Formation of methane-related authigenic carbonates within the bioturbated zone — An example from the upwelling area off Vietnam

In the biologically high-productive area off Vietnam authigenic carbonates formed in the bioturbated zone while methane was oxidized microbially under anaerobic conditions. Open crustacean burrows (Spongeliomorpha) connected to the seafloor acted as conduits for methane as suggested by intense cementation of burrow walls having a light C-isotope signature. Although seawater circulated within these open, inhabited tubes, conditions favorable for anaerobic oxidation of methane (AOM) occurred < 1 mm away from the tubes. Trace fossils document a gradual stiffening and induration of the material finally reaching hardground condition (indicated by Trypanites). Cementation started at several spots that coalesced later to form the nodule. Carbon isotopes indicate a biogenic methane source (δ13Ccarb reaching − 49 to − 40‰ V-PDB), while higher δ13C values imply mixing with carbon from other sources that is very likely within the bioturbated zone. The cement is generally fine grained (< 5 μm); calcite forms about 3/4 of the cement, dolomite about 1/4, and aragonite a minor proportion. High δ18O values of + 5 ± 1‰ reflect the general 18O-enriched isotope signature of the bottom water in South China Sea especially during glacial times, but influence of water released from decaying gas hydrate cannot be excluded. The studied nodule was found on top of a slump that displaced it.