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  • 1. Baggesen, Nanna S.
    et al.
    Davie-Martin, Cleo L.
    Seco, Roger
    Holst, Thomas
    Rinnan, Riikka
    Bidirectional Exchange of Biogenic Volatile Organic Compounds in Subarctic Heath Mesocosms During Autumn Climate Scenarios2022In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 127, no 6Article in journal (Refereed)
    Abstract [en]

    Biogenic volatile organic compound (BVOC) flux dynamics during the subarctic autumn are largely unexplored and have been considered insignificant due to the relatively low biological activity expected during autumn. Here, we exposed subarctic heath ecosystems to predicted future autumn climate scenarios (ambient, warming, and colder, dark conditions), changes in light availability, and flooding, to mimic the more extreme rainfall or snowmelt events expected in the future. We used climate chambers to measure the net ecosystem fluxes and bidirectional exchange of BVOCs from intact heath mesocosms using a dynamic enclosure technique coupled to a proton-transfer-reaction time-of-flight mass spectrometer (PTR?ToF?MS). We focused on six BVOCs (methanol, acetic acid, acetaldehyde, acetone, isoprene, and monoterpenes) that were among the most dominant and that were previously identified in arctic tundra ecosystems. Warming increased ecosystem respiration and resulted in either net BVOC release or increased uptake compared to the ambient scenario. None of the targeted BVOCs showed net release in the cold and dark scenario. Acetic acid exhibited significantly lower net uptake in the cold and dark scenario than in the ambient scenario, which suggests reduced microbial activity. Flooding was characterized by net uptake of the targeted BVOCs and overruled any temperature effects conferred by the climate scenarios. Monoterpenes were mainly taken up by the mesocosms and their fluxes were not affected by the climate scenarios or flooding. This study shows that although autumn BVOC fluxes on a subarctic heath are generally low, changes in future climate may strongly modify them.

  • 2. Bayer, T. K.
    et al.
    Gustafsson, E.
    Brakebusch, M.
    Beer, C.
    Future carbon emission from boreal and permafrost lakes are sensitive to catchment organic carbon loads2019In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 124, no 7, p. 1827-1848Article in journal (Refereed)
    Abstract [en]

    Abstract Carbon storage, processing, and transport in freshwater systems are important components of the global carbon cycle and sensitive to global change. However, in large-scale modeling this part of the boundless carbon cycle is often lacking or represented in a very simplified way. A new process-oriented lake biogeochemical model is used for investigating impacts of changes in atmospheric CO2 concentrations and organic carbon loading from the catchment on future greenhouse gas emissions from lakes across two boreal to subarctic regions (Northern Sweden and Alaska). Aquatic processes represented include carbon, oxygen, phytoplankton, and nutrient dynamics leading to CO2 and CH4 exchanges with the atmosphere. The model is running inside a macroscale hydrological model and may be easily implemented into a land surface scheme. Model evaluation demonstrates the validity in terms of average concentration of nutrients, algal biomass, and organic and inorganic carbon. Cumulative annual emissions of CH4 and CO2, as well as pathways of CH4 emissions, also compare well to observations. Model calculations imply that lake emissions of CH4 may increase by up to 45% under the Representative Concentration Pathway 8.5 scenario until 2100, and CO2 emissions may increase by up to 80% in Alaska. Increasing organic carbon loading to the lakes resulted in a linear response in CO2 and CH4 emissions across both regions, but increases in CO2 emissions from subarctic lakes in Sweden were lower than for southern boreal lakes, probably due to the higher importance of imported vegetation-?generated? inorganic carbon for CO2 emission from subarctic lakes.

