Physical and acoustic sediment properties of sediment cores from the eastern equatorial Atlantic

To reconstruct the deep-water circulation for the last 3.5 Ma from deep-sea sediments of the eastern equatorial Atlantic, sea floor morphology, sub-bottom reflectors and the echo character have been mapped on the basis of 3.5 kHz records and sediment cores. Physical properties of sediments and synthetic seismograms derived from them enable us to decipher reflector sequences in environments of pelagic, current-resuspended and turbidity sedimentation. The individual reflectors originate from carbonate dissolution, hiatus, coarse sand layers and interferences. Those which are related to carbonate dissolution and hiatus provide evidence of water-mass boundaries by their distribution. Five phases of different deep-water circulation can be seen in the record of th elast 3.5 Ma, and these are related to climate history: 1. Between 3.7 Ma and 2.2 Ma a strong deep-water circulation indicates a northward flow of bottom water below 4200 m (AABW = Antarctic-Bottom Water) and a southward flow of deep-water above 4200 m (NADW = North-Atlantic Deep Water). 2. Between 1.6 and 1.4 Ma a southward flow of bottom water below 4500 m and a diminished southward flow above 4500 m can be detected. This water-mass geometry can be interpreted by an expansion of the NADW-masses and a displacement of the AABW-masses during the same time. 3. Since 1.4 Ma a northward flow of a bottom-water current developed again. This current flow created a leeside sediment ridge in the southern part of the Kane Gap and furrows in the northern part of it. 4. Between 400,000 and 200,000 yrs B. P. the oceanic and atmospheric circulation increased. The strengthened oceanic circulation caused and increase in carbonate dissolution, which is documented by a traceable reflector from 2800 m to 4500 m water depth. At the same time an increase of the atmospheric circulation caused a drastic rise in the pelagic sediment accumulation (> 100 %) through an intensification of upwelling. This runs parallel with a higher oceanic productivity in the northern equatorial divergence zone and an enhanced supply of fluvial and probably eolian sediments from the Senegal and Guinea. 5. Before 10,000 yrs B. P. an erosive northward flowing bottom-water current prevailed below 4500 m water depth. After 10,000 yrs B.P. the bottom-water flow was sluggish and non erosive.

Sedimentological and geophysical investigation of sediment cores from the Amundsen Sea

Ice loss from the marine-based, potentially unstable West Antarctic Ice Sheet (WAIS) contributes to current sea-level rise and may raise sea level by up to 3.3 to 5 meters in the future. Over the past few decades, glaciers draining the WAIS into the Amundsen Sea Embayment (ASE) have shown accelerated ice flow, rapid thinning and grounding-line retreat. However, the long-term context of this ice-sheet retreat is poorly constrained, limiting our ability to accurately predict future WAIS behaviour. Here we present a new chronology for WAIS retreat from the inner continental shelf of the eastern ASE based on radiocarbon dates from three marine sediment cores. The ages document a retreat of the grounding line to within ~93 km of its modern position before 11.7±0.7 kyr BP (thousand years before present). This early deglaciation is consistent with ages for grounding-line retreat from the western ASE. Our new data demonstrate that, other than in the Ross Sea, WAIS retreat in the ASE has not continued progressively since the Last Glacial Maximum. Furthermore, our results suggest that the grounding-line position in the ASE was predominantly stable throughout the Holocene, and that any episodes of fast retreat similar to that observed today must have been short-lived. Alternatively, today's rapid retreat was unprecedented during the Holocene. Therefore, the current ice loss must originate in recent changes in regional climate, ocean circulation or ice-sheet dynamics. Incorporation of these results into models is essential to produce robust predictions of future ice-sheet change and its contribution to sea-level rise.

