13C-depleted biphytanic diacids as tracers of past anaerobic oxidation of methane

13C-depleted biphytanic diacids containing 0–2 pentacyclic rings are abundant biomarkers in Cenozoic methane seep limestones. The strong 13C depletion (−132 to −96‰) suggests that they derive from euryarchaea metabolizing methane. They occur with other biomarkers typically associated with anaerobic oxidation of methane, such as crocetane, pentamethylicosane (PMI), archaeol, hydroxyarchaeol and glycerol dibiphytanyl glycerol tetraethers (GDGTs). In contrast to GDGT-derived biphytane hydrocarbons, the biphytanic diacids appear to be almost exclusively derived from methanotrophic euryarchaea.