Radiocarbon dating

The physics of decay and origin of carbon 14 for the radiocarbon dating
1: Formation of Carbon-14.
2: Decay of Carbon-14.
3: The “equal” equation is for living organisms, and the unequal one is for non-living ones, in which the C-14 then decays (hence the 2).
From: Wikimedia Commons

We can indirectly date glacial sediments by looking at the organic materials above and below glacial sediments. Radiocarbon dating provides the age of organic remains that overly glacial sediments. It was one of the earliest techniques to be developed, during the 1940s. Radiocarbon dating works because an isotope of carbon, 14C, is constantly formed in the atmosphere by interaction of carbon isotopes with solar radiation and free neutrons. Living organisms absorb carbon (for example, we breathe it in). This carbon is therefore present in their bodies and bones. Upon death, no more 14C is absorbed and it starts to decay. By measuring the amount of 14C  in an organism, we can ascertain when it died. The short half-life of 14C means that it does not work for organisms that died after about 40,000 years ago.

In the figure right, the production of radio-active carbon is demonstrated. Here, 7 protons and 7 neutrons (N) plus one neutron form an isotope of carbon, with 8 neutrons and 6 protons[1].

This radioactive 14C isotope eventually decays to the stable element 14N, where 8 neutrons plus 6 protons (14C) decay to 7 neutrons to 7 protons (N + β). This decay is by beta transformation, with the emission of βparticles[1]. These 14C atoms are rapidly oxidised into carbon dioxide (12CO2), and are then absorbed by living organisms and oceans.

In Antarctica, where organic remains are rare, this usually means dating microscopic marine organisms in glaciomarine muds that overly glacial tills and sediments on the continental shelf[2-4]. Radiocarbon dating marine organisms has added complications in Antarctica, because around the Antarctic continent old deep ocean currents up well. These currents are contaminated with ‘old’ carbon, meaning that marine organisms alive today have a radio-carbon age of about 1200 years[5, 6].

Rates of radiocarbon production vary through time, in a quasi-periodic manner[1]. It is therefore necessary to distinguish between radiocarbon years (14C) and calendar years. These two ages can be reconciled using calibration against a chronology of calendar years. Tree ring data has been widely used to calibrate the timescales, as tree rings provide an annual calendar year, and the wood can be radiocarbon dated to provide a calibration.

References


1.            Lowe, J.J. and M.J.C. Walker, 1997. Reconstructing Quaternary Environments. 2nd Edition. Harlow, England: Prentice Hall. 446.

2.            Graham, A.G.C. and J.A. Smith, 2012. Palaeoglaciology of the Alexander Island ice cap, western Antarctic Peninsula, reconstructed from marine geophysical and core data. Quaternary Science Reviews, 35(0): 63-81.

3.            Heroy, D.C. and J.B. Anderson, 2007. Radiocarbon constraints on Antarctic Peninsula Ice Sheet retreat following the Last Glacial Maximum (LGM). Quaternary Science Reviews, 26(25-28): 3286-3297.

4.            Hillenbrand, C.-D., M. Melles, G. Kuhn, and R.D. Larter, 2012. Marine geological constraints for the grounding-line position of the Antarctic Ice Sheet on the southern Weddell Sea shelf at the Last Glacial Maximum. Quaternary Science Reviews, 32(0): 25-47.

5.            Davies, B.J., M.J. Hambrey, J.L. Smellie, J.L. Carrivick, and N.F. Glasser, 2012. Antarctic Peninsula Ice Sheet evolution during the Cenozoic Era. Quaternary Science Reviews, 31(0): 30-66.

6.            Hall, B.L., G.M. Henderson, C. Baroni, and T.B. Kellogg, 2010. Constant Holocene Southern-Ocean 14C reservoir ages and ice-shelf flow rates. Earth and Planetary Science Letters, 296: 115-123.

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