Uta Brathauer

LATE QUATERNARY VARIATIONS IN SEA SURFACE TEMPERATURES AND THEIR RELATIONSHIP TO ORBITAL FORCING RECORDED IN THE SOUTHERN OCEAN (ATLANTIC SECTOR)

Late Quaternary summer sea surface temperatures (SSTs) have been derived from radiolarian assemblages in the East Atlantic sector of the Southern Ocean. In the subantarctic and the polar frontal zone, glacial SSTs (oxygen isotope stages 2, 4, 6, and 8) were 3°–5°C cooler than today, indicating northward displacements of the isotherms about 2°–4° of latitudes. During interglacials, SSTs almost reached modern levels (oxygen isotope stages 7 and 9) or exceeded them by 2°–3°C (oxygen isotope stages 1 and 5.5). In the subantarctic Atlantic Ocean, changes in SST and calcium carbonate content of the sediment precede variations in global ice volume in the range of the main Milankovitch frequencies. Comparisons with the timing of North Atlantic Deep Water (NADW) proxy records suggests that this early response in the subantarctic Atlantic Ocean is not triggered by the flux of NADW to the Southern Ocean.

Terrigenous sediment supply in the Scotia Sea (Southern Ocean): response to Late Quaternary ice dynamics in Patagonia and on the Antarctic Peninsula

Geochemical and mineralogical compositions of modern and Late Quaternary marine sediments from the Scotia Sea trace sources and transport paths of terrigenous sediment. We discuss downcore variations of compositional data of two sediment cores from the northern and southern Scotia Sea that correlate with fluctuations in magnetic susceptibility. Sediments were derived from very different sources at both localities, as revealed by contrasting clay-mineral assemblages. However, a common feature is the input of more basic and undifferentiated crustal material with the potential of high magnetic susceptibility during glacial periods, indicated by variable quartz/feldspar ratios and major, trace and rare earth elements. Terrigenous sediments mainly originate from nearby terrestrial sources or are introduced through interbasinal sediment transfer from adjacent seas. The observed temporal compositional variations have to be attributed to changes in the relative detrital contributions from the diverse source areas. Ice-mass extensions in southern Patagonia, on the Antarctic Peninsula and adjacent islands likely control the supply of glaciogenic detritus to the open ocean during times of glacial expansion, diluting the sediment input of interbasinal origin. Current transport is mainly responsible for sediment dispersal to the pelagic Scotia Sea and may amplify the glaciological source signals during glacial climate periods, because of a stronger wind forcing of the Antarctic Circumpolar Current.

Radiolarian‐based transfer function for the estimation of sea surface temperatures in the Southern Ocean (Atlantic Sector)

A new radiolarian-based transfer function for sea surface temperature (SST) estimations has been developed from 23 taxa and taxa groups in 53 surface sediment samples recovered between 35° and 72°S in the Atlantic sector of the Southern Ocean. For the selection of taxa and taxa groups ecological information from water column studies was considered. The transfer function allows the estimation of austral summer SST (December–March) ranging between −1 and 18°C with a standard error of estimate of 1.2°C. SST estimates from selected late Pleistocene squences were sucessfully compared with independent paleotemperature estimates derived from a diatom transfer function. This shows that radiolarians provide an excellent tool for paleotemperature reconstructions in Pleistocene sediments of the Southern Ocean.

Clay Mineral Fluctuations in Late Quaternary Sediments of the Southeastern South Atlantic: Implications for Past Changes of Deep Water Advection

Downcore clay mineral fluctuations in Late Quaternary sediment cores from the southeastern South Atlantic and adjoining Southern Ocean are of low amplitude. North of the Antarctic Circumpolar Current/Weddell Gyre boundary, small-scale variations, particularly of clay mineral ratios, essentially monitor cyclic changes of deep water advection in response to climatic oscillations.

Calibration of Cycladophora davisiana events versus oxygen isotope stratigraphy in the subantarctic Atlantic Ocean - a stratigraphic tool for carbonate-poor Quaternary sediments

We calibrated the Cycladophora davisiana abundances versus oxygen isotope stratigraphy back to220ka for the subantarctic Atlantic Ocean. The relative abundances of C. davisiana and 18Omeasurements of benthic and planktic foraminifera have been determined in two sediment cores.Oxygen isotope stratigraphy has been used to date the C. davisiana records and to assignSPECMAP ages to the C. davisiana events. Comparisons with an existing calibration from thesubantarctic Indian Ocean show, that the C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2occur synchronous within the errors of the oxygen isotope stratigraphy in the Indian and the Atlanticsectors of the Southern Ocean. Larger deviations occur only for events `b1 and `g. Furthermore,the long-term fluctuations in C. davisiana abundances have been studied in a sediment core coveringthe last 700kyr. Based on biostratigraphic extinction levels, ages for early Brunhes C. davisianaevents have been estimated. Major C. davisiana abundance maxima occur approximately every100ka in conjunction with glacial/interglacial cycles over the entire record.

The late Pleistocene South Atlantic and Southern Ocean surface waters - A summary of time slice and time series studies

Central to global climate evolution is the paleoceanographic development of the South Atlantic as it represents the passageway for inter-hemispheric heat exchange within global thermohaline circulation (THC). Processes in the adjacent Southern Ocean regulate the heat import into the South Atlantic via the Agulhas “warm water route” (WWR) and the Drake Passage “cold water route” (CWR), and amplify climate change through various feedback mechanisms and teleconnections. For paleoceanographic reconstruction an inventory of new data sets and methods is now available, allowing for the estimation of Pleistocene sea-surface water temperatures and sea-ice distribution on time-slices and time-series based on the calcareous and siliceous microfossil record. Reconstruction of the Last Glacial Maximum (LGM) reveals distinct cooling in the Southern Ocean (up to 4 – 6 °C) accompanied by an expansion of winter and summer sea ice, cooling in the African upwelling regimes (up to 10°C) and in the Equatorial Atlantic ( 4 – 5 °C), but the Subtropical Gyre region remains relatively warm and unchanged compared with the present. While the WWR was not strongly altered during the LGM, heat transport via the CWR was most probably much weaker. The reconstruction of time-slices representing a warm climate end-member at the onset of the last climate cycle documents a distinct lead of southern high-latitudes in global climate development that also affects the south-west African upwelling regions. It is at the Marine Isotope Stage (MIS) 6/MIS 5 transition when Southern Ocean surface temperatures reach maximum values and sea ice is at a minimum, marking a period of South Atlantic heat piracy. During the isotopic minimum of MIS 5.5, the tropical South Atlantic was slightly colder than at present, likely the result of an enhanced poleward heat export. Time-series studies from key areas document that climate variability related to orbital forcing is overprinted by THC changes driven by meltwater injections into the North Atlantic and the Southern Ocean, changes in atmospheric circulation and greenhouse gas concentration, as well as sea ice that amplify climate change at global, hemispheric and regional scales. The study of centennial-scale variability during interglacial optima, such as MIS 5.5 and MIS 11, suggests that the presence of large ice sheets, meltwater events, changes in greenhouse gas concentration and sea-ice distribution are not the only prerequisite to trigger millennial-centennial-scale variability, but that another external agent, changes in solar irradiance, must be considered as an important factor in climate development.