Respiration in a shale rhizosphere dominated by deep root respiration, not oxidation of petrogenic organic carbon

Abstract:

Sedimentary rock containing organic carbon (OC) is exposed to oxidizing conditions through uplift, channel incision, and erosion. Recent studies highlight the role of rivers and floodplains in petrogenic OC oxidation. However, less is known about the processes operating inside hillslopes. In actively weathering and eroding hillslopes underlain by OC-rich bedrock, the pace and amount of oxidation is challenging to constrain and interactions between modern carbon cycling and petrogenic OC oxidation are poorly known. Here, we present the results of a multi-year field study documenting deep CO₂ fluxes throughout a weathering profile in the actively uplifting argillites of the Northern California Coast Ranges. In a 16 m deep weathering profile, solid phase OC is depleted from 0.37% in parent rock to 0.12% in weathered bedrock at the base of the rhizosphere, with much of the depletion occurring in the unsaturated zone between 6 and 10 m. Incubation experiments on solid phase samples of weathered and unweathered bedrock indicate that respiration rates of petrogenic OC could be up to 0.27𝛍gC/gRock/hr, or 17 gC/m²day. Sampling of water and gases throughout the weathering profile, afforded by a unique vadose zone monitoring system (VMS), reveal significant production of CO₂ to 8 m depth in both wet and dry seasons. This CO₂ is diffusively transported from the weathered bedrock to the soil at rates as high as 0.87 gC/m²day. To evaluate the extent to which petrogenic OC versus modern OC is the source of this respiration, radiocarbon analysis of gas samples at 1.5 m depth intervals throughout the profile was conducted in both wet and dry seasons. The results reveal that the respired CO₂ is likely entirely modern, which is consistent with the deep rooting (up to 14 m) and deep root water uptake occurring at the site. These findings suggest that a deep, modern organic carbon cycle co-occurs with petrogenic OC oxidation, and that despite vigorous carbon cycling deep in the profile, petrogenic OC persists within bedrock chips that arrive in soils, up to 0.29% OC. Our work demonstrates the importance of deep root processes in driving subsurface carbon dynamics, though more work is required to fully constrain the contribution of petrogenic OC including the role of dissolved and particulate export of petrogenic OC in weathering profile evolution.