  • 3. Burke, S. A.
    et al.
    Wik, M.
    Lang, A.
    Contosta, A. R.
    Palace, M.
    Crill, P. M.
    Varner, R. K.
    Long-Term Measurements of Methane Ebullition From Thaw Ponds2019In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 124, no 7, p. 2208-2221Article in journal (Refereed)
    Abstract [en]

    Arctic regions are experiencing rapid warming, leading to permafrost thaw and formation of numerous water bodies. Although small ponds in particular are considered hot spots for methane (CH4) release, long‐term studies of CH4 efflux from these surfaces are rare. We have collected an extensive data set of CH4 ebullition (bubbling) measurements from eight small thaw ponds (<0.001 km2) with different physical and hydrological characteristics over four summer seasons, the longest set of observations from thaw ponds to date. The measured fluxes were highly variable with an average of 20.0 mg CH4 · m−2 · day−1 (median: 4.1 mg CH4 · m−2 · day−1, n = 2,063) which is higher than that of most nearby lakes. The ponds were categorized into four types based on clear and significant differences in bubble flux. We found that the amount of CH4 released as bubbles from ponds was very weakly correlated with environmental variables, like air temperature and atmospheric pressure, and was potentially more related to differences in physical characteristics of the ponds. Using our measured average daily bubble flux plus the available literature, we estimate circumpolar thaw ponds <0.001 km2 in size to emit between 0.2 and 1.0 Tg of CH4 through ebullition. Our findings exemplify the importance of high‐frequency measurements over long study periods in order to adequately capture the variability of these water bodies. Through the expansion of current spatial and temporal monitoring efforts, we can increase our ability to estimate CH4 emissions from permafrost pond ecosystems now and in the future.

  • 4. Faucherre, Samuel
    et al.
    Jørgensen, Christian Juncher
    Blok, Daan
    Weiss, Niels
    Siewert, Matthias Benjamin
    Bang-Andreasen, Toke
    Hugelius, Gustaf
    Kuhry, Peter
    Elberling, Bo
    Short and Long-Term Controls on Active Layer and Permafrost Carbon Turnover Across the Arctic2018In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 123, no 2, p. 372-390Article in journal (Refereed)
    Abstract [en]

    Decomposition of soil organic matter (SOM) in permafrost terrain and the production of greenhouse gases is a key factor for understanding climate change‐carbon feedbacks. Previous studies have shown that SOM decomposition is mostly controlled by soil temperature, soil moisture, and carbon‐nitrogen ratio (C:N). However, focus has generally been on site‐specific processes and little is known about variations in the controls on SOM decomposition across Arctic sites. For assessing SOM decomposition, we retrieved 241 samples from 101 soil profiles across three contrasting Arctic regions and incubated them in the laboratory under aerobic conditions. We assessed soil carbon losses (Closs) five times during a 1 year incubation. The incubated material consisted of near‐surface active layer (ALNS), subsurface active layer (ALSS), peat, and permafrost samples. Samples were analyzed for carbon, nitrogen, water content, δ13C, δ15N, and dry bulk density (DBD). While no significant differences were observed between total ALSS and permafrost Closs over 1 year incubation (2.3 ± 2.4% and 2.5 ± 1.5% Closs, respectively), ALNS samples showed higher Closs (7.9 ± 4.2%). DBD was the best explanatory parameter for active layer Closs across sites. Additionally, results of permafrost samples show that C:N ratio can be used to characterize initial Closs between sites. This data set on the influence of abiotic parameter on microbial SOM decomposition can improve model simulations of Arctic soil CO2 production by providing representative mean values of CO2 production rates and identifying standard parameters or proxies for upscaling potential CO2 production from site to regional scales.

  • 5. Granfors, Anna
    et al.
    Ahnoff, Martin
    Mills, Matthew M.
    Abrahamsson, Katarina
    Organic iodine in Antarctic sea ice: A comparison between winter in the Weddell Sea and summer in the Amundsen Sea2014In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 119, no 12, p. 2276-2291Article in journal (Refereed)
    Abstract [en]

    Recent studies have recognized sea ice as a source of reactive iodine to the Antarctic boundary layer. Volatile iodinated compounds (iodocarbons) are released from sea ice, and they have been suggested to contribute to the formation of iodine oxide (IO), which takes part in tropospheric ozone destruction in the polar spring. We measured iodocarbons (CH3I, CH2ClI, CH2BrI, and CH2I2) in sea ice, snow, brine, and air during two expeditions to Antarctica, OSO 10/11 to the Amundsen Sea during austral summer and ANT XXIX/6 to the Weddell Sea in austral winter. These are the first reported measurements of iodocarbons from the Antarctic winter. Iodocarbons were enriched in sea ice in relation to seawater in both summer and winter. During summer, the positive relationship to chlorophyll a biomass indicated a biological origin. We suggest that CH3I is formed biotically in sea ice during both summer and winter. For CH2ClI, CH2BrI, and CH2I2, an additional abiotic source at the snow/ice interface in winter is suggested. Elevated air concentrations of CH3I and CH2ClI during winter indicate that they are enriched in lower troposphere and may take part in the formation of IO at polar sunrise.

