Dinoflagellate cyst evidence for Quaternary palaeoceanographic change in the northern Scotia Sea, South Atlantic Ocean

A dinoflagellate cyst record has been examined from two cores recovered from the crest and margins of a sediment drift in water depths of 3500-4500 m in the northern Scotia Sea, South Atlantic Ocean. 46 dinoflagellate cyst analyses have been conducted, covering a time-span ranging from the Holocene down to MIS 6, representing about 160 ka. This provides a resolution of approximately 3500 years. The sediments are predominantly fine-grained contourites and diatom-rich hemipelagites, capped by sandy-silty contourites rich in the planktonic foraminifer Neogloboquadrina pachyderma. The cores can be subdivided into four dinoflagellate cyst units, supported by diatom and radiolarian biostratigraphy, biogenic barium geochemistry, oxygen isotopes and magnetic susceptibility curves. The youngest dinoflagellate cyst unit was found only in the core from the drift crest, and has been dated at between 4380 and 12275 yr BP by radiocarbon dating. The unit is characterised by autotrophic dinoflagellate cysts with similarities to modem cysts from the region, although the assemblages display some marked internal variability that may suggest rather unstable Holocene oceanographic conditions. The next downhole unit was found in both cores and provided radiocarbon ages of 14 580 yr BP and 16 840 yr BP. Recovered cyst assemblages suggest deposition within or near to maximum sea ice limits, corresponding to a northward shift of the Antarctic Convergence during the Last Glacial Maximum within oxygen Marine Isotope Stage (MIS) 2. The final two units lack radiocarbon age control. The next downhole unit is characterised by heterotrophic dinoflagellate cysts such as round, brown Protoperidinium spp. and Selenopemphix antarctica, indicating deposition within maximum sea ice limits. This unit has a wide age range within MIS 5a-d including MIS 6. Toward the base of this unit the assemblage contains autotrophic dinoflagellate cysts such as Impagidium spp., Protoceratium reticulatum and Spiniferites spp., indicative of warmer, interglacial conditions, suggesting a retreat of the Antarctic Convergence and interpreted as MIS 5e, the last interglacial. MIS 3-4 are not resolved by the dinoflagellate cysts. The oldest unit recovered is marked by a return to heterotrophic dinoflagellate cysts with deposition in the presence of seasonal sea ice and open water, suggesting the Antarctic Convergence was now northward of the core site. The dinoflagellate cyst assemblages recovered from the cores illustrate the glacial-interglacial dynamism between two important biogeographical boundaries; the Antarctic Convergence. to the north and the maximum sea ice limit toward the south.

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