Chapter 8

Chronology Building: How to Get a Date

Outline

• Relative Dating• Absolute Dating• What Do Dates Mean?• The Check, Please• Dating in Historical Archaeology

Dating

• Absolute dates are expressed as specific units of scientific measurement —days, years, centuries, or millennia.

• Relative dates express relationships or comparisons: – Stepped pyramid at Saqqara in Egypt

is earlier than Khufu’s pyramid– Historic settlement of Williamsburg is

later than the pueblos of Chaco Canyon.

Keys to Relative Dating

1. The law of superposition.2. The index fossil concept.

Index Fossil Concept

• The idea that strata containing similar fossil assemblages are of similar age.

• This concept enables archaeologists to characterize and date strata within sites using distinctive artifact forms that research shows to be diagnostic of a particular period of time.

Potsherd Frequencies from Pueblo San Cristobal, New

MexicoDepth

Corrugated ware

Biscuit ware

Type I Type II Type III Total

1st foot

57(36.7)10

(6.5)2

(1.3)81

(52.2)5 (3.2)

155

2nd foot

116(31.3)17

(4.6)2

(.01)230 (62)

6 (1.6)371

3rd foot

27 (15.3) 2 (1.1)10

(5.7)134

(76.1)3 (1.7) 176

4th foot

28 (21.3) 4 (3)6

(4.5)93

(70.9)0 (0) 131

Potsherd Frequencies from Pueblo San Cristobal, New

MexicoDepth

Corrugated ware

Biscuit ware

Type I Type II Type III Total

5th foot

60 (17.3)15

(4.3)2

(.01)268

(77.6)0 (0)

345

6th foot

75 (18.6) 21 (5.2) 8 (1.9)297

(73.8)1(.01) 402

7th foot

53 (23.1) 10 (4.3)40

(17.5)126 (55)

0 (0) 229

Potsherd Frequencies from Pueblo San Cristobal, New

MexicoDepth

Corrugated ware

Biscuit ware

Type I Type IIType

IIITotal

8th foot

56 (24.6) 2 (.01)118

(51.9)51

(22.4)0 (0) 227

9th foot

93 (45.4) 1 (.01)107

(52.5)3 (1.4) 0 (0) 204

10th foot

84 (54.4) 1 (.01)69

(44.8)0 (0) 0 (0) 154

Total 649 83 364 1,283 15 2,39 4

Seriation Diagram: Nelson’s San Cristobal Potsherd

Frequencies.

Seriation

• A relative dating method.• Orders artifacts based on the

assumption that one cultural style slowly replaces an earlier style over time.

• With a master seriation diagram, sites can be dated based on their frequency of several artifact styles.

Seriation Diagram: Illumination in Pennsylvania

1850 - 1950

Methodology of Tree-Ring Dating

• Archaeologist digs up a sample of charcoal or wood with at least 20 rings.

• The sample is sent to the appropriate lab with contextual data.

• An analyst will cut or sand the sample down so the rings are easily visible, and the widths are then measured individually.

• The lab analyst tries to match the sample to the appropriate portion of the regional sequence.

Tree-ring Chronology

Radiocarbon Dating: How It Works

• There are three principal isotopes of carbon—12C, 13C, and 14C.

• Only one 14C atom exists for every trillion atoms of 12C in living material.

• 14C is produced in the upper atmosphere, where cosmic radiation creates neutrons that replace one of nitrogen’s protons (14N) to create 14C.

Radiocarbon Dating: How It Works

• 14C oxidizes and forms carbon dioxide, which is dispersed throughout the atmosphere by stratospheric winds.

• About 98% of all 14C enters the oceans; plants take up the rest through photosynthesis.

• From plants, it enters herbivores, and then carnivores. All organic life contains radioactive carbon.

Radiocarbon Dating: How It Works

• As long as an organism is alive, the amount of 14C in it remains in equilibrium with the atmosphere.

• Once the organism dies, the amount of 14C in its body begins to decrease.

• After 5730 years, Cambridge half-life, half of the 14C available in a sample will have converted to 14N.

• Radiocarbon dating is good for organic remains no more than 45,000 years old.

Radiocarbon Dating: How It Works

Example: • A sample contains 100 atoms of 14C.• After 5730 years, 50 of these atoms

have decayed into 14N.• After another 5730 years, half of the

remaining 50 14C atoms have converted to 14N, leaving only 25 14C atoms.

• After another 5730 years this would be halved again to about 12 14C atoms.

Radiocarbon Dating: Problems

• Photosynthetic pathways – The chemical process through

which plants metabolize carbon.– The three pathways discriminate

against carbon-13 differently so that similarly aged plants using different pathways may produce different radiocarbon ages.

