Abstract:
In order to reconstruct changes in paleo-productivity, paleo-sea-surface temperatures, and terrigenous/marine organic carbon input and their relationship to climate change, specific biomarkers (n-alkanes, fatty acids, alkenones, sterols), Rock-Eval pyrolysis data, as well as accumulation rates of organic carbon were determined in sediment samples from ODP Sites 1089 and 1093 (Atlantic sector of the Southern Ocean) for the time interval from Marine Oxygen Isotope Stages (MIS) 12 to 5e (i.e., about 450 to 100 ka). The results can be summarized as follows: - In general, the concentrations of the marine proxies brassicasterol and C (sub 37) -alkenones correlate well with the S2- and TOC-values. However, a correlation between TOC and specific terrigenous biomarker is not obvious. This indicates that TOC is predominantly of marine origin. - Furthermore, the results of this study suggest that long-chain fatty acids (n = 22 - n = 30) in the Site 1089 and Site 1093 region can be attributed to a marine rather than to a terrestrial source. - Based on the biomarker data, marine organic carbon was significantly enriched during glacial stages. Estimated paleoproductivity reached values of about 50 g C m (super -2) y (super -1) during peak interglacials (e.g., MIS 5e and the lowermost MIS 11), which is close to the modern productivity measured in the area of investigation (Site 1089). During glacial intervals, productivity increased to values of about 120-190 g C m (super -2) y (super -1) . These glacial/interglacial changes are explained by a northward shift of the high-productivity zone during glacial. - Multiple Taper Method (MTM) and Wavelet Analysis (WA) applied to the datasets of organic compounds of Site 1089 reveal cyclicities, which are consistent with frequencies detected in to the paleoclimatic record from the Vostok ice core and the foraminiferal delta (super 18) O record of the reversed SPECMAP stack. - The results from the applied spectral analyses indicate that paleoproductivity in the subantartic ocean show eccentricity-, obliquity- and precession-related periodicities. Based on the relationship between paleoproductivity values and the position of the high-productivity zone it is probable that changes on Milankovitch time scale affected not only the variability of marine productivity but also the associated position of the high-productivity zone. - Spectral analyses of the marine records show significant variations with sub-Milankowitch periodicities between 14 kyr and 2 kyr, partly in agreement with the Vostok Delta T record. According to the results of the WA high-frequency oscillations probably occur over the whole analysed time window. - The obeserved variations in alkenone-SST indicate significant oceanographic and climatic changes between 450 and 100 ka. During interglacials, alkenone-SST were abour 6 degrees C warmer than during glacials. - During climate optima (stage 11, 9, 7 and 5), SST exceeded present SST at Site 1089. The highest temperature (23 degrees C) at this site was observed during MIS 5 (at 128 ka). Southern Ocean temperatures during MIS 11 seem not to be warmer than during subsequent interglacials. - Furthermore, a gradual warming in the Southern Ocean suring glacial stages was found. The coldest SST were observed in MIS 12 (12 degrees C at 434 ka) and in the upper MIS 11 (12 degrees C at 375 ka and 389 ka). The paleoproductivity values, whih were strongly anticorrelated with the SST record at Site 1089, show a glacial maximum during MIS 12 (190 g C m (super -2) y (super -1) ) and a glacial minimum during MIS 2 (110 g C m (super -2) y (super -1) ). This is also in agreement with the estimated accumulation rates of TOC. The increase of the paleoproductivity towards older sediments indicates that the northward displacement of the Antarctic Circumpolar Current (ACC) reached its maximum during MIS 12. the biomarker data support the hypothesis that the gradual warming reflects a weakening impact of cold water from the ACC and an enhanced Agulhas leakage during glacial stages (MIS 12-MIS 6). - For MIS 10, a warm SST anomaly could be detected. The SST anomaly might be caused by changes in the ocean circulation, as the biomarker data indicate a contemporary northwards displacement of the North Atlantic Deep Water (NADW) and a reduction of the Agulhas leakage current to the Cape Basin during this event. This SST anomaly might be cuased by a resumption of the thermohaline circulation, triggered by a switch from lower to higher amplitude in the insolation forcing.