Plain English Summary

Our understanding Hg cycling in the environment is built upon the assumption that surface soils are the most important in terrestrial ecosystems. Our study shows that weathered bedrock also plays an important role. The weathering of sedimentary rocks were net sources of Hg while weathered igneous and metamorphic rocks were net sinks for Hg. In addition, weathered bedrock holds far more Hg than soils, which is not taken into account in our current models.

Abstract

Mercury sequestration in regolith (soils + weathered bedrock) is an important ecosystem service of the critical zone. This has largely remains unexplored, due to difficulty of sample collection and the assumption that Hg is predominantly sequestered within the surface soils (here we define as 0 – 0.3 m). We measured Hg concentrations and inventories in weathering profiles at six Critical Zone Observatories (CZOs): Boulder Creek in the Front Range of Colorado, Calhoun in the South Carolina Piedmont, Eel River in coastal northern California, Luquillo in tropical montane forest of Puerto Rico, Shale Hills of valley and ridges of central Pennsylvania, and Southern Sierra in the Sierra Nevada range of California. Surface soils had higher Hg concentrations than the deepest regolith samples, except for Eel River, which had lower Hg concentrations in surface soils compared to regolith. Using Ti normalization, CZOs with < 12 % rock‐derived Hg (Boulder Creek, Calhoun, and Southern Sierra) had Hg peaks between 1.5 – 8.0 m in depth. At CZOs with > 50% rock‐derived Hg, Eel River Hg concentrations and pools were greatest > 4.0 m in the weathering profile while Luquillo and Shale Hills had peaks at the surface that diminished within 1.0 cm of the surface. Hg and TOC were only significantly correlated in regolith at Luquillo and Shale Hills CZOs, suggesting Hg sorption to organic matter may be less dominant than clays or Fe(II)‐sulfides in deeper regolith. Our results demonstrate the importance of Hg sequestration in deep regolith, below typical soil sampling depths.

Citation

Richardson, Justin B., Arnulfo A. Aguirre, Heather L. Buss, A. Toby O'Geen, Xin Gu, Daniella M. Rempe, and Daniel deB. Richter (2018): Mercury sourcing and sequestration in weathering profiles at six Critical Zone Observatories. Global Biogeochemical Cycles, 32(10):1542-1555. DOI: 10.1029/2018GB005974

This Paper/Book acknowledges NSF CZO grant support.