Terrestrial methane cycle perturbations during the onset of the Paleocene-Eocene Thermal Maximum

Terrestrial methane (CH₄) emissions may have increased during the Paleocene-Eocene Thermal Maximum (PETM; ca. 56 Ma) and promoted additional warming, especially in the high latitudes. Although there is evidence for increased CH₄ cycling in a single Northern Hemisphere site, whether enhanced methane cycling was globally widespread is unknown because there have been no subsequent investigations. The mechanism of CH₄ release is also unknown because a direct comparison between temperature and CH₄ cycling has so far not been possible. Here we use biomarkers to reconstruct temperature change and CH₄ cycling in a new PETM-aged succession in New Zealand. Our results indicate that the stable carbon isotopic composition (δ¹³C) of bacterial hopanoids decreased to very low values (–60‰) during the onset of the PETM, indicating enhanced consumption of CH₄. These values are much lower than found in modern wetlands and suggest a major perturbation of the CH₄ cycle during the onset of the PETM. Low hopanoid δ¹³C values do not persist into the early Eocene, despite evidence for elevated temperatures. This indicates that the terrestrial CH₄ cycle operates differently during transient compared to gradual warming events. Enhanced CH₄ cycling during the PETM may help to resolve the temperature data-model mismatch in the high latitudes and could yield higher estimates of Earth system sensitivity than expected from CO₂ alone.