Peatland climate-methane feedbacks in warming periods were governed by pH and hydrology

Abstract Methane is emitted naturally from peatlands, but controls on those emissions and their response to global warming remain debated. Previous research attempted to assess this by exploring how methane cycling responded to past warming events, with lipid biomarker signatures for methanotrophy (the 13 C-depletion of bacterially-derived hopanoids, δ 13 C hop ) revealing changes to the methane cycle during the Paleocene–Eocene Thermal Maximum (PETM). However, the lack of modern analogues has hindered interpretation of those changes. We present new δ 13 C hop data from a suite of tropical peatlands, including the discovery of exceptionally 13 C-depleted hopanoids, with values approaching those of the PETM. Across all sites, δ 13 C hop decreases with increasing pH, likely co-occurring with a shift from predominantly hydrogenotrophic to acetoclastic methanogenesis pathways. Modern sites with the lowest δ 13 C hop have elevated methane emissions, higher Type I to Type II methanotroph ratios, and higher methanogen proportions, linking hopanoid isotopic depletion to enhanced methane production and flux. These insights allow re-evaluation of geological records, with low PETM δ 13 C hop also indicating intensified peatland methane emissions. Such processes evidently acted as positive feedbacks in the past and could do so in the future – not only as a direct response to warming but indirectly via changes to peatland hydrology and geochemistry.