Lipid Biomarkers and Carbon Isotopic Composition from Authigenic Carbonates and Seep Sediments from the US Mid-Atlantic Margin

Abstract:

Anaerobic oxidation of methane (AOM), mediated by Archaea and sulfate-reducing bacteria, is common in continental margin sediment and can result in authigenic carbonate precipitation. A lipid biomarker study was undertaken in Mid-Atlantic submarine canyons, focusing specifically on Baltimore and Norfolk canyons, to determine biomarker variability of carbonate rock and the associated sediment in cold seep communities dominated by chemosynthetic mussels, Bathymodiolus childressi. Preliminary 16S metagenomic results confirm the presence of free-living sulfur-reducing bacteria and methantrophic endosymbiotic bacteria in the mussels. Depleted d¹³C values in both the mussel tissue (-63 ‰) and authigenic carbonates (-48 ‰) support methanotrophy as the dominant nutritional pathway and AOM as the main driver of carbonate precipitation. In addition, paired ¹⁴C and ²³⁰Th dates are highly discordant, reflecting dilution of the ¹⁴C pool with fossil hydrocarbon derived carbon. Seep and canyon sediment, as well as authigenic carbonates, were collected and analyzed for a suite of biomarkers, including sterols, alcohols, alkanes and fatty acids, as well as δ¹³C values of select biomarkers, to elucidate pathways of organic matter cycling. A comparison of terrestrial biomarker signatures (e.g., n-alkane carbon preference index and C₂₃ / (C₂₃ + C₂₉) values, HMW n-alkanes and C29 sterols) suggests that terrestrial inputs dominate the submarine canyon surface sediment, whereas seep sediment is predominantly marine autochthonous (i.e., cholesterol and 5α-cholestanol). Lipid biomarker profiles (e.g., n-alkanes in the C₁₅ to C₃₃ range) from authigenic carbonates mirror those found in the seep sediment, suggesting that the organisms mediating carbonate precipitation on the seafloor are characteristic of the assemblages present in the sediment at these sites. With widespread methane leakage recently discovered along the Atlantic Margin, the presence of AOM-mediated carbonate precipitation documented in Baltimore and Norfolk canyons suggests that AOM may play a key role in long-term storage of methane and the carbon cycle along the Atlantic Margin.