Biological and physical processes structuring deep-sea surface sediments in the Scotia and Weddell Seas, Antarctica

Five deep-sea transects were sampled in the Scotia Sea and Weddell Sea from the Drake Passage to the South Sandwich Islands, Antarctica, with depths ranging from 767 to 6333 m. A combination of sediment surface and sediment profile imaging was used to characterize the physical and biological processes structuring the benthic boundary layer and surface sediments from the sediment–water interface (SWI) to as much as 17 cm below the SWI. The factors structuring surface sediments were related to distance from Antarctic landmasses, which are the main source of sediment, and deposition of water-column or sea-ice primary productivity. Stations near the continental shelf and slope tended to have coarse (granule to cobble) sediments derived from melting sea-ice. Input of fine sediments was estimated to be low at these stations because many of the coarse sediment grains appeared to be manganese coated. At the deepest stations in the Ona Basin, South Shetland Trench, and South Sandwich Trench, sediment appeared to be primarily hemipelagite silt-clay but were varved with as many as four distinct color laminations. These layers varied in thickness fom 0.6 to 10.6 cm and may have been related to turbidite flows. While geology and physical processes that deliver sediment to the bottom were the primary factors dictating substratum characteristics, biological processes were, to varying degrees, important in structuring the SWI and sediment fabric. Biological processes, primarily in the form of bioturbation, were most prominent at fine-grained stations but were also obvious even on rock substratum on top of the Shackleton Fracture Zone. There were gradients between the degree of bioturbation and type of biogenic structure present with depth. At depths <2000 m biogenic structures were more diverse and dominated (>75% of the sediment bioturbated) the structure of surficial sediments at 5 of 13 stations. At depths >2000 m, biological processes were overall less diverse but dominated at 8 of 11 stations.