.

Distribution of Cave Lions (Panthera leo spelea ) in Eurasia During the Wurm/Weichsel/Wisconsin Fullglacial

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Distribution of Cave Lions (Panthera leo spelea ) in Asia During the Wurm/Weichsel/Wisconsin Fullglacial

Distribution of Cave Lions (Panthera leo atrox ) in North America During the Wurm/Weichsel/Wisconsin Fullglacial

Eucladoceros

Common Name: Bush-Antlered Deer

Scientific Name: Eucladoceros sp.

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Related to: No Other Living Species

Current Status: Extinct.

Distribution: Found throughout Eurasia during the early part of the Late Pleistocene. No known living close relatives.

Distribution of Bush-Antlered Deer (Eucladoceros sp.) in Eurasia During the Wurm/Weichsel/Wisconsin Fullglacial

Distribution of Bush-Antlered Deer (Eucladoceros sp.) in Asia During the Wurm/Weichsel/Wisconsin Fullglacial

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Giant Cheetah

Common Name: Giant Cheetah

Scientific Name: Acinonyx pardinensis

Related to: modern cheetah (A. jubatus)

Current Status: Extinct

Distribution: Found in Europe, Africa, and southern Asia during the last major glacial stage. There were closely related species in North America during the same time period. Old World descendants, found only in Africa and southern Asia, are currently facing extinction.

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Distribution of Giant Cheetahs (Acinonyx pardinensis ) in Eurasia During the Wurm/Weichsel/Wisconsin Fullglacial

Distribution of Giant Cheetahs (Acinonyx pardinensis ) in Asia During the Wurm/Weichsel/Wisconsin Fullglacial

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Glyptodon

Common Name: Giant Armadillo

Scientific Name: Gylptodon sp.

Related to: Very distantly related to the much smaller armadillo common throughout South and Central America, and Texas.

Current Status: Extinct.

Distribution: Glyptodonts proper were found throughout South America from the Tertiary period through the Pleistocene. A close relative, the Glyptotherium, was found in Central America and southern North America during the Late Pleistocene. Both became extinct at the close of the Late Pleistocene.

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Distribution of Giant Armadillo (Gylptodon sp.) in South America During the Wurm/Weichsel/Wisconsin Fullglacial

Distribution of Giant Armadillo (Gylptodon sp.) in North America During the Wurm/Weichsel/Wisconsin Fullglacial

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Mammoth

Common Name: Wooley mammoth

Scientific Name: Mammuthus primigenius

Related to: No modern relatives. Last shared a common ancestor with modern elephants (Primelephas gomphotheriodes ) about 5 millions years ago.

Current Status: Extinct

Distribution: Common in Europe, northern Asia and much of North America during the last major glacial stage. Species experienced rapid decline and eventual extinction during the last phase of the last glacial maximum in all parts of the world.

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Distribution of Wooley Mammoths (Mammuthus primigenius ) in Eurasia During the Wurm/Weichsel/Wisconsin Fullglacial

Distribution of Wooley Mammoths (Mammuthus primigenius ) in Asia During the Wurm/Weichsel/Wisconsin Fullglacial

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Distribution of Wooley Mammoths (Mammuthus primigenius ) in North America During the Wurm/Weichsel/Wisconsin Fullglacial

Mastodon

Common Name: Mastodon

Scientific Name: Mastodon americanus

Related to: No modern relatives. Last shared a common ancestor with modern elephants and mastodons sometime before 10 millions years ago.

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Current Status: Extinct

Distribution: Found primarily in North and South America during the last major glacial stage. Family experienced rapid decline and eventual extinction during the end of the Pleistocene in all parts of the world.

Distribution of Mastodons (Mastodon americanus ) in North America During the Wurm/Weichsel/Wisconsin Fullglacial

Distribution of Mastodons (Mastodon americanus ) in South America During the Wurm/Weichsel/Wisconsin Fullglacial

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Megatherium

Common Name: Giant Ground Sloth

Scientific Name: Megatherium americanum

Related to: No Modern Species.

Current Status: Extinct.

