Intensification of the terrestrial methane cycle during hyperthermal intervals of the Meso- and Cenozoic

Wetlands and lakes represent the largest natural source of methane to Earth’s atmosphere, where this powerful greenhouse gas influences Earth’s radiative budget. The flux of methane from wetlands and lakes to the atmosphere ultimately depends on the balance between methanogens that produce methane and methanotrophs that consume methane. However, the balance of these biological processes and hence the operation of the terrestrial methane cycle in the geological past are poorly constrained. 

To address this problem, I will present novel biomarker data that record the relative contribution of methanotrophs to the bacterial pool in ancient wetlands and lakes. I will use a unique dataset that consist of >400 samples from across the world and which span most of the Cenozoic, including key hyperthermals like the PETM and ETMs, as well as Toarcian OAE hyperthermal. The aim is to explore the operation of the terrestrial methane cycle during different climate state, including hyperthermals that are characterized by rapid environmental change. 

The data show that the contribution of methanotrophs to the terrestrial bacterial pool has been remarkably stable through time, including across major climatic events like the K/Pg boundary, the Eocene – Oligocene transition, and the mid-Miocene climatic optimum. These results indicate that the terrestrial methane cycle is robust to long-term climatic perturbations and does not operate fundamentally different during greenhouse periods. However, during hyperthermals such as the PETM and the T-OAE, etc, the data indicate a significant perturbation of the terrestrial methane cycle. This means that transient warming events have the potential to destabilize this key biogeochemical cycle, which suggests that the terrestrial methane cycle will be impacted by anthropogenic climate change.