The Relationship Between Variations in Rock-Magnetic Properties and Grain Size of Sediments from ODP Hole 645C

Abstract

The potential use of anhysteretic remanence/magnetic susceptibility (ARM/X) to indicate the relative var­ iations of marine sediments is examined. Samples from Ocean Drilling Program (ODP) Hole 645C were measured for low-field magnetic susceptibility at two frequencies (0.47 and 4.7 kHz), given an anhysteretic remanence using a peak alternating magnetic (AC) field of 0.05 mT, and separated into the percentage sand, silt, and clay fractions. Results show little frequency dependence of low-field magnetic susceptibility and a direct relationship betweem ARM/X and clay concentration. This paper seeks to determine if rapidly measured rock-mag­ netic parameters can be used as proxy indicators of changes in the size distribution of marine sediments. We refer to the size distribution of the magnetic fraction as the dis­ tribution, whereas the overall size distribution of both the mag­ netic and nonmagnetic fraction is described as the grain-size distribution. King et al. (1982) demonstrated the utility of the ratio of an­ hysteretic remanence to magnetic susceptibility (ARM/X) as an indicator of relative particle-size variations in magnetite. High (low) ARM/X values indicate the presence of finer (coarse-­ grained magnetite particles. Magnetic minerals are subject to the same processes that transport and deposit nonmagnetic minerals, with the exception of environments where either in-situ deposi­ tion, e.g., by magnetotactic bacteria (Kirschvink and Chang, 1984; Petersen et al., 1986), or post-depositi onal dissolution by reduction diagenesis (Karlin and Levi, 1983, 1985; Canfield and Berner, 1987) are important processes. If in-situ deposition or reduction diagenesis are not important, and the magnetic min­ erals and nonmagnetic minerals have a similar size distribution in the source areas, then we hypothesize that variations in the relative particle size of magnetic minerals will reflect similar variations in grain size. King et al.'s results (1982) indicated that the ARM/X technique should be useful for distinguishing vari­ ations in the proportion of fine to coarse material. Therefore, we would expect to see the largest variations of ARM/X in sedi­ ments characterized by significant fluctuations in grain size. Here, we report the results of an experiment comparing rela­ tive variations (percentages of sand, silt, and clay) with ARM/X values obtained from samples from ODP Hole 645C. We chose Hole 645C because (1) the dominant sedimen­ tary processes are ice rafting and direct settling through the wa­ ter column, (2) the source of the sediment is confined to the sur­ rounding continental areas (Canada and Greenland), and (3) sedi­ mentary sequences consist of alternating coarse-grained (sandy muds and muddy sands) with fine-grained (silty-muds and muddy silts) units. A relationship between these units and whole-core magnetic susceptibility has been established (Srivastava, Arthur,