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

Abstract

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 <12% rock‐derived Hg (Boulder Creek, Calhoun, and Southern Sierra) had Hg peaks between 1.5 and 8.0 m in depth. At 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.

Shale weathering profiles show Hg sequestration along a New York–Tennessee transect

“…Despite its importance, Hg sequestration in the regolith (including soil, weathered bedrock, and bedrock) has been poorly constrained. As demonstrated by Schuster et al, (2018) and Richardson et al, (2018), soils and weathered rock can act as a large Hg reservoir, which can potentially alter the local and regional Hg cycling by sequestering or releasing Hg to the atmosphere or river waters. In particular, shale weathering can release Hg to rivers with negative impacts on recreational fishing and fish consumption, which are popular activities throughout the Great Lake and Appalachian regions of the USA (Vaughan & Russell, 2015).…”

“…Because most soil pits are not deep enough to access the underlying lithology, their connection is often overlooked. In one of the few studies to examine diverse soil to bedrock relationships, Richardson et al, (2018) observed that Hg concentrations are very low in most crystalline igneous and metamorphic rocks are \ 10 ng/g, but increased throughout the profile showing net accumulation and sequestration of Hg from the atmosphere. However, Richardson et al, (2018) also showed that sedimentary rocks can be Hg-rich and be a potential source for Hg to the critical zone.…”

“…In one of the few studies to examine diverse soil to bedrock relationships, Richardson et al, (2018) observed that Hg concentrations are very low in most crystalline igneous and metamorphic rocks are \ 10 ng/g, but increased throughout the profile showing net accumulation and sequestration of Hg from the atmosphere. However, Richardson et al, (2018) also showed that sedimentary rocks can be Hg-rich and be a potential source for Hg to the critical zone. In studies conducted by Roulet et al, (1998) and Grimaldi et al, (2008), soils derived from quartz-kaolinite sedimentary rocks had soil Hg concentrations [ 100 ng/g at depths below 2.0 m. Roulet et al, (1998) attributed the high soil Hg concentrations on the retention by Al and Fe oxides formed during pedogenesis.…”

“…For this reason, younger, less developed soils may not sequester as much Hg as older, more developed soils. As soils undergo pedogenesis, they typically accumulate organic matter and precipitate Fe oxides, both of which are important for Hg adsorption and sequestration in temperate forested soils (Gabriel & Williamson, 2004;Richardson et al, 2013;Richardson et al, 2018;Schwesig & Matzner, 2001;Yu et al, 2014). Thus, it is essential to understand the role of both bedrock sourcing and pedogenesis on Hg sequestration to understand the role of soil as an Hg sink or source.…”

“…In some systems, high geogenic Hg in shales and other sedimentary rocks can be inherited by the overlying soils. As a prime example, the soils derived from argillite me ´lange present at Eel River Critical Zone Observatory in California were substantially enriched in geogenic Hg (Tsui et al, 2014;Richardson et al, 2018). However, it can accumulate Hg as shales have high surface area due to their clay-sized fraction and diagenetic phyllosilicates, geogenic organic matter, and geogenic Fe oxides (Hosterman & Whitlow, 1981;Fu et al, 2015).…”