Radiocarbon Dating: Problems

• Reservoir effect – When organisms take in carbon from

a source that is depleted of or enriched in 14C relative to the atmosphere.

– Such samples may return ages that are considerably older or younger than they actually are.

Radiocarbon Dating: Problems

• De Vries effects – Fluctuations in the calibration curve

produced by variations in the atmosphere’s carbon-14 content.

– Causes radiocarbon dates to calibrate to more than one calendar age.

Recommended Sample Sizes for Radiocarbon and AMS

DatingConventional

(Grams)Ams

(Milligrams)

Charcoal 10–30 20–50

Wood 15–100 20–100

Dung 10–30 20–100

Peat 10–30 30–100

Seeds n/a 20–50

Recommended Sample Sizes for Radiocarbon and AMS

DatingConventional

(Grams)Ams

(Milligrams)

Organic sediments

200–2000 2–10 grams

Bone/antler 200 2–10 grams

Shell 20–100 50–100

Pollen n/a 15

Water n/a 1 liter

Calibration curve

Trapped Charge Dating

Trapped Charge Dating Techniques

• Thermoluminescence– Used on anything mineral heated to more

than 500° C. (ceramics, burnt stone artifacts)• Optically stimulated luminescence

– Used to date sediments. The age is the time elapsed between the last time a few moments exposure to sunlight reset the clock to zero and the present.

• Electron spin resonance– Can be used to date teeth that are beyond

the range of radiocarbon dating.

Dosimeter

• A device to measure the amount of gamma radiation emitted by sediments.

• It is normally buried in a stratum for a year to record the annual dose of radiation.

• Dosimeters are often a short length of pure copper tubing filled with calcium sulfate.

Techniques For Dating the Formation of a Layer of Rock

• Potassium-argon dating– Monitors the decay of potassium (K-

40) into argon gas (Ar-40). • Argon-argon dating

– A high-precision method for estimating the relative quantities of argon-39 to argon-40 gas.

– Used to date volcanic ashes between 500,000 and several million years.

Summary of Absolute Dating Methods

Technique Target material Accuracy

Carbon-14Any organic

material; carbon is the most common.

To 45,000 BP

Thermo-luminescence

Ceramics, burnt stone

Perhaps back to 300,000

years

OSLQuartz, feldspars in

eolian sands

Perhaps back to 300,000

years

Summary of Absolute Dating Methods

Technique Target material Accuracy

Electronic spin

resonance

Tooth enamel, burned stone tools,

corals, shells

10,000 to 300,000 or more years

Potassium-argon

Volcanic ash200,000 to

several million years

Argon-argon Volcanic ash200,000 to

several million years

Old Wood Problem

• A potential problem with radiocarbon (tree-ring) dating in which old wood has been scavenged and re-used in a later archaeological site.

• The resulting date is not a true age of the associated activity.

Mean Ceramic Date

• Statistical technique for combining the median age of manufacture for temporally significant pottery types to estimate the age of a feature or site.

• Based on Noël Hume’s A Guide to Artifacts in Colonial America, the model includes selected ceramic types defined by attributes of form, decoration, surface finish, and hardness.

Quick Quiz

1. ________ dates are expressed as specific units of scientific measurement —days, years, centuries, or millennia, _______ dates express relationships or comparisons.

Answer: absolute, relative

• Absolute dates are expressed as specific units of scientific measurement —days, years, centuries, or millennia, relative dates express relationships or comparisons.

2. The ___ _____ _____ enables archaeologists to characterize and date strata within sites using distinctive artifact forms that research shows to be diagnostic of a particular period of time.

Answer: Index Fossil Concept

• The Index Fossil Concept enables archaeologists to characterize and date strata within sites using distinctive artifact forms that research shows to be diagnostic of a particular period of time.

3. Problems associated with radiocarbon dating include:

A. The chemical process through which plants metabolize carbon

B. The Reservoir effectC. The De Vries effectsD. B and C onlyE. A, B, and C.

Answer: E

3. Problems associated with radiocarbon dating include the chemical process through which plants metabolize carbon, the Reservoir effect and the De Vries effects.

4. Electron spin resonance can be used to date teeth that are beyond the range of radiocarbon dating.

A. TrueB. False

Answer: A. True

• Electron spin resonance can be used to date teeth that are beyond the range of radiocarbon dating.

5. With Trapped Charge Dating, sites can be dated based on their frequency of several artifact styles.

A. TrueB. False

Answer: B.False

• With a master seriation diagram, sites can be dated based on their frequency of several artifact styles.