Description: Six sediment cores from the Eurasian Basin were studied to determine and understand climatically driven changes of Arctic Ocean basins. Detailed time control of sediments for the last 45 kyr is based on accelerator mass spectrometry (AMS) C14-dating of biogenic carbonate (N. pachyderma, left coiling). The most important results from our study are summarized as follows. From 45 to 13.5 ka low sedimentation rates prevailed (0.35 cm/kyr). They increased drastically at the transition from the last glacial to interglacial (Termination Ia, 13.5 ka) leading into high Holocene sedimentation rates (1.06 cm/kyr). Low carbonate concentrations (〈 4%) prevailed from 13.5 to 9 ka at Termination I. Decreased salinities can be expected for Termination la (Zahn et al., 1985, Jones & Keigwin, 1988, Mienert et al., 1989) due to glacial meltwater influence possibly accompanied by sea ice melting. As a result of the freshwater influence, productivity of planktic foraminifers decreased and this, in turn, resulted in a drastic decrease in carbonate concentration during Termination Ia. Although carbonate concentration varies only between 0 and 9%, it distinctly changes both the compressional-wave velocity (from 1485 to 1510 m/s) and the wave attenuation (from 0.1 to 0.45 dB/m/kHz) in the sediment. Climatically driven changes in magnetic susceptibility have proved to be a valuable paleoclimatic tool for intercore correlations. Our results indicate that the same general conclusions are valid for pelagic environments of both Atlantic and Arctic Ocean basins.

Description: To reconstruct the deep-water circulation for the last 3.5 Ma from deep-sea sediments of the eastern equatorial Atlantic, sea floor morphology, sub-bottom reflectors and the echo character have been mapped on the basis of 3.5 kHz records and sediment cores. Physical properties of sediments and synthetic seismograms derived from them enable us to decipher reflector sequences in environments of pelagic, currentresuspended and turbidity sedimentation. The individual reflectors originate from carbonate dissolution, hiatuses, coarse sand layers and interferences. Those which are related to carbonate dissolution and hiatuses provide evidence of water-mass boundaries by their distribution. Five phases of different deep-water circulation can be seen in the record of the last 3.5 Ma, and these are related to climate history: 1. Between 3.7 Ma and 2.2 Ma a strong deep-water circulation indicates a northward flow of bottom water below 4200 m (AABW = Antarctic - Bottom Water) and a southward flow of deep-water above 4200 m (NADW = North-Atlantic Deep Water). 2. Between 1.6 and 1.4 Ma a southward flow of bottom water below 4500 m and a diminished southward flow above 4500 m can be detected. This water-mass geometry can be interpreted by an expansion of the NADW-masses and a displacement of the AABW-masses during the same time. 3. Since 1.4 Ma a northward flow of a bottom-water current developed again. This current flow created a leeside sediment ridge in the southern part of the Kane Gap and furrows in the northern part of it. 4. Beetween 400,000 and 200,000 yrs B. P. the oceanic and atmospheric circulation increased. The strengthened oceanic circulation caused an increase in carbonate dissolution, which is documented by a traceable reflector from 2800 m to 4500 m water depth. At the same time an increase of the atmospheric circulation caused a drastic rise in the pelagic sediment accumulation (〉 100%) through an intensification of upwelling. This runs parallel with a higher oceanic productivity in the northern equatorial divergence zone and an enhanced supply of fluvial and probably eolian sediments from Senegal and Guinea. 5. Before 10,000 yrs B.P. an erosive northward flowing bottom-water current prevailed below 4500 m water depth. After 10,000 yrs B.P. the bottom-water flow was sluggish and non erosive.