B095-0006
Influence of anaerobic oxidation of methane on the precipitation of the seep carbonates of the Moreno Formation, Panoche Hills, California

Tuesday, 15 December 2020
Poster
Daniela Osorio Rodriguez1, Kalen Rasmussen2, Anaïs Roussel3, Camila Areias de Oliveira4, Miquela Ingalls5, Woodward W Fischer6, Victoria J Orphan1, Alex L Sessions1 and Russell Scott Shapiro7, (1)California Institute of Technology, Pasadena, CA, United States, (2)Colorado School of Mines, Civil and Environmental Engineering, Golden, CO, United States, (3)Georgetown University, Washington, DC, United States, (4)Universidade Federal Fluminense, Department of Geochemistry, Niteroi, Brazil, (5)Penn State University, Department of Geosciences, State College, PA, United States, (6)Caltech, Pasadena, CA, United States, (7)California State University Chico, Chico, CA, United States
Abstract:
The Moreno Formation (Paleocene, Danian) preserves an ancient methane seep system that formed within a forearc basin and now is exposed in the Panoche Hills, Diablo Range. Here we present petrographic observations in suite with carbonate carbon (δ13C), oxygen (δ18O) , and clumped (∆47) isotope analyses, in order to (1) constrain the formation chemistry and early diagenetic history of the Moreno seep carbonates, and (2) determine the influence of anaerobic oxidation of methane (AOM) on carbonate precipitation. A paragenetic sequence was developed based on the relationships of different petrographic fabrics and phases. δ18O values are indicative of minimal meteoric influence, and thus, the δ13C and ∆47 values likely retain information about the primary and early diagenetic chemistry. Together, they suggest that micrite formed from mixed marine and methane dissolved inorganic carbon (DIC) sources. Early diagenetic cements best record methane sourced DIC and other fabrics show a progressive marine influence. Additionally, ∆47 clumped-isotope thermometry shows that the Moreno seep carbonates formed out of equilibrium with respect to their ambient temperatures, and a ∆47 mixing model provides potential insight into AOM-related carbonate precipitation behavior. A kinetic isotope effect associated with precipitation from an AOM system yields ∆47 disequilibrium values toward unrealistically warm temperatures (low ∆47). Finally, δ13C of bulk organic matter suggests that preserved organic carbon is derived primarily from photosynthesis rather than methanotrophs. Overall, we show that the pairing of the aforementioned techniques is critical to unveil the unique paragenetic history of methane seep carbonates. Isotopic fingerprints of the Moreno Formation seeps imply that AOM-derived DIC was incorporated in carbonates, and the observed phases originated by biological and chemical processes.
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