Dating TechniquesFour CategoriesRadio-isotope methodsPaleomagnetic methodsOrganic/inorganic chemical methodsBiological 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 MethodBased on disintegration of unstable nucleiNegatron decay (n p+ + b- + energy)
Positron decay (p+ n + b+ + energy)
Alpha decay (AX A-4Y + He)
Radioactivity-ConceptsHalf-life (t1/2 ): N= N0/2Mean life: t=1/lActivity: # radioactive disintegrations/sec (dps)Specific activity: dps/wt. or dps/volUnits: Becquerel (Bq)=1 dps
Decay Rates: Ln (No/N) = lt t = t*Ln (No/N)
To be a useful for dating, radio-isotopes must:be measurable have known rate of decayhave appropriate t1/2 have known initial concentrationsbe a connection between event and radioisotope
Radioactivity-based DatingQuantity 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 Dating12C: 42*1012; 13C: 47*1010; 14C: 62 tonst1/2 = 5730 yr l= 1.0209*10-4/yrFormed in the atmosphere:14N + 1n 14C + 1HDecay: 14C 14N + b-
W.F. Libbys discovery of radiocarbonS. Korffs discovery: cosmic rays generate ~2 neutrons/cm2sec14C formed through nuclear reaction.14C readily oxidizes with O2 to form 14CO2Libbys t1/2 = 5568 yr.
Conventional Radiocarbon DatingCurrent t1/2 = 573040 yrt=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/gAstandard in 2000 = 0.225 Bq/g
Conventional Radiocarbon datingActivity of 14C needs to be normalized to the abundance of carbon: D14C: normalized valueD14C() = d14C 2(d13C+25)(1+d13C/103)d14C() = (1-Asample/Astandard)*103Radiocarbon age = 8033*ln(1+ D14C/103)
Conventional Radiocarbon datingPrecision has increasedRadiocarbon disintegration is a random process. If date is 5000100:68% chance is 4900-510099% chance is 4700-5300
Radiocarbon dating-CorrectionsRadiocarbon can be corrected by using tree-ring chronology.Radiocarbon dates can then be converted into Calendar years (cal yr).
Radiocarbon dating-ProblemsTwo 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-radiocarbonNuclear testing significantly increased D14C
Bomb 14C can be used as a tracer
Radiocarbon dating-conclusionPrecise 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-isotopesK-Ar40K simultaneously decays to 40Ca and 40Ar(gas)t1/2=1.3*109 yr (useful for rocks >500 kyrAmount of 40Ar is time-dependentProblems: Assumes that no 40Ar enters or leaves the systemLimited to samples containing K
Other radio-isotopesUranium series236U and 238U decay to 226Ra and 230ThU is included in carbonate lattice (e.g., corals)Age determined on the abundance of decay products Problems: Assumes a closed systemAssumes 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-ApplicationsArchaeological artifactsHeating (>500oC) re-sets TL signal to zeroUsed for dating pottery and baked sediments SedimentsExposure to sunlight re-sets the clockUsed for dating loess, sand dunes, river sand.
TL-ProblemsDifferent response to ionization# lattice defectssaturationIncomplete re-setting Water can absorb radiationUnknown amount of ionization
Fission-Track Dating238U can decay by spontaneous fissionSmall 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