Chronology of marine sediments by the racemization reaction of aspartic acid in planktonic foraminifera

Racemization reaction rate constants of aspartic acid (kASP) were obtained and compared in bulk foraminifera and in a dissolution resistant foraminiferal species Pulleniatina obliquiloculata obtained from marine sediments through ca. 20,000 yrBP in the western equatorial Pacific. The average kAsp values were 0.63 × 10−5 yr−1 for bulk foraminifera and 0.72 × 10−5 yr−1 for P. obliquiloculata. This disparity in kAsp values could be attributed to the presence of dissolution vulnerable foraminifera in bulk foraminifera. It is believed that the leaching of racemized free amino acid (having high D/L values) when the calcareous test was subjected to dissolution through geological time could have led to a lower value of kAsp in bulk foraminifera. This study thus demonstrates that it is important to analyze a single species of planktonic foraminifera resistant to dissolution in order to avoid spurious changes in kAsp values. If this condition were satisfied, aspartic acid racemization is shown to be extremely useful, not only for determining the geological age of marine sediment, but also to assess the quality of the sedimentary core stratigraphy.