(Table 2) Radiocarbon ages on Arctic box and multicores

Paleo-sea-ice history in the Arctic Ocean was reconstructed using the sea-ice dwelling ostracode Acetabulastoma arcticum from late Quaternary sediments from the Mendeleyev, Lomonosov, and Gakkel Ridges, the Morris Jesup Rise and the Yermak Plateau. Results suggest intermittently high levels of perennial sea ice in the central Arctic Ocean during Marine Isotope Stage (MIS) 3 (25-45 ka), minimal sea ice during the last deglacial (16-11 ka) and early Holocene thermal maximum (11-5 ka) and increasing sea ice during the mid-to-late Holocene (5-0 ka). Sediment core records from the Iceland and Rockall Plateaus show that perennial sea ice existed in these regions only during glacial intervals MIS 2, 4, and 6. These results show that sea ice exhibits complex temporal and spatial variability during different climatic regimes and that the development of modern perennial sea ice may be a relatively recent phenomenon.

Sedimentological, mineralogical, biogeochemical and micropaleontological parameters on sediment core PS69/849-2, Burton Basin, MacRobertson Shelf, East Antarctica

A multi-proxy study including sedimentological, mineralogical, biogeochemical and micropaleontological methods was conducted on sediment core PS69/849-2 retrieved from Burton Basin, MacRobertson Shelf, East Antarctica. The goal of this study was to depict the deglacial and Holocene environmental history of the MacRobertson Land-Prydz Bay region. A special focus was put on the timing of ice-sheet retreat and the variability of bottom-water formation due to sea ice formation through the Holocene. Results from site PS69/849-2 provide the first paleo-environmental record of Holocene variations in bottom-water production probably associated to the Cape Darnley polynya, which is the second largest polynya in the Antarctic. Methods included end-member modeling of laser-derived high-resolution grain size data to reconstruct the depositional regimes and bottom-water activity. The provenance of current-derived and ice-transported material was reconstructed using clay-mineral and heavy-mineral analysis. Conclusions on biogenic production were drawn by determination of biogenic opal and total organic carbon. It was found that the ice shelf front started to retreat from the site around 12.8 ka BP. This coincides with results from other records in Prydz Bay and suggests warming during the early Holocene optimum next to global sea level rise as the main trigger. Ice-rafted debris was then supplied to the site until 5.5 cal. ka BP, when Holocene global sea level rise stabilized and glacial isostatic rebound on MacRobertson Land commenced. Throughout the Holocene, three episodes of enhanced bottom-water activity probably due to elevated brine rejection in Cape Darnley polynya occured between 11.5 and 9 cal. ka BP, 5.6 and 4.5 cal. ka BP and since 1.5 cal. ka BP. These periods are related to shifts from warmer to cooler conditions at the end of Holocene warm periods, in particular the early Holocene optimum, the mid-Holocene warm period and at the beginning of the neoglacial. In contrast, between 7.7 and 6.7 cal. ka BP, brine rejection shut down, maybe owed to warm conditions and pronounced open-water intervals.

