Deep root activity overprints weathering of petrogenic organic carbon in shale

Earth and Planetary Science Letters
By: , and 



The oxidation of organic carbon in sedimentary bedrock (petrogenic OC, OCpetro) is increasingly recognized as a potential source of CO2 to the atmosphere. Recent studies provide evidence for the mobilization and oxidation of OCpetro in sedimentary bedrock during rock weathering. However, the mechanisms and rates remain uncertain, particularly where overlying soils and vegetation drive contemporaneous oxidation of recently fixed organic carbon. Here, we quantify OCpetro weathering across a 16 m shale depth profile in a steep, rapidly eroding forested hillslope in the Northern California Coast Ranges. We report solid and gas phase radiocarbon and stable isotope analyses of samples extracted from specialized in-situ samplers, and a supporting laboratory incubation experiment of the shale regolith. OCpetro is removed from the weathered bedrock at a rate of approximately 0.12 gC/m3yr, which is orders of magnitude lower than the rate of OCpetro oxidation we achieved in the laboratory with crushed samples (557.1 gC/m3/yr). This disparity occurs despite high O2(g) content across the depth profile, indicating that physical accessibility of OCpetro can regulate oxidative weathering. There is no direct radiocarbon evidence of OCpetro oxidation in CO2(g) across the upper 13 m of the weathering profile during both wet and dry seasons. Instead, vadose zone CO2(g) production at the site is dominated by respiration of recently fixed carbon associated with deep rooting. OCpetro is clearly mobilized across the vadose zone during weathering in this rapidly eroding, oxygen-rich, biologically dynamic hillslope, but at rates far below what can be measured given the contribution of root-derived CO2(g).

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Publication type Article
Publication Subtype Journal Article
Title Deep root activity overprints weathering of petrogenic organic carbon in shale
Series title Earth and Planetary Science Letters
DOI 10.1016/j.epsl.2023.118048
Volume 607
Year Published 2023
Language English
Publisher Elsevier
Contributing office(s) Geosciences and Environmental Change Science Center
Description 118048, 12 p.
Country United States
State California
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