Organic matter preservation through the oxygen-deficient zone of the NE Arabian Sea as discerned by organic carbon: mineral surface area ratios

摘要

It is debated whether there is a causal or evencorrelative relationship between the presence of a minimum inbottom water oxygen (BWO) concentration and the preservation oforganic matter in underlying marine sediments. One relation shipthat has not been examined in this regard is the relationshipbetween sedimentary organic carbon content and mineral surfacearea. Normalization of organic matter content to mineral surfacearea eliminates the mass-based problems associated withtraditional weight percent carbon measurements, and thus mayallow better evaluation of carbon preservation. We measuredorganic carbon to mineral surface area ratios (OC:SA) on 26sediment samples from along the NE edge of the Arabian Sea, andon 21 hydrodynamically sorted fractions (using split-flow lateraltransport thin [SPLITT] fractionation) from three of these stations.Samples spanned from above to well below the minimum in localBWO concentration. Sediments deposited under the oxygenminmum had OC:SA ratios in excess of 1.1mg OC m-2, indicativeof enhanced preservation. Abioturbated sediment at the edge of theoxygen minimum (BWO~35μM) also had a high OC:SA ratio.With the exception of the anomalous sample along the slope, allsamples from shallower and deeper than the oxygen-depleted watermass (BWO>35μM) have OC:SA ratios that fall within thetypically observed range (0.5-1.1mg OC m-2). While further workwill be necessary to discern what causes this relationship, our dataindicate that organic matter preservation (as estimated by OC:SA)is enhanced within the general locale of the BWO minimum in NEArabian Sea sediments. If this relationship proves to be robust,repeatable and stable over time in oter modern BWO minimumzones, surface area- normalized organic carbon loadings may be auseful paleoceanographic proxy for past BWO levels.