Sedimentology and age determinations on core PS2813-1

Die Rekonstruktion des Einflusses von Strömungen und glazialmarinen Prozessen auf das Sedimentationsgeschehen am Kontinentalhang der Antarktischen Halbinsel im westlichen Weddellmeer basiert auf sedimentologischen und geophysikalischen Daten eines Kolbenlotkerns. Der Sedimentkern wurde während des Fahrtabschnitts ANT-XIV/3 mit dem FS "Polarstern" aus einer mächtigen Levee-Struktur eines Rinnen-Rückensystems gewonnen. Es wurden sedimentologische sowie sedimentphysikalische Untersuchungen an dem Kernmaterial durchgeführt. Die texturellen Änderungen im Kern und die Variationen der gemessenen Parameter ermöglichen eine lithofazielle Gliederung und stratigraphische Einstufung der Sedimentabfolge. Die untersuchten Sedimente umfassen den Zeitraum der vier letzten Klimazyklen bis heute und repräsentieren die Ablagerungsbedingungen von mehr als 340 000 Jahren. Vier Faziestypen wurden unterschieden, die sowohl glaziale als auch interglaziale Ablagerungsräume charakterisieren. (1) Die überwiegend groblaminierten Sedimentabfolgen wurden der Laminitfazies zugeordnet. Unter glazialen Umweltbedingungen kam es infolge schwacher Bodenströmungen zur Ablagerung feinkörniger, laminierter, strömungsbetonter Sedimente. (2) Strukturlose, sehr homogene Sedimentabfolgen des Kems beschreiben einen weiteren, den Kaltzeiten zugeordneten, Faziestyp, der durch geringe Variationen in den Sedimenteigenschaften charakterisiert ist. (3) Kernabschnitte, die weitgehend strukturlos sind bzw. leichte Bioturbationen und relativ viel eistransportiertes Material aufweisen, wurden als IRD-Fazies bezeichnet. Sie repräsentiert den Übergang vom Glazial zum Interglazial, in dem sich das Schelfeis und die Meereisbeckung zurückzogen. In den Sedimenten kam es infolge der gesteigerten Kalbungsrate zur Anreicherung der Eisfracht. (4) Die relativ biogenreichen, hellen Ablagerungen wurden der interglazialzeitlichen Karbonatfazies zugeteilt. Der signifikant erhöhte Anteil planktischer Foraminiferen weist auf eine gesteigerte Bioproduktivität im Oberflächenwasser hin, die aus verstärkten jahreszeitlichen Schwankungen der Meereisbedeckung resultiert. Die betrachteten Sedimentationsprozesse, wie biologische Produktivität, Umlagerungsprozesse durch Meeresströmungen, gravitativer Sedimenttransport und Eistransport, sind das Abbild komplexer Wechselwirkungen aus Meeresspiegelschwankungen, Änderungen ozeanographischer Bedingungen und der Vereisungsdynamik. Das Sedimentationsgeschehen im Untersuchungsgebiet wurde folglich durch die Variationen der vorherrschenden Umweltbedingungen bestimmt. Im Glazial kam es unter einer geschlossenen Meereisbedeckung zur Ablagerung feinkörniger, geschichteter Sedimente. Vorwiegend Turbiditströmungen kontrollierten das Sedimentationsgeschehen innerhalb des betrachteten Rinnen-Rückensystems. Unter dem Einfluß der Coriolis-Kraft und wahrscheinlich einer Konturströmung wurden die suspendierten, feinkörnigen Partikel aus dem zentralen Bereich der Rinne verdriftet und über dem nördlichen Uferwall abgelagert. Höherenergetische gravitative Prozesse beeinflußten das Sedimentationsgeschehen episodisch und sind durch gut sortierte Ablagerungen mit erhöhten Gehalten im Mittel- bis Grobsiltbereich dokumentiert. Höhere Sedimentationsraten in den Glazialen trugen verstärkt zur Bildung des Uferwalls bei. Die Ablagerungen der ebenfalls glazialzeitlichen homogenen Fazies belegen unterschiedliche Ablagerungsbedingungen und eine Verschiebung der dominierenden Prozesse. Während des Übergangs vom Glazial zum Interglazial nahm die Bodenwasserbildungsrate durch das Aufschwimmen des Schelfeises zu, wodurch die Strömungsintensität gesteigert wurde. Eine verstärkte Eisbergaktivität wird durch die Anreichung des IRD-Materials dokumentiert. Während interglazialer Zeiten ermöglichten offen-marine Bedingungen im Südsommer eine leicht erhöhte biologische Produktivität, so daß der Ablagerungsraum durch die Sedimentation biogener Komponenten verstärkt beeinflußt wurde.

