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1. Effects of long-term climate trends on the methane and CO2 exchange processes of Toolik Lake, Alaska

   https://doi.org/10.3389/fenvs.2022.948529  

   Eugster, Werner ; DelSontro, Tonya ; Laundre, James A. ; Dobkowski, Jason ; Shaver, Gaius R. ; Kling, George W. ( September 2022 , Frontiers in Environmental Science)

   Methane and carbon dioxide effluxes from aquatic systems in the Arctic will affect and likely amplify global change. As permafrost thaws in a warming world, more dissolved organic carbon (DOC) and greenhouse gases are produced and move from soils to surface waters where the DOC can be oxidized to CO 2 and also released to the atmosphere. Our main study objective is to measure the release of carbon to the atmosphere via effluxes of methane (CH 4 ) and carbon dioxide (CO 2 ) from Toolik Lake, a deep, dimictic, low-arctic lake in northern Alaska. By combining direct eddy covariance flux measurements with continuous gas pressure measurements in the lake surface waters, we quantified the k 600 piston velocity that controls gas flux across the air–water interface. Our measured k values for CH 4 and CO 2 were substantially above predictions from several models at low to moderate wind speeds, and only converged on model predictions at the highest wind speeds. We attribute this higher flux at low wind speeds to effects on water-side turbulence resulting from how the surrounding tundra vegetation and topography increase atmospheric turbulence considerably in this lake, above the level observed over large ocean surfaces. We combine this process-level understanding of gas exchange with the trends of a climate-relevant long-term (30 + years) meteorological data set at Toolik Lake to examine short-term variations (2015 ice-free season) and interannual variability (2010–2015 ice-free seasons) of CH 4 and CO 2 fluxes. We argue that the biological processing of DOC substrate that becomes available for decomposition as the tundra soil warms is important for understanding future trends in aquatic gas fluxes, whereas the variability and long-term trends of the physical and meteorological variables primarily affect the timing of when higher or lower than average fluxes are observed. We see no evidence suggesting that a tipping point will be reached soon to change the status of the aquatic system from gas source to sink. We estimate that changes in CH 4 and CO 2 fluxes will be constrained with a range of +30% and −10% of their current values over the next 30 years. 

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2. The Importance of Spring Mixing in Evaluating Carbon Dioxide and Methane Flux From a Small North‐Temperate Lake in Wisconsin, United States

   https://doi.org/10.1029/2021JG006537  

   Gorsky, A. L. ; Lottig, N. R. ; Stoy, P. C. ; Desai, A. R. ; Dugan, H. A. ( December 2021 , Journal of Geophysical Research: Biogeosciences)

   In limnological studies of temperate lakes, most studies of carbon dioxide (CO₂) and methane (CH₄) emissions have focused on summer measurements of gas fluxes despite the importance of shoulder seasons to annual emissions. This is especially pertinent to dimictic, small lakes that maintain anoxic conditions and turnover quickly in the spring and fall. We examined CO₂and CH₄dynamics from January to October 2020 in a small humic lake in northern Wisconsin, United States through a combination of discrete sampling and high frequency buoy and eddy covariance data collection. Eddy covariance flux towers were installed on buoys at the center of the lake while it was still frozen to continually measure CO₂and CH₄across seasons. Despite evidence for only partial turnover during the spring, there was still a notable 19‐day pulse of CH₄emissions after lake ice melted with an average daytime flux rate of 8–30 nmol CH₄m⁻²s⁻¹. Our estimate of CH₄emissions during the spring pulse was 16 mmol CH₄m⁻²compared to 22 mmol CH₄m⁻²during the stratified period from June to August. We did not observe a linear accumulation of gases under‐ice in our sampling period during the late winter, suggesting the complexity of this dynamic period and the emphasis for direct measurements throughout the ice‐covered period. The results of our study help to better understand the magnitude and timing of greenhouse gas emissions in a region expected to experience warmer winters with decreased ice duration.

    

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3. Carbon Dioxide and Methane Flux in a Dynamic Arctic Tundra Landscape: Decadal‐Scale Impacts of Ice Wedge Degradation and Stabilization

   https://doi.org/10.1029/2020GL089894  

   Wickland, K. P. ; Jorgenson, M. T. ; Koch, J. C. ; Kanevskiy, M. ; Striegl, R. G. ( November 2020 , Geophysical Research Letters)

   Ice wedge degradation is a widespread occurrence across the circumpolar Arctic causing extreme spatial heterogeneity in water distribution, vegetation, and energy balance across landscapes. These heterogeneities influence carbon dioxide (CO₂) and methane (CH₄) fluxes, yet there is little understanding of how they effect change in landscape‐level carbon (C) gas flux over time. We measured CO₂and CH₄fluxes in an area undergoing ice wedge degradation near Prudhoe Bay, Alaska, and combined with repeat imagery analysis to estimate seasonal landscape‐level C flux response to geomorphic change. Net CO₂and CH₄emissions changed by −25% and +42%, respectively, resulting in a 14% increase in seasonal CO₂‐C equivalent emissions over 69 years as ice wedge degradation formed water‐filled troughs. The dynamic ice wedge degradation/stabilization process can cause significant changes in CO₂and CH₄fluxes over time, and the integration of this process is important to forecasting landscape‐level C fluxes in permafrost regions abundant in ice wedges.

