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+ + b- + energy) – Positron decay (p+ n + b+ + energy) – Alpha decay (AX A-4Y + He) Radioactivity-Concepts • Half-life (t1/2 ): N= N0/2 • Mean life: t=1/l • Activity: # radioactive disintegrations/sec (dps) • Specific activity: dps/wt. or dps/vol • Units: Becquerel (Bq) =1 dps • Decay Rates: Ln (No/N) = lt t = t*Ln (No/N) To be a useful for dating, radioisotopes 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 • l= 1.0209*10-4/yr • Formed in the atmosphere: 14N + 1n 14C • Decay: 14C 14N + b- + 1H 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: • D14C: “normalized value” D14C(‰) = d14C –2(d13C+25)(1+d13C/103) d14C(‰) = (1-Asample/Astandard)*103 • Radiocarbon age = 8033*ln(1+ D14C/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 D14C 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