Sources of fine-grained magnetic minerals in sediments: a problem revisited

摘要

Progress in the discrimination of fine-grained magnetic minerals in sediments and sediment sources using measurements of magnetic susceptibility at two frequencies and of magnetic remanence generated by a combination of direct and alternating currents is summarized. The former measurements yield values for low field and frequency-dependent susceptibility (χ_(lf) and χ_(fd), respectively). The latter generate an Anhysteretic Remanent Magnetisation (ARM) that, when normalized to the DC bias field, can be represented as the susceptibility of ARM (χ_(ARM)). Using quotients derived from χ_(lf), χ_(fd) and χ_(ARM) to generate bi-logarithmic plots, it is possible to discriminate between assemblages dominated by magnetic minerals formed through pedogenesis and those arising from the presence of magnetotactic bacteria that use chains of magnetic particles to seek optimum environments in surface sediments. Sediments to which both types of magnetic mineral contribute occupy positions between the envelopes of values typical of soil-derived and magnetosome-derived assemblages. Fire also appears to give rise to a distinctive envelope of values indicative of smaller magnetic grain sizes, whilst the signature of extracellular magnetite is indicative of the finest grain sizes here recorded.