    

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4. Continuous Dynamics of Dissolved Methane Over 2 Years and its Carbon Isotopes (δ 13 C, Δ 14 C) in a Small Arctic Lake in the Mackenzie Delta

   https://doi.org/10.1029/2020JG006038  

   McIntosh Marcek, Hadley A. ; Lesack, Lance F. W. ; Orcutt, Beth N. ; Wheat, C. Geoff ; Dallimore, Scott R. ; Geeves, Kimberley ; Lapham, Laura L. ( March 2021 , Journal of Geophysical Research: Biogeosciences)

   Seasonally ice‐covered permafrost lakes in the Arctic emit methane to the atmosphere during periods of open‐water. However, processes contributing to methane cycling under‐ice have not been thoroughly addressed despite the potential for significant methane emission to the atmosphere at ice‐out. We studied annual dissolved methane dynamics within a small (0.2 ha) Mackenzie River Delta lake using sensor and water sampling packages that autonomously and continuously collected lake water samples, respectively, for two years at multiple water column depths. Lake physical and biogeochemical properties (temperature; light; concentrations of dissolved oxygen, manganese, iron, and dissolved methane, including stable carbon, and radiocarbon isotopes) revealed annual patterns. Dissolved methane concentrations increase under‐ice after electron acceptors (oxygen, manganese, and iron oxides) are depleted or inaccessible from the water column. The radiocarbon age of dissolved methane suggests a source from recently decomposed carbon as opposed to thawed ancient permafrost. Sources of dissolved methane under‐ice include a diffusive flux from the sediments and may include water column methanogenesis and/or under‐ice hydrodynamic controls. Following ice‐out, the water column only partially mixes allowing half of the winter‐derived dissolved methane to be microbially oxidized. Despite oxidation at depth, surface water was a source of methane to the atmosphere. The greatest diffusive fluxes to the atmosphere occurred following ice‐out (75 mmol CH₄m⁻² d⁻¹) and during a mixing episode in mid‐July, likely driven by a storm event. This study demonstrates the importance of fine‐scale temporal sampling to understand dissolved methane processes in seasonally ice‐covered lakes.

    

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5. Radiative forcing of methane fluxes offsets net carbon dioxide uptake for a tropical flooded forest

   https://doi.org/10.1111/gcb.14615  

   Dalmagro, Higo J. ; Zanella de Arruda, Paulo H. ; Vourlitis, George L. ; Lathuillière, Michael J. ; de S. Nogueira, José ; Couto, Eduardo G. ; Johnson, Mark S. ( April 2019 , Global Change Biology)

   Wetlands are important sources of methane (CH₄) and sinks of carbon dioxide (CO₂). However, little is known about CH₄and CO₂fluxes and dynamics of seasonally flooded tropical forests of South America in relation to local carbon (C) balances and atmospheric exchange. We measured net ecosystem fluxes of CH₄and CO₂in the Pantanal over 2014–2017 using tower‐based eddy covariance along with C measurements in soil, biomass and water. Our data indicate that seasonally flooded tropical forests are potentially large sinks for CO₂but strong sources of CH₄, particularly during inundation when reducing conditions in soils increase CH₄production and limit CO₂release. During inundation when soils were anaerobic, the flooded forest emitted 0.11 ± 0.002 g CH₄‐C m⁻² d⁻¹and absorbed 1.6 ± 0.2 g CO₂‐C m⁻² d⁻¹(mean ± 95% confidence interval for the entire study period). Following the recession of floodwaters, soils rapidly became aerobic and CH₄emissions decreased significantly (0.002 ± 0.001 g CH₄‐C m⁻² d⁻¹) but remained a net source, while the net CO₂flux flipped from being a net sink during anaerobic periods to acting as a source during aerobic periods. CH₄fluxes were 50 times higher in the wet season; DOC was a minor component in the net ecosystem carbon balance. Daily fluxes of CO₂and CH₄were similar in all years for each season, but annual net fluxes varied primarily in relation to flood duration. While the ecosystem was a net C sink on an annual basis (absorbing 218 g C m⁻²(as CH₄‐C + CO₂‐C) in anaerobic phases and emitting 76 g C m⁻²in aerobic phases), high CH₄effluxes during the anaerobic flooded phase and modest CH₄effluxes during the aerobic phase indicate that seasonally flooded tropical forests can be a net source of radiative forcings on an annual basis, thus acting as an amplifying feedback on global warming.

    

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