Gaseous elemental mercury emissions in the Yangbajing geothermal field, Tibetan Plateau

Abstract

Mercury (Hg) is a volatile, toxic trace metal. Gaseous elemental mercury (Hg⁰) is a prevalent form of Hg in the atmosphere and is considered a dangerous pollutant with various adverse effects on both humans and the environment. The continental subduction zone, where volcanism is particularly active, is an important and unique area where the mercurriferous belt is found. In this work, a quantitative estimation of the Hg⁰ emissions in the Yangbajing (YBJ) geothermal field, which is located in the Indian–Asian continental subduction zone, was performed. According to our analysis, soil diffuse emission and fumarole degassing were the main forms of Hg⁰ emissions in the YBJ geothermal field. Significant differences in Hg⁰ emissions were observed between the northern and southern areas of the YBJ geothermal field. The Hg⁰ emission fluxes from soil diffuse emissions in the northern and southern areas, as well as fumarole degassing in the southern area, were 107.5–31,954.2 ng m⁻² d⁻¹, 2.8–11,039.3 ng m⁻² d⁻¹, and 28.7–73,229.4 ng m⁻² d⁻¹, respectively. Furthermore, the Hg⁰ emissions in both the northern and southern areas were spatially distributed along the NE-trending fault. This finding demonstrated the influence of geological tectonic setting on the Hg⁰ emissions in the geothermal field. In addition to the Hg⁰ flux, the gas compositions, carbon isotopes, microbial community abundance, and diversity of bacteria and archaea in the northern and southern areas exhibited significant differences. These differences indicated that Hg⁰ emissions in the northern and southern areas originated from magmatic and hydrothermal systems, respectively. The Hg⁰ emissions in the YBJ geothermal field are attributable to magmatic and hydrothermal degassing, thermal release, and microbial activities. Our work indicated that continental subduction zones are important areas of Hg⁰ degassing. Future research should focus on Hg⁰ emissions in continental subduction zones.