Sedimentological investigations and age model on sediment core PS2561-2

Benthic d13C values (F. wuellerstorfi), kaolinite/chlorite ratios and sortable silt median grain sizes in sediments of a core from the abyssal Agulhas Basin record the varying impact of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) during the last 200 ka. The data indicate that NADW influence decreased during glacials and increased during interglacials, in concert with the global climatic changes of the late Quaternary. In contrast, AABW displays a much more complex behaviour. Two independent modes of deep-water formation contributed to the AABW production in the Weddell Sea: 1) brine rejection during sea ice formation in polynyas and in the sea ice zone (Polynya Mode) and 2) super-cooling of Ice Shelf Water (ISW) beneath the Antarctic ice shelves (Ice Shelf Mode). Varying contributions of the two modes lead to a high millennial-scale variability of AABW production and export to the Agulhas Basin. Highest rates of AABW production occur during early glacials when increased sea ice formation and an active ISW production formed substantial amounts of deep water. Once full glacial conditions were reached and the Antarctic ice sheet grounded on the shelf, ISW production shut down and only brine rejection generated moderate amounts of deep water. AABW production rates dropped to an absolute minimum during Terminations I and II and the Marine Isotope Transition (MIS) 4/3 transition. Reduced sea ice formation concurrent with an enhanced fresh water influx from melting ice lowered the density of the surface water in the Weddell Sea, thus further reducing deep water formation via brine rejection, while the ISW formation was not yet operating again. During interglacials and the moderate interglacial MIS 3 both brine formation and ISW production were operating, contributing various amounts to AABW formation in the Weddell Sea.

(Table 1) Age determination of sediment core JT96-09PC

he oxygen minimum zone (OMZ) off Vancouver Island was more oxygen depleted relative to modern conditions during the Allerød (~13.5 to 12.6 calendar kyr) and again from ~11 to 10 kyr. The timing of OMZ intensification is similar to that seen throughout the North Pacific, although the onset appears to have been delayed by ~1500 years off Vancouver Island. Radiocarbon dating of coeval benthic and planktonic foraminifera shows that between 16.0 and 12.6 kyr the age contrast between surface and intermediate waters (920 m depth) off Vancouver Island was similar to, or slightly less than, that today. There is no evidence of an increased age difference (i.e., decreased ventilation) during the deglaciation, particularly during the Allerød. However, sedimentary marine organic carbon concentration and mass accumulation rate increased substantially in the Allerød, suggesting that increased organic matter export was the principal cause of late Pleistocene OMZ intensification off Vancouver Island.

Geochemistry and age models of sediments from the Celtic margin

The sedimentary sections of three cores from the Celtic margin provide high-resolution records of the terrigenous fluxes during the last glacial cycle. A total of 21 14C AMS dates allow us to define age models with a resolution better than 100 yr during critical periods such as Heinrich events 1 and 2. Maximum sedimentary fluxes occurred at the Meriadzek Terrace site during the Last Glacial Maximum (LGM). Detailed X-ray imagery of core MD95-2002 from the Meriadzek Terrace shows no sedimentary structures suggestive of either deposition from high-density turbidity currents or significant erosion. Two paroxysmal terrigenous flux episodes have been identified. The first occurred after the deposition of Heinrich event 2 Canadian ice-rafted debris (IRD) and includes IRD from European sources. We suggest that the second represents an episode of deposition from turbid plumes, which precedes IRD deposition associated with Heinrich event 1. At the end of marine isotopic stage 2 (MIS 2) and the beginning of MIS 1 the highest fluxes are recorded on the Whittard Ridge where they correspond to deposition from turbidity current overflows. Canadian icebergs have rafted debris at the Celtic margin during Heinrich events 1, 2, 4 and 5. The high-resolution records of Heinrich events 1 and 2 show that in both cases the arrival of the Canadian icebergs was preceded by a European ice rafting precursor event, which took place about 1-1.5 kyr before. Two rafting episodes of European IRD also occurred immediately after Heinrich event 2 and just before Heinrich event 1. The terrigenous fluxes recorded in core MD95-2002 during the LGM are the highest reported at hemipelagic sites from the northwestern European margin. The magnitude of the Canadian IRD fluxes at Meriadzek Terrace is similar to those from oceanic sites.

Physical properties and age determination of sediment cores from the Hinlopen/Yermak Megaslide north of Spitsbergen, Arctic Ocean

Integrated interpretation of multi-beam bathymetric, sediment-penetrating acoustic (PARASOUND) and seismic data show a multiple slope failure on the northern European continental margin, north of Spitsbergen. The first slide event occurred during MIS 3 around 30 cal. ka BP and was characterised by highly dynamic and rapid evacuation of ca. 1250 km**3 of sediment from the lower to the upper part of the continental slope. During this event, headwalls up to 1600 m high were created and ca. 1150 km**3 material from hemi-pelagic sediments and from the lower pre-existing trough mouth fan has been entrained and transported into the semi-enclosed Sophia Basin. This megaslide event was followed by a secondary evacuation of material to the Nansen Basin by funnelling of the debris through the channel between Polarstern Seamount and the adjacent continental slope. The main slide debris is overlain by a set of fining-upward sequences as evidence for the associated suspension cloud and following minor failure events. Subsequent adjustment of the eastern headwalls led to failure of rather soft sediments and creation of smaller debris flows that followed the main slide surficial topography. Discharge of the Hinlopen ice stream during the Last Glacial Maximum and the following deglaciation draped the central headwalls and created a fan deposit of glacigenic debris flows.

Planktic foraminifers, oxygen- and carbon isotopic compositions, and radiocarbon age in bottom sediments from Core AMK4-316GC

In a sediment core AMK4-316 (460 cm long) on the basis of radiocarbon, oxygen isotope, and lithological data climatostratigraphy is established for time interval about 145 ka. The method of factor analysis and spline interpolation applied to data on distribution of planktic foraminifera species has allowed to reconstruct average annual and seasonal temperatures and salinity at the surface and at depth 100 m. The optimum of the Last Interglaciation (5e) is characterized by maximal temperatures, low amplitudes of seasonal fluctuations, and by increased thickness of the upper homogeneous layer. The glacial hydrological mode has arisen here 115 ka ago. Coolings outstripped appropriate events of the global continental glaciation. Minimal average annual temperatures (4-4.5°C) are reconstructed for 47-45, 42, 36, 29-30, and 10 ka. For 50-30 ka interval numerous strong temperature fluctuations that reflect migrations of the polar front are established. Maximal differences of salinity at the surface and depth 100 m showing influence of melting waters were in the beginning of deglaciations (135 and 20 ka) and repeatedly arose in 50-30 ka interval. The Last Glacial Maximum (18 ka) is characterized by the lowest salinity but not by a peak of low temperatures at the surface. Surface temperature was lowered up to 10 ka. Average annual surface temperature of the Holocene optimum was 2°C above the modern one and 2°C below temperature in the Interglaciation optimum (5e), thickness of the upper homogeneous layer exceeded 100 m.