METAMORPHIC CO2 EMISSIONS OUTPACE CO2 DRAWDOWN FROM SILICATE WEATHERING IN A YOUNG, CARBONATE MOUNTAIN BELT

Chemical weathering at the Earth’s surface can sequester atmospheric CO₂ into the rock record by weathering silicate minerals, or release CO₂ into the atmosphere through organic carbon oxidation and sulfuric acid weathering of carbonates. Recently, a number of datasets have characterized the balance of carbon sources and sinks from rock weathering in mountain ranges. The emerging view of the global carbon cycle is that weathering in mountain ranges composed of sedimentary or metamorphic rocks are considered to be a carbon source, in the absence of significant biospheric organic carbon burial. However, most carbon balances do not consider deep weathering sources associated with subduction processes. Studies from different mountain ranges (e.g. Himalaya, Northern American Cordillera) suggest that CO₂ degassing from metamorphic decarbonation may outpace CO₂ consumption by chemical weathering near the Earth’s surface. For global models to account for the effects of metamorphic CO₂ degassing within the full carbon balance, further case studies are required.

We present a case study from the Central Apennine Mountains, an active mountain range that exposes both carbonate and mixed carbonate-siliciclastic lithologies. We assess the surficial and deep sources of dissolved solutes and isotopes from two major rivers and their tributaries in the Central Apennines, and estimate their impact on the regional inorganic carbon budget. With an inverse modeling approach, we constrain the lithologies being weathered and apportion the sources of acid within the landscape. Our regional-scale estimates of inorganic carbon exchange with the atmosphere from 1) silicate weathering, 2) sulfuric acid weathering of carbonates, and 3) metamorphic CO₂ degassing reveal that metamorphic CO₂ is an important, regional source of rock-derived carbon in Central Apennine rivers and that CO₂ release from metamorphic decarbonation reactions outpace CO₂ drawdown from silicate weathering. In contrast, sulfuric acid weathering is a relatively minor source of rock-derived CO₂ in this landscape. The results of this study suggest that both near-surface and deep sources of CO₂ must be considered when constructing the inorganic carbon budget of an active orogen, and that metamorphic CO₂ should be accounted for in global carbon cycle models.