Tracing the source and transport of Hg during pedogenesis in strongly weathered tropical soil using Hg isotopes

Soil is one of the largest reservoirs and re-emission sources of mercury (Hg) on Earth's surface. The accumulation and remobilization of Hg during pedogenesis affect how Hg is transported from terrestrial to aquatic environment and biota, but these processes are poorly understood. Here we present Hg concentration and isotope ratios of soil and bedrock samples from a latosol profile formed through intense weathering of Cenozoic basalts in Zhanjiang, Guangdong Province, China, to trace the sources, transport and transformation processes of Hg during pedogenesis, and to gain insights on the potential impact of tropical soils on global Hg cycling. The entire soil profile shows negative delta Hg-202 (-2.97 parts per thousand +/- 0.08 parts per thousand to -2.54 parts per thousand +/- 0.08 parts per thousand) and negative Delta Hg-199 (-0.68 parts per thousand +/- 0.04 parts per thousand to -0.43 parts per thousand +/- 0.04 parts per thousand), with a gradual downward positive shift of Delta Hg-199 below 35 cm. An isotopic mixing model based on Delta Hg-199 suggests a dominant Hg input from atmospheric Hg (Hg-Atm) and limited contribution (<36%) of geogenic Hg (Hg-Geo) from the bedrock. The strongly negative Delta Hg-199 throughout the latosol profile suggests that Hg-Atm was likely subject to substantial photoreduction on soil surface and migrated downward after incorporation into soils. Moreover, there is a distinct positive shift of delta Hg-202 alongside a rapid decrease of Hg-Geo from the bedrock to the weakly weathered basalt, indicating a substantial loss of Hg-Geo with the preferential release of lighter isotopes during the initial weathering, likely due to the dissolution of primary minerals. In addition, the delta Hg-202 shows a negative correlation with soil pH at some horizons, attributable to the isotopic fractionation during Hg(II) speciation change in soil solution and the selective complexation/adsorption of different Hg(II) species onto mineral surfaces, which are affected by surface charge properties and thus soil pH. Overall, our results provide direct evidence for the accumulation and downward migration of Hg-Atm in tropical soils during pedogenesis, as well as the loss of Hg-Geo during the weathering of bedrock, which may serve as an underappreciated source of Hg in tropical regions and have a potential impact on the Hg isotope signatures in aquatic environments.