The racemization reaction of aspartic acid in bone has a half-life of 3,500 years at 25°C. The reaction is sensitive to temperature, and thus can be used to estimate the average temperature which a bone has experienced since it was deposited, if its age can be determined by an alternative method such as radiocarbon dating. We tried to estimate the palaeotemperatures for three kinds of fossils by measuring D/L aspartic acid ratios and radiocarbon ages of the XAD-2 treated collagen-hydrolysate fractions in the fossils. The D/L aspartic acid ratio was measured with a gas chromatograph, and radiocarbon dating was performed by a Tandetron AMS system at Nagoya University.
For four animal bone fossils collected from the shell mound excavated at Awazu submarine archeological site in Lake Biwa, Shiga, Japan, the average temperature was calculated to be 13.2±1.1°C, using the D/L aspartic acid ratio of 0.11 and 14C age of 4,500 BP [1]. The temperature roughly matches its present temperature, and it suggests that there have been no large temperature fluctuations of long duration. The result agrees with the fact that Lake Biwa could not have experienced large environmental change during last 4,500 years.
The radiocarbon age of a Mammoth molar fossil collected from Bykovsky Peninsula, eastern Siberia, was obtained to be 35,080±390 BP. The aspartic acid in the Mammoth molar, with too small D/L ratio, has hardly racemized since it was dead and buried. This result is well consistent with the fact that it was preserved in permafrost.
The ratio of D/L aspartic acid of 0.13 was obtained for a Naumann's elephant molar fossil collected from the bottom of the Uwa Sea, Ehime, Japan. The value gives the average temperature of 4.1±0.1°C, using the radiocarbon age of 43,870±450 BP [2]. The Uwa Sea exists in the Seto Inland Sea, which was a land before 10,000 years, in a glacial period, and has been in the present state since 8,000 years ago. Therefore, the surrounding temperature of the molar fossil could have changed. The temperature of 4°C could indicate the actual temperature, to which the fossil has been exposed.
[1] Minami and Nakamura, Nucl. Instr. and Meth. B172, 462-468 (2000), [2] Minami et al., Nucl. Instr. and Meth. B223-224, 302-307 (2004)
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See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)