GSA Connects 2022 meeting in Denver, Colorado

Paper No. 235-21
Presentation Time: 9:00 AM-1:00 PM

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


ALLMAN, Lindsi and STEWART, Emily, Department of Earth, Ocean, and Atmospheric Science, Florida State University, 1011 Academic Way, Tallahassee, FL 32304, Tallahassee, FL 32310

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 d13Corg and d13Ccarb 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 d13Corg 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 d13Corg (shifting about +17‰), TOC, and wt% carbonate. Trends of lightening d13Corg 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). d13Ccarb values become heavier both below and above the sill with changes of about +3‰ and +5‰, respectively.