ALBERT

Description: Morphometric measurements of Fragilariopsis kerguelensis valves from a two-year time series covering the period from November 2002 to October 2004 collected by a sediment trap at 800m below the ocean surface which was moored near 54° S 140° E in ca. 2300 m water depth, close to the top of the Australia-Antarctica mid-ocean ridge.

Description: During the PROMESS campaign (summer 2004) a borehole (PRGL1) was drilled in the upper slope of the Gulf of Lions. Previous studies showed that the deposition of the sedimentary units at borehole PRGL1 was determined by changes in the discharge of the Rhone river, induced by sea level and climate variability with a strong 100 ka imprint. However, there are no detailed studies that focus on the glacial/interglacial transitions (i.e. Terminations) that define 100 ka Quaternary ice-age cycles. Here we present a high-resolution record of terrestrial long chain n-alkanes (C21 to C33) from MIS 3 to MIS 11 in order to monitor changes in the sediments deposited on the upper slope of the Gulf of Lions with the aim to provide new insight into the climatic events occurring during Terminations in the western Mediterranean Sea. Principal component analysis grouped n-alkane on the basis of the degree of maturation of organic matter (fresh/degraded) that allowed to monitor past variability in the source of the sediment deposited in the upper slope as a consequence of sea level and climate change. This information was used to estimate the impact that northern ice-sheet variability had on the western Mediterranean basin. As sea level started to fall, fresh terrestrial organic matter increased owing to the seaward migration of Rhone's mouth. Simultaneously, the decrease in sea level exposed old and reworked organic material from the Rhone's catchment, including the delta plain and the continental shelf, that was eroded and transported into our study site. This resulted in an increase in the proportion of degraded terrestrial organic matter together with sea level decrease. Aside from sea level changes, the proportion of fresh vs degraded organic matter seems to be affected by the amount of continental runoff, with an increasing proportion of fresh terrestrial organic matter occurring at times of reduced continental runoff. During the last glacial, the increase of fresh terrestrial organic matter was contemporaneous with Heinrich events (HEs), suggesting dry and cold conditions on the western Mediterranean basin. These dry and cold millennial-scale episodes were reproduced during Terminations II, III and IV preceding flood events, most probably due to weakening/reinforcement of Atlantic Meridional Overturning Circulation (AMOC) that caused dry/humid conditions in western Mediterranean.

Description: A marine sediment core retrieved from the middle continental slope of the northwestern Barents Sea was analyzed for its geochemical (alkenones) and micropalaeontological (diatoms, coccolithophores and dinocyst) content in order to reconstruct the evolution of upper ocean conditions and ice-sheet dynamics during the last 25 kyr. Additionally, quantitative reconstructions of sea surface conditions (temperature, salinity and sea-ice cover extent) were conducted based on the best analogue technique applied to dinocyst assemblages and on the alkenone unsaturation index. The sediment core contains a post Last Glacial Maximum depositional sequence unaffected by stratigraphic discontinuities. Low salinity and laminated sediments after 20 cal kyr BP, indicate a massive settling of meltwater sediment-laden plumes from the initial melting of the Svalbard-Barents Sea Ice Sheet on Western Svalbard. First record of measurable alkenones, together with a drop of the number of months of sea-ice cover and increase in SSTs suggests an intensification of the influx of Atlantic waters into the study area at ~ 15 cal kyr BP representing the termination of the last glacial period and onset of the Bølling interstadial. The first occurrence of diatoms and increase in the abundance of all microfossils marked the onset of the Holocene at 11.2 cal kyr BP when modern-type sea surface conditions were rapidly established in Western Svalbard. Reconstructions based on dinocyst data and alkenone unsaturation index suggest relatively warm and stable temperatures between 9.9 and 8.9 kyr BP and a decrease of SSTs from 4.2 cal kyr BP to present coinciding with the Holocene Thermal Maximum and the decrease of summer insolation in the high latitude northern hemisphere, respectively.

Description: An array of four sediment trap moorings recorded the particulate flux across the Antarctic Circumpolar Current (ACC) at 170 °W, between November 1996 and January 1998, as part of the US JGOFS-Antarctic Environment and Southern Ocean Process Study (AESOPS) program. The trap locations represent sampling within the Polar Frontal Zone, the Antarctic Polar Front, the Antarctic Zone and the Southern Antarctic Zone. Here we report observations from 1000 m below the sea-surface compared to seafloor and surface water distributions. Sub-sample splits from each trap were obtained and total diatom flux and species composition were determined. The diatom fluxes were quantified using both a dilution and a 'spike' method to allow for the rapid repeatability of measurements. Diatom flux was found to be highly seasonal across the ACC particularly at higher latitudes. Marine snow aggregates of intact diatom cells and chains were the major components of the biogenic flux. Siliceous particle size was noted to decrease with increasing latitude, which could be aligned with a shift of the diatom assemblage to small-size species/sea-ice affiliated species. A 'double-structured' diatom flux was recorded at the location of the Antarctic Polar Front trap, with a shift in the diatom assemblage from larger to smaller diatoms in the second flux episode. The sediment trap assemblage shows deviations from the surface water assemblage, while surface sediment samples indicate that significant dissolution occurs after 1000 m and at the sediment-water interface. Estimation of diatom biovolumes across the ACC shows that large diatoms have the potential to greatly impact biogenic fluxes to the ocean interior despite their low fluxes. Small species of the genus Fragilariopsis could potentially export as much Corg as Fragilariopsis kerguelensis near the retreating ice edge. However, their low abundance in the surface sediments also suggests that these diatoms are a shallow export species.