Sedimentology on sediment cores from the eastern Weddell Sea, Antarctica

Sediment cores from nine sites along a profile on the Antarctic continental margin off Kapp Norvegia were analysed sedimentologicaly. The carbonate and organic carbon content, grain size distribution, composition of the coarse fraction and clay minerals were determined. d18O- and d13C-isotope ratios were also measured. The distribution of ice rafted debris was determined by a new method. Sedimentation-rates were obtained from 230Th- and 14C-analyses. A segregation into seven different sediment facies was made possible by different sedimentological parameters, which can be attributed to different sedimentation environments and conditions. Thr profile can be divided morphologicaly into shelf, upper continental slope, slope terrace and lower continental slope. The paratill facies is deposited on the shelf during an interglacial phase and consists mainly of ice rafted sediments. A portion of the fine fraction is being carried away by the antarctic coastel current. The sedimentation rate lies between 0 and 3 cm/1000 a. The coarse grained deposits of the upper, relatively steep continental slope, were specified as a rest sediment. Current and gravity sediment transport are responsible for the intensive sorting of ice rafted material coming from the shelf. The fine sediment is carried away by currents while sand and silt are deposited as small turbidites on the slope terrace. The morainic facies only appears at the base of the upper continental slope and defines ice advances, beyond the shelf margin. The facies mainly consists of transported shelf sediments. The interglacial facies, deposited during the interglacial phases on the continental slope, are characterized by high proportions of ice raft, coarse mean grain size, low content of montmorillonite and a carbonate content, which mainly originates from planktonic foraminifera (N. pachyderma). At the central part of the slope the sedimentation rate is at its lowest (2 cm/1000 a) and increases to 3-4 cm/1000 a towards the sea, due to high production of biogenic components and towards the continent due to an increasing input of terrigenous material. Sedimentary conditions during glacial times are depicted in the glacial facies by a low content of ice rafted debris, a lower mean grain size and a high content of montmorillonite. Biogeneous components are absent. The sedimentation rate is generally about 1 cm/1000a. A transition facies is deposited during the transition from glacial to interglacial conditions. Typical for this facies, with a terrigenous composition similar to the interglacial facies, is a high content of radiolaria. The reason for the change of plankton from a siliceous to a carbonacous fauna may have been the changing hydrography caused by the sea ice. The surge facies is deposited at the continental margin under the ice shelf and is a sediment exclusively delivered by currents. With the aid of this facies it was, for the first time possible to prove the existence of Antarctic ice surges, an aspect wh ich has been discussed for the past 20 years.

Terrigenous sedimentation of sediment core GeoB6007-2

Variations in deposition of terrigenous fine sediments and their grain-size distributions from a high-resolution marine sediment record offshore northwest Africa (30°51.0'N; 10°16.1'W) document climate changes on the African continent during the Holocene. End-member grain-size distributions of the terrigenous silt fraction, which are related to fluvial and aeolian dust transport, indicate millennial-scale variability in the dominant transport processes at the investigation site off northwest Africa as well as recurring periods of dry conditions in northwest Africa during the Holocene. The terrigenous record from the subtropical North Atlantic reflects generally humid conditions before the Younger Dryas, during the early to mid-Holocene, as well as after 1.3 kyr BP. By contrast, continental runoff was reduced and arid conditions were prevalent at the beginning of the Younger Dryas and during the mid- and late Holocene. A comparison with high- and low-latitude Holocene climate records reveals a strong link between northwest African climate and Northern Hemisphere atmospheric circulation throughout the Holocene. Due to its proximal position, close to an ephemeral river system draining the Atlas Mountains as well as the adjacent Saharan desert, this detailed marine sediment record, which has a temporal resolution between 15 and 120 years, is ideally suited to enhance our understanding of ocean-continent-atmosphere interactions in African climates and the hydrological cycle of northern Africa after the last deglaciation.