Aspartic acid geochronology of sediments from the ocean margin off of India

Abstract

Aspartic acid racemization was studied in the clay and coarse fractions of sediments from southwestern Bay of Bengal (Core 1) in order to understand the role of terrestrial organic matter in the amino acid geochronology of Indian marginal sediments. Conventional radiocarbon ages were determined for the coarse carbonate fraction (>25 μm) of sediments, the core covering an age back to ∼40 kyr BP. Aspartic acid racemization rate constants ( k asp) were calculated for clay and coarse fractions of bulk sediments using a first order kinetic model and three horizons with constant k asp values were identified, corresponding to the periods from 0 to 5.5 kyr, 5.5 to 19 kyr and >19 kyr. Best fit models of high significance were derived from the relationship between aspartic acid D/ L ratio and calibrated radiocarbon ages as an alternative to racemization rate constants. To test the applicability of these first order kinetic and logarithmic models derived for Core 1, they were used to calculate ages for another sediment core (Core 2), calculated aspartic acid ages being consistent with radiocarbon ages. However, the logarithmic model gave better results than the first order kinetic model for older samples, suggesting that the logarithmic model is a better choice for the amino acid geochronology of Indian margin sediments. For both sediment fractions, aspartic acid ages based on the logarithmic model gave similar ages, consistent with radiocarbon ages and suggesting that there is no significant difference in aspartic acid racemization between both fractions of bulk sediment. As a result, the clay fraction can also be used for amino acid dating of Indian margin sediments.