Paleoproductivity changes off Lützow-Holm Bay in the Antarctic Ocean during the past 650 kyrs

The Southern Ocean has played a significant role in the global climate system during the geologic past, even in the present-day. For example, it has been proposed that primary production was higher and nutrient utilization in surface waters was more efficient in the glacial Southern Ocean than today, effectively lowering the glacial atmospheric CO2 concentration. In order to resolve the causes and processes of atmospheric CO2 change, important is to understand mechanisms and processes of sub-systems in the Antarctic Cryosphere such as a change of biological productivity, sea surface temperature, surface water frontal system, sea-ice distribution, and East Antarctic ice sheet during the glacial-interglacial climate cycle. We collected a piston core off Lützow-Holm Bay (LHB-3PC, 66.0S, 40.0E, WD 4469 m) in the Indian Sector of the Southern Ocean during the R/V Hakuho-maru cruise KH07-4 Leg 3. Sediments of core LHB-3PC are mainly composed of diatomaceous clay. Age model of core LHB-3PC was established by diatom and radiolarian biostratigraphy and a graphic correlation between grain size variation of magnetic minerals and Marine Isotope Stage (MIS). Based on the lithology and non-destructive measurements, relative higher density and silty clay sediments were deposited during the glacial period. The silty clay layers are characterized by clearly upward fining sequence, and the base of each layer is clearly defined as a sharp boundary over lighter-colored pelagic sediments. These lithologic features indicate that the silty clay layers were deposited as a result of down-slope turbidity transport from the continental margin of Antarctica. Thus, the turbidites were formed during the glacial periods due to glacial advances on the continental shelf edge. Concentrations of total organic carbon (TOC) ranged from 0.1 to 0.4 wt% at core LHB-3PC, except for each turbidite layers. TOC increased at the interglacials. High biogenic opal contents also occurred during the interglacials, indicating that the marine productivity was enhanced at the interglacial periods in the high-latitudinal Southern Ocean. Carbon isotopes of bulk organic matters ranged from -26 to -22 permil, and isotope values increased during the interglacial stages. The high carbon isotopes of organic matter in the sinking particles in the Breid Bay, Antarctica were attributed to high growth rate of phytoplankton in the euphotic layer, which produced the diatom bloom during the early summer (Handa et al., 1992; Hayakawa et al., 1996). Therefore, high carbon isotope of sediment organic matter in core LHB-3PC suggests that the phytoplankton growth rate increased during the interglacial period in the Antarctic surface water. In contrast, marine productivity was significantly restricted during the glacials. The lowered glacial productivity was also supported by the decrease in amount of siliceous microfossils such as diatom and radiolarian at core LHB-3PC. Finally, the diatom assemblages demonstrate that the surface water over the core LHB-3PC was not covered by perennial sea ice during the glacials.