Periodical breakdown of the Arabian Sea oxygen minimum zone caused by deep convective mixing

Abstract

AbstractThe northern Arabian Sea is at present characterized by a pronounced oxygen minimum zone (OMZ) with oxygen concentrations reaching values as low as 2 µM between 150 and 1250 m. This intense mid-water OMZ results from high annual organic particle fluxes and a moderate rate of thermocline ventilation. Sediment studies have shown that the intensity of the northern Arabian Sea OMZ has fluctuated on Milankovitch and sub-Milankovitch time scales, in conjunction with changes in either surface water productivity or thermocline ventilation. Here we evaluate the role of convective mixing in the periodical breakdown of the OMZ by reconstructing the density gradient for periods showing a well-ventilated water column. For this reason we reconstructed sea surface temperatures and salinities for the last 70 ka based on alkenone thermometry and δ18O analyses on planktic and benthic foraminifers. For the studied time span thermocline ventilation by intermediate water formation in the northern Arabian Sea is a viable mechanism to explain observed fluctuations in the intensity of the OMZ. We postulate that the necessary decrease in the vertical density gradient during well-ventilated periods resulted from intensified winter monsoonal winds in combination with effects caused by glacio-eustacy.