Date/heure
Date(s) - 24 janvier 2023
13 h 00 min - 14 h 00 min
Catégories
Lithology and Tectonics: Building the inorganic carbon budget of a young mountain range
Erica Erlanger, Aaron Bufe, Guillaume Paris, Ilenia D’Angeli, Luca Pisani, Preston Kemeny, Jessica Stammeier, Negar Haghipour, Niels Hovius
Résumé: Mountain building has classically been linked with CO2 drawdown from chemical weathering of silicate minerals in the critical zone, although recent views on mountain building recognize the importance of rock-derived CO2 emissions from the weathering of petrogenic organic carbon and inorganic carbonate. However, the focus on near-surface weathering reactions within the orogenic carbon factory does not consider the emission of metamorphic CO2 from subduction processes in the deep crust and mantle. Such deep carbon sources could dwarf the surficial drawdown and release of carbon, in particular in actively extending mountain ranges that subduct large volumes of carbonate rock. Thus, accounting for weathering processes at depth and in the critical zone in parallel is crucial to fully assess how mountain-range uplift impacts the carbon cycle. Here, we quantify the exchange of CO2 between rock and the atmosphere from subduction-related processes and from critical zone weathering reactions in two major river systems in the central Apennine Mountains of Italy. The catchments straddle a geodynamic gradient across the subduction zone that is expressed as changes in surface heat flow and crustal thickness, whereas climatic boundary conditions are relatively constant. At the regional scale, we find that metamorphic CO2 sources outpace critical zone inorganic carbon sources and sinks by 2 orders of magnitude above a window in the subducting slab that is characterized by high heat flow and low crustal thickness, and could have driven efficient degassing over the last 2 Ma. In contrast, surficial weathering processes dominate the carbon budget where crustal thickness is greater and heat flow is lower. Importantly, the difference in metamorphic degassing fluxes across the geodynamic gradient is multiple orders of magnitude larger than the difference in critical zone weathering fluxes. Thus, modulations of metamorphic decarbonation reactions are the most efficient process by which tectonics can regulate the inorganic carbon cycle in the Apennines. Both near-surface and deep sources of CO2 must be considered when constructing the carbon budget of orogenic systems that include the subduction of carbonate rock.