Abstract

Major and selected trace element (Al, Fe, Mg, K, Ba, Mn, V, Cu, Ni, and Ti) records were analyzed in a core recovered in the Amirante passage (equatorial Indian Ocean) in order to monitor the sedimentary response to pelagic productivity in this area during the last 500 kyr. Geochemical proxies such as bulk mass accumulation rate (bulk MAR), and Al-normalized major and trace element contents and MARs were compared to a biological marker of paleoproductivity: the radiolarian index (TSRI). Special attention is paid to the barium record, due to its potential reliability as a paleo-productivity proxy. However, barium may also be present in the sediment in minerals not related to surface productivity. Thus, a sequential leaching procedure was carried out to determine the main Ba-carrier fraction in the sediment, and especially the amount of biogenic barium. The sedimentation in the Amirante Passage is characterized by a low bulk MAR and low terrigenous input. Distribution of Ba within the different fractions of the sediment indicates that both terrigenous input and biologic activity are the main sources of Ba in the sediment. Barium cannot, thus, be used as a single paleoproductivity proxy. The bulk MAR variations are directly related to variations in the TSRI, reflecting a direct link between surface water productivity and pelagic sedimentation at this site. The bulk MAR record exhibits high values during glacial and interglacial intervals from 500 ka to 238 ka, and consistently low values from 238 ka to 38 ka, suggesting a period of low surface productivity between the 8-7 and the 3-2 isotope stage transitions. This pattern does not correspond to a marked change in wind strength, and a shift in the circulation of the surface currents system is hypothesized to explain such an extended low productivity interval.

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