Molybdenum isotope fractionation in the critical zone

Abstract:

Mo isotope fractionation during soil development was studied across three pedogenic gradients encompassing a range of controlling factors. These include redox conditions, organic matter content, Fe and Mn oxy(hydr)oxide content, mineral composition, degree of weathering, pH, type and amount of atmospheric inputs, age, climate, and underlying rock type. Soil profiles from the island of Maui along a precipitation gradient ranging from 850 to 5050 mm mean annual precipitation show a decrease in average soil d⁹⁸Mo from -0.04±0.11‰ at the driest, most oxic site, which is indistinguishable from the basalt parent material (-0.09±0.08‰), to -0.33±0.10‰ at the wettest, most reducing site. A suite of 6 Icelandic soils display a broad trend with heavier d⁹⁸Mo values in soil horizons that are more weathered and with higher organic matter content, with values up to +1.50±0.09‰ in a Histosol. Selective extractions of Mo from different soil components indicate that the association with organic matter and silicate or Ti-oxide residue dominates retention of Mo in these soils, with adsorption on Fe and Mn oxy(hydr)oxides playing a lesser role. Across all basaltic soils, d⁹⁸Mo values are lighter in soils that exhibit the most net Mo loss relative to the parent material, and d⁹⁸Mo values are heavier in soils that exhibit net Mo gains. A well-drained soil and saprolite profile in the Luquillo Mountains of Puerto Rico also shows heavier d⁹⁸Mo values than the parent material, up to +0.71±0.10‰ with an integrated profile average of +0.28±0.10‰ ‰ versus -0.01±0.10‰ in the quartz diorite bedrock, but an overall moderate loss of 28% of Mo relative to bedrock. The Luquillo saprolite is unfractionated from bedrock in the deeper part of the profile, however, indicating that rock weathering dissolution processes and secondary clay formation do not fractionate Mo isotopes, and that the heavy Mo isotope values in the upper, most weathered part of the profile reflect atmospheric inputs, biological cycling, or sorption-desorption processes. The presence of both light and heavy Mo (relative to parent material) across all sites and within individual soil profiles suggests that it is normal for multiple Mo fractionation mechanisms to operate under the open-system conditionsof soils.