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

Understanding whether vigorous bottom currents redistribute biogenic compon ents coming from the surface water is critical to evaluating the results fr om 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 thic kness maps. We use the Th-230 method to estimate the contribution of advect ed sediments to the bulk sediment and rare-earth elements (REE) and trace-e lement compositions to determine the possible differences in sedimentary or igin of the transported material. Magnetic susceptibility and "focusing fac tor" distributions suggest that (a) accumulation and sediment redistributio n on the contourite drift have occurred throughout the last 40 ka, (b) the lateral transport of biogenous and detrital material represents 50-90% of t he 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 particle s were mainly of volcanogenic origin, i.e., from the Crozet and Kerguelen s lopes. The more significant contribution from the volcanic sources during t he last glacial maximum is consistent with the action of the Antarctic Circ umpolar Current-Circumpolar Deep Water (ACC-CDW). In addition, the formatio n of this tongue would be strongly linked to the long-term interactions bet ween the Antarctic Bottom Water (AABW) and the ACC-CDW. (C) 2000 Elsevier S cience Ltd. All rights reserved.