Wetland methane feedback during the early Eocene hyperthermals

The early Eocene was marked by a series of rapid global warming events known as hyperthermals, offering insights into the planet’s biogeochemical responses to extreme climate conditions. While increased terrestrial methane (CH₄) cycling has been recorded during the Paleocene−Eocene thermal maximum (PETM; ca. 56 Ma), investigations into terrestrial CH₄ cycle perturbations during subsequent smaller-magnitude hyperthermals have been lacking. Consequently, the question of whether all hyperthermals exhibited a common terrestrial CH₄ feedback remains unanswered. The Fushun coal seam, a recently characterized expanded wetland deposit in northeast China, preserves records of four early Eocene hyperthermals, presenting a unique opportunity to study CH₄ cycling responses to varying degrees of elevated warming. Here, we employed biomarkers to reconstruct wetland CH₄ cycling within the terrestrial succession. Our findings indicate a significant strengthening of microbial CH₄ cycling during all hyperthermals, as evidenced by a high 3β-methylhopane index and highly negative hopane δ¹³C values (<−45‰). These results suggest that wetland CH₄ emissions possibly acted as a common positive feedback mechanism during each hyperthermal event, sustaining elevated temperature levels. Furthermore, our data suggest a scaling of methane-climate feedback with the magnitudes of carbon isotope excursions, with more dramatic carbon cycle perturbations likely amplifying CH₄ cycle intensity and, consequently, CH₄ fluxes.