Ferromanganese crusts as indicators for paleoceanographic events in the NE Atlantic |
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Authors: | A Koschinsky P Halbach J R Hein A Mangini |
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Institution: | 1. Freie Universit?t Berlin, Fachrichtung Rohstoff- und Umweltgeologie, Malteserstr. 74-100, D-12249, Berlin, Germany 2. U.S. Geological Survey, 345 Middlefield Road, ms 999, 94025, Menlo Park, California, USA 3. Heidelberger Akademie der Wissenschaften, Im Neuenheimer Feld 366, D-69120, Heidelberg, Germany
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Abstract: | Hydrogenetic ferromanganese crusts reflect the chemical conditions of the sewater from which they formed. Fine-scale geochemical
analysis of crust layers in combination with age determinations can therefore be used to investigate paleoceanographic changes
which are recorded in geochemical gradients in the crusts. At Tropic seamount (off northwest Africa), uniform crust growth
influenced by terrigenous input from the African continent occurred during approximately the past 12 Ma. Phosphatization of
these crusts is minor. In contrast, crusts from Lion seamount, located between Madeira and the Portuguese coast, display a
much more variable growth history. A pronounced increase in Ni, Cu, and Zn is observed in some intervals of the crusts, which
probably reflects increased surface productivity. A thick older phosphatized generation occurs in many samples. Hydrographic
profiles indicate that Mediterranean outflow water (MOW) may play an important role in the composition of these crusts.10Be dating of one sample confirms that the interruption of the MOW during the Messinian salinity crisis (6.2–5 Ma ago) resulted
in changes in element composition. Sr-isotope dating of the apatite phase of the old crust generation has been carried out
to obtain a minimum age for the older generation of Atlantic crusts and to determine whether crust phosphatization in the
Atlantic can be related to phosphatization episodes recorded in Pacific crusts. The preliminary data show that the old phosphatized
crust generation might be as old as approximately 30–40 Ma. |
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Keywords: | Ferromanganese crusts Phosphatization Paleoceanographic events NE Atlantic Mediterranean outflow water Messinian salinity crisis |
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