Last glacial sea surface temperatures and sea‐ice extent in the Southern Ocean (Atlantic‐Indian sector): A multiproxy approach

Abstract

[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.

Antarctic lakes suggest millennial reorganizations of Southern Hemisphere atmospheric and oceanic circulation

“…We suggest, as have others before us (i.e., 29-31), a strong coupling between Antarctica and the southern temperate regions, and that during the LGM the polar cell and probably sea ice expanded (32,33) and westerly winds and oceanic fronts moved north (31,34,35). This scenario would have made it more difficult for snow-bearing storms to penetrate the distance to the Dry Valleys.…”

The timing and cause of glacial advances in the southern mid-latitudes during the last glacial cycle based on a synthesis of exposure ages from Patagonia and New Zealand

“…Reconstructions from south of the Polar Front suggest that Antarctic sea ice duration in the southwest Atlantic was at its greatest during the last glacial cycle between ca. 32 and 22 ka, consistent with decreasing temperatures in Antarctica (Gersonde, 2003;Allen et al, 2011;Crosta et al, 2004). Maximum summer sea ice retreated to south of 61°S by ca.…”

Polar stratification: A critical view from the Southern Ocean

“…the last glacial period (Figs. 1 and 5; Gersonde and Zielinski, 2000;Gersonde et al, 2003Gersonde et al, , 2005. Therefore, the observed glacial decline in biological productivity in the area corresponding to the modern POOZ, e.g.…”

“…With the emergence of the hypothesis that intense polar stratification may have acted as an effective mechanism for shutting down CO 2 outgassing in the glacial Southern Ocean (Elderfield and Rickaby, 2000;Sigman and Boyle, 2000;Stephens and Keeling, 2000), watercolumn stratification during the summer months was taken into account as a possible consequence of the expansion of seasonal sea-ice coverage (Frank et al, 2000;Gersonde et al, 2003;Schneider-Mor et al, 2005). Most workers compared proxies for palaeoproductivity ( δ 13 C ratios in diatomaceous organic matter, MAR Opal , Ba bio ) with proxies for nutrient utilization (δ 15 N ratios in organic matter, δ 30 Si ratios in diatom valves) and inferred oceanic stratification for times with low productivity and increased nutrient utilization (e.g.…”