  • 6. Harms, Tamara K.
    et al.
    Rocher-Ros, Gerard
    Godsey, Sarah E.
    Emission of Greenhouse Gases From Water Tracks Draining Arctic Hillslopes2020In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 125, no 12, article id e2020JG005889Article in journal (Refereed)
    Abstract [en]

    Experimental and ambient warming of Arctic tundra results in emissions of greenhouse gases to the atmosphere, contributing to a positive feedback to climate warming. Estimates of gas emissions from lakes and terrestrial tundra confirm the significance of aquatic fluxes in greenhouse gas budgets, whereas few estimates describe emissions from fluvial networks. We measured dissolved gas concentrations and estimated emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from water tracks, vegetated depressions that hydrologically connect hillslope soils to lakes and streams. Concentrations of trace gases generally increased as ground thaw deepened through the growing season, indicating active production of greenhouse gases in thawed soils. Wet antecedent conditions were correlated with a decline in CO2 and CH4 concentrations. Dissolved N2O in excess of atmospheric equilibrium occurred in drier water tracks, but on average water tracks took up N2O from the atmosphere at low rates. Estimated CO2 emission rates for water tracks were among the highest observed for Arctic aquatic ecosystems, whereas CH4 emissions were of similar magnitude to streams. Despite occupying less than 1% of total catchment area, surface waters within water tracks were an estimated source of up to 53?85% of total CH4 emissions from their catchments and offset the terrestrial C sink by 5?9% during the growing season. Water tracks are abundant features of tundra landscapes that contain warmer soils and incur deeper thaw than adjacent terrestrial ecosystems and as such might contribute to ongoing and accelerating release of greenhouse gases from permafrost soils to the atmosphere.

  • 7. Jammet, Mathilde
    et al.
    Crill, Patrick
    Dengel, Sigrid
    Friborg, Thomas
    Large methane emissions from a subarctic lake during spring thaw: Mechanisms and landscape significance2015In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 120, no 11, p. 2289-2305Article in journal (Refereed)
    Abstract [en]

    The ice-cover season and subsequent spring thaw are thought to be of particular importance for the biogeochemical cycle of northern lakes and wetlands. Yet the magnitude of their methane emissions during an entire cold season is uncertain due to scarce measurements. While wetlands are known to be the highest natural emitters of methane, emissions from northern lakes are an uncertain component of terrestrial carbon budgets. To evaluate the importance of methane emissions from a subarctic lake during winter and spring, surface methane fluxes were recorded with the eddy covariance method in a subarctic fen-type wetland and in an adjacent shallow lake, from freezeup to complete ice out. The fen was a steady emitter of methane throughout winter. While no detectable flux was observed from the ice-covered lake surface during winter, it was the largest methane source of the landscape in spring, with a cumulative release 1.7-fold higher than at the fen, accounting for 53% of annual lake emissions. The high temporal resolution of the measurements allowed making a direct link between breakdown of the temperature stratification after ice breakup and the highest release of methane from the lake surface. A sediment upwelling at the end of the thaw season likely contributed to these emissions. We suggest that, unlike wetlands, shallow seasonally ice-covered lakes can have their highest methane emission potential in the cold season, likely dominating the spring methane release of subarctic landscapes with high lake coverage.

  • 8. Jansen, J.
    et al.
    Thornton, B. F.
    Jammet, M. M.
    Wik, M.
    Cortés, A.
    Friborg, T.
    MacIntyre, S.
    Crill, P. M.
    Climate-Sensitive Controls on Large Spring Emissions of CH4 and CO2 From Northern Lakes2019In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 124, no 7, p. 2379-2399Article in journal (Refereed)
    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.

