Abstract
Impressive, several meters high tubular concretions in shallow marine calcareous sands and sandstones represent part of the well-exposed, subsurface plumbing network of an Early Eocene methane seep system in the Balkanides foreland (Pobiti Kamani area, Varna, NE Bulgaria). An integrated approach, including petrography, inorganic geochemistry and lipid biomarker analyses was used to reconstruct the evolution of pore fluids and cementation conditions during tube formation and particularly, the role of methane-related carbonate diagenesis. Host sediment lithification from marine pore waters was perturbed soon after deposition by oxidation of predominantly microbial methane causing pervasive cementation by a 13C-poor, homogeneous calcite cement ( δ 13C values as low as − 44.5‰ V-PDB). The importance of microbially mediated anaerobic oxidation of methane (AOM) is confirmed by extremely 13C-depleted archaeal biomarkers ( δ 13C values as low as − 123‰ V-PDB). A suite of macrocyclic dialkyl glycerol diethers (MDGD-0 to -2) and sn-3-hydroxyarchaeol comprises a characteristic trait of the Eocene tubular concretions and might represent molecular fossils of so far unknown methane-oxidizing archaea (ANME). Subsurface calcite cementation surrounding the ascending methane plume, resulted from the changing pore water chemistry in response to AOM and could have, on a local scale, been encouraged by the concurrent alteration of detrital feldspar. Fluctuating δ 13C (up to − 8‰ V-PDB) and δ 18O (− 0.5 to − 9‰ V-PDB) signatures within a single tubular sandstone concretion are at least partly the consequence of isotopic resetting during late meteoric water circulation.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.