Ulrich Zielinski

Diatom distribution in Southern Ocean surface sediments (Atlantic sector): Implications for paleoenvironmental reconstructions

Abstract A study of 230 surface sediment samples collected in the Atlantic sector of the Southern Ocean between the southernmost Weddell Sea and the Subtropical Zone documents the modern distribution of diatoms revealing patterns of paleoenvironmental significance. Estimations of diatom valves per gram dry sediment display numbers of (50–200) × 106 in the zone of high opal burial located between the mean position of the winter sea ice edge and the Polar Front and maximum values of greater than 200 × 106 in the near-shore sedimentary basins off the Antarctic Peninsula. Lowest diatom concentrations and assemblages strongly affected by dissolution were encountered in the Weddell Basin. Despite alteration of the diatom assemblages prior to their incorporation into the sediment record, the biogeographic distribution and the abundance pattern of most of the 35 studied diatom species shows a close relationship with the surface hydrography (water temperature). These relationships can be used to estimate past surface water temperatures based on statistical treatments of the assemblages or on simple relations of species occurrences in the geological record. Another close link occurs between the distribution of sea ice and sea ice related diatoms.

Quaternary surface water temperature estimations: Calibration of a diatom transfer function for the Southern Ocean

The quantitative diatom analysis of 218 surface sediment samples recovered in the Atlantic and western Indian sector of the Southern Ocean is used to define a base of reference data for paleotemperature estimations from diatom assemblages using the Imbrie and Kipp transfer function method. The criteria which justify the exclusion of samples and species out of the raw data set in order to define a reference database are outlined and discussed. Sensitivity tests with eight data sets were achieved evaluating the effects of overall dominance of single species, different methods of species abundance ranking, and no-analog conditions (e.g., Eucampia Antarctica) on the estimated paleotemperatures. The defined transfer functions were applied on a sediment core from the northern Antarctic zone. Overall dominance of Fragilariopsis kerguelensis in the diatom assemblages resulted in a close affinity between paleotemperature curve and relative abundance pattern of this species downcore. Logarithmic conversion of counting data applied with other ranking methods in order to compensate the dominance of F. kerguelensis revealed the best statistical results. A reliable diatom transfer function for future paleotemperature estimations is presented.

Plio–Pleistocene diatom biostratigraphy from ODP Leg 177, Atlantic sector of the Southern Ocean

Abstract Seven sites were drilled during Ocean Drilling Program Leg 177 in the Atlantic sector of the Southern Ocean (SO) on a transect over the Antarctic Circumpolar Current from the Subantarctic to the Antarctic Zone. At four sites sediments were recovered with a Pliocene/Pleistocene sediment package of up to 580 m allowing the refinement of previous diatom zonation concepts. Samples were analyzed on stratigraphic distribution and abundance of diatom species. A refined diatom biozonation tied to the geomagnetic polarity record is proposed. For the middle and late Pleistocene two zonations applicable to the northern and southern area of the SO were constructed, considering different latitudinal distributions of biostratigraphic diatom marker species. The southern zonation for the Pleistocene relies on the occurrence of species of the genus Rouxia, R. leventerae and R. constricta n. sp. as well as on a revised last occurrence datum (LOD) of Actinocyclus ingens (0.38 Ma, late marine isotope stage (MIS) 11). The use of these new stratigraphic marker species refines the temporal resolution for biostratigraphic age assignment to up to 0.1 Myr. In particular the LOD of R. leventerae as an indicator for the MIS 6/5 boundary (Termination II) will improve future dating of carbonate-free Antarctic sediments. These new data were obtained from sediments of Sites 1093 and 1094 (Antarctic Zone). The northern zonation for the middle and late Pleistocene time interval is based on the Pleistocene abundance pattern of Hemidiscus karstenii which was already proposed by previous investigations (e.g. Gersonde and Barcena, 1998 ). One new species (R. constricta) and two new combinations (Fragilariopsis clementia, Fragilariopsis reinholdii) are proposed in this study.

New software package available for quantitative paleoenvironmental reconstructions

The Imbrie and Kipp transfer function method (IKM) and the modern analog technique (MAT) are accepted tools for quantitative paleoenvironmental reconstructions. However, no uncomplicated, flexible software has been available to apply these methods on modern computer devices. For this reason the software packages PaleoToolBox, MacTransfer, WinTransfer, MacMAT, and PanPlot have been developed. The PaleoToolBox package provides a flexible tool for the preprocessing of microfossil reference and downcore data as well as hydrographic reference parameters. It includes procedures to randomize the raw databases; to switch specific species in or out of the total species list; to establish individual ranking systems and their application on the reference and downcore databasessemi; and to convert the prepared databases into the file formats of IKM and MAT software for estimation of paleohydrographic parameters.

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.

Parmales species (siliceous marine nanoplankton) in surface sediments of the Weddell Sea, Southern Ocean: indicators for sea ice environment?

Abstract The distribution pattern of the Parmales, a recently described order of siliceous marine nanophytoplankton, is described and quantified in surface sediments of the Atlantic sector of the Southern Ocean. Additional samples were investigated from the Bransfield Strait and the eastern Bellingshausen Sea. Highest abundances occurred in nearshore areas of the southern and southeastern Weddell Sea. Due to the distribution of Parmales species, which is strictly linked to these neritic areas, influenced most of the year by sea ice, it is suggested that the presence of Parmales species may be an indicator of sea ice influence probably useful in later down core studies.