ALBERT

Description: We present a new proxy, the oxygen isotope measurements (d18O) of radiolarian opal from sediment cores in the Subantarctic Zone (SAZ) and Permanent Open Ocean Zone (POOZ) of the Atlantic Southern Ocean. The SAZ core records of radiolarian and foraminiferal d18O are well correlated. In the POOZ core, only radiolarian 18O shifts to distinctly lower values during the glacial (when this core was in the Sea Ice Zone (SIZ)), while relevant foraminiferal and diatomaceous d18O records exhibit no such shift. This suggests a seasonal salinity change derived from the melting of snow accumulated on sea ice. We propose that a northward relocation of the Southern Ocean zonal system and enhanced water-vapor transport from lower latitudes forced the development of a stratified sea-surface in the northern SIZ during late glacial spring time. This raises unexpected aspects for numerical modeling of past atmospheric coupling and moisture transport, as well as on glacial Southern Ocean stratification and its implication for ocean/atmosphere gas exchange, water mass generation and productivity.

Description: The presence of a complex bedform arrangement on the sea-floor of the continental shelf in the western Amundsen Sea Embayment, West Antarctica, indicates a multi-temporal record of flow related to the activity of one or more ice streams in the past. Mapping and division of the bedforms into distinct landform assemblages reveals their time-transgressive history, which implies that bedforms can neither be considered part of a single down-flow continuum nor a direct proxy for palaeo-ice velocity, as suggested previously. A main control on the bedform imprint is the geology of the shelf, which is divided broadly between rough bedrock on the inner shelf, and smooth, dipping sedimentary strata on the mid-to-outer shelf. Inner shelf bedform variability is well preserved, revealing information about local, complex basal-ice conditions, meltwater flow, and ice dynamics over time. These details, which are not apparent at the scale of regional morphological studies, indicate that past ice streams flowed across the entire shelf, at times, and often had onset zones that lay within the interior of the Antarctic Ice Sheet today. In contrast, highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice-stream flow, and may be a truer representation of fast palaeo-ice flow in these locations. A revised model for ice streams on the shelf captures complicated multi-temporal bedform patterns associated with an Antarctic palaeo-ice stream for the first time, and confirms a strong substrate control on this ice stream system that drained the West Antarctic Ice Sheet during the Late Quaternary.

Description: The first full water column hafnium isotopic compositions of Atlantic seawater have been obtained at seven locations from the Labrador Sea to the Drake Passage. Despite subpicomolar concentrations in seawater, a precision of the Hf isotopic measurements of 〈0.7 epsilon-Hf units was achieved. An overall epsilon-Hf range between -3.1 in the Labrador Sea and +4.4 in Antarctic bottom water was determined, the distribution of which broadly reflects continental weathering inputs. Within particular water column profiles, significant differences of up to 4 epsilon-Hf units occur. Combined with Nd isotope data of the same samples, it is evident that the Hf isotopic composition of seawater is too radiogenic for a given Nd isotopic composition and that the largest difference between expected and measured Hf isotopic compositions in seawater occurs near the oldest continental crust in the Labrador Sea. This corroborates the previous proposition, which was mainly based on ferromanganese crust data, that the Hf isotopic composition of seawater is controlled by incongruent weathering of continental crust and possibly, to some extent, by hydrothermal contributions. Hafnium concentrations in the ocean do not increase along the deep ocean conveyer indicating an oceanic residence time of only a few hundred years, which is significantly shorter than previously assumed. The Hf isotopic composition of past seawater can therefore serve as a proxy for short distance, basin scale mixing processes and the regime and intensity of nearby continental weathering processes.

Description: The effect of volcanic activity on submarine hydrothermal systems has been well documented along fast- and intermediate-spreading centers but not from slow-spreading ridges. Indeed, volcanic eruptions are expected to be rare on slow-spreading axes. Here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated, apparently magmatically robust segment center on the slow-spreading southern Mid-Atlantic Ridge near 5°S. Three high-temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase-separated, vapor-type fluids. Exit temperatures at one of the fields reach up to 407°C, at conditions of the critical point of seawater, the highest temperatures ever recorded from the seafloor. Fluid and vent field characteristics show a large variability between the vent fields, a variation that is not expected within such a limited area. We conclude from mineralogical investigations of hydrothermal precipitates that vent-fluid compositions have evolved recently from relatively oxidizing to more reducing conditions, a shift that could also be related to renewed magmatic activity in the area. Current high exit temperatures, reducing conditions, low silica contents, and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source, probably related to a young volcanic eruption event nearby, in which basaltic magma is actively crystallizing. This is the first reported evidence for direct magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge.

