Dating%20Techniques

1
Dating Techniques

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

2
(No Transcript)
3
(No Transcript)
4

• Relative dating
• Chronological succession (e.g.,
  dendrochronology).
• Synchronous events (e.g. volcanic ash).
• Absolute dating
• Recognition of time-dependent processes (e.g.,
  radioactivity).

5
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)

6
Radioactivity-Concepts

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

7

• Decay Rates
• Ln (No/N) lt
• t tLn (No/N)

8
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

9
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).

10
Radiocarbon Dating

• 12C 421012 13C 471010 14C 62 tons
• t1/2 5730 yr
• l 1.020910-4/yr
• Formed in the atmosphere
• 14N 1n 14C 1H
• Decay
• 14C 14N b-

11
(No Transcript)
12
W.F. Libbys discovery of radiocarbon

• S. Korffs discovery cosmic rays generate 2
  neutrons/cm2sec
• 14C formed through nuclear reaction.
• 14C readily oxidizes with O2 to form 14CO2
• Libbys t1/2 5568 yr.

13
Conventional Radiocarbon Dating

• Current t1/2 573040 yr
• t8033Ln(Asample/Astandard), where Aactivity.
• 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

14
Conventional Radiocarbon dating

• Activity of 14C needs to be normalized to the
  abundance of carbon
• D14C normalized value
• D14C() d14C 2(d13C25)(1d13C/103)
• d14C() (1-Asample/Astandard)103
• Radiocarbon age 8033ln(1 D14C/103)

15
Conventional Radiocarbon dating

• Precision has increased
• Radiocarbon disintegration is a random process.
• If date is 5000100
• 68 chance is 4900-5100
• 99 chance is 4700-5300

16
(No Transcript)
17
Radiocarbon dating-Problems
18
Radiocarbon dating-Corrections

• Radiocarbon can be corrected by using tree-ring
  chronology.
• Radiocarbon dates can then be converted into
  Calendar years (cal yr).

19
(No Transcript)
20
(No Transcript)
21
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.

22
(No Transcript)
23
(No Transcript)
24
Deviation of the initial radiocarbon activity.
25
(No Transcript)
26
Bomb-radiocarbon
Nuclear testing significantly increased D14C
27
Bomb 14C can be used as a tracer
28
(No Transcript)
29
(No Transcript)
30
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.

31
(No Transcript)
32
Other Radio-isotopes

• K-Ar
• 40K simultaneously decays to 40Ca and 40Ar(gas)
• t1/21.3109 yr (useful for rocks gt500 kyr
• Amount of 40Ar is time-dependent
• Problems
• Assumes that no 40Ar enters or leaves the system
• Limited to samples containing K
• U-series

33
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.

34
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

35
TL-Applications

• Archaeological artifacts
• Heating (gt500oC) 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.

36
TL-Problems

• Different response to ionization
• lattice defects
• saturation
• Incomplete re-setting
• Water can absorb radiation
• Unknown amount of ionization

37
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 (gt200 kyr)
• Problem tracks can heal over time