Jennifer L. Druhan
Corey Lawrence
Daniella M. Rempe
Alison K. Tune
2023
<p><span>The oxidation of organic carbon in sedimentary bedrock (petrogenic OC, OC</span><i><sub><span class="math"><span id="MathJax-Element-1-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mi is="true">p</mi><mi is="true">e</mi><mi is="true">t</mi><mi is="true">r</mi><mi is="true">o</mi></mrow></msub></math>"><span class="MJX_Assistive_MathML">petro</span></span></span></sub></i><span>) is increasingly recognized as a potential source of CO</span><sub>2</sub><span> to the atmosphere. Recent studies provide evidence for the mobilization and oxidation of OC</span><i><sub><span class="math"><span id="MathJax-Element-2-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mi is="true">p</mi><mi is="true">e</mi><mi is="true">t</mi><mi is="true">r</mi><mi is="true">o</mi></mrow></msub></math>"><span class="MJX_Assistive_MathML">petro</span></span></span></sub></i><span> 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 OC</span><sub><i><span class="math"><span id="MathJax-Element-3-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mi is="true">p</mi><mi is="true">e</mi><mi is="true">t</mi><mi is="true">r</mi><mi is="true">o</mi></mrow></msub></math>"><span class="MJX_Assistive_MathML">petro</span></span></span></i></sub><span> 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. OC</span><sub><i><span class="math"><span id="MathJax-Element-4-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mi is="true">p</mi><mi is="true">e</mi><mi is="true">t</mi><mi is="true">r</mi><mi is="true">o</mi></mrow></msub></math>"><span class="MJX_Assistive_MathML">petro</span></span></span></i></sub><span> is removed from the weathered bedrock at a rate of approximately 0.12 gC/m</span><sup>3</sup><span>yr, which is orders of magnitude lower than the rate of OC</span><i><sub><span class="math"><span id="MathJax-Element-5-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mi is="true">p</mi><mi is="true">e</mi><mi is="true">t</mi><mi is="true">r</mi><mi is="true">o</mi></mrow></msub></math>"><span class="MJX_Assistive_MathML">petro</span></span></span></sub></i><span> oxidation we achieved in the laboratory with crushed samples (557.1 gC/m</span><sup>3</sup><span>/yr). This disparity occurs despite high O</span><i><sub><span class="math"><span id="MathJax-Element-6-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mn is="true">2</mn><mo stretchy="false" is="true">(</mo><mi is="true">g</mi><mo stretchy="false" is="true">)</mo></mrow></msub></math>"><span class="MJX_Assistive_MathML">2(g)</span></span></span></sub></i><span> content across the depth profile, indicating that physical accessibility of OC</span><sub><i><span class="math"><span id="MathJax-Element-7-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mi is="true">p</mi><mi is="true">e</mi><mi is="true">t</mi><mi is="true">r</mi><mi is="true">o</mi></mrow></msub></math>"><span class="MJX_Assistive_MathML">petro</span></span></span></i></sub><span> can regulate oxidative weathering. There is no direct radiocarbon evidence of OC</span><i><sub><span class="math"><span id="MathJax-Element-8-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mi is="true">p</mi><mi is="true">e</mi><mi is="true">t</mi><mi is="true">r</mi><mi is="true">o</mi></mrow></msub></math>"><span class="MJX_Assistive_MathML">petro</span></span></span></sub></i><span> oxidation in CO</span><i><sub><span class="math"><span id="MathJax-Element-9-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mn is="true">2</mn><mo stretchy="false" is="true">(</mo><mi is="true">g</mi><mo stretchy="false" is="true">)</mo></mrow></msub></math>"><span class="MJX_Assistive_MathML">2(g)</span></span></span></sub></i><span> across the upper 13 m of the weathering profile during both wet and dry seasons. Instead, vadose zone CO</span><i><sub><span class="math"><span id="MathJax-Element-10-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mn is="true">2</mn><mo stretchy="false" is="true">(</mo><mi is="true">g</mi><mo stretchy="false" is="true">)</mo></mrow></msub></math>"><span class="MJX_Assistive_MathML">2(g)</span></span></span></sub></i><span> production at the site is dominated by respiration of recently fixed carbon associated with deep rooting. OC</span><i><sub><span class="math"><span id="MathJax-Element-11-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mi is="true">p</mi><mi is="true">e</mi><mi is="true">t</mi><mi is="true">r</mi><mi is="true">o</mi></mrow></msub></math>"><span class="MJX_Assistive_MathML">petro</span></span></span></sub></i><span> 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 CO</span><i><sub><span class="math"><span id="MathJax-Element-12-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub is="true"><mrow is="true" /><mrow is="true"><mn is="true">2</mn><mo stretchy="false" is="true">(</mo><mi is="true">g</mi><mo stretchy="false" is="true">)</mo></mrow></msub></math>"><span class="MJX_Assistive_MathML">2(g)</span></span></span></sub></i><span>.</span></p>
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10.1016/j.epsl.2023.118048
en
Elsevier
Deep root activity overprints weathering of petrogenic organic carbon in shale
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