Hannes Grobe

Benthic foraminiferal assemblages from the eastern South Atlantic Polar Front region between 35° and 57°S: Distribution, ecology and fossilization potential

Abstract Surface sediment samples taken with a multiple corer from the Polar Front region along two profiles in the eastern South Atlantic sector between 35° and 57°S have been investigated for their benthic foraminiferal content. Live fauna was differentiated from the death assemblage. Based on the dead assemblages, potential fossil assemblages (dead fauna reduced by non-resistant agglutinants) were calculated for comparison with upper Neogene core material. The distribution of the potential fossil assemblages defined byQ-mode principal component analysis mirrors the Recent hydrographic, as well as the trophic situation in the eastern South Atlantic (ocean circulation and productivity). Two Southern Component Bottom Water Faunas reflect the northward flow of Antarctic Bottom Water (AABW) and Lower Circum-Polar Deep Water (LCDW): an assemblage dominated by the arenaceousMultifidella nodulosa below the Calcite Compensation Depth (CCD), and aNuttallides umbonifer dominated assemblage above the CCD. In the region of highest bio-silicious primary productivity, south of the Polar Front and north of the winter sea-ice limit, these faunas are overprinted by a Southern High-productivity Fauna, dominated by infaunal species such asPullenia bulloides,Melonis pompilioides andM. zaandamae. North Atlantic Deep Water (NADW) meets the Circum-Polar Deep Water (CDW) at intermediate depth, as reflected in the distribution of the Northern Component Deep Water Fauna, dominated byEpistominella exigua. The high-productivity assemblage overprinting this fauna, the Northern High-productivity Fauna, is dominated by the shallow endobenthicBulimina aculeata. A Strong Bottom-current Fauna, dominated byAngulogerina angulosa, characterizes areas between 400 and 900 m water depth with sandy sediment, independent from water mass characteristics and food supply.

Organic carbon, carbonate, and clay mineral distributions in eastern central Arctic Ocean surface sediments

Abstract Results from a detailed sedimentological investigation of surface sediments from the eastern Arctic Ocean indicate that the distribution of different types of sediment facies is controlled by different environmental processes such as sea-ice distribution, terrigenous sediment supply, oceanic currents, and surface-water productivity. In comparison to other open-ocean environments, total organic carbon contents are high, with maximum values in some deep-basin areas as well as west and north of Svalbard. In general, the organic carbon fraction is dominated by terrigenous material as indicated by low hydrogen index values and high C/N ratios, probably transported by currents and/or sea ice from the Eurasian Shelf areas. The amount of marine organic carbon is of secondary importance reflecting the low-productivity environment described for the modern ice-covered Arctic Ocean. In the area north of Svalbard, some higher amounts of marine organic matter may indicate increased surface-water productivity controlled by the inflow of the warm Westspitsbergen Current (WSC) into the Arctic Ocean and reduced sea-ice cover. This influence of the WSC is also supported by the high content of biogenic carbonate recorded in the Yermak Plateau area. The clay mineral distribution gives information about different source areas and transport mechanisms. Illite, the dominant clay mineral in the eastern central Arctic Ocean sediments, reaches maximum values in the Morris-Jesup-Rise area and around Svalbard, indicating North Greenland and Svalbard to be most probable source areas. Kaolinite reaches maximum values in the Nansen Basin, east of Svalbard, and in the Barents Sea. Possible source areas are Mesozoic sediments in the Barents Sea (and Franz-Josef-Land). In contrast to the high smectite values determined in sea-ice samples, smectite contents are generally very low in the underlying surface sediments suggesting that the supply by sea ice is not the dominant mechanism for clay accumulation in the studied area of the modern central Arctic Ocean.

