CAUTIONARY NOTE ON MAGNETIC GRAIN-SIZE ESTIMATION USING THE RATIO OF ARM TO MAGNETIC SUSCEPTIBILITY

摘要

The ratio of anhysteretic remanent magnetization (ARM) to magnetic susceptibility (chi) is widely used for rapid estimation of magnetic grain-size variations. On interpreting ARM/chi variations, no significant influence of interactions among magnetic grains on ARM acquisition is usually assumed implicitly, although some previous experiments using artificial sediments and biogenic magnetites opposed this assumption. Our rock-magnetic study of deep-sea surface sediments from the Pacific ocean showed that ARM/chi depends strongly on chi, the concentration of magnetic minerals, and on the R value of Cisowski [1981], an index of the strength of magnetic interactions. This result demonstrates that ARM/chi can be influenced significantly by magnetic interactions in natural sediments. We corrected for the influence of the magnetic interactions using the observed concentration dependence of ARM/chi. Regional variations of ARM/chi values before the correction were geologically unnatural in terms of magnetic grain-size variations. Previous interpretations of ARM/chi variations on magnetic grain-size without consideration of magnetic interactions may not be valid. [References: 18]