Role of pedogenesis in distribution of magnetic susceptibility in two California chronosequences

Abstract

Pedons from two soil chronosequences from coastal terraces in northern California were examined and sampled and samples were analyzed to determine the contribution of normal pedogenic processes to the distribution of mass magnetic susceptibility (χ) in soils. Commonly, χ distribution in soils and the formation of susceptibility enhancement in near-surface soil layers are attributed in large part to the in situ conversion of nonferrimagnetic iron oxides and oxyhydroxides to microcrystalline ferrimagnetic minerals. Solubilization of Fe 2+ and subsequent precipitation also contribute to enhancement. We show that χ enhancement occurred throughout the eluvial and in some illuvial horizons to depths as great as 100 cm in these soils. Eluvial horizon χ and χ enhancement increased with increasing soil age. The enhancement of magnetic susceptibility resulted from the accumulation of pedogenic ferrimagnetics and the preferential accumulation of both inherited magnetite and pedogenic maghemite. We use the differential solubility of inherited magnetite and maghemite in citrate-bicarbonate-dithionite and X-ray diffraction to distinguish between the two minerals. Although magnetite is a relatively more durable soil mineral, it did weather and the total amount of magnetite decreased as soil age increased. Sonicated sand grains, free of clay or Fe coatings contributed to susceptibility enhancement. Silt- and clay-sized grains provided the larger part of illuvial horizon χ. Illuviation to or formation of clay-sized ferrimagnetic particles in illuvial horizons probably reduced the observed eluvial enhancement. Enhancement of eluvial horizons and χ distribution throughout the soil oprofiles can be explained by common soil forming processes.