Significance of clay mineral assemblages in the Antarctic Ocean

Abstract Typical examples from different morphological and geological settings in the Antarctic Ocean are reviewed in order to discuss the value of clay mineral assemblages for reconstructing the glacial history of Antarctica, the paleoceanographic history of the Antarctic Ocean and the sedimentary processes at the Antarctic continental margin. The significance of clay minerals for paleoenvironmental reconstructions strongly varies with both the position of the sites under investigation and the age of the sediments. In late Mesozoic to Paleogene sediments clay mineral assemblages are sensitive tools for reconstructing climatic conditions. For example, the shift from smectite-dominated assemblages to illite- and chlorite-dominated assemblages in the earliest Oligocene clearly documents the transition from chemical weathering conditions under a warm and humid climate to physical weathering under cooler conditions. Submarine elevations such as Maud Rise and Kerguelen Plateau give the best record for direct paleoclimatic and paleoceanographic studies. At the proximal sites of the continental slope and shelf, as well as in the deep sea, the paleoclimatic information normally is masked by a variety of processes resulting in sediment redistribution. At those sites, in contrast, the clay mineral assemblages bear a wealth of information on different sedimentary processes. After the establishment of a continental East Antarctic ice sheet, physical weathering prevailed. Variations in the clay mineral records predominantly reflect the influence of different sediment sources resulting from different glacial, hydrographic or gravitational transport processes. Because these sedimentation processes are generally linked to climatic variations, the clay mineral assemblages in most of the Neogene and Quaternary sediments provide indirect paleoclimatic information. The processes are best documented in the clay mineral composition in those areas where changes in source regions with distinct petrographic differences are expected and where distances from the source region are low.

Palaeoproductivity at the Antarctic continental margin: opal and barium records for the last 400 ka

Abstract Records of biogenic opal and barium were measured in sediment cores at the Antarctic continental margin in the area of the Weddell, Lazarev and Cosmonaut seas. These records provide a qualitative and quantitative tool to estimate changes in palaeoproductivity over the last 400 ka. The stratigraphy of the investigated cores is calibrated to a lithostratigraphy, adjusted to a stable isotope record from the eastern Weddell Sea, which is supported by a Th-dating method. We present evidence that interglacial productivity along the Antarctic continental margin is twice as high compared to subantarctic sites near South Orkney. A glacial/interglacial pattern with high productivity during peak warm stages can be observed back to 400 ka. High interglacial productivity is linked to a reduced sea-ice coverage, which is regulated by the heat flux introduced by North Atlantic Deep Water (NADW) to the Antarctic Ocean. Generally, good correlations between the barium and opal records of the sediment cores indicate that dissolution of opal in the water column and the sediment does not obscure the surface productivity signal. Therefore, in this area biogenic opal in combination with other proxies, can be used for palaeoproductivity estimates. Palaeoproductivity is also assessed quantitatively from the barium record using the approaches of Dymond et al. (Dymond, J., Suess, E., Lyle, M., 1992. Barium in deep-sea sediments: a geochemical proxy for paleoproductivity. Paleoceanography 7, 163–181) and Francois et al. (Francois, R., Honjo, S., Manganini, S.J., Ravizza, G.E., 1995. Biogenic barium fluxes to the deep sea: implications for paleoproductivity reconstructions. Glob. Biochem. Cycles 9(2), 289–303). Palaeoproductivity rates obtained by both methods show a good temporal correspondence. In peak warm stages, higher values are computed with the approach of Dymond et al. (1992). Though some barium may be provided by lateral advection of material, as indicated by Th data near South Orkney, both methods provide values, which are representative of a high-productivity area. They are drastically reduced during glacial times. The extent and duration of sea-ice coverage and the persistence of coastal polynyas is considered to be of primary importance in controlling the flux of biogenic material to sediments of the Antarctic continental margin.

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.

