Molybdenum sources and isotopic composition during early stages of pedogenesis along a basaltic climate transect

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 (based on 98Mo/95Mo relative to NIST SRM 3134) are heavier relative to bedrock, suggesting weathering processes preferentially retain light Mo isotopes, however, the mechanisms governing this process in soils are poorly understood. We investigated soil Mo abundance and isotopic composition as a function of climate along a well-constrained climosequence on a 10kyr lava flow in Hawaii receiving 660 to 2100mmyear−1 mean annual precipitation (MAP). We assessed Mo abundance and isotopic composition as a function of soil organic matter (OM) content, short-ranged-ordered (SRO) iron (Fe) (oxyhydr)oxide abundance, and Mo loss/gain. We find net accumulation of Mo across all soils (up to +139% gain) that is positively correlated with increasing MAP. The highest Mo gains are in surface soil horizons, and are correlated with high OM content. The isotopic composition of soil Mo, deviates from underlying basaltic bedrock, which ranges from δ98Mo values of −0.11‰ to −0.26‰. Soil Mo isotopic values are lightest at the dry sites (δ98Mo=−0.63‰) and become heavier with increasing precipitation (up to δ98Mo=+0.03‰). Samples with the heaviest δ98Mo values are from horizons with the largest net gains of Mo relative to bedrock and have higher OM content. In order to further constrain Mo fluxes into and out of the soil system, we measured Mo isotope ratios in local rainwater (average δ98Mo=+1.11‰), groundwater (average δ98Mo=+0.14‰), and vegetation (δ98Mo values between −0.18‰ and +0.64‰). Based on these data, we propose that Mo in these soils is substantially augmented by additions from precipitation, volcanic fog, and potentially anthropogenic inputs of Mo, and that retention of these inputs within soils is likely related to Mo–OM interactions. The large atmospheric inputs of Mo strongly modulate the δ98Mo values of dissolved Mo fluxes from soil to values only slightly heavier than bedrock. These patterns illustrate the potential for Mo as a tracer of atmospheric inputs and pedogenic processes in soils and help elucidate the mechanisms that drive heavy δ98Mo values in the global riverine Mo flux.