Towards Understanding Temporal and Spatial Patterns of Molybdenum in the Critical Zone

Molybdenum (Mo) is a redox-sensitive element that has been used to constrain paleo-oxygen conditions in marine sediments and could potentially be used similarly in soils. Sedimentary Mo ultimately comes from the terrestrial weathering of rocks during soil development (pedogenesis), but the mechanisms controlling Mo loss and mobility in soils are not well known. Here we present Mo concentration data from soils and specific soil pools across two climatic gradients on the Hawaiian Islands, the Maui climate gradient (MCG) and the Kona climate gradient (KCG). The MCG is a well-established precipitation gradient that exhibits a decrease in soil Eh and Fe content and an increase in organic matter (OM) content with increasing rainfall. We expected to find lower Mo concentrations and lower Mo mobilization in highly reducing (greater rainfall) soils as a result of Fe reduction and near complete loss of Fe-oxyhydroxides. However, we observe higher weak acid leachable Mo concentrations at higher rainfall sites. Since high rainfall coincides with a loss of Fe by reductive dissolution and the accumulation of OM, suggesting a shift from Fe-oxyhydroxide control of Mo at low rainfall sites to predominately OM-control of Mo at high rainfall sites. Selective chemical extractions indicate that Mo concentrations are two orders of magnitude higher in the OM bound pool versus the Fe(Al) bound pool, corroborating the importance of Mo-OM interactions on overall soil Mo pools. We also find evidence for increasing Mo retention in wetter soils along the KCG, which may be a result of increasing OM and Fe and Al oxyhydroxide content. An alternate explanation is that higher rainfall contributes sufficiently greater marine-derived atmospheric input of Mo. The fact that there are consistent patterns with rainfall on both climate gradients illustrate the potential for Mo as a tracer of pedogenic and redox processes in soils and highlight the importance of constraining controls on Mo fluxes.