Dating Techniques

• Four Categories– Radio-isotope methods– Paleomagnetic methods– Organic/inorganic chemical methods– Biological methods

• Relative dating:– Chronological succession (e.g., dendrochronology).

– Synchronous events (e.g. volcanic ash).• Absolute dating:

– Recognition of time-dependent processes (e.g., radioactivity).

Radio-isotopic Method

• Based on disintegration of unstable nuclei– Negatron decay (n p+ + - + energy)

– Positron decay (p+ n + + + energy)

– Alpha decay (AX A-4Y + He)

Radioactivity-Concepts

• Half-life (t1/2 ): N= N0/2• Mean life: =1/• Activity: # radioactive disintegrations/sec (dps)• Specific activity: dps/wt. or dps/vol• Units: Becquerel (Bq) =1 dps

• Decay Rates: Ln (No/N) = t

t = Ln (No/N)

To be a useful for dating, radio-isotopes must:

• be measurable • have known rate of decay• have appropriate t1/2 • have known initial concentrations• be a connection between event and

radioisotope

Radioactivity-based Dating

• Quantity of the radio-isotope relative to its initial level (e.g., 14C).

• Equilibrium /non-equilibrium chain of radioactive decay (e.g., U-series).

• Physical changes on sample materials caused by local radioactive process (e.g., fission track).

Radiocarbon Dating

• 12C: 42*1012; 13C: 47*1010; 14C: 62 tons• t1/2 = 5730 yr• = 1.0209*10-4/yr• Formed in the atmosphere:

14N + 1n 14C + 1H• Decay:

14C 14N + -

W.F. Libby’s discovery of radiocarbon

• S. Korff’s discovery: cosmic rays generate ~2 neutrons/cm2sec

• 14C formed through nuclear reaction.

• 14C readily oxidizes with O2 to form 14CO2

• Libby’s t1/2 = 5568 yr.

Conventional Radiocarbon Dating

• Current t1/2 = 5730±40 yr

• t=8033*Ln(Asample/Astandard), where A:activity.• Oxalic acid is the standard (prepared in 1950).• Dates reported back in time relative to 1950

(radiocarbon yr BP).• Astandard in 1950 = 0.227 Bq/g

• Astandard in 2000 = 0.225 Bq/g

Conventional Radiocarbon dating

• Activity of 14C needs to be “normalized” to the abundance of carbon:

• 14C: “normalized value”14C(‰) = 14C –2(13C+25)(1+13C/103)14C(‰) = (1-Asample/Astandard)*103

• Radiocarbon age = 8033*ln(1+ 14C/103)

Conventional Radiocarbon dating

• Precision has increased• Radiocarbon

disintegration is a random process.

• If date is 5000±100:• 68% chance is 4900-5100• 99% chance is 4700-5300

Radiocarbon dating-Problems

Radiocarbon dating-Corrections

• Radiocarbon can be corrected by using tree-ring chronology.

• Radiocarbon dates can then be converted into “Calendar years” (cal yr).

Radiocarbon dating-Problems

• Two assumptions:– Constant cosmic ray intensity.– Constant size of exchangeable carbon reservoir.

• Deviation relative to dendrochronology due to:– Variable 14C production rates.– Changes in the radiocarbon reservoirs and rates of

carbon transfer between them.– Changes in total amount of CO2 in atmosphere,

hydrosphere, and atmosphere.

Deviation of the initial radiocarbon activity.

Bomb-radiocarbon

Nuclear testing significantly increased 14C

Bomb 14C can be used as a tracer

Radiocarbon dating-conclusion

• Precise and fairly accurate (with adequate corrections).

• Useful for the past ~50,000 yr.• Widespread presence of C-bearing

substrates.• Relatively small sample size (specially for

AMS dates).• Contamination needs to be negligible.

Other Radio-isotopes

• K-Ar– 40K simultaneously decays to 40Ca and 40Ar(gas)– t1/2=1.3*109 yr (useful for rocks >500 kyr– Amount of 40Ar is time-dependent– Problems:

• Assumes that no 40Ar enters or leaves the system• Limited to samples containing K

• U-series

Other radio-isotopes

• Uranium series– 236U and 238U decay to 226Ra and 230Th– U is included in carbonate lattice (e.g., corals)– Age determined on the abundance of decay

products – Problems:

• Assumes a closed system• Assumes known initial conditions.

Thermo-luminescence (TL)

• TL is light emitted from a crystal when it is heated.

• TL signal depends on # e- trapped in the crystal.• Trapped e- originate from radioactive decay of

surrounding minerals.• TL signal is proportional to time and intensity.• Useful between 100 yr and 106 yr

TL-Applications

• Archaeological artifacts– Heating (>500oC) re-sets TL signal to zero– Used for dating pottery and baked sediments

• Sediments– Exposure to sunlight re-sets the “clock”– Used for dating loess, sand dunes, river sand.

TL-Problems

• Different response to ionization– # lattice defects– saturation

• Incomplete re-setting • Water can absorb radiation• Unknown amount of ionization

Fission-Track Dating

• 238U can decay by spontaneous fission• Small “tracks” are created on crystals

(zircon, apatite, titanite) and volcanic glass.• Track density is proportional to U-content

and to time since the crystal formed.• Useful for dating volcanic rocks (>200 kyr)• Problem: tracks can “heal” over time