Carbon dioxide emissions from the oxidative weathering of sedimentary rocks in highly erosive settings

Abstract

Chemical weathering of sedimentary rocks is an important control on the composition of Earth’s surface system and climate over geological timescales, via two main pathways: i) the oxidation of petrogenic organic carbon (OCpetro); and ii) the oxidation of sulfide minerals coupled to the dissolution of carbonate minerals. Both processes are characterized by the release of carbon dioxide (CO2) and consumption of oxygen (O2). However, the corresponding present-day weathering fluxes and their environmental controls are only partly constrained due to a lack of in situ measurements. This research presents direct measurements of CO2 release that were performed in steep, rapidly eroding terrains of the Waiapu catchment, North Island, New Zealand, and two neighbouring catchments in the French southern Alps that are part of the Draix-Bléone observatory. For this, the tools for studying the gaseous exchange of CO2 and O2 during oxidative weathering are refined. It is shown that direct measurements using drilled gas accumulation chambers are feasible during short-term (≥ 1 week) field campaigns in remote areas. In addition, the gaseous movement of CO2 and O2 in shallow weathering zones can be quantified, which supports upscaling local findings to the wider landscape. Following this approach, high CO2 emissions are reported that identify steep terrains as hotspots of oxidative weathering in the geological carbon cycle. Furthermore, the contributions to the bulk weathering fluxes from OCpetro oxidation and sulfide oxidation coupled to carbonate dissolution are assessed with radiocarbon and stable carbon isotope analyses. For this, a microbial impact on the oxidation of OCpetro is discussed. Based on repeated fieldtrips across multiple seasons, important environmental controls on the size and source of in situ CO2 emissions are identified, including temperature, precipitation and the chemical composition of the bedrock. Overall, the oxidative weathering fluxes and their environmental controls presented here provide a basis for future models of Earth’s geochemical and climatic history. It is highlighted that these now need to consider a positive climate feedback by the response of oxidative weathering fluxes to warming.