Lack of enhanced preservation of organic matter in sediments under the oxygen minimum on the Oman Margin
The impingement of oxygen minima on continental margins is widely thought to promote the accumulation of sedimentary facies enriched in well-preserved organic matter. It is shown here, however, that such a relationship does not clearly apply to the productive Oman Margin in the Arabian Sea, which hosts one of the most severe oxygen minima in the oceans. Measurements made on the 0-1 cm depth interval from fourteen box cores collected from the outer shelf-upper continental slope area off Oman show that (1) deposited organic matter is overwhelmingly of marine origin,(2) there is no significant correlation between the abundance of sedimentary organic carbon (C org) and the bottom- water O 2 concentration, (3) there is no relation between the sedimentary C org:N ratio and bottom-water O 2, (4) there is no correlation between the hydrogen index (HI) of the organic matter and bottom water oxygen. There are, however, significant correlations between the C org:N ratio and the I:C org, Cr:Al, and Zr:Al ratios, as well as between the C org:N ratio and the hydrogen index. These relationships are interpreted as being the result of winnowing and attendant reworking which produce a progressive increase in the C org:N ratio and a decrease in the HI and the I:C org ratio as organic matter becomes progressively degraded. Commensurate rises in the Cr:Al and Zr:Al ratios result from the increased proportion of heavy minerals associated with winnowed lag deposits. Overall, these data suggest that the bottom water oxygen concentration has little effect in governing either the distribution or the degree of preservation of organic matter on this margin. Thus, the generally high but spatially variable C org content of the sediments on the Oman Margin may not reflect the occurrence of an oxygen minimum but instead be the result of a high settling flux of organic matter, supported by monsoon-driven upwelling, and post-depositional redistribution of the organic material by hydrodynamic influences.