  • 9. Lundin, Erik J.
    et al.
    Giesler, Reiner
    Persson, Andreas
    Thompson, Megan S.
    Karlsson, Jan
    Integrating carbon emissions from lakes and streams in a subarctic catchment2013In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 118, no 3, p. 1200-1207Article in journal (Refereed)
    Abstract [en]

    Northern inland waters emit CO2 and CH4 to the atmosphere but the importance of these emissions is poorly understood due to a lack of integrated catchment-scale estimates of carbon (C) emissions from lakes and streams. In this study we quantified the annual emission of CO2 and CH4 from 27 lakes and 23 stream segments in a 15 km2 subarctic catchment in northern Sweden. All lakes and streams were net sources of C to the atmosphere on an annual basis. Streams dominated (96%) the aquatic CO2 emission while lakes (61%) dominated the aquatic CH4 emission. Total aquatic C emission from the catchment was estimated to be 9.1 g C m−2 yr−1 (98% as CO2). Although streams only accounted for 4% of the aquatic area in the catchment, they accounted for 95% of the total emission. The C emissions from lakes and streams were considerably larger than previously reported downstream waterborne export of C from the catchment, indicating that the atmospheric losses of C in the aquatic systems are an important component in the catchment C balance.

  • 10. Mzobe, P.
    et al.
    Yan, Y.
    Berggren, M.
    Pilesjö, P.
    Olefeldt, D.
    Lundin, E.
    Roulet, N. T.
    Persson, A.
    Morphometric Control on Dissolved Organic Carbon in Subarctic Streams2020In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 125, no 9, article id e2019JG005348Article in journal (Refereed)
    Abstract [en]

    Climate change has the potential to alter hydrological regimes and to expand saturated areas in permafrost environments, which are important sources of organic carbon. The sources, transfer zones, and delivery mechanisms of carbon into the stream network are controlled by the morphometric properties of the catchment; however, the utility and limitations of these properties as predictors of dissolved organic carbon concentrations have rarely been systematically evaluated. This study tested the relationships between 18 morphometric indicators and observed dissolved organic carbon (DOC) concentrations in the Stordalen catchment, Sweden. Geospatial and explorative statistics were combined to assess the topographical, areal, and linear indicators influencing the distribution of DOC in the catchment. The results suggest that catchment morphometric indicators can be used as proxies to predict DOC concentrations along a longitudinal continuum in subarctic climate regions (R2 up to 0.52). Morphometry indicators that best served as predictors of DOC concentration in the model were as follows: relief, slope length and steepness factor (LS-factor), sediment transport capacity, and catchment area. Due to the influence that catchment form exerts in DOC spatial patterns and processing, a morphometric approach can serve as a first approximation of DOC spatial patterns within a catchment. The initial step in identifying carbon sources based on the catchment topography has the potential to allow for quick and multilevel comparison within and between catchments.

  • 11. Olid, C.
    et al.
    Zannella, A.
    Lau, D. C. P.
    The Role of Methane Transport From the Active Layer in Sustaining Methane Emissions and Food Chains in Subarctic Ponds2021In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 126, no 3, article id e2020JG005810Article in journal (Refereed)
    Abstract [en]

    Groundwater discharge from the seasonally thawed active layer is increasingly recognized as an important pathway for delivering methane (CH4) into Arctic lakes and streams, but its contribution to CH4 emissions from thaw ponds and its influence on the trophic support and nutritional quality of pond food chains remains unexplored. We quantified the transport of CH4 from the active layer through groundwater discharge into thaw ponds in a subarctic catchment in northern Sweden, using radon (222Rn) as groundwater tracer. We analyzed stable isotopes and fatty acids of pond macroinvertebrates to evaluate the potential effects of groundwater-mediated CH4 inputs on the aquatic food chains. Our results indicate that active layer groundwater discharge flows are nontrivial (range 6%–46% of pond volume per day) and the associated CH4 fluxes (median 339 mg C m−2day−1, interquartile range [IQR]: 179–419 mg C m−2 day−1) can sustain the diffusive CH4 emissions from most of the ponds (155 mg C m−2 day−1, IQR: 55–234 mg C m−2 day−1). Consumers in ponds receiving greater CH4 inputs from the active layer had lower stable carbon (C) isotope signatures that indicates a greater trophic reliance on methane oxidizing bacteria (MOB), and they had lower nutritional quality as indicated by their lower tissue concentrations of polyunsaturated fatty acids. Overall, this work links physical (CH4 transport from the active layer), biogeochemical (CH4 emission), and ecological (MOB-consumer interaction) processes to provide direct evidence for the role of active layer groundwater discharge in CH4 cycling of subarctic thaw ponds.

