Organic matter (OM) buried in sediments at the Oman margin in the Arabian Sea has been characterised at the molecular level. For the first time, replicate analyses of lipid biological markers (biomarkers) in sediments from approximate water depths of 400, 800, 1000 and 1250 m have been carried out in conjunction with analyses of TOC, TN, grain size, the stable isotopes of carbon (*13C) and nitrogen (*15N), and total hydrolysable amino acids (THAA).

The main aims of this research were to:
1) elucidate the sources of OM to the sediments.
The high level of primary production in the overlying waters is reflected by the lipid distributions in these sediments, but the biomarkers do not provide evidence for intense re-working by zooplankton in the euphotic zone. This may reflect rapid disaggregation of large faecal particles in the water column. Zooplankton markers may be better preserved in sediments underlying deeper waters. Bacterial biological markers allow a tentative distinction between a pelagic and benthic signal, the former increasing in relative importance with increasing water depth. The observed trend may also mirror the intensity of the OMZ at the margin.

2) to gain a better understanding of the processes that control OM preservation within an oxygen depleted environment.
Most accrued data underpin the hypothesis that oxygen is not the controlling factor in the preservation of carbon at the Oman Margin. OM deposition is, however, controlled by grain size which is in turn a function of the physical dynamics of the water column. Physical events such as slumping can be of importance in the preservation of OM at the margin.
Changes in OM composition that can be attributed to steady state diagenesis are limited and only evident in two of the twelve cores analysed down-slope. This is perplexing, since TOC and TN contents of the sediments are high and might be thought attractive for bacterial degradation. One explanation is that the indigenous OM is re-worked in the water column, prior to deposition. OM degradation state appears to decrease down-slope which may be coupled with a decrease in water column residence time.

3) assess the impact of benthic fauna on the quality, quantity and distribution of OM within marine sediments.
The high density of benthic macrofauna (polychaetes) present at this margin are thought to have a substantial impact on the distribution of biomarkers in the sediments and on the recycling and preservation of OM at the 800 m site.
The crustacean Encephaloides armstrongi was observed in extremely high numbers in a band from approximately 600 to 1000 m, reaching a maximum of 137 individuals m-2 at a water depth of approximately 1000 m. An ophiuroid (Ophiolimna antartic) was also present in abundance at this site. OM concentrations at the 1000 m site are severely depleted and biomarkers have a distribution indicative of invertebrates. It is clear that these megafauna are directly influencing the quality and quantity of OM buried in these, organic rich sediments.

This research presents the first evidence that benthic fauna control the deposition of OM in marine sediments.