Diatom and hydrocarbon analysis of sediment core NBP00-03-JPC38

In this study, we present a unique high-resolution Holocene record of oceanographic and climatic change based on analyses of diatom assemblages combined with biomarker data from a sediment core collected from the Vega Drift, eastern Antarctic Peninsula (EAP). These data add to the climate framework already established by high-resolution marine sedimentary records from the Palmer Deep, western Antarctic Peninsula (WAP). Heavy sea ice conditions and reduced primary productivity were observed prior to 7.4 ka B.P. in relation with the proximity of the glacial ice melt and calving. Subsequent Holocene oceanographic conditions were controlled by the interactions between the Westerlies-Antarctic Circumpolar Current (ACC)-Weddell Gyre dynamics. A warm period characterized by short seasonal sea ice duration associated with a southern shift of both ACC and Westerlies field persisted until 5 ka B.P. This warm episode was then followed by climate deterioration during the middle-to-late Holocene (5 to 1.9 ka B.P.) with a gradual increase in annual sea ice duration triggered by the expansion of the Weddell Gyre and a strong oceanic connection from the EAP to the WAP. Increase of benthic diatom species during this period was indicative of more summer/autumn storms, which was consistent with changes in synoptic atmospheric circulation and the establishment of low- to high-latitude teleconnections. Finally, the multicentennial scale variability of the Weddell Gyre intensity and storm frequency during the late Holocene appeared to be associated with the increased El Niño-Southern Oscillation frequency.

Carbonate mineralogy and stable isotope composition of cool-water bryozoans from sediments of ODP Leg 182 sites

The carbon and oxygen isotopic compositions of selected bryozoan skeletons from upper Pleistocene bryozoan mounds in the Great Australian Bight (Ocean Drilling Program Leg 182; Holes 1129C, 1131A, and 1132B) were determined. Cyclostome bryozoans, Idmidronea spp. and Nevianipora sp., have low to intermediate magnesian calcite skeletons (1.5-10.0 and 0.9-6.4 molar percentage [mol%] MgCO3, respectively), but a considerable number include marine cements. The cheilostome Adeonellopsis spp. are biminerallic, principally aragonite, with some high magnesian calcite (HMC) (6.6-12.1 mol% MgCO3). The HMC fraction of Adeonellopsis has lower d13C and similar d18O values compared with the aragonite fraction. Reexamination of modern bryozoan isotopic composition shows that skeletons of Adeonellopsis spp. and Nevianipora sp. form close to oxygen isotopic equilibrium with their ambient water. Therefore, changes in glacial-interglacial oceanographic conditions are preserved in the oxygen isotopic profiles. The bryozoan oxygen isotopic profiles are correlated well with marine isotope Stages 1-8 in Holes 1129C and 1132B and to Stages 1-4(?) in Hole 1131A. The horizons of the bryozoan mounds that yield skeletons with heavier oxygen isotopic values can be correlated with isotope Stages 2, 4(?), 6, and 8 in Hole 1129C; Stages 2 and 4(?) in Hole 1131A; and Stages 2, 4, 6, and 8 in Hole 1132B. These results provide supporting evidence for a model for bryozoan mound formation, in which the mounds were formed during intensified upwelling and increased trophic resources during glacial periods.

Relative abundances of stratigraphically useful planktonic foraminifers from ODP Leg 167 sites

Late Neogene biostratigraphy of planktonic foraminifers has been investigated from 13 sites cored during Ocean Drilling Program Leg 167 off the coast of California. The planktonic foraminiferal biostratigraphy of six of these sites is presented here at higher stratigraphic resolution for the interval that encompasses the late early Pliocene through the Quaternary (~3.5 Ma to present day). The sites form a transect along the California margin from 31°N to 41°N within the California Current system. A new planktonic foraminiferal zonation has been established largely on evolutionary changes within the Neogloboquadrina plexus, supported by other taxa. A total of eight zones are recognized, most of which are broadly applicable throughout the region, thus providing a biostratigraphic zonation of the sequence at ~0.5-m.y. intervals. The new zonation appears to be unique to the California Current system. The diversity of planktonic foraminiferal assemblages during the late Neogene appears to have remained relatively constant despite large-scale paleoclimatic change. The assemblages are consistently dominated by few taxa that almost always include the neogloboquadrinids and Globigerina bulloides. Low diversity and high dominance of the assemblages favored these and other taxa well adapted to upwelling systems exhibiting high seasonal surface ocean variability. Apparently the oceanographic conditions that favor such assemblages have persisted at least for the duration of the late Neogene (~3.5 Ma to present day). The biostratigraphically important forms have been illustrated with scanning electron micrographs.

Isotope ratios, IRD and planktonic foraminifera content during marine isotope stage 5 of sediment cores MD99-2303, MD99-2304 and MD95-2010

Variable climatic and oceanographic conditions characterized the last interglacial at high northern latitudes, probably related to changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). The magnitudes of these changes are comparable to the Holocene variability, and were thus significantly subdued compared to glacial climate changes. A thermal optimum occurred during the early part of the interglacial, followed by a period of reduced Atlantic inflow to the northernmost Nordic Seas. Subsequently, a new period with increased strength of the AMOC occurred. Significant amounts of Ice-Rafted Debris (IRD) were deposited in the northernmost Nordic Seas before any major change of the global ice volume. This implies an early onset of local ice sheet growth, probably the result of enhanced inflow of Atlantic water to the northernmost Nordic Seas contemporary with a Northern Hemisphere summer insolation minimum. Contrasting sea-land conditions provided large moisture fluxes towards land, giving rise to rapid, early glacial growth. Throughout the glacial part of Marine Isotope Stage (MIS) 5, millennial-scale cold events occurred along the axis of the warm water transport, from the subtropics all the way to the northernmost Nordic Seas. Correlation of IRD events from sites in the Fram Strait, on the Voring Plateau, and in the North Atlantic provides evidence that the major Northern Hemisphere ice sheets at times responded coherently to the same forcing. The widespread distribution of these events highlights the importance of the oceanic influence on the regional climate system.

