Nature of organic matter in surface sediments from the Pakistan continental margin and the deep Arabian Sea: amino acids

The role of organic carbon input, sedimentation rate and bottom water oxyge nation (BWO) for organic matter (OM) degradation was investigated using sur face sediments collected from above, within and below the oxygen minimum zo ne (OMZ) within varying sedimentary regimes along the Pakistan continental margin. For this, samples from 47 box cores and multi cores were analyzed f or their bulk component (organic carbon, total nitrogen, carbonate, opal) a nd amino acid concentrations, The results are compared to plankton net samp les from the Arabian Sea, sinking particles collected at the Pakistan conti nental margin, cultured sedimentary deep-sea bacteria, and data from the de ep Arabian Sea basins. High organic carbon and amino acid concentrations we re found in the laminated sediments of the OMZ, in bioturbated sediments im mediately below the OMZ, and below regions of high productivity in the west ern Arabian Sea. The contribution of amino acids to organic carbon and tota l nitrogen as well as the molar contribution of certain non-protein amino a cids do not mirror these differences in OC contents. No clear trend in rela tion to sediment structure or water depth is observed. In contrast to these established indicators of OM degradation, the molar contribution of the ar omatic amino acid tyrosine (Tyr) exhibits a distinct pattern. Its contribut ion is relatively high in sediments at depths above the OMZ and further inc reases towards the center of the OMZ. Below the OMZ, it decreases continuou sly with increasing water depth. In addition, for samples from similar dept hs and bottom water oxygen contents Tyr shows systematic differences betwee n varying sedimentary regimes. The lack of a distinct trend for the establi shed indicators does not support the idea of a predominant control of BWO o n quantitative OM preservation. Nevertheless, the variability of Tyr in rel ation to water depth or the sedimentary regime suggests that (i) the produc tivity-related OC input, (ii) the bulk accumulation rate, and (iii) BWO inf luence the alteration of the sedimentary OM by controlling its oxygen expos ure rime. (C) 1999 Elsevier Science Ltd. All rights reserved.