Late Pliocene-Quaternary history of the northwestern Indian Ocean: an organic geochemistry perspective

The residue organic carbon (OC) content and stable isotopic compositions of OC and nitrogen for sediment samples from the Ocean Drilling Program (ODP) Leg 117, Sites 724 and 725, have been used to establish the history of the monsoon wind regime and its effect on coastal upwelling and primary productivity, in the northern Indian Ocean since the late Pliocene. Generally, residue OC content at Site 724 is greater than 2%; whereas at Site 725 it is less than 1%. Sediments deposited prior to the onset of the Pleistocene contain larger amounts of OC that have relatively depleted stable carbon and nitrogen isotope values. High primary productivity, reduced microbial activity as a result of the intrusion of highly saline waters from the Red Sea and the Persian Gulf, and/or enhanced preservation, produced these larger amounts of OC and relative depletion. -- The amount of organic matter preserved near the Pliocene-Pleistocene boundary is low due to decreased coastal upwelling, productivity and weak southwest monsoon winds. The lower amounts of organic matter preserved during the Pleistocene-Holocene period are generally associated with more positive stable nitrogen isotope values, owing to an intensification of the oxygen minimum zone and denitrification since the early Pleistocene. A correlation of the residue OC content with oxygen isotope stages, at Site 724, indicates slightly better preservation during interglacial periods due to increased coastal upwelling, primary productivity and/or preservation. Generally, stable nitrogen isotope compositions are relatively enriched at Site 725 than at Site 724. This may be attributed to the stable oxygen minimum zone and intense denitrification at shallower depths. A correlation of the OC isotope values with oxygen isotope stages indicates an enrichment in ¹³C during glacial periods. -- Based on stable isotope ratios, terrestrial input of organic matter into the Oman Margin is minimal owing to the lack of major rivers and low productivity in the source regions