Stable Isotopes Reveal the Contribution of Glacier Melting to Mercury Budget in Tibetan Rivers

Abstract

Rapid retreat of glaciers accompanied by the release of mercury (Hg) in the Tibetan Plateau (TP), known as the “Asian water tower”, threatens the water security of billions of people living downstream in China and Southwest Asia. It is thus crucial to identify the sources and transformations of Hg in TP ecosystems. Here we measured Hg isotopes in water samples collected from two adjacent TP rivers, the Upper Mekong River (UMR) and the Upper Salween River (USR), to better constrain Hg geochemistry and its export under climate changes. We found similar δ²⁰²Hg and Δ²⁰⁰Hg variations in the two rivers, which reflect a similarly mixed contribution of atmospheric precipitation and catchment input to both rivers. However, in general, USR shows notably higher Δ¹⁹⁹Hg than UMR. As the catchment of USR has a larger glacier-covered area than UMR, we propose that the higher Δ¹⁹⁹Hg of USR water samples reflects a higher glacier Hg input, which is typically characterized by extremely positive Δ¹⁹⁹Hg. Our study indicates that increasing glacier melting, vegetation greening and precipitation induced by climate changes may enhance Hg export from TP, potentially endangering the fragile Tibetan ecosystem and human health.