The operation of the terrestrial methane cycle during the Cenozoic and Mesozoic, including hyperthermals such as the PETM and T-OAE

The terrestrial realm is the largest natural source of methane to Earth’s atmosphere, where this powerful greenhouse gas influences Earth’s radiative budget. The terrestrial methane cycle is therefore a key biologically mediated process with the flux of methane to the atmosphere ultimately depending on the balance between methanogens that produce methane and methanotrophs that consume methane in wetlands and lakes. However, these 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 a large number of ancient wetlands and lakes. I will use a unique dataset that consist of 100s of samples from across the world and which span most of the Cenozoic, as well as Toarcian OAE. The aim is to explore the past operation of the terrestrial methane cycle, including during periods of rapid environmental change.

The data show that the contribution of methanotrophs to the terrestrial bacterial pool has been remarkable 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 transient warming events, e.g. the Paleocene Eocene Thermal Maximum (PETM), 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 might have implications for how the terrestrial methane cycle will respond to anthropogenic climate change.