Coupled variations in V-Fe abundances and isotope compositions in latosols: Implications for V mobilization during chemical weathering

Abstract

Weathering is a key process controlling V transport from the continental crust to the hydrosphere. To understand V behavior during chemical weathering, we investigated the concentrations and isotope compositions of V in a latosol profile developed from gabbros in southern China. Despite the significant redistribution of V in the profile, the δ51V values display a narrow range from −0.77‰ to −0.94‰, which is similar to the values of weathered gabbro (−0.88 ± 0.03‰) and fresh gabbro (−0.79 ± 0.06‰) within analytical uncertainty. Iron concentrations and isotope compositions were also analyzed to further determine how V migrates in the profile. Vanadium and Fe concentrations in the profile are positively correlated, suggesting a coupled V and Fe redistribution along the profile. The δ56Fe values of the latosols also show limited variation from 0.04 ± 0.03‰ to 0.14 ± 0.02‰. These observations indicate that V and Fe isotopes are not significantly fractionated during the chemical weathering of gabbros.Based on the observations of the latosols, we propose that soluble ferrous Fe is released from silicate minerals during weathering in oxic environments and oxidized to insoluble ferric Fe in situ, forming Fe (oxyhydr)oxide colloids/particles. The V (+3) and V (+4) ions are released from the minerals and rapidly oxidized to soluble V (+5), which is quantitatively adsorbed on the surfaces of the colloidal Fe (oxyhydr)oxides. Sequentially, Fe (oxyhydr)oxide colloids/particles are transported by water flow through the latosol profile. This study provides important insight into V isotope and V behavior during chemical weathering and helps constrain the V isotope compositions of weathered upper continental crust and river water, and this insight is critical for understanding how V is transferred from the land to the ocean.