Description: Die Rekonstruktion der glaziomarinen Sedimentationsprozesse am antarktischen Kontinentalrand des westlichen Bellingshausenmeeres erfolgte durch die sedimentologische Auswertung eines 962 cm langen Schwerelotkernes aus 3594 m Wassertiefe. Der Kern wurde während des Fahrtabschnittes ANT-XI/3 mit dem FS "Polarstern" vom Scheitel einer Sediment- "Drift" gezogen. An dem Sedimentkern wurde eine lithologische Beschreibung, sowie sedimentologische Untersuchungen und sedimentphysikalische Messungen durchgeführt. Anhand der Ergebnisse konnten signifikante Änderungen in der Zusammensetzung und Struktur der Sedimente erkannt, und drei Faziestypen unterschieden werden. Die Faziestypen charakterisieren jeweils glaziale oder interglaziale Zeiträume. Der größte Teil der Sedimentabfolge gehört der Laminitfazies an. Dabei handelt es sich um feinlaminierte Sedimentabschnitte, die vorwiegend aus feinkörnigen, terrigenen Komponenten zusammengesetzt sind. In die feinlaminierten Abschnitte sind vereinzelte, wenige Milimeter bis Zentimeter mächtige Siltlagen eingeschaltet. Die biogenen Anteile sind gering, Anzeichen für Bodenleben fehlen völlig. Die Manganfazies wird von authigen gebildeten Mangankonkretionen dominiert, die jeweils diskrete Lagen bilden. Dabei handelt es sich zum einen um Mikromanganknollen und -krusten und zum andern um manganhaltige Gangfüllungen. Biogene und terrigene Anteile sind in diesem Faziestyp unbedeutend. Die Biogenfazies ist von strukturlosen und stark bioturbierten Sedimenten gekennzeichnet. In diesen Sedimentabschnitten ist der hohe Anteil an Eisfracht (IRD) und die erhöhten Gehalte an Kalziumkarbonat und Opal in der Sandfraktion markant. Die stratigraphische Einordnung des Sedimentkernes erfolgte über die von Grobe & Mackensen (1992) entwickelte Lithostratigraphie, mit deren Einheiten die Faziestypen des Sedimentkernes korreliert werden konnten. Dabei ergaben sich zwei mögliche Altersmodelle und ein Basisalter von ca. 250.000 Jahren. Anhand der stratigraphischen Fixpunkte wurden Sedimentationsraten des Gesamtsedimentes und Akkumulationsraten des Kalziumkarbonates, des Biogenopals und des organisch gebundenen Kohlenstoffes berechnet. Dabei wurde gezeigt, daß lediglich das Kalziumkarbonat und der Biogenopal als Anzeiger für biologische Produktion dienen können, wobei Lösungsprozesse in der Wassersäule und im Sediment eine große Rolle spielen. Der Gehalt an organisch gebundenem Kohlenstoff ist in dem Sedimentkern nur erhaltungsbedingt zu erklären. Die Sedimentationsprozesse der einzelnen Faziestypen sind von den Eisverhältnissen, der biologischen Produktion, dem gravitativen Transport und der Umlagerung durch Meeresströmungen abhängig. Die Auswirkung der einzelnen Faktoren ist jeweils unterschiedlich ausgeprägt und wirkt sich spezifisch auf die einzelnen Parameter aus. In den Glazialen hatte ein Vorstoß des Schelfeises über die Schelfkante zur Anlieferung großer Sedimentmassen geführt, die über gravitativen Transport den Kontinentalhang hinunter transportiert wurden. Die Feinfracht wurde über parallel zum Kontinentalhang laufende Konturströme westwärts transportiert und in der Larninitfazies der Driftkörper abgelagert. Am Ende der Glaziale kam es zur Sedimentation der Manganfazies. Die geringen Sedimentationsraten am Kamm der Sedimentdrift kamen aufgrund reduzierter Intensität der Konturströme und fehlender Umlagerung von Schelfsedimenten in Folge rückschreitender Schelfeisrnassen zustande. In den Interglazialen kam es durch den aufsteigenden Meeresspiegel zum Aufschwimmen des Schelfeises. Der damit verbundene Abbau der Eisrnassen über dem Schelf, hatte eine hohe Sedimentation von IRD zur Folge. Mit fortschreitendem Interglazial kam es in Zeiten nur saisonaler Meereisbedeckung zu verstärkter biologischer Produktion und zur Sedimentation biogenen Materials.

