Investigating past methane cycle perturbations through the lens of novel polyfunctionalised hopanoids

Abstract

Enhanced aerobic methanotrophy has been inferred in wetland environments during past warm climates, notably the Paleocene-Eocene Thermal Maximum (PETM; ∼56 Ma). This is based on a negative carbon isotope excursion in bacterial-derived hopanes (d13Chop). However, these compounds have multiple source organisms and their d13Chop can also reflect changes in the metabolic pathway used to assimilate carbon. In contrast, bacteriohopanepolyols (BHPs) are polyfunctionalised hopanoids that may be a more diagnostic tracer of methane oxidation, but their preservation potential is unclear. In this study, we aim to: (i) determine the preservation of novel BHPs during the PETM; and (ii) explore the potential of BHPs as a proxy to reconstruct past methane cycle dynamics. Overall, our study presents the most diverse bacteriohopanepolyol profile found thus far in sedimentary deposits beyond 50 million years ago, suggesting excellent preservation. Interestingly, a peak in 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol) coincides with the most negative d13Chop value during the PETM. An increase in aminopentol during the PETM was also previously observed, further supporting the utility of aminopentol as a proxy for methane cycling during transient warming events.

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Identifiers

ORCID for E.H. Hollingsworth: orcid.org/0000-0002-7067-6837

ORCID for G.N. Inglis: orcid.org/0000-0002-0032-4668

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