Chemical weathering of sedimentary rocks as a source of carbon dioxide to the atmosphere

Abstract

The geological carbon cycle maintains sustainable conditions for the Earth’s habitability through weathering and erosion. This research focuses on two chemical weathering processes: i) rock organic carbon oxidation and ii) carbonate dissolution by sulfuric acid, both resulting in carbon dioxide (CO2) release to the atmosphere. The long-term and modern-day fluxes and environmental controls navigating chemical weathering reactions are investigated in small (<1 km2), densely monitored catchments of two critical zone observatories: i) the Susquehanna Shale Hills (Pennsylvania, USA) and ii) the Draix-Bléone (Provence, France). In the low erosive setting of the Susquehanna Shale Hills observatory, the trace element rhenium (Re) is studied alongside total organic carbon and radiocarbon data to estimate the long-term rock organic carbon oxidation rates. This study confirms that rock organic carbon weathering is a supply-limited process. In addition, substantial anthropogenic perturbation of the surface Re cycle is observed at this site. In the Draix-Bléone observatory an unprecedented high-temporal resolution stream water sampling campaign took place in two nearby catchments. The catchments are contrasting in physical erosion rates and variable extent of vegetation cover, allowing to investigate controls on present day rock organic carbon oxidation rates and inorganic carbon balance. In the highly erosive and sparsely vegetated catchment the CO2 release rates are largely dominated by carbonate dissolution by sulfuric acid. Furthermore, strong seasonality in dissolved SO4-2 /HCO3- ratios and Re concentrations and in streams is observed, which is interpret as a seasonal signal in sulfuric acid production and rock organic carbon oxidation, respectively. These findings suggest a temperature control on CO2 release rates through chemical weathering reactions, which is most pronounced in highly erosive and sparse vegetation setting. Overall, this research provides novel insights on environmental controls on chemical weathering rates and proposes a positive climatic feedback on chemical weathering of sedimentary rocks, which carries important implications for future climate studies.