Paleo-export production, terrigenous flux and sea surface temperatures around Tasmania: Implications for glacial/interglacial changes in the Subtropical Convergence zone

Abstract

The Tasmanian Gateway, focus of ODP Leg 189, is a key oceanographic area within the Southern Ocean. Our investigations concentrate on the last -500,000 years of sedimentation at Sites 1168 (western Tasmanian margin), 1170 and 1171 (Tasman Rise), and 1172 (East Tasman Plateau). A suite of geochemical proxy data reflecting paleo-export production, terrigenous flux, and sea surface temperature, constrain temporal and spatial variations in surface water masses and oceanographic frontal systems over these sites. Interglacial periods were commonly of low productivity and less affected by terrigenous matter supply, suggesting that the position of the Subtropical Convergence remained south of Tasman Rise. Only during early MIS 11 and MIS 9 over the southern Tasman Rise, and during MIS 7 over the northern Tasman Rise, did enhanced marine productivity, combined with an enhanced terrigenous flux, indicate waxing influence of subantarctic waters. During glacial MIS 2, marine productivity and terrigenous flux increased significantly at Sites 1168, 1170, and 1171 implying that the Subtropical Convergence moved northward to ∼42°S west of Tasmania. East of Tasmania, the presence of the East Australian Current caused the Subtropical Convergence to remain south of East Tasman Plateau. Glacial MIS 6 appears to have been different from MIS 2. The Subtropical Convergence stayed north of East Tasman Plateau, but clearly south of Site 1168 on the western Tasmanian margin. Strongly enhanced marine productivity and terrigenous flux during MIS 10 and 12 at Sites 1168, 1170, and 1172 suggest the dominant influence of subantarctic waters and the position of the Subtropical Convergence north of East Tasman Plateau. At South Tasman Rise, in contrast, the reduced terrigenous flux implies that Site 1171 moved outside the belt of westerly winds. Marine productivity ceased at that time mainly due to iron limitation.