Methane is a potent greenhouse gas, whose atmospheric sources and climate feedbacks during the last deglaciation (time period ranging from 14,700 to 9,500 years ago), are not well known. Ice cores collected from Greenland demonstrate that abrupt (decadal-scale) increases in North Atlantic temperature and precipitation coincided with an abrupt rise in atmospheric methane concentration (AMC). The origin of this climate/methane coupling likely reflects a strong teleconnection between terrestrial ecosystems and abrupt climate change that is at least hemispheric in extent. While low latitude wetlands are generally accepted to be the major source of atmospheric methane during the last glacial termination, the interpolar methane gradient, an indicator of the latitudinal distribution of methane sources computed from the methane concentration difference between contemporaneous Greenland and Antarctic ice cores, suggests that a new northern high latitude source contributed up to 41% of the new global methane emissions during 11,500 to 9,500 years ago. The source of the increase in northern AMC remains the subject of much debate. Several hypotheses have been put forth to explain the rise in AMC, including the release of methane clathrates, northern peatland development, and the formation of thermokarst (permafrost thaw) lakes. Radiocarbon ages of methane in the Greenland ice cores further constrained the AMC increase to a source(es) dominated by contemporary (not radiocarbon-depleted) methane-carbon, such as wetlands.

The overarching goal of this project was to integrate paleoecology, climatology, biogeochemistry, geomorphology, and geology to understand the timing, magnitude, and contribution of previously-unaccounted-for, extensive land areas poleward of 30 °N flooded by ice-sheet melt and sea-level rise to abrupt increases in northern atmospheric methane sources during the last deglaciation. We built upon our existing syntheses of northern peatland and lake databases to include deglacial-period methane emissions from terrestrial environments that were rapidly inundated by ice-sheet melt. These include previously exposed continental shelves and terrestrial ecosystems that transitioned to coastal wetlands (including mudflats and deltas) and proglacial lakes formed by impoundment of meltwater in front of ice sheets. We hypothesized that methane emissions from these extensive, flooded terrestrial ecosystems constituted a significant feedback between sea level rise and climate through the warming effect of atmospheric methane. These sources of contemporary methane would be consistent with polar ice core records.

During the past 20,000 years, more than 2.8 million square kilometers of previously unglaciated coastal, near-shore areas of today's Bering, Chukchi, Laptev, and East Siberian seas were flooded due to the melting of the Laurentide, Cordilleran, and Eurasian ice sheets. This now-inundated shelf and land area (an area larger than Greenland), is known as ‵Beringia‵. We estimated methane emissions from coastal wetlands in Beringia for the past 20,000 years by applying modern-day emissions to the extent of reconstructed coastal wetland areas based on an extended geospatial analysis of inundated coastal areas during sea level rise. Maximum emissions (1.6-7.5 Tg CH4/year) 14,000 years ago preceded peak emissions from northern peatlands and thermokarst lakes, better aligning with polar ice core reconstructions of the northern source.

While large proglacial lakes collectively flooded millions of square kilometers in the northern hemisphere over the last deglacial period, we focused our reconstruction on potential methane emissions from a single, large proglacial lake, Lake Agassiz in North America. Combining paleorecords of sediment organic geochemistry, sediment carbon accumulation, and paleolake area and bathymetry with observations of present-day proglacial lake methane emissions (including those based on novel remote sensing approaches), we estimated that Lake Agassiz contributed 0.4-2.7 Tg CH4/year during the last deglaciation. Although poor constraints of past global proglacial lake areas and morphologies currently prevent extrapolation of our results, we suggest that these systems were likely an additional source of methane (adding to coastal wetlands, thermokarst lake and glacial lake emissions) during the last deglacial transition that require further study.

Knowledge of past methane emissions from newly inundated land areas by sea-level rise, ice-sheet melt, wetland expansion, permafrost thaw, and associated lake formation provides understanding of global climate feedbacks that will likely accelerate in arctic regions. This knowledge is critical to answering larger societal questions on the role of northern systems in global environmental change and to the validation of Earth system models needed to project future climate states. Senior scientists on the project trained and supervised a number of graduate and undergraduate students and postdoctoral scholars. Project participants gave presentations at universities and to the general public on issues of arctic lakes, coastal wetlands, permafrost, glaciers, ice sheets, and climate-change feedbacks during the last deglaciation and Holocene, and contributed to multiple peer-reviewed articles summarizing the state of knowledge and research on these topics.

Last Modified: 01/06/2025
Modified by: Katey M Walter Anthony