Benthic Foraminiferal Assemblages and the δ13C-Signal in the Atlantic Sector of the Southern Ocean: Glacial-to-Interglacial Contrasts

We use benthic foraminiferal assemblages and benthic δ13C to interpret glacial-to-interglacial contrasts in two gravity cores from the lower bathyal Antarctic continental margin at 69°S, and the abyssal Agulhas Basin at 43°S. As a recent analogue, sediment surface samples from an eastern Atlantic Ocean and Weddell Sea transect between 20°–70°S are discussed. In the investigated area, benthic foraminiferal assemblages reflect both ocean circulation and surface productivity. Also at most stations from a belt with seasonally high surface productivity between 48°S–55°S, the δ13C values of epibenthic Cibicidoides spp., including F. wuellerstorfi are depleted relative to the bottom water δ13C∑CO2 and hence do not follow the 1:1 relationship established for more northern areas. This bears implications for the interpretation of large glacial-interglacial 513C shifts from the Southern Ocean: Significant parts of this shift can be caused by a northward migration of high productivity belts associated with the Polar Front and the winter sea-ice limit rather than indicating nutrient-rich glacial Southern Ocean deep and bottom water.

Late Quaternary glacial history and short-term ice-rafted debris fluctuations along the East Greenland continental margin

Abstract High-resolution stable oxygen and carbon isotope and sedimentological investigations were carried out on four west-east profiles at the East Greenland continental margin between 68° and 75°N. The sediment cores represent distinct glacial/interglacial palaeoclimatic episodes over the past 190 ka. Based on oxygen isotope stratigraphy and AMS 14C dating, our data can be well correlated with the global climate record. However, there are some excursions from the global climate curve suggesting a local/regional overprint by meltwater events of the Greenland Ice Sheet, especially at the beginning of isotope stage 3 and during Termination 1. Distinct high-amplitude variations in supply of ice-rafted debris (IRD) indicate repeated advances and retreats of the Greenland Ice Sheet, causing fluctuations in the massive production and transport of icebergs into the Greenland Sea. During the last 190 ka, a number of IRD peaks appear to be correlated with cooling cycles observed in the GRIP Greenland Ice Core. Drastic events in iceberg discharge along the East Greenland continental margin recurred at very short intervals of 1000–3000 years (i.e. much more frequently than the about 10 000 years associated with Heinrich events), suggesting short-term collapses of the Greenland Ice Sheet on these time-scales. These late Weichselian Greenland Ice Sheet oscillations appear to be in phase with those in the Barents Sea area. Maximum flux rates of terrigenous (ice-rafted) material were recorded at the continental slope between about 21 and 16 ka, which may correspond to the maximum (stage 2) extension of glaciers on Greenland. The beginning of Termination I is documented by a distinct shift in the oxygen isotopes and a most prominent decrease in flux of IRD at the continental slope caused by the retreat of continental ice masses.

Holocene glacimarine sedimentation, inner Scoresby Sund, East Greenland: the influence of fast-flowing ice-sheet outlet glaciers

Abstract Holocene glacimarine sedimentation associated with fast-flowing outlet glaciers draining the Greenland Ice Sheet is investigated using sedimentary and acoustic data from inner Scoresby Sund, East Greenland. Sedimentation in inner Scoresby Sund is dominated by three processes which are influenced by differences in proximity to fast-flowing outlet glaciers, extent of glacier-ice cover and fjord bathymetry: (1) sediment-gravity flow, principally in the form of turbidity currents and debris flows; (2) suspension sedimentation from turbid meltwater plumes; and (3) iceberg rafting. These processes result in texturally and sedimentologically heterogeneous lithofacies. Proportionally, fine-grained muds (laminated, stratified and massive facies) dominate cores recovered from inner Scoresby Sund, accounting for 80% of the total, whereas diamict facies account for only 15%. Abundant fine-grained muds demonstrate that meltwater flux and sedimentation is significant in this high Arctic glacimarine environment, in settings proximal to fast-flowing outlet glaciers. With increasing distance from these glacier termini, muds are replaced progressively by iceberg-rafted, coarse-grained sediment. The dominance of this iceberg-rafted sediment in outer Scoresby Sund reflects both its more distal location from fast-flowing glacier termini, and the high calving flux associated with these ice masses. Laminated muds deposited by turbidity currents and suspension settling from overflow plumes in inner Scoresby Sund are similar to lithofacies produced in temperate and subpolar glacimarine systems. This implies a similarity in sedimentation processes and resulting facies across a wide spectrum of climatically-, glaciologically- (fast-flowing and non fast-flowing ice-masses) and oceanographically-variable glacimarine settings. Recognition of laminated, fine-grained facies in the geological record therefore does not necessarily indicate a temperate palaeo-glacial setting. However, the predominance of iceberg-rafted diamict facies in ice-distal sedimentary records suggests the former presence of relatively cold environmental conditions in which iceberg-sedimentation played a dominant role.

