ALBERT

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: 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: 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.

Description: Authigenic phosphatic laminites enclosed in phosphorite crusts from the shelf off Peru (10°01' S and 10°24' S) consist of carbonate fluorapatite layers, which contain abundant sulfide minerals including pyrite (FeS2) and sphalerite (ZnS). Low d34Spyrite values (average -28.8 per mill) agree with bacterial sulfate reduction and subsequent pyrite formation. Stable sulfur isotopic compositions of sulfate bound in carbonate fluorapatite are lower than that of sulfate from ambient sea water, suggesting bacterial reoxidation of sulfide by sulfide-oxidizing bacteria. The release of phosphorus and subsequent formation of the autochthonous phosphatic laminites are apparently caused by the activity of sulfate-reducing bacteria and associated sulfide-oxidizing bacteria. Following an extraction-phosphorite dissolution-extraction procedure, molecular fossils of sulfate-reducing bacteria (mono-O-alkyl glycerol ethers, di-O-alkyl glycerol ethers, as well as the short-chain branched fatty acids i/ai-C15:0, i/ai-C17:0 and 10MeC16:0) are found to be among the most abundant compounds. The fact that these molecular fossils of sulfate-reducing bacteria are distinctly more abundant after dissolution of the phosphatic laminite reveals that the lipids are tightly bound to the mineral lattice of carbonate fluorapatite. Moreover, compared with the autochthonous laminite, molecular fossils of sulfate-reducing bacteria are: (1) significantly less abundant and (2) not as tightly bound to the mineral lattice in the other, allochthonous facies of the Peruvian crusts consisting of phosphatic coated grains. These observations confirm the importance of sulfate-reducing bacteria in the formation of the phosphatic laminite. Model calculations highlight that organic matter degradation by sulfate-reducing bacteria has the potential to liberate sufficient phosphorus for phosphogenesis.

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.