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Carbon trace gas dynamics in subarctic lakes
Stockholm University, Faculty of Science, Department of Geological Sciences.ORCID iD: 0000-0001-5965-7662
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Northern lakes are important sources of greenhouse gases carbon dioxide and methane to the atmosphere. Emissions are expected to increase as the climate continues to warm. Even so, lake carbon budgets are currently poorly constrained. This is in part because of a limited understanding of the processes that govern the flux. This thesis focuses on the physical and biogeochemical drivers of carbon trace gas emissions from three small, post-glacial lakes situated within the Stordalen Mire, a subarctic peatland underlain by thawing permafrost in northern Sweden. A unique, multiyear dataset is used to quantify the importance of different emission pathways – ebullition, turbulence-driven diffusion and release from storage – on short and long timescales. In summer and on seasonal to interannual timescales, emissions are robust functions of thermal energy input. Short-term storage-and-release cycles are governed by kinetic drivers, such as turbulence fuelled by wind shear and, to a lesser extent, by thermal convection. In winter, when the lakes are ice-covered, persistent anoxia and density-driven currents enable methane accumulation at rates exceeding summer emissions. Release at ice-off in spring can constitute the majority of annual methane emissions and scales predictably with ice-cover season length, except in warm winters when snowmelt displaces lake water. Most lake flux studies focus on the warmest summer months and omit the spring efflux, as well as emissions in the colder ice-free months which, because of the well-known temperature-dependency of carbon cycling processes, tend to be low. The latter sampling bias may lead to a substantial overestimation of the ice-free flux in regional and global lake emission budgets. Temperature proxies, potentially combined with gas transfer models, can efficiently gap-fill colder months to arrive at a more representative flux estimate, but important feedbacks, such as lake degassing with increasing wind speed, must be taken into account. The mechanisms emerging from intense study of the Stordalen lakes are likely to be found in a majority of northern lakes, which are small, seasonally ice-covered and of post-glacial origin. However, because gas transfer velocity and temperature sensitivity are spatiotemporally variable, field observations remain essential for the development and calibration of models, and to predict future emissions.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University , 2020. , p. 56
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper ; 379
Keywords [en]
lakes, methane, carbon dioxide, fluxes, gas transfer, proxy, climate change
National Category
Geochemistry
Research subject
Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-176269ISBN: 978-91-7797-945-6 (print)ISBN: 978-91-7797-946-3 (electronic)OAI: oai:DiVA.org:su-176269DiVA, id: diva2:1373486
Public defence
2020-01-22, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Manuscript.

Available from: 2019-12-19 Created: 2019-11-27 Last updated: 2019-12-18Bibliographically approved
List of papers
1. Climate‐Sensitive Controls on Large Spring Emissions of CH4 and CO2 From Northern Lakes
Open this publication in new window or tab >>Climate‐Sensitive Controls on Large Spring Emissions of CH4 and CO2 From Northern Lakes
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2019 (English)In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 124, no 7, p. 2379-2399Article in journal (Refereed) Published
Abstract [en]

Northern lakes are important sources of the climate forcing trace gases methane (CH4) and carbon dioxide (CO2). A substantial portion of lakes' annual emissions can take place immediately after ice melt in spring. The drivers of these fluxes are neither well constrained nor fully understood. We present a detailed carbon gas budget for three subarctic lakes, using 6 years of eddy covariance and 9 years of manual flux measurements. We combine measurements of temperature, dissolved oxygen, and CH4 stable isotopologues to quantify functional relationships between carbon gas production and conversion, energy inputs, and the redox regime. Spring emissions were regulated by the availability of oxygen in winter, rather than temperature as during ice‐free conditions. Under‐ice storage increased predictably with ice‐cover duration, and CH4 accumulation rates (25 ± 2 mg CH4‐C·m−2·day−1) exceeded summer emissions (19 ± 1 mg CH4‐C·m−2·day−1). The seasonally ice‐covered lakes emitted 26–59% of the annual CH4 flux and 15–30% of the annual CO2 flux at ice‐off. Reduced spring emissions were associated with winter snowmelt events, which can transport water downstream and oxygenate the water column. Stable isotopes indicate that 64–96% of accumulated CH4 escaped oxidation, implying that a considerable portion of the dissolved gases produced over winter may evade to the atmosphere.

National Category
Geosciences, Multidisciplinary
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-176144 (URN)10.1029/2019JG005094 (DOI)
Funder
EU, FP7, Seventh Framework Programme, 282700Swedish Research Council, 2013‐5562Swedish Research Council, 2007‐4547
Available from: 2019-11-25 Created: 2019-11-25 Last updated: 2019-12-18Bibliographically approved
2. Drivers of diffusive lake CH4 emissions on daily to multi-year time scales
Open this publication in new window or tab >>Drivers of diffusive lake CH4 emissions on daily to multi-year time scales
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(English)In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189Article in journal (Other academic) Submitted
Abstract [en]

Lakes and reservoirs are important emitters of climate forcing trace gases. Various environmental drivers of the flux, such as temperature and wind speed, have been identified, but their relative importance remains poorly understood. Here we use an extensive field dataset to disentangle physical and biogeochemical controls on the turbulence-driven diffusive flux of methane (CH4) on daily to multi-year timescales. We compare 8 years of floating chamber fluxes from three small, shallow subarctic lakes (2010–2017, n = 1306) with fluxes computed using 9 years of surface water concentration measurements (2009–2017, n = 606) and a small-eddy surface renewal model informed by in situ meteorological observations. Chamber fluxes averaged 6.9 ± 0.3 mg m−2 d−1 and gas transfer velocities (k600) from the chamber-calibrated surface renewal model averaged 4.0 ± 0.1 cm h−1. We find robust (R2 ≥ 0.93, p < 0.01) Arrhenius-type temperature functions of the CH4 flux (Ea' = 0.90 ± 0.14 eV) and of the surface CH4 concentration (Ea' = 0.88 ± 0.09 eV). Chamber derived gas transfer velocities tracked the power-law wind speed relation of the model (k ∝ u3/4). While the flux increased with wind speed, during storm events (U10 ≥ 6.5 m s−1) emissions were reduced by rapid water column degassing. Spectral analysis revealed that on timescales shorter than a month emissions were driven by wind shear, but on longer timescales variations in water temperature governed the flux, suggesting emissions were strongly coupled to production. Our findings suggest that accurate short- and long term projections of lake CH4 emissions can be based on distinct weather- and climate controlled drivers of the flux.

National Category
Geosciences, Multidisciplinary Environmental Sciences Climate Research
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-176228 (URN)10.5194/bg-2019-322 (DOI)
Funder
Swedish Research Council, 2007-4547Swedish Research Council, 2013-5562Swedish Research Council, 2015-06020NERC - the Natural Environment Research Council, NSERC RGPIN-2017-04059
Available from: 2019-11-25 Created: 2019-11-25 Last updated: 2020-01-03
3. Temperature proxies as a solution to biased sampling of lake methane emissions
Open this publication in new window or tab >>Temperature proxies as a solution to biased sampling of lake methane emissions
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(English)Manuscript (preprint) (Other academic)
National Category
Geosciences, Multidisciplinary Climate Research Environmental Sciences
Research subject
Geochemistry
Identifiers
urn:nbn:se:su:diva-176231 (URN)
Funder
Swedish Research Council, 2007-4547Swedish Research Council, 2013-5562Swedish Research Council, 2015-06020NERC - the Natural Environment Research Council, NSERC RGPIN-2017-04059
Available from: 2019-11-25 Created: 2019-11-25 Last updated: 2019-12-18Bibliographically approved

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