Temperature and hydrological controls on carbon dioxide release from weathering of sedimentary rocks

The amount of carbon contained in sedimentary rocks is vast, with ~1.3×10⁷ PgC stored as organic matter and ~6.5×10⁷ PgC as carbonate minerals. Combined, this is ~130,000 times the carbon content of the pre-industrial atmosphere. Erosion can expose these rocks to Earth's oxygenated surface and oxidative weathering processes, releasing carbon dioxide (CO₂) from the lithosphere to the atmosphere by: i) the oxidation of rock-derived organic matter, and/or ii) when sulfuric acid (produced by the oxidation of sulfide minerals, e.g., pyrite) reacts with carbonate minerals. The combined CO₂ flux from weathering of sedimentary rocks plays a central role in the global carbon cycle over millennial timescales and longer, yet we have few measurements and the climatic controls on the CO₂ emissions have not been assessed.

Here we monitored CO₂ production during the oxidative weathering of sedimentary rocks in the Laval catchment of the Draix-Bleone Critical Zone Observatory, near Draix, Alpes de Haute Provence, France. We use newly-designed chambers which allow us to quantify in-situ CO₂ fluxes (Soulet et al., 2018, Biogeosciences), and have monitored them seasonally from December 2016 to May 2019. In order to partition the source of the CO₂ between carbonate minerals and rock organic matter, we sampled the CO₂ using zeolite traps and measured its carbon stable isotopes (δ¹³C) and radiocarbon activity (F¹⁴C).

Measured CO₂ fluxes ranged widely between ~10 and ~1200 gC m^-2 yr^-1 depending on the chamber and season. In all of the chambers, we observed a ~10-fold increase in the CO₂ fluxes from winter to summer months. For chambers where we have measured the δ¹³C and F¹⁴C of CO₂, this appears to be for both the dissolution of carbonate minerals (~70% of total flux) and the oxidation of rock organic matter (~30%). After rainfall events, we find that the measured CO₂ fluxes decline rapidly, but return to pre-event levels over hours to days. Otherwise, the CO₂ fluxes are positively correlated with the temperature in the weathering chamber. These are the first field-based measurements and show large seasonal changes in CO₂ emissions from weathering of sedimentary rocks. The CO₂ fluxes approach those measured in soils and they appear to have a similar sensitivity to changes in temperature.