Variations of oxygen-minimum and primary productivity recorded in sediments of the Arabian Sea

Two deep-sea sediment cores from the northeastern and the southeastern Arab ian Sea were studied in order to reconstruct the palaeoenvironments of the past glacial cycles. Core 136KL was recovered from the high-productivity ar ea off Pakistan within the modern oxygen-minimum zone (OMZ), By contrast, m odem primary productivity at the site of MD900963 close to Maldives is mode rate and bottom waters are today well oxygenated. For both cores, we recons tructed the changes in palaeoproductivity using a set of biomarkers (alkeno nes, dinosterol and brassicasterol); the main result is that primary produc tivity is enhanced during glacial stages and lowered during interstadials. The proxies associated with productivity show a 23 kyr cyclicity correspond ing to the precession-related insolation cycle. Palaeoredox conditions were studied in both cores using a new organic geochemical parameter (C-35/C-31 -n-alkane ratio) developed by analysing surface sediments from a transect a cross the OMZ off Pakistan, The value of this ratio in core 136KL shows man y variations during the last 65 kyr, indicating that the OMZ was not stable during this time: it disappeared completely during Heinrich-and the Younge r Dryas events, pointing to a connection between global oceanic circulation and the stability of the OMZ. The C-35/C-31 ratio determined in sediments of core MD900963 shows that bottom waters remained rather well oxygenated o ver the last 330 kyr, which is confirmed by comparison with authigenic meta l concentrations in the same sediments. A zonally averaged, circulation-bio geochemical ocean model was used to explore how the intermediate Indian Oce an responds to a freshwater flux anomaly at the surface of the North Atlant ic. As suggested by the geochemical time series, both the abundance of Sout hern Ocean Water and the oxygen concentration are significantly increased i n response to this freshwater perturbation, (C) 1999 Elsevier Science B.V. All rights reserved.