Diagenetic Dissolution of Biogenic Magnetite in Surface Sediments of the Benguela Upwelling System |
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Authors: | K Hilgenfeldt |
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Institution: | Fachbereich Geowissenschaften, Universit?t Bremen, Postfach 33 0440, D-28334 Bremen, Germany e-mail: khilgenfeldt@uni-bremen.de Fax: +49-421-2187008, DE
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Abstract: | Intense primary biologic productivity in the surface waters of the Benguela upwelling system provides a high supply of organic
matter to the sea floor at the continental slope off Namibia and sustains extreme concentrations of magnetite producing bacteria
in the top sediment layers. Biogenic magnetite is thus by far the dominant carrier of the magnetic signal in these deposits
also because of a very minor input of terrigenous ferrimagnetic minerals. Reducing conditions in the sediment column cause
a selective dissolution of the bacterial magnetite fraction just a few centimeters below the main mineralization horizon.
This diagenetic process is documented in detail by high-resolution rock magnetic analyses and transmission electron microscopy.
Concentration dependent and grain-size sensitive magnetic parameters, such as susceptibility, laboratory imparted remanences,
and hysteresis data, reveal a significant drop in ferrimagnetic mineral content within the upper 10 cm of the sediments accompanied
by a gradual downward coarsening of the ferrimagnetic mineral assemblage from primarily magnetic single-domain particles in
the top centimeters to multi-domain grains in deeper strata. Electron microscope observations enable both an unequivocal identification
of bacterial magnetite on the basis of shape and grain-size and to trace dissolution effects on the biogenic magnetic mineral
component to depth.
Received: 9 November 1998 / Accepted: 10 October 1999 |
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Keywords: | Marine sediments Biogenic magnetite Early diagenesis Magnetite dissolution Rock magnetic properties |
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