Geoscience Reference
In-Depth Information
Appendix
Dating techniques
Radiocarbon dating
Radiocarbon dating relies upon the decay of
14
C, relative to the concentration of
12
C, within an organism after its death.
14
Cisproduced in the upper atmosphere by the
bombardment of
14
Nbycosmic radiation. The
14
Cisthen transported to the Earth's surface by
atmospheric activity such as storms and becomes fixed in the biosphere. It becomes attached
to complex organic molecules through photosynthesis in plants and animals ingesting those
plants in turn to absorb the
14
C along with
12
C and
13
C. When the organism dies, the ratio of
14
C within its carcass begins to gradually decrease. The
14
C half-life is 5730 years which means
that half of the
14
Cpresent at the time of death will decay over this time period.
The ratio of
14
Cto
12
Cinthe atmosphere has not remained constant over time. This is due
to variations in the intensity of the cosmic radiation bombardment of the Earth, and changes
in the effectiveness of the Van Allen belts and the upper atmosphere to deflect that
bombardment. Comparisons between radiocarbon dates and those from other independent
techniques such as dendrochronology and coral growth ring chronologies have allowed
thedevelopment of calibration tables that outline these variations over time. Radiocarbon
dates are calibrated against these tables to convert the result from radiocarbon to calendar
years.
Cosmogenic nuclide dating
The method is based upon the same idea as radiocarbon dating of
14
Cproduced in
the atmosphere. Cosmogenic nuclides are produced by the interactions of cosmic rays with
atoms in a soil or rock surface to produce
3
He,
10
Be,
14
C,
21
Ne,
26
Al and
36
Cl.
36
Cl for example,
is produced by spallation reactions of
39
K and
40
Ca, and by the activation of
35
Ca after
interactions with cosmic rays. The accumulation rate of these nuclides is proportional to the
cosmic ray flux and to the concentration of target nuclides in the surface material. Thus, the
concentration of cosmogenic nuclides in sampling data can be used to determine the length
of time the sample has spent at or near Earth's surface. The production rates of the nuclides
are low, thus they can only be detected using high-resolution mass spectrometers. Accelerator
268
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