A new rock‐magnetic approach to selecting sediments for geomagnetic paleointensity studies: Application to paleointensity for the last 4000 years

Abstract

The hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization (DRM/ARM) for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches. One approach is a detailed rock‐magnetic examination of the implicit assumptions of the DRM/ARM method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions. Sediments are ‘uniform’ with respect to DRM/ARM ratio if they contain magnetite in the 1–15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20–30 times the minimum concentration. The DRM/ARM ratios of these sediments should provide estimates of relative geomagnetic paleointensity. Relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility (X ARM ) versus low‐field susceptibility (X) and the size range ≃ 1–15 μm is approximately identified by high‐field hysteresis parameters. A rock‐magnetic evaluation of LeBoeuf Lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study. The second approach to evaluating the DRM/ARM ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data. A comparison between the LeBoeuf Lake estimates and Thellier‐Thellier results from the western United States supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations.