CARBON AND SULFUR CYCLING IN CRETACEOUS COLD SEEPS RECORDED IN THE PARAGENESIS AND GEOCHEMISTRY OF DIAGENETIC CARBONATES

The Teepee Buttes in Colorado comprise a network of Late Cretaceous fossiliferous carbonate accumulations hosted by the Pierre Shale. An ancient methane seep origin for the formation of these features along pronounced structural trends in the Western Interior Seaway is well established by previous carbonate-carbon isotope data, which show values as light as ca. -50 per mil, and organic biomakers that point to extensive aerobic and anaerobic methanotrophy. For the first time, we have expanded the analytical palette applied to the buttes to include a detailed isotope study of sulfur preserved as carbonate-associated sulfate within the diagenetic carbonates, including a comparison of concretionary masses within and beyond the influence of the seeps. Our goal is to constrain the principal loci and intensity trends for bacterial sulfate reduction as products of the syntrophic consortium that mediates anaerobic oxidation of methane and as calibrated through our studies of modern seeps in the Gulf of Mexico. We also expect to delineate micro- and macrofaunal symbiotic linkages driven by chemosynthetic (sulfide oxidizing) bacterial communities and, through analysis of shell material, to identify these activities within specific organisms, such as lucinid bivalves. Consistent with the benthic faunal populations, C-S-Fe analysis of host shales points to oxygen in the bottom waters.

As a backdrop for our systematic macrofaunal paleoecology and related study of syntrophic C and S cycling, our work expands on previous efforts to include a detailed paragenetic deconstruction of the carbonate fabrics, which is aided by extensive high-resolution C isotope and trace element analysis. The C-O isotope work and elemental analyses are delineating early formed carbonates characterized by depleted C isotopes, comparatively heavier O, lower Fe contents, and higher Sr and Mg. Although there is some overlap, phases linked to later diagenetic fluids show heavier and lighter C and O isotope properties, respectively, and similarly opposite Fe, Sr, and Mg behavior. Botryoidal cements, pelsparites, and yellow calcite dominate the early diagenetic forms. Blocky white sparry cements formed later, and micrite is dominantly a product of later micritization of early fabrics and correspondingly shows broad geochemical properties.