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

Abstract

It is debated whether there is a causal or even correlative relationship between the presence of a minimum in bottom water oxygen (BWO) concentration and the preservation of organic matter in underlying marine sediments. One relationship that has not been examined in this regard is the relationship between sedimentary organic carbon content and mineral surface area. Normalization of organic matter content to mineral surface area eliminates the mass-based problems associated with traditional weight percent carbon measurements, and thus may allow better evaluation of carbon preservation. We measured organic carbon to mineral surface area ratios (OC:SA) on 26 sediment samples from along the NE edge of the Arabian Sea, and on 21 hydrodynamically sorted fractions (using split-flow lateral transport thin [SPLITT] fractionation) from three of these stations. Samples spanned from above to well below the minimum in local BWO concentration. Sediments deposited under the oxygen minimum had OC:SA ratios in excess of 1.1 mg OC m −2, indicative of enhanced preservation. A bioturbated sediment at the edge of the oxygen minimum (BWO ∼35 μM) also had a high OC:SA ratio. With the exception of the anomalous sample along the slope, all samples from shallower and deeper than the oxygen-depleted water mass (BWO>35 μM) have OC:SA ratios that fall within the typically observed range (0.5–1.1 mg OC m −2). While further work will be necessary to discern what causes this relationship, our data indicate that organic matter preservation (as estimated by OC:SA) is enhanced within the general locale of the BWO minimum in NE Arabian Sea sediments. If this relationship proves to be robust, repeatable and stable over time in other modern BWO minimum zones, surface area-normalized organic carbon loadings may be a useful paleoceanographic proxy for past BWO levels.