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.南大洋（大西洋东部海域）表层海洋水文特征的变化，可通过同位素地球化学与微古生物学研究进行重建。本研究共对南大西洋南部海域采集的75份高质量多管沉积物表层样品，以及沿横穿南极绕极流（Antarctic Circumpolar Current）的子午向断面获取的3根第四纪沉积物岩芯开展了分析。 本研究将24种有孔虫（foraminifera）物种及形态类型的稳定氧同位素组成检测结果，与近表层海洋水文特征进行了对比。不同类群的有孔虫被划分为4组，分别栖息于上层水体的不同深度。栖息于浅水环境（如G. bulloides、N. pachyderma）与较深水环境（如G. inflata）的有孔虫物种间的δ¹⁸O差值，可反映上层250米水体中实测的温度梯度。因此，栖息深浅不同的有孔虫之间的δ¹⁸O差值，可作为南极绕极流表层水体垂直温度梯度的指示指标，且该指标不受冰盖体积变化的影响。 本研究对表层沉积物样品中的所有浮游有孔虫（planktonic foraminifera）进行了计数，并筛选出27个物种及形态类型，以构建用于统计分析的参考数据集。通过R型与Q型主成分分析，研究将浮游有孔虫分别划分为4个与5个组合群落。上述组合群落的地理分布主要与表层海水温度相关。Q型主成分分析得到的5个组合群落（因子）可解释原始数据97.1%的方差。基于转换函数（transfer function）技术，通过对Q型因子与实测表层海水环境平均参数进行多元回归分析，本研究构建了一组回归方程。该新型转换函数可用于估算浮游有孔虫古组合群落对应的古表层海水季节温度，估算精度约为±1.2℃。该编号为F75-27-5的转换函数，尤其适用于南极绕极流大西洋东部海域的环境条件重建。 在过去14万年中，现今北亚南极带（PS2076-1/3）的重建表层海水温度波动幅度可达夏季7.5℃、冬季8.5℃。在现今极锋带（PS1754-1），冰期与间冰期之间的温度波动表现为夏季2.5~8.5℃、冬季1.0~5.0℃的低温区间。与现代相比，过去14万年中北亚南极带的计算氧同位素温度梯度更低，这指示上层水体混合作用较强。在极锋带，冰期6、4、2阶段的氧同位素温度梯度同样较低；而在间冰期5、3阶段及全新世（Holocene），温度梯度与现代几乎一致，这指示当时上层水体混合作用与现代相近。 古盐度通过结合δ¹⁸O数据与通过转换函数估算得到的古温度数据进行重建。尤其在现今极锋带（PS1754-1）与南极带（PS1768-8）中，可检测到盐度最高下降4‰的短期变化。此次表层海水盐度的显著下降，指示末次冰期结束后约16500~13000年前的南半球冰消期起始阶段，融水输入量显著增加。 环境参数重建结果显示，过去14万年中南极绕极流东部海域的锋面系统位置仅发生了小幅变化。亚热带锋（Subtropical Front）与亚南极锋（Subantarctic Front）的平均位置在冰期与间冰期之间移动了约3个纬度；南极极锋（Antarctic Polar Front）则移动了约4个纬度。但在氧同位素阶段16以及14~12阶段，南纬41°处的冷表层海水温度重建结果显示，该锋面系统格局发生了显著改变。此时期亚热带锋可能向北移动了多达7个纬度。