A magnetic signature of bottom current erosion

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To characterize the magnetic signal associated with bottom current erosion, we have conducted downcore measurements of magnetic texture and fabric in three cores containing known erosional hiatuses. Using biostratigraphic and magnetostratigraphic analysis, erosional hiatuses were identified for three cores from two current-dominated environments: the northern Bermuda Rise and the south Indian Basin. The cores were contiguously subsampled and the magnetic susceptibility ( K), the anisotropy of magnetic susceptibility (AMS) and the anhysteretic susceptibility ( K ARM) were measured. The AMS parameters h and q were used to describe the relative magnitude and the shape, respectively, of the representative susceptibility ellipsoid. The concentration-independent and dimensionless ratio K ARM/ K was used to indicate relative changes in magnetic grain size. Hiatuses in the three cores were marked by unusually high values of h and q indicating a greater development of internal fabric with a more linear fabric representation; depressed values of K ARM/ K indicated a coarser mean magnetic grain size. The magnitude of this signal, however, did not have a simple relationship with hiatus duration. t-test results statistically confirmed the existence of an erosional signature. To enhance the applicability of this technique, a quantitative discriminant model was constructed and calibrated using the magnetic characteristics of erosion. Since this model was constructed from samples of known ‘erosional’ and ‘non-erosional’ character, the discriminant functions could then be used as reference equations to identify erosion in other cores. Biostratigraphic and oxygen isotopic data from a 14 m core from the northern Bermuda Rise, extending back to isotopic stage 8 (∼ 250 ka BP), indicate two short intervals of low accumulation rate which, for this region, are most probably attributable to bottom current erosion. Magnetic data for this core were entered into the reference discriminant model and the two zones of suspected erosion were classified by the functions as being highly ‘crosional’ in character. Zones of probable erosion were identified at several other levels in the core. The timing of these short-term crosional events was found to coincide closely with glacial intervals, suggesting that bottom currents active on the northern Bermuda Rise were invigorated periodically during glacial maxima at c, 18, 58, 64, 85, 130–140, 165–180, 225, and 250 ka BP.