Collaborative Research: Towards a mechanistic prediction of methane ebullition fluxes from northern peatlands

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Peatlands cover only about 3% of the Earth's land area, but are disproportionately important in producing methane, a strong greenhouse gas. Peatlands yield an estimated 5-10% of all methane to the atmosphere and are also recognized as an important reservoir in the global carbon cycle, accounting for about 33% of the global soil carbon. This project serves the national interest and NSF's mission by promoting the progress of scientific understanding of the mechanisms that regulate the release of this potent greenhouse gas to the atmosphere. The fundamental issue addressed by this project relates to the mechanisms and hydrological factors that regulate the sudden (episodic) release of gaseous methane to the atmosphere. These releases are driven by changes in water level and atmospheric pressure that encourages upward bubble transport/release of methane. This project will generate new understanding of how changes in water level, atmospheric pressure and peat fabric lead to sudden releases of methane that far exceed previously held theories on methane release. Although the project is focused on a boreal peatland it will impact our understanding of methane dynamics in other climates, including sub-tropical systems such as the Everglades, and Artic systems. The project includes summer research experiences for the participation of minority students in field geoscience research. Two full-time graduate students and one postdoctoral scientist will be involved this project. Results of the work will be disseminated through student led presentations at national/international meetings and articles submitted to international journals.The contribution of peatlands to the atmospheric CH4 burden remains unclear in large part due to incomplete understanding of the ebullition pathway. Oxidation of dissolved methane reduces the release of methane by diffusion, but the transit time of bubbles released via ebullition is too short for extensive oxidation to occur, i.e. ebullition releases increase the greenhouse gas potential of peatlands. This project will advance understanding of ebullition by coupling new, innovative measurement strategies to physical model development. This integration of measurement and modeling will permit a fundamental step forward towards a more quantitative understanding of CH4 ebullition from peatlands. Two hypotheses will be tested: H1: The frequency and size of ebullition events from peatlands can be predicted from CH4 production rates and pressure changes within the peat column when measurable properties related to the peat structure (and strength) are incorporated into model fitting parameters; H2: Ebullition from the peatland is regulated by a threshold related to the accumulated gas volume, measurable physical properties related to the peat strength, and how gas coalesces within the peat column (e.g. dispersed bubbles versus a few large 'bubbles'). Measurements will be performed in Caribou Bog, a multi-unit peatland located in Maine. Volumetric gas content will be monitored using ground penetrating radar, whereas ebullition fluxes will be monitored using a combination of acoustic ebullition sensors, time-lapse imaging of gas traps, hydraulic heads in piezometers and chamber measurements using a fast methane analyzer. Pore water CH4 samples will be acquired using mini piezometer nests. An existing ebullition model describing gas bubble expansion will be combined with an invasion percolation approach to describe the transport of CH4 between multiple peat layers by both diffusion in the pore water and ebullition between layers. Although the proposed model does not explicitly incorporate the geomechanical properties of peat, model predictions for maximum gas contents will be compared with key measurable geomechanical properties that may control ebullition.