Downcore diagenetic changes in organic matter and implications for paleoproductivity estimates

Particulate amino acid (AA) content of a 41.85 m long IMAGES core (MD01-2379), collected in 560 m water depth offshore from Northwest Australia, was analyzed to study organic matter (OM) degradation and preservation through glacial–interglacial intervals. A comparison of stable oxygen isotope (δ18O) records of the planktonic foraminifera Globigerinoides ruber with the SPECMAP δ18O profile, suggests a core bottom age of about 486 ky, and an average sedimentation rate of about 9.4 cm ky− 1. Total hydrolysable amino acid (THAA) concentrations are the lowest (240 μg g− 1) in the deeper part of the core, and the highest (1820 μg g− 1) at about 80 cm below the core top. The glacial–interglacial fluctuations in THAA are clearly visible in the upper half of the core, but less prominent in the lower half. An exponential decrease in THAA with increasing depth implies that AAs are decomposed/recycled even after their rapid decomposition at the water/sediment interface. Depth distribution profiles of organic carbon (OC), total nitrogen and THAA indicate long term degradation of OM in deeply buried sediments. An equation has been defined to correct for the loss of OC due to this degradation. Corrected values of OC concentration are significantly higher than those actually measured. We suggest that these corrected values more accurately reflect the OC initially deposited on the sea floor. Thus, primary productivity estimates based on the ‘corrected’ OC content and its interpretation in the light of logarithmic degradation/preservation of labile OM have potential to improve current estimation of paleoproductivity and its fluctuation with glacial–interglacial variations.