[1] On the basis of the quantitative study of diatom, radiolarian, and planktic foraminiferal assemblages, we estimated summer sea surface temperature (SSST) and sea-ice extent at 50 sediment core localities in the Atlantic and western Indian sector of the Southern Ocean to reconstruct the last glacial environment at the GLAMAP (18 to 15 ka, equal to 21,500 to 18,000 calendar (cal) years BP) and EPILOG (19.5 to 16.0 ka, equal to 23,000 to 19,000 cal years BP) time slices. Stratigraphic identification of the time slices was accomplished by a combination of AMS 14 C measurements, benthic isotope, and siliceous microfossil abundance records. While the SSST estimates reveal greater surface water cooling than reconstructed by CLIMAP [1981], reaching a maximum in the area of the present Subantarctic Zone, the sea-ice reconstruction indicates that CLIMAP overestimated the expansion of the Antarctic sea-ice field, especially for austral summer. During winter the seaice field was expanded by 60-70% compared to the present. Last glacial summer sea surface isotherms indicate a northward shift of the zonal bands of the Antarctic Circumpolar Current and a relative expansion of the cold water realm south of the Subantarctic Front by $5°in latitude. This coincides with a northward displacement of the zone of enhanced biogenic silica deposition and iceberg occurrence. As a result of northward expansion of Antarctic cold waters and a relatively small displacement of the Subtropical Front, thermal gradients were steepened during the last glacial in the area of the present Subtropical Front. The northward displacement of Antarctic cold waters and the related deflection of Southern Ocean waters along the eastern boundary of South America may have resulted in a weakened ''cold water route'' across the Drake Passage. In contrast, the transport of warm and salty surface water from the Indian into the Atlantic Ocean via the ''warm water route'' was not blocked allowing continuous but reduced import of heat into the South Atlantic.
Abstract. The quantitative diatom analysis of 218 surface sediment samples recovered in the Atlantic and western Indian sector of the Southern Ocean is used to define a base of reference data for paleotemperature estimations from diatom assemblages using the Imbrie and Kipp transfer function method. The criteria which justify the exclusion of samples and species out of the raw data set in order to define a reference database are outlined and discussed. Sensitivity tests with eight data sets were achieved evaluating the effects of overall dominance of single species, different methods of species abundance ranking, and no-analog conditions (e.g., Eucampia antarctica) on the estimated paleotemperatures. The defined transfer functions were applied on a sediment core from the northern Antarctic zone. Overall dominance of Fragilariopsis kerguelensis in the diatom assemblages resulted in a close affinity between paleotemperature curve and relative abundance pattern of this species downcore. Logarithmic conversion of counting data applied with other ranking methods in order to compensate the dominance of F. kerguelensis revealed the best statistical results. A reliable diatom transfer function for future paleotemperature estimations is presented.
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