Description: High-resolution stable oxygen and carbon isotope analyses and detailed sedimentological and geochemical investigations were performed in order to i) reconstruct the paleoclimate and paleoceanography of the Greenland Sea associated with late Quaternary glacial-interglacial cycles, and ii) to link the terrestrial and deep-sea climatic records. The reconstruction of the paleoenvironmental history of the East Greenland margin and the correlation between the terrestrial and deep sea records are major objectives of the ESF-PONAM-Programme (European Science Foundation - Polar North Atlantic Margins). For this study 16 gravity and 2 box cores were recovered along the East Greenland continental margin between 69°N and 72°N on three W-E transects running from the shelf to the deep sea. The glaciomarine sediments recovered from the heavily ice-covered East Greenland continental margin reflect changes associated with the glacial/interglacial climatic cycles of the last 240 ka. The glaciomarine sediments are characterised by a dominance of terrestrially derived components and a lower content of biogenic components. Glaciomarine sedimentation processes, terrigenous sediment input, and biogenic productivity in the study area are strongly influenced by fluctuations in the extent of the Greenland Ice Sheet, extent of the sea-ice cover, rate of iceberg drifting, meltwater input, and changes in the East Greenland Current (EGC). The relatively low carbonate content (〈10 %) and the dominant occurrence of N. pachyderma sin. (〉95 %) throughout the sediment sequences indicate a low biological productivity in the surface water resulting from the extensive sea-ice cover and the strong influence of cold and low-saline polar waters of the EGC. An increase in the surface-water productivity, on the other hand, occurred during certain periods within interglacial and glacial stages. This indicates that the sea ice along the Western margin of the Greenland Sea was at least seasonally reduced during these time intetvals. Based on the accumulation rates of the coarse terrigenous matter (〉63 µm) and amounts of IRD, the advance and retreat of East Greenland glaciers over the past 200 ka can be correlated with those postulated from the terrestrial records. At least five repeated advances and retreats of glaciers beyond the coastline are proposed between the late Early to Middle Weichselian (65-61, 59-51, 48-42, 35-31, and 28-25 ka). Maximum fluxes of IRD recorded along the continental margin between 21 and 16 ka, reflect the maximum extent of East Greenland glaciers probably reaching the shelf break at that time. The stable oxygen isotope records measured on the planktonic foraminifer N. pachyderma sin. reveal some excursions from the global climate record due to a local andlor regional overprint through meltwater supply andlor cold water masses of the EGC. Distinct meltwater events are documented during Terminations II and l and at the beginning of Stage 3 resulting from the collapse of the Greenland Ice Sheet. The early period of all glacial stages (i.e. 716, 514, and 312) was subjected to an abrupt and rapid build-up of a sea-ice cover. Hence, a distinct decrease in the carbonate content, the low number of planktonic foraminifers, and light d13C values reflect the strong reduction in the C02 exchange between the atmosphere and ocean, and the surface-water productivity, resulting from a meltwater cap andlor an extensive sea-ice cover. The onset of Termination l is characterised by a distinct shift towards light d180 values, a dramatic decrease in the IRD-flux, and a marked increase in organic matter, indicating the rapid retreat of East Greenland glaciers and a reduced sea-ice cover. According to distinct shifts toward light d180 and heavy d13C values of N. pachyderma sin. and O. umbonatus, the present-day circulation Patterns of surface- and deep-water masses were probably established between 7.4 and 6.1 ka. This is very similar to the timing estimated from studies On microfossil assemblages of the Greenland Sea. In particular, the distinct IRD peaks correlate with the fluctuations of the Greenland Ice Sheet during the last two glacial-interglacial cycles. Most of the major IRD peaks correspond to periods of cooling of air temperatures over Greenland. During the interval between 225 and 60 ka, the IRD peaks are in phase (at the 23-kyr orbital processional cycle) with maximum Summer Insolation at 70°N This suggests that the Greenland Ice Sheet may have experienced a predominantly 23-kyr cycle of growth and decay, and therefore, collapsed and discharged large volumes of icebergs to the Greenland Sea when Summer insolation reached its maxima. During the last glacial period, there is a strong correlation between major pulses in the supply of IRD, and the Bond Cycles and the Heinrich Events recorded in the GRIP ice core and North Atlantic deep-sea sediments. Furthermore, the higher frequency of IRD events on millennial scales matches the cooling phase of the abrupt Dansgaard-Oeschger Cycles recorded in the GRIP ice core. Consequently, the apparent evidence of millennial scale IRD events in the North Atlantic and the GIN Sea suggests coherent fluctuations of the large northern hemisphere ice sheets (i.e. the Fennoscandian/Barents Sea and Laurentide/Greenland ice sheets) during the last glacial period.

