How Particle Size Influences Oxidation of Ancient Organic Matter during Weathering of Black Shale

Abstract

Weathering continuously converts rock to regolith at Earth’s surface while regulating the atmospheric concentrations of CO2 and O2. Shale weathering is of particular interest because shale, the most abundant rock type exposed on continents, stores much of the ancient organic carbon (OCpetro) buried in rocks. Using geochemical and mineralogical analysis combined with neutron scattering and imaging, we investigated the weathering profile of OCpetro in saprock in a black shale (Marcellus Formation) in the Ridge and Valley Appalachians in Pennsylvania, U.S.A. Consistent with the low erosion rate of the landscape, we discovered that Marcellus is completely depleted in carbonate, plagioclase, and pyrite in saprock below the soil layer. On the contrary, only ∼60% of OCpetro was depleted in saprock. By comparing the pore structure of saprock to bedrock and samples combusted to remove organic matter (OM), we confirmed that the large particles of OM are preferentially depleted, leaving elongated pores of tens to hundreds of micrometers in length, while the smaller particulates of OM (ranging from ∼5 to ∼200 nm) are largely preserved during weathering. The retarded weathering of small OM particles is attributed to their close association with mineral surfaces in the shale matrix. The texture of OM in shale is underappreciated as an important factor that controls porosity generation and the weathering rate of OCpetro.

Low rates of rock organic carbon oxidation and anthropogenic cycling of rhenium in a slowly denuding landscape

“…A decoupling of S loss from OC petro loss has been reported in other shale weathering profiles (Gu & Brantley, 2022; Jaffe et al, 2002), with the S oxidation front being deeper than the coupled Re and OC petro oxidation. The deeper S front than OC petro oxidation front presumably reflects the faster reaction kinetics of pyrite (Gu & Brantley, 2022; Petsch et al, 2000). However, this assumes that O 2 in air and water has access to phases in the subsurface.…”

“…[Color figure can be viewed at wileyonlinelibrary.com] its weathered products with clay minerals may protect them from further oxidation (Gu & Brantley, 2022;Hemingway et al, 2019).…”

“…Over long time periods, carbon dioxide (CO 2 ) emissions through volcanism, carbonate weathering by sulfuric acid, and oxidation of rock‐derived organic carbon (petrogenic, OC petro ) are countered by CO 2 sequestration through silicate weathering, carbonate formation, and burial of biospheric organic carbon (OC bio ) (Hilton & West, 2020). The CO 2 emissions from OC petro oxidation (Gu & Brantley, 2022; Petsch, 2014) and carbonate weathering by sulfuric acid (Burke et al, 2018; Torres et al, 2017) may be of similar magnitude to the release of CO 2 from volcanic degassing (79 ± 9 MtC yr −1 , Plank & Manning, 2019), yet the global rates of OC petro oxidation remain poorly constrained (Petsch, 2014).…”

“…First, low denudation rates (< 0.5 mm yr −1 ) are common over the majority of Earth's continents, and these settings contribute to ~60% of global chemical denudation (Larsen et al, 2014). Second, low erosion rates can lead to the development of thick weathering profiles, helping to reveal reaction fronts of OC petro and sulfide oxidation (Brantley et al, 2013; Gu & Brantley, 2022; Petsch et al, 2000; Wildman, 2004) and track the mobility of Re during weathering (Jaffe et al, 2002). Finally, settings with lower chemical denudation could be more sensitive to Re inputs from anthropogenic sources (Miller et al, 2011; Rahaman et al, 2012), helping us understand the modern‐day perturbation of the Re cycle (Sen & Peucker‐Ehrenbrink, 2012).…”

Virtual Special Issue of ACS Earth and Space Chemistry in Honor of Prof. Michael F. Hochella, Jr.

“…In addition, the three Hochella Symposium organizers were invited by Prof. Joel Blum, Editor-in-Chief of ACS Earth and Space Chemistry , to organize and serve as Guest Editors of a virtual special issue of this journal in honor of Mike Hochella. A total of 20 peer-reviewed manuscripts were accepted for publication in this virtual special issue (), with topics ranging from mineral/water interface chemistry and mineral/organic matter nanoparticles to organic matter/soil chemistry and microbial geochemistry studies. − These papers highlight the importance of nanogeoscience, mineral/aqueous solution interface chemistry, biogeochemistry, and organic matter/soil chemistry in understanding the Earth system.…”