Oceanographic Conditions in the Deepest Parts of the Kattegat, Scandinavia, Revealed Through Recent Benthic Foraminifera and Hydrography

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The Kattegat has, since the 1980s, been subject to extensive investigations focusing on various aspects of the marine environment. The most important reasons for this concentration of work are marine eutrophication, periodically occurring oxygen deficiency in the bottom-water and subsequent benthic mortality. In this study a previously not discussed sill area separating two deep areas in the Kattegat has been identified. In the contexts of periodically occurring oxygen deficiency in the southern Kattegat, it is most relevant to study the environmental conditions in this area. Hydrographic data from the area inside and outside the sill area suggest that the deep-water on both sides of the sill do communicate. No evidence for stagnation in the deep area inside the sill was found. In addition to hydrography data, benthic foraminifera has been used as environmental tracers. Two characteristic benthic foraminiferal faunas have been defined one representing water depths; exceeding 80m and one representing shallower depths. These faunas occur on both sides of the sill area. Two investigated deep depressions within the deep area inside the sill, the Alkor Depth (138m) and Poseidon Depth (150m) are marked by a fauna that is identical with that in the so-called Kattegat Trench. This suggests that the depressions are not more stagnant, if stagnant at all, than the surrounding Kattegat Trench and the Djupa Rännan Trench located outside the sill area. The information generated in this study is essential for the understanding of the processes resulting in oxygen deficiency in the deep water of the southern Kattegat.