Last 10000 years Variation in the Intensity of OMZ-Core Reconstructed from Sediment of the Eastern Arabian Sea

Abstract

The Arabian Sea Oxygen Minimum Zone (OMZ) is one of the most intense of the world ocean OMZs witnessing denitrification, hence, has implications to global climate. In order to understand the Holocene variations in the OMZ of this region, a sediment core (BP3-GCR3) from a water depth of 500 m was retrieved, which is presently swept by nearly anoxic waters of the core of the OMZ. The sedimentary oxide-bound Mn, a reliable redox proxy, exhibits a gradual decrease from ∼14 ppm during the early-Holocene to ∼8 ppm during the late-Holocene, suggesting a relatively less intense OMZ during the former period. The concentration of a suite of other redox-sensitive elements (Fe, Co and Ce) associated with the dispersed sedimentary oxide particulates also show a gradual and coherent decrease through the Holocene suggesting reduction of dissolved oxygen (DO) in the core of the OMZ towards the modern times. This intensification of OMZ seems to be a result of increased export production coupled with decreasing ventilation of thermocline waters through the Holocene. A one-time positive shift is observed at ∼5.5 ka BP in the organic carbon content (OC) from ∼2 % to ∼10 % in core-top. This shift in OC content is also marked by rapid decrease of CaCO3 by ∼20 %, suggesting enhanced carbonate dissolution due to increased anoxia in the OMZ core caused by increased export production in the late Holocene.