Diatom and pore water data from two piston cores from the central sub-basin and one from the western sub-basin of Bransfield Strait in the northern Antarctic Peninsula were used to elucidate the depositional mechanism of a layered diatom ooze unit observed in all three cores. The diatom ooze unit, formed by the alternation of diatom ooze layers and terrigenous layers, is enriched in organic carbon, biogenic silica, sulfide sulfur content and depleted in pore water sulfate concentration. This depletion of pore water sulfate in the diatom ooze interval is indicative of development of reducing micro-environment in which bacterially mediated sulfate reduction occurred. The negative relationship between the organic carbon and sulfate contents, however, indicates that sulfate reduction was taking place but does not control organic carbon preservation in the diatom ooze interval. Rather, the overwhelming dominance of intact Chaetoceros resting spores in the diatom ooze layer indicates rapid sedimentation of the diatom spores as a result of repetitive late summer blooms at the sea-ice margin on the Bransfield shelf at around 2500 yrs BP when persistent sea-ice margin might have existed on the shelf and/or shelf break. During the cold period, underflows were probably caused by cooling of the Bransfield shelf water, and these flows probably played an important role for concentrating the summer diatom blooms to produce the diatom ooze layer recorded in the sub-basins of the Bransfield Strait. Intervening terrigenous layers were mostly deposited during the winter and represent the input of reworked detrital clay by stronger bottom flows in non-bloom conditions under the sea ice.
