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Drivers of diffusive lake CH4 emissions on daily to multi-year time scales
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper.ORCID-id: 0000-0001-5965-7662
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper.ORCID-id: 0000-0002-5640-6419
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2020 (Engelska)Ingår i: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 17, nr 7, s. 1911-1932Artikel i tidskrift (Refereegranskat) Published
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.

Ort, förlag, år, upplaga, sidor
2020. Vol. 17, nr 7, s. 1911-1932
Nationell ämneskategori
Multidisciplinär geovetenskap Miljövetenskap Klimatforskning
Forskningsämne
geokemi
Identifikatorer
URN: urn:nbn:se:su:diva-176228DOI: 10.5194/bg-17-1911-2020OAI: oai:DiVA.org:su-176228DiVA, id: diva2:1372736
Forskningsfinansiär
Vetenskapsrådet, 2007-4547Vetenskapsrådet, 2013-5562Vetenskapsrådet, 2015-06020NERC - the Natural Environment Research Council, NSERC RGPIN-2017-04059Tillgänglig från: 2019-11-25 Skapad: 2019-11-25 Senast uppdaterad: 2020-05-04Bibliografiskt granskad
Ingår i avhandling
1. Carbon trace gas dynamics in subarctic lakes
Öppna denna publikation i ny flik eller fönster >>Carbon trace gas dynamics in subarctic lakes
2020 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: Department of Geological Sciences, Stockholm University, 2020. s. 56
Serie
Meddelanden från Stockholms universitets institution för geologiska vetenskaper ; 379
Nyckelord
lakes, methane, carbon dioxide, fluxes, gas transfer, proxy, climate change
Nationell ämneskategori
Geokemi
Forskningsämne
geokemi
Identifikatorer
urn:nbn:se:su:diva-176269 (URN)978-91-7797-945-6 (ISBN)978-91-7797-946-3 (ISBN)
Disputation
2020-01-22, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (Engelska)
Opponent
Handledare
Anmärkning

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted.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

Tillgänglig från: 2019-12-19 Skapad: 2019-11-27 Senast uppdaterad: 2020-05-25Bibliografiskt granskad

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Av författaren/redaktören
Jansen, JoachimThornton, Brett F.Crill, Patrick M.
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Institutionen för geologiska vetenskaper
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Biogeosciences
Multidisciplinär geovetenskapMiljövetenskapKlimatforskning

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