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