Evidence for strong sediment redistribution by bottom currents along the southeast Indian ridge

Abstract

Understanding whether vigorous bottom currents redistribute biogenic components coming from the surface water is critical to evaluating the results from paleoenvironmental reconstructions based on sediment accumulation rates in the Southern Ocean. A large contourite drift along the southern flank of the Southeast Indian Ridge (SEIR) is recognized in published sediment thickness maps. We use the 230 Th method to estimate the contribution of advected sediments to the bulk sediment and rare-earth elements (REE) and trace-element compositions to determine the possible differences in sedimentary origin of the transported material. Magnetic susceptibility and “focusing factor” distributions suggest that (a) accumulation and sediment redistribution on the contourite drift have occurred throughout the last 40 ka, (b) the lateral transport of biogenous and detrital material represents 50–90% of the input at the foot of the SEIR, and (c) transport was even higher during glacial periods. Both REE profiles and trace-element ratios (La n /Tb n and Th/Sc) reveal that during the last glacial period, the terrigenous particles were mainly of volcanogenic origin, i.e., from the Crozet and Kerguelen slopes. The more significant contribution from the volcanic sources during the last glacial maximum is consistent with the action of the Antarctic Circumpolar Current-Circumpolar Deep Water (ACC-CDW). In addition, the formation of this tongue would be strongly linked to the long-term interactions between the Antarctic Bottom Water (AABW) and the ACC-CDW.