Wetland methane feedback during the early Eocene hyperthermals

Abstract 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.