Seasonal changes in sediment core PS1795-2

The Weddell Sea and the associated Filchner-Rønne Ice Shelf constitute key regions for global bottomwater production today. However, little is known about bottom-water production under different climate and icesheet conditions. Therefore, we studied core PS1795, which consists primarily of fine-grained siliciclastic varves that were deposited on contourite ridges in the southeastern Weddell Sea during the Last Glacial Maximum (LGM). We conducted high-resolution X-ray fluorescence (XRF) analysis and grain-size measurements with the RADIUS tool (Seelos and Sirocko, 2005, doi:10.1111/j.1365-3091.2005.00715.x) using thin sections to characterize the two seasonal components of the varves at sub-mm resolution to distinguish the seasonal components of the varves. Bright layers contain coarser grains that can mainly be identified as quartz in the medium-to-coarse silt grain size. They also contain higher amounts of Si, Zr, Ca, and Sr, as well as more ice-rafted debris (IRD). Dark layers, on the other hand, contain finer particles such as mica and clay minerals from the chlorite and illite groups. In addition, Fe, Ti, Rb, and K are elevated. Based on these findings as well as on previous analyses on neighbouring cores, we propose a model of enhanced thermohaline convection in front of a grounded ice sheet that is supported by seasonally variable coastal polynya activity during the LGM. Accordingly, katabatic (i.e. offshore blowing) winds removed sea ice from the ice edge, leading to coastal polynya formation. We suggest that glacial processes were similar to today with stronger katabatic winds and enhanced coastal polynya activity during the winter season. Under these conditions, lighter coarser-grained layers are likely glacial winter deposits, when brine rejection was increased, leading to enhanced bottom-water formation and increased sediment transport. Vice versa, darker finer-grained layers were then deposited during less windier season, mainly during summer, when coastal polynya activity was likely reduced.

Corrected index properties for ODP Leg 127 and 128 sites

During Legs 127 and 128, we found a systematic error in the index property measurements, in that the wet bulk density, grain density, and porosity did not satisfy well-established interrelationships. We have found that an almost constant difference exists between the weight of water lost during drying and the volume of water lost. This discrepancy is independent of volume or water content of the sample. The water losses should be equal because the density of water is close to 1.0 g/cm**3. The pycnometer wet volume measurement has been identified as the source of the systematic error. The wet volume on average is 0.2 cm**3 too low. For the rare cases when the water content is negligible, there is no offset. The source of the wet volume error results from the partial vapor pressure of water in the pycnometer cell. Newly corrected tables of index properties measured during Legs 127 and 128 are included. The corrected index properties are internally consistent. The data are in better agreement with theoretical models that relate the index properties to other physical properties, such as thermal conductivity and acoustic velocity. In future, a standard volume sampler should be used, or the wet volume should be calculated from the dry volume and the water loss by weight.

Sedimentological and paleomagnetic investigations on two gravity cores and ODP sites 113-690 and 693 off Kapp Norvegia, Antarctic continental margin

Oligocene to Quaternary sediments were recovered from the Antarctic continental margin in the eastern Weddell Sea during ODP Leg 113 and Polarstern expedition ANT-VI. Clay mineral composition and grain size distribution patterns are useful for distinguishing sediments that have been transported by ocean currents from those that were ice-rafted. This, in turn, has assisted in providing insights about the changing late Paleogene to Neogene sedimentary environment as the cryosphere developed in Antarctica. During the middle Oligocene, increasing glacial conditions on the continent are indicated by the presence of glauconite sands, that are interpreted to have formed on the shelf and then transported down the continental slope by advancing glaciers or as a result of sea-level lowering. The dominance of illite and a relatively high content of chlorite suggest predominantly physical weathering conditions on the continent. The high content of biogenic opal from the late Miocene to the late Pliocene resulted from increased upwelling processes at the continental margin due to increased wind strength related to global cooling. Partial melting of the ice-sheet occurred during an early Pliocene climate optimum as is shown by an increasing supply of predominantly current-derived sediment with a low mean grain size and peak values of smectite. Primary productivity decreased at ~ 3 Ma due to the development of a permanent sea-ice cover close to the continent. Late Pleistocene sediments are characterized by planktonic foraminifers and biogenic opal, concentrated in distinct horizons reflecting climatic cycles. Isotopic analysis of AT. pachyderma produced a stratigraphy which resulted in a calculated sedimentation rate of 1 cm/k.y. during the Pleistocene. Primary productivity was highest during the last three interglacial maxima and decreased during glacial episodes as a result of increasing sea-ice coverage.

Sedimentological investigations of a sediment core profile from the continental slope of the South Orkney Islands

Sediment patterns such as texture, composition, and facies from three selected areas of the Antarctic continental margin of the Weddell Sea are discussed in relation to environmental variations of the Quaternary hydrosphere and kryosphere. Advance and retreat of ice shelves as well as oscillations in sea ice coverage are reflected by particular sediment facies. The distribution of ice-rafted detritus tracks the Antarctic Coastal Current, and the Weddell Sea Bottom water contour current can be recognized by its distinctive winnowing and erosion pattern. Distribution and abundance of biogenic sediment components are mainly controlled by duration of sea ice coverage reflecting the long-term climatic evolution.

