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

Abstract

The role of organic carbon input, sedimentation rate and bottom water oxygenation (BWO) for organic matter (OM) degradation was investigated using surface sediments collected from above, within and below the oxygen minimum zone (OMZ) within varying sedimentary regimes along the Pakistan continental margin. For this, samples from 47 box cores and multi cores were analyzed for their bulk component (organic carbon, total nitrogen, carbonate, opal) and amino acid concentrations. The results are compared to plankton net samples from the Arabian Sea, sinking particles collected at the Pakistan continental margin, cultured sedimentary deep-sea bacteria, and data from the deep Arabian Sea basins. High organic carbon and amino acid concentrations were found in the laminated sediments of the OMZ, in bioturbated sediments immediately below the OMZ, and below regions of high productivity in the western Arabian Sea. The contribution of amino acids to organic carbon and total nitrogen as well as the molar contribution of certain non-protein amino acids do not mirror these differences in OC contents. No clear trend in relation to sediment structure or water depth is observed. In contrast to these established indicators of OM degradation, the molar contribution of the aromatic amino acid tyrosine (Tyr) exhibits a distinct pattern. Its contribution is relatively high in sediments at depths above the OMZ and further increases towards the center of the OMZ. Below the OMZ, it decreases continuously with increasing water depth. In addition, for samples from similar depths and bottom water oxygen contents Tyr shows systematic differences between varying sedimentary regimes. The lack of a distinct trend for the established indicators does not support the idea of a predominant control of BWO on quantitative OM preservation. Nevertheless, the variability of Tyr in relation to water depth or the sedimentary regime suggests that (i) the productivity-related OC input, (ii) the bulk accumulation rate, and (iii) BWO influence the alteration of the sedimentary OM by controlling its oxygen exposure time.