Bacteriohopanepolyols reveal a methane cycle perturbation during the paleocene-eocene thermal maximum

Methane emissions from wetlands have been inferred to increase during past ‘hothouse’ states, notably the Paleocene-Eocene Thermal Maximum (PETM; ∼56 Ma), likely amplifying global warming. Evidence was based upon the stable carbon isotopic composition of bacterial-derived hopanes and hopenes (δ 13 C hop ), and a shift towards lower values (−50 to −76 ‰) in multiple PETM-aged lignites. However, δ 13 C hop could reflect a mixture of sources and/or changes in the metabolic pathway used by methanotrophs to assimilate carbon. In contrast, bacteriohopanepolyols (BHPs) are more diagnostic, but their preservation potential in pre-Quaternary-aged sediments remains poorly constrained. Here, we determine the preservation of BHPs during the PETM and explore their utility as a proxy to reconstruct past methane cycle dynamics. To achieve this, we examine the presence of BHPs in a mixed terrestrial and marine sedimentary sequence from New Zealand. We identify 16 BHPs, 6 of which are reported for the first time in deposits as old as the PETM. The prevalence of complex BHPs, such as bacteriohopane-31,32,33,34-tetrol cyclitol ether (BHT-CE), reflects their exceptional preservation. Although this appears to be selective, varying between depositional environments. Independent of the preservation state, the relative abundance of a BHP associated with Type I methanotrophs, 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), increases during the PETM. This coincides with the most negative δ 13 C hop value, supporting the utility of aminopentol as a proxy for aerobic methanotrophy . Overall, our study presents the greatest diversity of BHPs found thus far in deposits older than ∼ 50 Ma and demonstrates the opportunity to apply BHPs to unravel past changes in the methane cycle.