Molybdenum isotopes and mass balance during early stages of pedogenesis

Abstract:

Molybdenum (Mo) is an essential micronutrient and redox sensitive trace metal that has the potential to be a tracer of pedogenic processes. Globally, riverine δ⁹⁸Mo values are elevated relative to bedrock, suggesting weathering processes preferentially retain light Mo isotopes, however, the mechanisms governing this process in soils are poorly understood. To elucidate these mechanisms, we studied seven soil profiles developed on a 10ka lava flow in Hawaii receiving 600 to 2000 mm mean annual precipitation. We assessed Mo abundance and isotopic composition as a function of soil organic matter (OM) content, iron (Fe) and manganese (Mn) (oxyhydr)oxide abundance, and Mo loss/gain. We found net accumulation of Mo across all sites (+48% to +289%) that was positively correlated with increasing precipitation, OM content, and Fe and Mn (oxyhydr)oxide content and inversely correlated with soil depth. Thus, the highest Mo gains are in the wettest surface soil horizons, which also have high OM content. Selective extractions of surface soils indicate that 13% to 40% of mobile Mo is predominately associated with OM; whereas Mo associated with Fe and Mn (oxyhydr)oxides is an order of magnitude lower (0.6% to 6%). The isotopic composition of soil Mo deviated from parent material values (δ⁹⁸Mo ~-0.15‰). Mo isotopic values were lightest at the dry sites (δ⁹⁸Mo values of -0.29‰ to -0.63‰) and become heavy with increasing precipitation (δ⁹⁸Mo -0.2‰ to +0.3‰). At all sites, the surface horizons were isotopically heavy relative to the subsurface horizons, and samples with the heaviest δ⁹⁸Mo values corresponded with horizons that have gained Mo and have higher OM content. Subsurface Mo isotopic values are lighter than bedrock isotopic composition and may reflect associations with Fe and Mn (oxyhydr)oxides. In order further to constrain Mo fluxes into and out of the soil system, we measured Mo isotopes in local rainwater, groundwater, and vegetation. Based on this data, we propose that the gains in Mo relative to parent material are attributed to exogenous inputs of Mo and the preservation of these inputs depends strongly on Mo-OM interactions. These patterns illustrate the potential for Mo as a tracer of pedogenesis in soils and help elucidate the mechanisms that drive heavy δ⁹⁸Mo values in the global riverine Mo flux.