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.

The research goals for this project were to integrate paleoecology, biogeochemistry, geomorphology, and geology to understand the timing, magnitude and overall contribution of northern cold-climate peatlands and lakes to rapid increases in AMC during the last deglaciation. In order to achieve this goal, we first synthesized new and existing peatland, thermokarst-lake, kettle-lake and other post-glacial lake initiation data and methane and carbon cycling data. Second, in order to get the most complete picture of the peatland/thermokarst-lake forcing on climate, it was important to consider the currently inundated shelf area around the continents that was exposed during the last glacial maximum lowstand and likely hosted additional peatlands and lakes. This was done through a combination of analysis of spatial datasets as well as compiling core data from published records.

We compiled new databases (1207 lake records and 1063 buried peatland records) of lake and peatland locations, origins, landscape attributes, and past climate patterns to examine both spatial and temporal patterns of lake and peatland formation across the northern high latitudes with the goal of identifying major controls over and drivers of these ecosystems’ formation since the last deglaciation and their carbon cycling feedbacks to climate. Quantitative agreement between modeling and observations showed (1) peak pan-Arctic lake methane emissions preceded peak peatland emissions but lagged abrupt deglacial increases in AMC; (2) peatland and lakes formed on the extensive continental shelves likely contributed to the abrupt increase, but emission estimates have high uncertainties due to few paleorecords for the now inundated region; (3) Regional trends in the timing of permafrost aggradation and thaw and associated methane emissions in North American and Eurasian peatland ecosystems can be inferred based on plant macrofossils and peat properties using peat core records; and (4) northern lake and peatland carbon sequestration followed strong methane emissions in these evolving systems, serving as a mechanism of climate cooling during the Holocene. Finally, (5) our analysis of paleoenvironmental and present-day records indicates that the permafrost carbon feedback from methane associated with thermokarst-lake development was likely larger during the early Holocene when higher atmospheric temperatures in the Arctic [1.6 ± 0.8 ° C warmer than present during the Holocene Thermal Maximum, 11,000 to 9,000 years ago] combined with initially less physical protection of permafrost soils by vegetation and lower-relief landscapes, culminated in more thermokarst activity than at present. However, the permafrost carbon feedback has potential to be much larger in the future. Terrestrial Arctic warming of 4 to 6 °C (RCP4.5) and >7 °C (RCP8.5) projected to occur this century will be unprecedented for the Holocene and is anticipated to accelerate gross lake area growth and the permafrost carbon feedback.

Knowledge of past methane emissions from peatland 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, peatlands, permafrost 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: 08/30/2018
Modified by: Katey M Walter