Stable isotope analysis of Porites coral core from the Andaman Islands

North Atlantic climate variations are reflected in sedimentary records from the northern Indian Ocean in which two basins, the Arabian Sea and the Bay of Bengal, are strongly affected by the monsoon. Contrary to the Bay of Bengal the Arabian Sea plays an important role in the global marine nitrogen cycle. In its mid-water oxygen minimum zone (OMZ) bioavailable fixed nitrogen is reduced to nitrogen gas (NO3- - > N2), whereas oxygen concentrations are slightly above the threshold of nitrate reduction in the OMZ of the Bay of Bengal. A coral colony (Porites lutea) growing south of Port Blair on the Andaman Islands in the Bay of Bengal was studied for its response to changes in the monsoon system and its link to temperature changes in the North Atlantic Ocean, between 1975 and 2006. Its linear extension rates, d13C and d18O values measured within the coral skeleton reveal a strong seasonality, which seems to be caused by the monsoon-driven reversal of the surface ocean circulation. The sampling site appears to be influenced by low salinity Bay of Bengal Water during the NE monsoon (boreal winter) and by the high salinity Arabian Sea Water during the SW monsoon in summer. The high salinity Arabian Sea Water circulates along with the Summer Monsoon Current (S-MC) from the Arabia Sea into the Bay of Bengal. Decreasing d18O and reconstructed salinity values correlate to the increasing SSTs in the North Atlantic Ocean indicating a reduced influence of the S-MC at the sampling site in the course of northern hemispheric warming. During such periods oxygen-depletion became stronger in the OMZ of the Arabian Sea as indicated by the sedimentary records. A reduced propagation of oxygen-depleted high salinity Arabian Sea Water into the Bay of Bengal could be a mechanism maintaining oxygen concentration above the threshold of nitrate reduction in the OMZ of the Bay of Bengal in times of global warming.

Organic carbon geochemistry of ODP Leg 121 samples

Organic petrological and geochemical analyses were performed on samples cored on Broken Ridge and Ninetyeast Ridge in the Central Indian Ocean during Leg 121. Organic carbon (Corg) contents are less than 1% in each individual sample and average Corg values calculated for larger stratigraphic units are less than 0.2%. Generally, there is more organic matter in Cretaceous sediments than in Tertiary. In the Cretaceous, the bulk of the organic matter consists of terrigenous debris, but a significant contribution of marine-derived organic matter was found in some samples, especially in the early Maestrichtian on Broken Ridge (Site 754). The youngest Pliocene-Pleistocene sediments at Site 758 (northern part of Ninetyeast Ridge) contain a significant amount of clastic material transported to the site by the (distal) Bengal Fan. In these sediments, Corg contents of up to 0.9% were measured and are due to the inflow of terrigenous organic debris. Corg values are positively correlated with bulk sediment accumulation rates (i.e., sediments contain more organic matter at times of faster deposition). The size of terrigenous organic particles is generally small in all sediments. The extremely small number of particles in the Cretaceous sediments at Site 758 and their smaller grain size, compared to the Cretaceous sediments on Broken Ridge, indicate that Cretaceous surface water paleocurrents flowed from southeast to northwest in the Proto-Indian Ocean. In the central Indian Ocean, sediments deposited above the carbonate compensation depth consist of nannofossil and foraminiferal oozes. In contrast to Corg values, calcite contents in the sediments are negatively correlated with bulk sediment accumulation rates (i.e., carbonate oozes were deposited only during times of extremely slow sedimentation). Therefore, older sediments deposited in the young and still narrow Indian Ocean accumulated faster and are less carbonate-rich than Neogene sediments, although carbonate accumulation rates were higher.