Germanium sequestration by soil: Targeting the roles of secondary clays and Fe-oxyhydroxides

Germanium is a trace element that behaves similarly to silicon and shows promise as a tracer of silicate weathering processes. Sequestration of Ge by clay minerals is thought to be the dominant mechanism responsible for Ge/Si fractionation during weathering. In some environments, Ge may be sequestered by iron-oxyhydroxides. We examine Ge behavior in soils along a well-characterized climate gradient on the Hawaiian island of Maui to clarify the relationship between Fe geochemistry and soil Ge / Si ratios. All of the sites are developed on basaltic lavas and tephras approximately 400 ky old, are highly Si-depleted, and have Ge / Si ratios higher than parent material. Sites experiencing less than ∼3300 mm rain/yr host oxidized, highly weathered basaltic soils, in which Fe is present as secondary Fe-oxyhydroxide minerals. In contrast, the wetter sites (> 3340 mm/yr) show extensive Fe reduction, mobilization, and loss. We show that Ge sequestration is independent of Fe redox behavior along the gradient, and thus precipitation of Fe-oxyhydroxides is not a major factor contributing to Ge / Si fractionation during weathering. Instead, elevated soil Ge / Si ratios reflect partitioning of Ge into secondary clay minerals, plus additional retention of Ge by refractory non-silicates.