Late Quaternary glacial-interglacial changes in sediment composition at the East Greenland continental margin and their paleoceanographic implications

Stable isotope records, and sedimentological and organic-geochemical investigations of marine sediments from the east Greenland Sea at 70°N provide important information about glacial-interglacial variations of paleoenvironments through the last 225 kyr.The oxygen isotope records established on the planktonic foraminifer N. pachyderma sin. show some excursions from the global climate pattern, probably due to local/regional overprint by meltwater supply. The cold, low-saline East Greenland Current and fluctuations in sea-ice covering were a crucial element controlling the carbonate production in the subsurface/surface water column in the east Greenland Sea over the last 225 kyr. The beginning of Termination Ia is AMS 14C dated at about 15.8 kyr B.P. and interpreted as a Greenland Ice Sheet meltwater signal. The stage 2/3 boundary is dated at about 25 kyr B.P. The timing of the onset of the last deglacial meltwater event is about 800 years earlier than that of the Barents Shelf Ice Sheet meltwater signal recorded in the Fram Strait.Several major pulses of increased supply of coarse-grained terrigenous material by glacio-marine processes occurred during the last 225 kyr. The supply of coarse-grained ice-rafted debris at the East Greenland continental slope reached maximum values during the last glacial maximum (stage 2/Weichselian, 15-19 kyr B.P.).The drastic climatic change and the gradual retreat of continental ice masses/glaciers during the last deglaciation (Termination I) are clearly documented in the marine sedimentary sequences from shelf and upper slope environments. This process resulted in distinctly decreased supply and deposition of ice-rafted debris in the open shelf-upper slope environments. During Termination I, the sea-ice cover also decreased, causing an increase in surface-water productivity, indicated by increased organic carbon and biogenic opal deposition.

Sediment redistribution versus paleoproductivity change: Weddell Sea margin sediment stratigraphy and biogenic particle flux of the last 250,000 years deduced from 230Thex, 10Be and biogenic barium profiles

High resolution 230Thex and 10Be and biogenic barium profiles were measured at three sediment gravity cores (length 605–850 cm) from the Weddell Sea continental margin. Applying the 230Thex dating method, average sedimentation rates of 3 cm/kyr for the two cores from the South Orkney Slope and of 2.4 cm/kyr for the core from the eastern Weddell Sea were determined and compared to δ18O and lithostratigraphic results. Strong variations in the radionuclide concentrations in the sediments resembling the glacial/interglacial pattern of the δ18O stratigraphy and the 10Be stratigraphy of high northern latitudes were used for establishing a chronostratigraphy. Biogenic Ba shows a pattern similar to the radionuclide profiles, suggesting that both records were influenced by increased paleoproductivity at the beginning of the interglacials. However, 230Thex0 fluxes (0 stands for initial) exceeding production by up to a factor of 4 suggest that sediment redistribution processes, linked to variations in bottom water current velocity, played the major role in controlling the radionuclide and biogenic barium deposition during isotope stages 5e and 1. The correction for sediment focusing makes the ‘true’ vertical paleoproductivity rates, deduced from the fluxes of proxy tracers like biogenic barium, much lower than previously estimated. Very low 230Thex0 concentrations and fluxes during isotope stage 6 were probably caused by rapid deposition of older, resedimented material, delivered to the Weddell Sea continental slopes by the grounded ice shelves and contemporaneous erosion of particles originating from the water column.