Deep-water sedimentary environments of the northwestern Weddell Sea and South Sandwich Islands, Antarctica

Abstract

Abstract Two transects have been sampled using short cores (multi and box), seabed photography, video sequences, and sediment profile images across the northwestern Weddell Sea and South Sandwich Forearc, Antarctica. A total of 12 core stations were examined for sediment structure, texture and composition to determine their depositional history. Four of the core stations from the Weddell Continental Slope, Abyssal Plain and South Sandwich Forearc and Trench were further examined for trace metal analysis (Ba, U, Mn and Pb) for sediment source and palaeoproductivity, and 210Pb for age profiles and depth of mixing by bioturbation. The 3055 m station at the base of the Weddell Continental Slope, recovered laminated muddy–silty contourites with a surface layer of hemipelagites with abundant ice-rafted debris, possibly originating from the 1995 Larsen A and 2002 Larsen B ice shelf disintegration. This is interpreted as a moderate-energy depositional environment influenced by Weddell Sea Bottom Water along-slope current, reworking sediment. Sediment sources to this station include the direct input of clastic material from ice with variation in sediment sources from in situ current winnowing and along-slope transport reflected in the Mn/Li and Ba/Li ratios. The 4760 m station on the Weddell Abyssal Plain recovered hemipelagites, interpreted as a low-energy depositional environment with sediment supplied both from direct ice-rafting and pelagic settling during productive seasonally ice-free periods. The 210Pb profiles indicate a sedimentation rate of 0.024 cm/year−1, with the 206/207Pb isotope ratios suggesting a principle sediment source from ice-rafting. The 2292 and 6326 m stations from the slope and floor of the South Sandwich Forearc recovered pelagic and turbiditic sediments. This is interpreted as a high-energy down-slope depositional environment, with high surface biogenic productivity and sediment supply leading to localised down-slope mass-movements on the slope and floor. High levels of Mn/Li and elevated U/Li ratios demonstrate a strong concentration of Mn in organic-poor sediment. The Mn redoxcline is present at 10 cm depth. The high Ba/Li (500 ppm) in the forearc floor core reflects some limited enhanced productivity. Source sediment for the forearc may, in part, be derived from the northwest Weddell Abyssal Plain, based on the 206/207Pb isotope ratios reflecting transport by Weddell Sea Bottom Water.