Aspartic acid racemization and age-depth relationships for organic carbon in Siberian permafrost

We have analyzed the degree of racemization of aspartic acid in permafrost samples from Northern Siberia, an area from which microorganisms of apparent ages up to a few million years have previously been isolated and cultured. We find that the extent of aspartic acid racemization in permafrost cores increases very slowly up to an age of similar to25,000 years (around 5 m in depth). The apparent temperature of racernization over the age range of 0-25,000 years, determined using measured aspartic acid racernization rate constants, is -19degreesC. This apparent racernization temperature, is significantly lower than the measured environmental temperature (-11 to -13degreesC) and suggests active recycling Of D-aspartic acid in Siberian permafrost up to, an age of around 25,000 years. This indicates that permafrost organisms are capable of repairing some molecular damage incurred while in a "dormant" state over geologic time.