COMPARATIVE-STUDY OF ORGANIC-MATTER PRESERVATION IN IMMATURE SEDIMENTS ALONG THE CONTINENTAL MARGINS OF PERU AND OMAN .1. RESULTS OF PETROGRAPHIC AND BULK GEOCHEMICAL DATA

Detailed petrographical and bulk geochemical investigations of organic matter (OM) have been performed on sediments deposited below or close to upwelling areas offshore Peru (ODP-Leg 112; Sites 679, 681. 688) a nd Oman (ODP-Leg 117; Sites 720, 723, 724) in order to obtain a quanti tative understanding of its accumulation and degradation. Microscopica l as well as nanoscopical investigations reveal that the OM in sedimen ts affected by upwelling mechanisms mainly (up to 98%) consists of uns tructured (amorphous) organic aggregates without any apparent biologic al structures. In sediments which are not or to a lesser extent affect ed by upwelling (Site 720) terrestrial OM predominates. Organic carbon (TOC) contents are highly variable and range between 9.8% in sediment s deposited below upwelling cells and 0.2% in sediments outside the up welling zone. The TOC/sulphur ratios of the sediments scatter widely. The samples from the deep-water locations (Sites 688 and 720), show C/ S-ratios of ''normal'' marine sediments, whereas at the other location s no correlation or even a negative correlation between sulphur and TO C concentration exists. In most of the upwelling-influenced sediments OM contains a significant amount of sulphur. The incorporation of sulp hur into the OM followed microbial sulphate reduction and occurred in the upper meters of the sedimentary column. Below, OM is still present in vast amounts and relatively hydrogen-rich, but is nevertheless non -metabolizable and becomes the limiting factor for bacterial sulphate reduction. According to mass balance calculations 90-99% of the OM pro duced in the photic zone was remineralized and 1-3% was consumed by mi crobial sulphate reduction. The aerobic and anaerobic processes have g reatly affected degradation and conservation of OM. Copyright (C) 1996 Elsevier Science Ltd.