DATING DEEP-LAKE SEDIMENTS USING AMINO ACID RACEMIZATION IN FOSSIL OSTRACODES

The long-term rate of racemization for amino acids preserved in fossil ostracode shells was determined using independently dated sediment cores from five deep lakes. The racemization rate for aspartic and glutamic acids in the common ostracode genus, Candona, was calibrated to 100,000 years, providing the basis for an age equation with a realistic age uncertainty of about ±20%. The new age equation can be applied to other hypolimnic settings because the temperature below the thermocline of deep lakes from middle- and high-latitude regions can reasonably be assumed to have remained close to 4°C. The sample size required for analysis is an order of magnitude less than for ¹⁴C dating and presents new opportunities to date deposits that are organic poor or suffer from large ¹⁴C reservoir effects. A total of 808 individual Candona shells were analyzed using reverse-phase HPLC from 81 stratigraphic levels in 10 cores from five lakes. The chronology for each core is based on ¹⁴C dating and other evidence published previously. Most of the samples were from Bear Lake, Utah/Idaho. Enantiomeric (D/L) ratios in ostracodes from Lake Constance and Lake Ammersee generally overlap with those in similar-age shells from Bear Lake. D/L ratios from Lake Issyk-kul and Lake Michigan are generally either somewhat lower or higher (respectively) than those in similar-age samples from Bear Lake. The offsets might reflect slight differences in bottomwater temperature histories among lakes. The rate of racemization determined from the deep-lake samples can also be compared with an entirely independent estimate of the racemization rate that was formulated previously based on the extent of racemization in shells heated in the laboratory, combined with shells dated by ¹⁴C, whose temperature history was assumed from weather data. The concordance of two independent estimates of the long-term racemization rates, especially for aspartic acid, lends confidence to the ages derived by amino acid geochronology. This work builds upon the valuable contributions of Glenn Goodfriend's research into racemization geochemistry.