TRACKING CARBON PRODUCTION IN SEDIMENTS DURINGEMPLACEMENT OF THE CENTRAL ATLANTIC MAGMATIC PROVINCE, FLORIDA

Large Igneous Provinces (LIPs) can cause dramatic changes to the Earth system. During the emplacement of LIPs, igneous sills intrude into surrounding sediment and initiate the release of methane and CO₂. In a high-grade metamorphic contact aureole, decarbonation may release nearly 100% of the sedimentary carbon. This flux of carbon to the Earth’s surface has been linked to environmental change, carbon cycle volatility, and mass extinction (Ganino & Arndt 2009; Davies et al. 2017; Heimdal et al., 2020, 2021). While relatively few studies use an observation-based approach to investigate carbon mobility, this project uses direct observation to quantify and track the release of methane and CO₂ from sediment adjacent to sills of the Central Atlantic Magmatic Province (CAMP) in Florida basement rock. Isotope measurements and petrographic analysis are used to track the flow of carbon, oxygen, and hydrogen rich fluids throughout the sample suite. This will ultimately give insight into carbon movement within sediments heated by CAMP sills, enabling the estimation of the net carbon flux to the surface environment and re-capture in overlying sediment. Preliminary results focus on a core drilled in the North Florida basement, Florida Geological Survey borehole W1789, that is comprised of organic-rich black shales with intruding basaltic sills. Isotopic measurements of d¹³C_org and d¹³C_carb as well as measurement of percentage of calcite and total organic carbon (TOC) content were made throughout the core, targeting areas near sills. The data show d¹³C_org values becoming isotopically lighter with decreasing TOC moving down toward the sill, until the rocks directly above (within < 1 m) show an opposite trend of increasing d¹³C_org (shifting about +17‰), TOC, and wt% carbonate. Trends of lightening d¹³C_org below and > 1 m above the sill imply the release of predominantly CO₂ gas instead of methane. There is an asymmetry in carbon capture evident at the top of each sill, shown by an increasing percentage of carbonate carbon and organic carbon in the samples just above each sill but not below. The high C content suggests that at least some of this carbon was added to both sedimentary and igneous precursors (i.e., it is not simply residual). d¹³C_carb values become heavier both below and above the sill with changes of about +3‰ and +5‰, respectively.