Mineral Magnetic Properties of Middle and Upper Pleistocene Sediments at Sites 883, 884, and 887, North Pacific Ocean

Abstract

Mineral magnetic studies have been conducted on middle and upper Pleistocene sediments from Holes 883D, 884D, and 887B in the North Pacific Ocean to assess their suitability for determinations of relative paleointensity of the geomagnetic field. Thermo-magnetic and high-field hysteresis data suggest that the magnetic mineralogy of the clay-rich sediments at Holes 883D and 884D is dominated by pseudosingle domain magnetite in a narrow grain-size range. The uniform grain size of the magnetic particles in the clay-rich sediment is consistent with sorting and transport by long-distance bottom currents. Coarse-grained magnetic minerals occur in high concentrations in restricted stratigraphic intervals where volcanic ashes occur. Apart from these intervals, the sediments meet the established criteria for relative paleointensity studies. Such studies are now in progress. The sediments at Hole 887B are not suitable for relative paleointensity determinations because of large variations in both grain size and concentration of magnetite. In particular, reductive diagenesis appears to have caused preferential dissolution of fine-grained magnetite in diatom-rich intervals in Hole 887B. Significant variation in the magnetic properties of diatom-, clay-, and tephra-rich parts of all the sequences preclude a straightforward interpretation o f tne magnetic susceptibility signal as a paleoclimate proxy as is the case in depositional environments that are dominated by climatically modulated CaCO 3 fluctuations.