Dating Techniques
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Transcript of Dating Techniques
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Dating Techniques
• Four Categories– Radio-isotope methods– Paleomagnetic methods– Organic/inorganic chemical methods– Biological methods
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• Relative dating:– Chronological succession (e.g., dendrochronology).
– Synchronous events (e.g. volcanic ash).• Absolute dating:
– Recognition of time-dependent processes (e.g., radioactivity).
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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)
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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
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• Decay Rates: Ln (No/N) = t
t = Ln (No/N)
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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
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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).
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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 + -
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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.
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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
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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)
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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
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Radiocarbon dating-Problems
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Radiocarbon dating-Corrections
• Radiocarbon can be corrected by using tree-ring chronology.
• Radiocarbon dates can then be converted into “Calendar years” (cal yr).
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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.
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Deviation of the initial radiocarbon activity.
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Bomb-radiocarbon
Nuclear testing significantly increased 14C
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Bomb 14C can be used as a tracer
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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.
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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
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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.
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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
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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.
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TL-Problems
• Different response to ionization– # lattice defects– saturation
• Incomplete re-setting • Water can absorb radiation• Unknown amount of ionization
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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