Tracking 'geo-respiration' of fossil rock carbon using trace metals and their isotopes

The chemical weathering of rocks has profound impacts on Earth's biogeochemistry, influencing surface processes from nutrient cycling to the carbon cycle and climate. The canonical view of the weathering control on climate has been that the dissolution of silicate minerals by carbonic acid moves carbon from the atmosphere to the geosphere, providing a negative feedback for atmospheric CO2 and stabilising
climate over geological time. However, recent work shows that oxidative weathering - particularly of shales - may release comparable volumes of carbon to the atmosphere as is consumed via silicate mineral dissolution, raising questions for the conventional weathering thermostat. Shale-derived CO2 release can occur through two key pathways i) oxidation of 'fossil' organic carbon contained within rocks, and ii) release of carbon from carbonate rocks via dissolution by sulfuric acid derived from sulfide oxidation. Together these processes facilitate 'georespiration' of rock carbon, transferring carbon to
the atmosphere, and potentially generating a positive weathering feedback for atmospheric CO2. The global CO2 fluxes from these processes and the factors affecting them remain poorly constrained, but
are essential to quantify in order to assess the net carbon budget of weathering and its role in mediating Earth climate. Key questions include the relative roles of erosion and climate in driving fluxes, as well as how oxidative fluxes behave under contrasting geomorphic conditions, and how this impacts net weathering CO2 fluxes. Further work is needed in order to understand how oxidative weathering reactions and their climate impacts are controlled in the present day, how they have changed in the past, and how they might change in the future.

This knowledge gap can be addressed through application of the Rhenium (Re) and Vanadium (V) trace metal and isotope systems as proxies for oxidative weathering reactions. Rhenium and Vanadium may be effective tools to track oxidative reactions due to their enrichment in petrogenic organic matter and sulfides - the materials undergoing oxidative weathering. Rhenium loss has been recorded in shale
soil profiles undergoing weathering, and has been connected to accompanying loss of petrogenic organic carbon. Stable isotope fractionation of Re (187Re and 185Re) has been predicted
during redox, and first observations of isotope fractionation between the solid and dissolved products of weathering have been made. Less is understood about the behaviour of V during weathering. As
a redox-sensitive element V is expected to interact with oxidative weathering processes, however there are only limited measurements from soils and rivers. The existing data suggest V is hosted in
silicates and sulfides, but its cycling behaviour following release during weathering is not well known. Vanadium stable isotopes (51V and 50V) are of interest because the vanadate-vanadyl redox couple
operates across the natural range of redox conditions at Earth's surface, and has showed some utility in tracking redox processes in magmatic systems. By further developing and combining Re and V and their isotopes, there is potential for a powerful tool to track the full suite of oxidative reactions and concomitant CO2 release occuring in the present day and through geological time.