HIGH-LATITUDE WETLANDS AS A SOURCE OF METHANE AND DRIVER OF SHORT- AND LONG-TERM CLIMATE CHANGE IN THE EOCENE GREENHOUSE WORLD

Although the greenhouse world of the Eocene lacked extensive ice sheets, there is ample evidence that climate and the carbon cycle varied regularly in response to short-term orbital forcings, with some changes being quite large (e.g. 'hyperthermals'). What remains uncertain is how small changes in solar insolation that are the basis of these forcings were amplified to have a global impact in an ice-free world. Using a combination of carbon isotope data from authigenic carbonates (provides evidence for methane production took place in Eocene) and incubation experiments using leaf litter (allows estimates of methane production to be made), we suggest that temperature-dependent methane emission from leaf litter at the surface of high-latitude wetlands provides a means of amplifying small temperature changes, and of modifying the carbon budget and carbon isotope ratio of earth surface reservoirs. When methane production rates are scaled to the landscape level, mass-balance modeling indicates that Eocene temperature and carbon isotope changes can be accounted for at orbital timescales, thus supporting a wetland-methane-temperature feedback hypothesis.

The Eocene is also characterized by long-term warming leading up to the Early Eocene Climatic Optimum (EECO), and gradual cooling from the EECO to the early Oligocene. It is possible that such temperature changes could also be driven in part by variable methane emissions from high-latitude +/- other wetlands, with the areal extent of these wetlands playing the critical role as a feedback mechanism. The expectation would be wetland expansion associated with increasing global temperatures, and wetland contraction associated with decreasing temperatures. Although present-day ice cover makes it difficult to map changes in the extent of high-latitude wetlands in detail, a general decrease can be inferred from the EECO to the early Oigocene.

Overall this research suggests an intriguing possibility: that methane production in high-latitude wetlands of the greenhouse played a feedback role in the climate system that is analogous to that played by high-latitude ice sheets of the present-day icehouse. Ongoing study of this possibility may lead to a better understanding of the Eocene as well as a better understanding of what a warmer, future world may look like.