Magnetic fabric and remanence analyses of cores from the U.S. continental margin and the Vema Channel

Abstract

Paleomagnetic analyses have been performed on 8 cc cubic samples from 20 gravity and piston cores (172 samples) taken at the base of the U.S. continental margin in the region of a Deep Western Boundary Undercurrent (DWBUC) and from 28 gravity and piston cores (484 samples) from the eastern flank on the Vema Channel. Downcore remanent magnetic (RM) and magnetic fabric measurements have been performed on these samples to examine the effects of depositional mechanisms on the magnetization of sediment at both sites. Vector diagrams after stepwise a.f. demagnetization to 50 mT for both suites of samples indicate variable RM stability above 8–10 cm depth. Below this depth, samples exhibit a stable, apparently primary RM after demagnetization to fields of 10 mT. These results indicate that if the RM is to be used to orient these deep-sea cores, samples should be chosen from greater than 10 cm depth in core and demagnetized to at least 10 mT. Fabric analyses, based on anisotropy of magnetic susceptibility (AMS) measurements, reveal primary sedimentary fabric in most cores from the U.S. continental margin, but anomalous fabric below 40–80 cm depth in some Vema Channel cores. This anomalous fabric is believed to be the result of soft-sediment deformation at the site. AMS long-axis alignments are essentially random for these Vema Channel cores. Under the DWBUC however, the dominant fabric appears to be normal to the current flow direction while cores from outside the DWBUC exhibit a long-axis AMS alignment which parallels bottom contours. The magnetic fabric parameter F s for core-top samples on the eastern flank of the Vema Channel exhibits high values directly under fast flowing Antarctic Bottom Water (AABW) and relatively low values under more sluggish water masses above AABW. These F s results are consistent with those reported for earlier Vema Channel studies and for results from DWBUC core-top samples.