Fig. 6
Hydrocarbon gas composition plotted in a modified Bernard diagram (C1/(C2 + C3) vs. δ13CC1 in ‰ V-PDB)41 indicates a thermogenic hydrocarbon source. The data plots along a mixing line (dashed line) of thermogenic and microbially derived methane (10% increments if not labelled otherwise). Endmembers are a headspace sample extracted from a pushcore containing residual CH4 hydrates within the Karambusel vent field (SO299 157_ROV-16) and from a Niskin bottle collected at a ~ 30 °C warm vent at the southern flank of Karambusel (SO299 026_ROV-nis2), and a seep sample from the Guaymas Basin (SO241 047_GC-700)39. Note that for Mussel Cliff only samples from below the sulphate-methane transition zone (i.e., where CH4 is least affected by microbial processes) are included.
