Marine productivity and terrigenous matter supply: the variability of the Subtropical Convergence around Tasmania during the last 500 ka
This study focussed on temporal and spatial changes in export productivity and terrigenous flux, in response to the varying oceanographical frontal system, in the Tasmanian Seaway since 500 ka. Four sites, drilled during ODP-Leg 189, were studied: Site 1168 (western Tasmanian margin), Sites 1170 and 1171 (Tasman Rise), and Site 1172 (East Tasman Plateau). Downcore variations in geochemical proxy data allowed to reconstruct the position of the oceanographical frontal system in the Southern Ocean since 500 ka. The paleoproductivity was assessed from the accumulation rates of carbonate, organic carbon, excess barium, and chlorins. For the first time chlorin was used in the Southern Ocean to reconstruct paleoproductivity. Siliciclastic material, by means of the aluminium, iron and titanium concentrations, was used as proxy for the supply of terrigenous matter. Their ratios provided evidence for the origin of the terrigenous material as being wind-blown dust. In general, the interglacial periods were characterised by less productivity and a lower terrigenous supply. The Subtropical Convergence was located south of 49°S during the interglacial isotopic stages 1 - 7. During marine isotopic stages 9 - 13, increased export productivity suggests a increased influence of Subantarctic Surface Water at the southern Site 1171. This indicates a position of the Subtropical Convergence and the zone of the westerlies more to the north. During the glacials, the frontal system was shifted to the north. The Subtropical Convergence reached as far north as between 42°S and 39°S east of Tasmania and between 44°S and 40°30’S west of Tasmania during the glacials. Reduced productivity during marine isotopic stages 8, 10 and 12 at the southern Site 1171 suggests that the Subantarctic Front was located to the north of Site 1171. The high glacial productivity in the nutrient-rich Subantarctic Surface Water was mainly caused by an increased supply of iron, delivered by strengthened westerly winds. These results indicate that the “Iron Hypothesis” is applicable for the studied area. It can be concluded that strong glacial/interglacial changes have determined the variations in the oceanographic frontal system in the Tasmanian Seaway. In general, the frontal system in the study area shifted to the south since 500 ka.
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