The control of weathering processes on riverine and seawater hafnium isotope ratios

Hafnium (176)Hf/(177)Hf isotope ratio variations in marine records are thought to reflect changes in continental weathering through time, but the behavior of Hf in rivers, and during weathering, is not well understood. Here, we present (176)Hf/(177)Hf data for rivers, bedrock, soils, and leaching experiments for the Moselle basin, Vosges, France. These data strongly suggest that the (176)Hf/(177)Hf composition of river waters is controlled by preferential dissolution of accessory phases (i.e., apatite, sphene) versus more resistant minerals (e.g., K-feldspar) and linked to the intensity of silicate weathering. Estimates for the global isotopic composition of riverine Hf suggest that the ocean Hf budget may be dominated by river input, and variations seen in marine records can be directly related to changes in silicate weathering intensity.