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A new seismostratigraphic model has been established within the Arctic Ocean adjacent to the East Siberian Shelf on the basis of multichannel seismic reflection data acquired along a transect at 81°N. Ages for the sedimentary units were estimated via links to seismic lines and drill site data of the US Chukchi Shelf, the Lomonosov Ridge, and the adjacent Laptev Shelf. Two distinct seismic units were mapped throughout the area and are the constraints for dating the remaining strata. The lower marker unit, a pronounced high-amplitude reflector sequence (HARS), is the most striking stratigraphic feature over large parts of the Arctic Ocean. It indicates a strong and widespread change in deposition conditions. Probably, it developed during Oligocene times when a reorientation of Arctic Plates took place, accompanied by the gradual opening of the Fram Strait, and a widespread regression of sea level. The top of the HARS likely marks the end of Oligocene/early Miocene (23 Ma). An age estimate for the base of the sequence is less clear but likely corresponds to base of Eocene (˜56 Ma). The second marked unit detected on the seismic lines parallels the seafloor with a thickness of about 200 ms two-way travel time (160 m). Its base is marked by a change from a partly transparent sequence with weak amplitude reflections below to a set of continuous high-amplitude reflectors above. This interface likely marks the transition to large-scale glaciation of the northern hemisphere and therefore is ascribed to the top Miocene (5.3 Ma).

The international multi-ship expedition ARCTIC91 was able to collect a seismic transect between the Morris Jesup Rise and the Yermak Plateau. These conjugate plateau structures in the south-western part of the Eurasian Basin bound one of the slowest mid-oceanic spreading ridges in the world, the Gakkel Ridge. The seismic data reveal a sedimentary cover that is thin relative to the age of the oceanic crust at 83"N, 10"E and 85"N, 15"W. Close to the plateaus, thicker sequences are evident (Morris Jesup Rise, 500m; Yermak Plateau, 1500m). The seismic lines on the Morris Jesup Rise reveal only a thin sedimentary cover of 0.2s TWT. In contrast, a layer with a thickness of almost 0.9 s TWT (1300 m) was found on the northernmost tip of the Yermak Plateau. The topography of the oceanic basement is very rough along the seismic lines, as could be expected at a slow spreading ridge. Depth variations of more than 1000m are typical. Hydrosweep swath mapping provides the first detailed 3-D image from the Arctic mid-ocean ridge system at 87"N, 60"E and 84"N, 0".

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The global climate evolution during Cenozoic times is characterized by the transformation from warm Paleogene oceans with low latitudinal and bathymetric thermal gradients into the more recent modes of circulation characterized by strong thermal gradients, oceanic fronts, cold deep oceans, and cold high-latitude surface waters.