Granite weathering profiles accumulate vegetation-derived mercury

Abstract

Weathering of silicate rocks, particularly granitic rocks, generates the pedosphere that is of crucial importance in functioning terrestrial ecosystems. The pedosphere hosts large amounts of mercury (Hg). The source of Hg in granite weathering products is currently poorly constrained, however. Here we conducted Hg isotopic analyses on two weathering profiles developed on granitic plutons in Central and South China. Soils from both profiles show a downward decrease in Hg concentration but consistently negative ∆199Hg values. Mercury in precipitation, granites, and vegetation display positive, near-zero, and negative ∆199Hg values, respectively. The negative ∆199Hg values in the two weathering profiles are consistent with estimates of vegetation-derived Hg, supporting that Hg in weathering profiles are dominantly sourced from vegetation. Hg in vegetation is transported into surface soil firmly bonded to organic matter and migrates downwards weathering profiles. This study provides insights into the biogeochemical cycle of Hg in the granite-weathering crust and helps to explain the negative ∆199Hg values observed in terrestrial-derived sedimentary rocks.