Downhole changes in the concentration, grain size, and mineralogy of the magnetic components of Holocene through upper Pleistocene continental slope sediments (Ocean Drilling Program Holes 903A and 904A) were analyzed to judge the primary detrital vs. the diagenetic origin of the magnetic signal. Natural remanent magnetization (NRM), magnetic susceptibility (k), anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM), and additional hysteresis measurements were performed for densely sampled intervals (ca. every 15 cm). Single-domain through small pseudosingle-domain magnetite is identified as the dominant remanence carrying mineral. However, relative changes of the rock-magnetic properties on a sample-by-sample basis appear to be more instructive in detecting diagenetic overprints of the signal than are determinations of absolute grain sizes. Major downhole changes in the rock-magnetic properties are dominated by the original detrital signal and clearly record differences of glacial and interglacial modes of deposition. Postdepositional dissolution and the authigenic growth of magnetite modulate the detrital magnetic mineralogy dominantly in well-defined sections, which represent the contact of unconformably deposited sediment packages. This diagenetic alteration of the magnetic signal is restricted to the finer ferrimagnetic fraction and can be detected by analyzing the grain selectivity of the ARM acquisition process. The alteration of the magnetic mineralogy occurred repeatedly during sediment diagenesis.

Date of initial receipt: 10 March 1995
Date of acceptance: 8 February 1996

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