OXIDATIVE WEATHERING CAUSES CHROMATE PRODUCTION AND REDISTRIBUTION IN INDONESIAN LATERITES

Chromate (Cr(VI)) is toxic, highly mobile, and is naturally produced through the oxidative weathering of Cr bearing rocks. Oxidative weathering of Cr is accompanied by Cr isotope fractionation, and this fractionation can be used to track the redox history of Earth’s surface. In an effort to determine the rates and controls of oxidative Cr weathering, we examined the distribution and speciation of Cr in the ultramafic laterite soils of Sulawesi Island, Indonesia. Our soil samples were collected from a near vertical drill core that penetrated from the topsoil through the 25m deep soil profile and into the underlying bedrock. After collection, the soils were freeze dried and separated into 2 mm and 75 µm size fractions. We then determined both the exchangeable (using a 1M CaCl_2­ leach) and tightly sorbed (extracted using a 0.1M PO₄^3- extraction) Cr (VI) pools in both size fractions.

Vertical profiles of Cr(VI), in both exchangeable and tightly bound pools, exhibit two peaks in concentration, implying significant Cr(VI) mobility within the soil profile. The tightly sorbed pool reached concentrations up to 570 ppm at 15m depth near the limonite-saprolite boundary. The exchangeable Cr(VI) reached concentrations up to 1.3 ppm at 16m, also near the limonite-saprolite boundary. Overall, our measurements document significant Cr(VI) production and mobility during oxidative weathering in these Indonesian laterites. Similar soils developed on ultramafic rocks in New Caledonia exhibit much higher concentrations of tightly bound Cr(VI), but undetectable exchangeable Cr(VI), implying that Cr weathering is sensitive to the specific soil environment. To further advance our knowledge of the complex interactions involved in Cr weathering, we plan to examine weathering of other ultramafic rock components, the speciation of Cr in our samples using X-ray spectroscopy, and to determine the isotopic fractionation associated with Cr redistribution and loss.