ON THE ORGANIC-CARBON MAXIMUM ON THE CONTINENTAL-SLOPE OF THE EASTERNARABIAN SEA

The sedimentary organic carbon maximum an the continental slope off we stern India is widely believed to be due to the preferential preservat ion of deposited organic matter at water depths where the intense oxyg en minimum intersects the sea floor. This region is considered to cons titute one of the modern analogues for the environment of formation of organic-rich sedimentary facies that are common in the geological rec ord. We critically examine the hypothesis that the oxygen minimum in t he eastern Arabian Sea is the site of enhanced organic matter accumula tion and preservation using analyses of suites of samples with wide ge ographical coverage along this margin. Organic carbon and nitrogen rea ch maximum concentrations between 200 and 1600 m depth, whereas the lo west dissolved oxygen contents in the oxygen minimum lie between 200 a nd 800 m depth. The C-organic/N ratios and the delta(13)C(organic) val ues show that the organic matter is overwhelmingly marine, and Rock-Ev al pyrolysis data demonstrate that the hydrogen indices of the sedimen ts are similar in the sediments accumulating within and outside the ox ygen minimum. Thus, the organic carbon maximum extends over a larger d epth range than the oxygen minimum (as is also evident on some other s lopes), and there is no evidence for preferential preservation of the organic matter within the oxygen minimum. The distribution of organic matter on the western Indian continental margin is controlled by (1) v ariations in supply (decreasing westward away from the centers of coas tal upwelling and also decreasing with increasing water depth), (2) di lution by other sedimentary components, and (3) the texture of the sed iments (coarser-grained sediments having lower carbon contents), which is controlled in turn by sediment supply and reworking. The evidence available suggests that the organic carbon maximum on this slope is no t related to the position of the oxygen minimum and, consequently, tha t oxygen minima cannot be used to explain the distribution of organic carbon at intermediate palaeodepths in the geological record.