Pelagic productivity changes in the equatorial area of the northwest Indian Ocean during the last 400,000 years

Abstract

Major and selected trace element (Al, Fe, Mg, K, Ba, P, Mn, V, Cu, Ni, and Ti) records were analyzed in two cores recovered from the northwestern Indian Ocean (core MD 90940 on the Madingley Rise and core MD 85668 in the Somali Basin) in order to monitor the sedimentary response to pelagic productivity in this area during the last 400,000 years. Geochemical proxies such as bulk mass accumulation rate (MAR), Al-normalized major and trace element contents, major and trace element MARs were compared to biologic markers of palaeoproductivity: a new radiolarian index (rad ratio), and the variations of foraminiferal and radiolarian assemblages. Special attention being paid to the barium record, a sequential leaching procedure determined the main Ba-carrier fraction in the sediment, and, thus, the amount of biogenic barium. The `blue ocean' sedimentation on the Madingley Rise (core MD 90940) is characterized by a low MAR and an insignificant terrigenous input. The bulk MAR variations are directly related to the variations of the rad ratio, reflecting a direct link between surface productivity and pelagic sedimentation at this site. In this sediment, barium is mainly found as barite and can be considered as a palaeoproductivity proxy. In the Somali Basin (core MD 85668), the terrigenous inputs are relatively higher, but the bulk MAR variations are still related to the variations of surface productivity. No barite was found and barium is mainly linked to the terrigenous input. A comparison of the two pelagic sites of the Madingley Rise and the Somali Basin points out the differences between the barium records at both sites. Despite the location of both cores in the pelagic domain, the geochemical record, and especially the barium one, is different from one core to the other. The comparison of the variations of the bulk MAR, which reflects the variations of the surface productivity, presents significant discrepancies between both sites. In the `blue ocean' (core MD 90940), the productivity peak values are mainly recorded during glacial intervals, but their maximum values are observed during interglacial isotope Stage 9. In the Somali Basin (core MD 85668), high bulk MAR periods occurred during glacial isotope stages. The interglacial episode of high productivity recorded on the Madingley Rise, the absence of significant variations of productivity at the 8/7 transition in the Somali basin, as well as the absence of a time lag between both records indicate that the surface productivity is, in this area, controlled more by the local variations of the palaeoceanographic patterns than the global climatic changes, e.g. the glacial/interglacial alternations.