Release and recapture: Limited carbon mobilization in sediment adjacent to Central Atlantic Magmatic Province sills, Florida, USA

Large Igneous Provinces (LIPs) are associated with dramatic changes to the Earth system; their rapid release of carbon may initiate catastrophic climatic changes causing mass extinction. This carbon release is often associated with volcanic emissions, yet metamorphic carbon production is poorly constrained. Metamorphic carbon devolatilization occurs as intrusions interact with host sediment, potentially releasing substantial sedimentary carbon. While this process has been modeled, few studies use an observational approach. Here, we apply geochemical analysis, petrographic analysis, Raman spectroscopy, vitrinite reflectance, and thermodynamic modeling to understand carbon devolatilization in sediment adjacent to Central Atlantic Magmatic Province basaltic intrusions using two boreholes that penetrate the Florida basement. We find geochemical and petrographic evidence for CO₂ release from carbonates below a 10 m thick sill, with upward fluid transport and recapture of mobilized CO₂ at the top of the sill. Ultimately, we estimate 220 mols/m² to 9900 mols/m² of carbon were mobilized from sedimentary carbonate. Uncertainties in the extent of re‑carbonation suggest that anywhere from 0 mols/m² to 7100 mols/m² may have escaped to the surface. We find little evidence of organic carbon mobility, due partly to the increased activity of CO₂ from carbonate devolatilization. Discrepancies between measured temperatures and 1D thermal models suggest complexities in heat transport through aqueous pore fluid flow and short (hundred to thousand year) heating timescales. These results highlight that further field-based study can help clarify the roles that heating timescales, fluid flow, sediment chemistry, and other factors play in releasing and capturing carbon during LIP emplacement.