Age models and summer sea surface temperature and winter sea ice concentration for the EPILOG-LGM time slice in the Pacific Southern Ocean

Sea surface temperatures and sea-ice extent are the most critical variables to evaluate the Southern Ocean paleoceanographic evolution in relation to the development of the global carbon cycle, atmospheric CO2 variability and ocean-atmosphere circulation. In contrast to the Atlantic and the Indian sectors, the Pacific sector of the Southern Ocean has been insufficiently investigated so far. To cover this gap of information we present diatom-based estimates of summer sea surface temperature (SSST) and winter sea-ice concentration (WSI) from 17 sites in the polar South Pacific to study the Last Glacial Maximum (LGM) at the EPILOG time slice (19,000-23,000 cal. years BP). Applied statistical methods are the Imbrie and Kipp Method (IKM) and the Modern Analog Technique (MAT) to estimate temperature and sea-ice concentration, respectively. Our data display a distinct LGM east-west differentiation in SSST and WSI with steeper latitudinal temperature gradients and a winter sea-ice edge located consistently north of the Pacific-Antarctic Ridge in the Ross sea sector. In the eastern sector of our study area, which is governed by the Amundsen Abyssal Plain, the estimates yield weaker latitudinal SSST gradients together with a variable extended winter sea-ice field. In this sector, sea-ice extent may have reached sporadically the area of the present Subantarctic Front at its maximum LGM expansion. This pattern points to topographic forcing as major controller of the frontal system location and sea-ice extent in the western Pacific sector whereas atmospheric conditions like the Southern Annular Mode and the ENSO affected the oceanographic conditions in the eastern Pacific sector. Although it is difficult to depict the location and the physical nature of frontal systems separating the glacial Southern Ocean water masses into different zones, we found a distinct temperature gradient in latitudes straddled by the modern Southern Subtropical Front. Considering that the glacial temperatures north of this zone are similar to the modern, we suggest that this represents the Glacial Southern Subtropical Front (GSSTF), which delimits the zone of strongest glacial SSST cooling (>4K) to its North. The southern boundary of the zone of maximum cooling is close to the glacial 4°C isotherm. This isotherm, which is in the range of SSST at the modern Antarctic Polar Front (APF), represents a circum-Antarctic feature and marks the northern edge of the glacial Antarctic Circumpolar Current (ACC). We also assume that a glacial front was established at the northern average winter sea ice edge, comparable with the modern Southern Antarctic Circumpolar Current Front (SACCF). During the glacial, this front would be located in the area of the modern APF. The northward deflection of colder than modern surface waters along the South American continent leads to a significant cooling of the glacial Humboldt Current surface waters (4-8K), which affects the temperature regimes as far north as into tropical latitudes. The glacial reduction of ACC temperatures may also result in the significant cooling in the Atlantic and Indian Southern Ocean, thus may enhance thermal differentiation of the Southern Ocean and Antarctic continental cooling. Comparison with temperature and sea ice simulations for the last glacial based on numerical simulations show that the majority of modern models overestimate summer and winter sea ice cover and that there exists few models that reproduce our temperature data rather well.

Water mass evolution of the Greenland Sea since late glacial times

Four sediment cores from the central and northern Greenland Sea basin, a crucial area for the renewal of North Atlantic deep water, were analyzed for planktic foraminiferal fauna, planktic and benthic stable oxygen and carbon iso- topes as well as ice-rafted debris to reconstruct the environ- mental variability in the last 23 kyr. During the Last Glacial Maximum, the Greenland Sea was dominated by cold and sea-ice bearing surface water masses. Meltwater discharges from the surrounding ice sheets affected the area during the deglaciation, influencing the water mass circulation. During the Younger Dryas interval the last major freshwater event occurred in the region. The onset of the Holocene interglacial was marked by an increase in the advection of Atlantic Wa- ter and a rise in sea surface temperatures (SST). Although the thermal maximum was not reached simultaneously across the basin, benthic isotope data indicate that the rate of overturn- ing circulation reached a maximum in the central Greenland Sea around 7ka. After 6-5ka a SST cooling and increas- ing sea-ice cover is noted. Conditions during this so-called "Neoglacial" cooling, however, changed after 3 ka, probably due to enhanced sea-ice expansion, which limited the deep convection. As a result, a well stratified upper water column amplified the warming of the subsurface waters in the central Greenland Sea, which were fed by increased inflow of At- lantic Water from the eastern Nordic Seas. Our data reveal that the Holocene oceanographic conditions in the Green- land Sea did not develop uniformly. These variations were a response to a complex interplay between the Atlantic and Polar water masses, the rate of sea-ice formation and melting and its effect on vertical convection intensity during times of Northern Hemisphere insolation changes.