Stable isotope record of foraminifera from South Atlantic sediments with reconstruction of paleotemperatures and paleosalinities

Changes in surface water hydrography in the Southern Ocean (eastern Atlantic sector) could be reconstructed on the basis of isotope-geochemical and micropaleontological studies. A total of 75 high quality multicorer sediment surface samples from the southern South Atlantic Ocean and three Quaternary sediment cores, taken on a meridional transect across the Antarctic Circumpolar Current, have been investigated. The results of examining stable oxygen isotope compositions of 24 foraminiferal species and morphotypes were compared to the near-surface hydrography. The different foraminifera have been divided into four groups living at different depths in the upper water column. The 8180 differences between shallow-living (e.g. G. bulloides, N. pachyderma) and deeper-dwelling (e. g. G. inflata) species reflect the measured temperature gradient of the upper 250 m in the water column. Thus, the 6180 difference between shallow-living and deeper-living foraminifera can be used as an indicator for the vertical temperature gradient in the surface water of the Antarctic Circumpolar Current, which is independent of ice volume. All planktonic foraminifera in the surface sediment samples have been counted. 27 species and morphotypes have been selected, to form a reference data Set for statistical purposes. By using R- and Q-mode principal component analysis these planktonic foraminifera have been divided into four and five assemblages, respectively. The geographic distribution of these assemblages is mainly linked to the temperature of sea-surface waters. The five assemblages (factors) of the Q-mode principal component analysis account for 97.l % of the variance of original data. Following the transferfunction- technique a multiple regression between the Q-mode factors and the actual mean sea-surface environmental parameters resulted in a set of equations. The new transfer function can be used to estimate past sea-surface seasonal temperatures for paleoassemblages of planktonic foraminifera with a precision of approximately ±1.2°C. This transfer function F75-27-5 encompasses in particular the environmental conditions in the Atlantic sector of the Antarctic Circumpolar Current. During the last 140,000 years reconstructed sea-surface temperatures fluctuated in the present northern Subantarctic Zone (PS2076-1/3) at an amplitude of up to 7.5°C in summer and of up to 8.5°C in winter. In the present Polarfrontal Zone (PS1754-1) these fluctuations between glacials and interglacials show lower temperatures from 2.5 to 8.5°C in summer and from 1.0 to 5.0°C in winter, respectively. Compared to today, calculated oxygen isotope temperature gradients in the present Subantarctic Zone were lower during the last 140,000 years. This is an indicator for a good mixing of the upper water column. In the Polarfrontal Zone also lower oxygen isotope temperature gradients were found for the glacials 6, 4 and 2. But almost similar temperature gradients as today were found during the interglacial stages 5, 3 and the Holocene, which implicates a mixing of the upper water column compared to present. Paleosalinities were reconstructed by combining d18O-data and the evaluated transfer function paleotemperatures. Especially in the present Polarfrontal Zone (PS1754-1) and in the Antarctic Zone (PS1768-8), a short-term reduction of salinity up to 4 %o, could be detected. This significant reduction in sea-surface water salinity indicates the increased influx of melt-water at the beginning of deglaciation in the southern hemisphere at the end of the last glacial, approximately 16,500-13,000 years ago. The reconstruction of environmental Parameters indicates only small changes in the position of the frontal Systems in the eastern sector of the Antarctic Circumpolar Current during the last 140,000 years. The average position of the Subtropical Front and Subantarctic Front shifted approximately three latitudes between interglacials and glacials. The Antarctic Polar Front shifted approximately four latitudes. But substantial modifications of this scenario have been interpreted for the reconstruction of cold sea-surface temperatures at 41Â S during the oxygen isotope stages 16 and 14 to 12. During these times the Subtropical Front was probably shified up to seven latitudes northwards.