Lake sediments from Store Koldewey, Northeast Greenland

A 2.9 m long sedimentary record was studied from a small lake, here referred to as Duck Lake, located at 76°25'N, 18°45'W on Store Koldewey, an elongated island off the coast of Northeast Greenland. The sediments were investigated for their geophysical and biogeochemical characteristics, and for their fossil chironomid assemblages. Organic matter began to accumulate in the lake at 9.1 cal. kyr BP, which provides a minimum age for the deglaciation of the basin. Although the early to mid-Holocene is known as a thermal maximum in East Greenland, organic matter accumulation in the lake remained low during the early Holocene, likely due to late plant immigration and lack of nutrient availability. Organic matter accumulation increased during the middle and late Holocene, when temperatures in East Greenland gradually decreased. Enhanced soil formation probably led to higher nutrient availability and increased production in the lake. Chironomids are abundant throughout the record after 9.1 cal. kyr BP and seem to react sensitively to changes in temperature and nutrient availability. It is concluded that relative temperature reconstructions based on biogeochemical data have to be regarded critically, particularly in the period shortly after deglaciation when nutrient availability was low. Chironomids may be a suitable tool for climatic reconstructions even in those high arctic environments. However, a better understanding of the ecology of chironomids under these extreme conditions is needed.

Geochemistry and sedimentation rates of ODP Leg 199 sites and of surface sediments in the eastern Pacific ocean

We present the first high-resolution organic carbon mass accumulation rate (MAR) data set for the Eocene equatorial Pacific upwelling region, from Sites 1218 and 1219 of the Ocean Drilling Program. A maximum Corg MAR anomaly appears at 41 Ma and corresponds to a high carbonate accumulation event (CAE). Independent evidence suggests that this event (CAE-3) was a time of rapid cooling. Throughout the Eocene, organic carbon burial fluxes were an order of magnitude lower than fluxes recorded for the Holocene. In contrast, the expected organic carbon flux, calculated from the biogenic barium concentrations for these sites, is roughly equal to modern. A sedimentation anomaly appears at 41 Ma, when both the measured and the expected organic carbon MAR increases by a factor of two-three relative to the background Eocene fluxes. The rain of estimated Corg and barium from the euphotic zone to the sediments increased by factors of three and six, respectively. We suggest that the discrepancy between the expected and measured Corg in the sediments is a direct consequence of the increased metabolic rates of all organisms throughout the Eocene oceans and sediments. This hypothesis is supported by recent work in ecology and biochemical kinetics that recognizes the fundamental basis of ecology as following from the laws of thermodynamics. This dependence is now elucidated as the Universal Temperature Dependence (UTD) "law" of metabolism and can be applied to all organisms over their biologically relevant temperature range. The general pattern of organic carbon and barium deposition throughout the Eocene is consistent with the UTD theory. In particular, the anomaly at 41 Ma (CAE-3) is associated with rapid cooling, an event that triggered slower metabolic rates for all organisms, slower recycling of organic carbon in the water and sediment column, and, consequently, higher deposition of organic carbon in the sediments. This "metabolism-based" scenario is consistent with the sedimentation patterns we observe for both Sites 1218 and 1219.

Biological Atlas of the Arctic Seas 2000: Physical oceanography, hydrochemistry, meteorology, and plankton of the Barents and Kara Seas

Presented are physical and biological data for the region extending from the Barents Sea to the Kara Sea during 158 scientific cruises for the period 1913-1999. Maps with the temporal distribution of physical and biological variables of the Barents and Kara Seas are presented, with proposed quality control criteria for phytoplankton and zooplankton data. Changes in the plankton community structure between the 1930s, 1950s, and 1990s are discussed. Multiple tables of Arctic Seas phytoplankton and zooplankton species are presented, containing ecological and geographic characteristics for each species, and images of live cells for the dominant phytoplankton species.