Bioturbation: minimal effects on the magnetic fabric of some natural and experimental sediments

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Magnetic fabric measurements have been performed on three suites of sediment samples to contrast biologically disturbed and undisturbed sediments. The first of these analyses includes samples from laminated, relatively undisturbed and bioturbated, totally disturbed horizons found in an Athabasca Oil Sand core. The second includes samples from biologically undisturbed sediments from Mammoth Cave in Kentucky. The third includes evaluation of the evolution of a magnetite horizon which was established by the author in December of 1982 in a biologically active tidal flat on Sapelo Island, Georgia. Changes in this magnetite horizon were monitored by periodic subsampling. Results from these suites of samples indicate that the measured magnetic fabric in bioturbated sediments may exhibit primary sedimentary characteristics similar to undisturbed sediments. Such primary indicators include the anisotropy of magnetic susceptibility (AMS) parameters V used by Graham, always greater than 45°, Q used by Rees and Hamilton, always less than 0.69, and a subhorizontal magnetic foliation plane. I infer that the presence of apparently primary fabric distributions in these bioturbated sediments indicates that physical, not biological factors are responsible for the observed fabric. These factors include dewatering and simple, low magnitude compaction. The data indicate that it cannot be assumed, a priori, that poor fabric distributions are the result of bioturbation. On the contrary, for the Sapelo Island experiments, the magnetic fabric actually improved with bioturbation and the Athabasca bioturbated magnetic fabric is very similar to that from the Athabasca laminated sediment. Data from the Sapelo Island, Georgia, tidal flat experiment are interpreted to indicate that long-axis AMS alignments develop within the sediment. Such alignments are the result of water movement through sediment which exhibits increased porosity due to high rates of burrowing. It is also observed that RM precision improves during the experiment in spite of bioturbation. © 1984.

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Earth and Planetary Science Letters

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