Distribution: M. americanum was common throughout South America during the Pliocene and Pleistocene. With a length of up to 5-6 meters, they were larger and more heavily built han modern elephants! A closely related genus (Eremotherium ) occurred in North America during the Pleistocene, but appears to have been largely restricted to the area that is now Florida.

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Distribution of Giant Ground Sloth (Megatherium americanum ) in South America During the Wurm/Weichsel/Wisconsin Fullglacial

Distribution of Giant Ground Sloth (Eremotherium sp.) in North America During the Wurm/Weichsel/Wisconsin Fullglacial

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Short-Faced Hyena

Common Name: Short-Faced Hyena

Scientific Name: Pachycrocuta brevirostris

Related to: Very distantly related to spotted hyenas found in Africa today.

Current Status: Extinct.

Distribution: The short-faced hyena was a common form of hunting hyena found in Eurasia during the Early-Middle Pleistocene. Distantly related to Chasmaporthetes ossifragus (the hunting hyena), the only hyena species to have migrated into North America. Both species became extinct slightly before the Late Pleistocene.

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Distribution of Short-faced Hyena (Pachycrocuta brevirostris ) in Eurasia During the Riss-Wurm/Eem/Sangamon Interglacial

This is the Sole Distribution of the Short-faced Hyena.

Smilodon

Common Name: Sabre-Toothed Tiger

Scientific Name: Smilodon populator (related to S. fatalis in North America during the Late Pleistocene)

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Related to: No living species

Current Status: Extinct

Distribution: Common throughout South America, with related sub-species throughout North America during the last major glacial stage. The genus Smilodon is part of the same family as the genus Megantereon - the dirk-toothed cats of Europe, Asia, and Africa from the same time period. Common throughout the Pleistocene, but became extinct after last full-glacial.

Distribution of Sabre-Toothed Tigers (Smilodontini ) in South America During the Wurm/Weichsel/Wisconsin Fullglacial

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Distribution of Sabre-Toothed Tigers (Smilodontini ) in North America During the Wurm/Weichsel/Wisconsin Fullglacial

Distribution of Dirk-Toothed Cats (Smilodontini ) in Northern Eurasia During the Wurm/Weichsel/Wisconsin Fullglacial

Distribution of Dirk-Toothed Cats (Smilodontini ) in Africa During the Wurm/Weichsel/Wisconsin Fullglacial

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Wooley Rhinoceros

Common Name: Wooley Rhinoceros

Scientific Name: Coelodonta antiquitatis

Related to: Distantly related to the rare Dicerorhinus sumatrensis, a Sumatran rhinoceros living today, but near extinction.

Current Status: Extinct.

Distribution: Common throughout northern Europe and Eastern Asia during the Late Pleistocene. Had identical ecological requirements as the wooley mammoth. The large frontal horn was probably for clearing heavy snow to find forage. Unlike the wooley mammoth, the wooley rhinoceros never migrated into North America

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Distribution of Wooley Rhinoceros (Coelodonta antiquitatis ) in Eurasia During the Wurm/Weichsel/Wisconsin Fullglacial

Distribution of Wooley Rhinoceros (Coelodonta antiquitatis ) in Asia During the Wurm/Weichsel/Wisconsin Fullglacial

The Ice Age 7 - Sea Level Changes

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The Ice Age 8 - Useful References

Bowen, D. Q. 1978 Quaternary Geology: A Stratigraphic Framework for Multidisciplinary Work. Pergamon Press, New York.

Butzer, Karl W. 1971 Environment and Archaeology: An Ecological Approach to Prehistory (Second Edition). Aldine-Atherton, New York.

Flint, Richard Foster 1957 Glacial and Pleistocene Geology. John Wiley & Sons, Inc., New York. (Sixth Printing 1967)

Nilsson, Tage

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1983 The Pleistocene: Geology and Life in the Quaternary Ice Age. Ferdinand Enke Verlag, Stuttgart.

The Ice Age 9 - Conclusion

This concludes this exercise. You may come back to this exercise at any time to lookup information about the Pleistocene that may have had an influence on the development of the hominids. If you have reviewed all of the information in this stack, you may return to the main menu.

CONTINUE DOWNLOADING

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