Mercury Sourcing and Sequestration in Weathering Profiles at Six Critical Zone Observatories

• Published in: Global biogeochemical cycles 2018-10, Vol.32 (10), p.1542-1555
• Main Authors: Richardson, Justin B., Aguirre, Arnulfo A., Buss, Heather L., Toby O'Geen, A., Gu, Xin, Rempe, Daniella M., Richter, Daniel de B.
• Format: Article
• Language: English
• Subjects: Bedrock; Catadromous fishes; Clay; Coastal inlets; Creeks; critical zone; Ecosystem services; Ecosystems; Eels; excess mercury; Hills; Iron; Marine fishes; Mercury; Mercury (metal); mercury cycling; mercury inventories; Metamorphic rocks; Montane environments; Mountain forests; Observatories; Organic matter; Profiles; Regolith; Ridges; Rivers; Sedimentary rocks; Shale; Shales; Soil; Soil depth; Soil sampling; Soil surfaces; Terrestrial ecosystems; Tropical climate; Tropical forests; Water depth; Weathering
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2018GB005974

Mercury sequestration in regolith (soils + weathered bedrock) is an important ecosystem service of the critical zone. This has largely remained unexplored, due to the difficulty of sample collection and the assumption that Hg is predominantly sequestered within 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 the tropical montane forest of Puerto Rico, Shale Hills of the 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 50% rock‐derived Hg, Eel River Hg concentrations and pools were greatest at >4.0 m in the weathering profile, while Luquillo and Shale Hills had peaks at the surface that diminished within 1.0 m of the surface. Hg and total organic C were only significantly correlated in regolith at Luquillo and Shale Hills CZOs, suggesting that 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. Plain Language Summary Our understanding of 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 was the net sources of Hg, while the weathered igneous and metamorphic rocks were the net sinks for Hg. In addition, weathered bedrock holds far more Hg than soils, which is not taken into account in our current models. Key Points Mercury concentrations and inventories in weathering profiles were explored at six Critical Zone Observatories Using Hg/Ti ratios, we found that >88% of the Hg concentration was not rock‐derived at Boulder Creek, Calhoun, and Southern Sierra CZOs Our study estimated that the top 10 m of regolith has Hg inventories two orders of magnitude greater than considered by current models