Description: The surface water hydrography along the western Iberian margin, as part of the North Atlantic's eastern boundary upwelling system, consists of a complex, seasonally variable system of equatorward and poleward surface and subsurface currents and seasonal upwelling. Not much information exists to ascertain if the modern current and productivity patterns subsisted under glacial climate conditions, such as during marine isotope stage (MIS) 2, and how North Atlantic meltwater events, especially Heinrich events, affected them. To help answer these questions we are combining stable isotope records of surface to subsurface dwelling planktonic foraminifer species with sea surface temperature and export productivity data for four cores distributed along the western and southwestern Iberian margin (MD95-2040, MD95-2041, MD99-2336, and MD99-2339). The records reveals that with the exception of the Heinrich events and Greenland Stadial (GS) 4 hydrographic conditions along the western Iberian margin were not much different from the present. During the Last Glacial Maximum (LGM), subtropical surface and subsurface waters penetrated poleward to at least 40.6°N (site MD95-2040). Export productivity was, in general, high on the western margin during the LGM and low in the central Gulf of Cadiz, in agreement with the modern situation. During the Heinrich events and GS 4, on the other hand, productivity was high in the Gulf of Cadiz and suppressed in the upwelling regions along the western margin where a strong halocline inhibited upwelling. Heinrich event 1 had the strongest impact on the hydrography and productivity off Iberia and was the only period when subarctic surface waters were recorded in the central Gulf of Cadiz. South of Lisbon (39°N), the impact of the other Heinrich events was diminished, and not all of them led to a significant cooling in the surface waters. Thus, climatic impacts of Heinrich events highly varied with latitude and the prevailing hydrographic conditions in this region.

Description: Radiolarian-based paleoceanographic reconstructions generally use the abundance of selected radiolarian species. However, the recent focus on the opal flux and the development of isotope measurements in biogenic opal and the organic matter embedded in it demands a better knowledge of the origin of the opal. We present here an estimation of the opal content of the skeleton of 63 radiolarian species from two sites in the Southern Ocean. The skeletons are modelled as associations of simple geometrical shapes, and the volume thus obtained is combined with opal density to obtain the amount of opal. These data are, thus, used to determine the most important opal carriers in the radiolarian assemblage in both cores.

Description: Seasonal depth stratified plankton tows, sediment traps and core tops taken from the same stations along a transect at 29°N off NW Africa are used to describe the seasonal succession, the depth habitats and the oxygen isotope ratios (delta18O(shell)) of five planktic foraminiferal species. Both the delta18O(shell) and shell concentration profiles show variations in seasonal depth habitats of individual species. None of the species maintain a specific habitat depth exclusively within the surface mixed layer (SML), within the thermocline, or beneath the thermocline. Globigerinoides ruber (white) and (pink) occur with moderate abundance throughout the year along the transect, with highest abundances in the winter and summer/fall season, respectively. The average delta18O(shell) of G. ruber (w) from surface sediments is similar to the delta18O(shell) values measured from the sediment-trap samples during winter. However, the delta18O(shell) of G. ruber (w) underestimates sea surface temperature (SST) by 2 °C in winter and by 4 °C during summer/fall indicating an extension of the calcification/depth habitat into colder thermocline waters. Globigerinoides ruber (p) continues to calcify below the SML as well, particularly in summer/fall when the chlorophyll maximum is found within the thermocline. Its vertical distribution results in delta18O(shell) values that underestimate SST by 2 °C. Shell fluxes of Globigerina bulloides are highest in summer/fall, where it lives and calcifies in association with the deep chlorophyll maximum found within the thermocline. Pulleniatina obliquiloculata and Globorotalia truncatulinoides, dwelling and calcifying a part of their lives in the winter SML, record winter thermocline (~180 m) and deep surface water (~350 m) temperatures, respectively. Our observations define the seasonal and vertical distribution of multiple species of foraminifera and the acquisition of their delta18O(shell).

Description: This paper documents the migration of the Polar Front (PF) over the Iberian margin during some of the cold climatic extremes of the last 45 ka. It is based on a compilation of robust and coherent paleohydrological proxies obtained from eleven cores distributed between 36 and 42°N. Planktonic delta18O (Globigerina bulloides), ice-rafted detritus concentrations, and the relative abundance of the polar foraminifera Neogloboquadrina pachyderma sinistral were used to track the PF position. These three data sets, compared from core to core, show a consistent evolution of the sea surface paleohydrology along the Iberian margin over the last 45 ka. We focused on five time slices representative of cold periods under distinct paleoenvironmental forcings: the 8.2 ka event and the Younger Dryas (two recent cold events occurring within high values of summer insolation), Heinrich events 1 and 4 (reflecting major episodes of massive iceberg discharges into the North Atlantic), and the Last Glacial Maximum (typifying the highest ice volume accumulated in the Northern Hemisphere). For each event, we generated schematic maps mirroring past sea surface hydrological conditions. The maps revealed that the Polar Front presence along the Iberian margin was restricted to Heinrich events. The sea surface conditions during the Last Glacial Maximum were close to those at present day, except for the northern sites which briefly experienced subarctic conditions.