  • 12. Pedersen, Emily Pickering
    et al.
    Elberling, Bo
    Michelsen, Anders
    Seasonal variations in methane fluxes in response to summer warming and leaf litter addition in a subarctic heath ecosystem2017In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 122, no 8, p. 2137-2153Article in journal (Refereed)
    Abstract [en]

    Methane (CH4) is a powerful greenhouse gas controlled by both biotic and abiotic processes. Few studies have investigated CH4 fluxes in subarctic heath ecosystems, and climate change-induced shifts in CH4 flux and the overall carbon budget are therefore largely unknown. Hence, there is an urgent need for long-term in situ experiments allowing for the study of ecosystem processes over time scales relevant to environmental change. Here we present in situ CH4 and CO2 flux measurements from a wet heath ecosystem in northern Sweden subjected to 16 years of manipulations, including summer warming with open-top chambers, birch leaf litter addition, and the combination thereof. Throughout the snow-free season, the ecosystem was a net sink of CH4 and CO2 (CH4 −0.27 mg C m−2 d−1; net ecosystem exchange −1827 mg C m−2 d−1), with highest CH4 uptake rates (−0.70 mg C m−2 d−1) during fall. Warming enhanced net CO2 flux, while net CH4 flux was governed by soil moisture. Litter addition and the combination with warming significantly increased CH4 uptake rates, explained by a pronounced soil drying effect of up to 32% relative to ambient conditions. Both warming and litter addition also increased the seasonal average concentration of dissolved organic carbon in the soil. The site was a carbon sink with a net uptake of 60 g C m−2 over the snow-free season. However, warming reduced net carbon uptake by 77%, suggesting that this ecosystem type might shift from snow-free season sink to source with increasing summer temperatures.

  • 13. Perryman, Clarice R.
    et al.
    McCalley, Carmody K.
    Malhotra, Avni
    Fahnestock, M. Florencia
    Kashi, Natalie N.
    Bryce, Julia G.
    Giesler, Reiner
    Varner, Ruth K.
    Thaw Transitions and Redox Conditions Drive Methane Oxidation in a Permafrost Peatland2020In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 125, no 3, article id e2019JG005526Article in journal (Refereed)
    Abstract [en]

    Permafrost peatlands are a significant source of methane (CH4) emissions to the atmosphere and could emit more CH4 with continued permafrost thaw. Aerobic methane-oxidizing bacteria may attenuate a substantial fraction of CH4 emissions in thawing permafrost peatlands; however, the impact of permafrost thaw on CH4 oxidation is uncertain. We measured potential CH4 oxidation rates (hereafter, CH4 oxidation) and their predictors using laboratory incubations and in situ porewater redox chemistry across a permafrost thaw gradient of eight thaw stages at Stordalen Mire, a permafrost peatland complex in northernmost Sweden. Methane oxidation rates increased across a gradient of permafrost thaw and differed in transitional thaw stages relative to end-member stages. Oxidation was consistently higher in submerged fens than in bogs or palsas across a range of CH4 concentrations. We also observed that CH4 oxidation increased with decreasing in situ redox potential and was highest in sites with lower redox potential (Eh < 10 mV) and high water table. Our results suggest that redox potential can be used as an important predictor of CH4 oxidation, especially in thawed permafrost peatlands. Our results also highlight the importance of considering transitional thaw stages when characterizing landscape-scale CH4 dynamics, because these transitional areas have different rates and controls of CH4 oxidation relative to intact or completely thawed permafrost areas. As permafrost thaw increases the total area of semiwet and wet thaw stages in permafrost peatlands, CH4 oxidation represents an important control on CH4 emissions to the atmosphere.

  • 14. Prowse, T.
    et al.
    Bring, A.
    Mard, J.
    Carmack, E.
    Holland, M.
    Instanes, A.
    Vihma, T.
    Wrona, F. J.
    Arctic Freshwater Synthesis: Summary of key emerging issues2015In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 120, no 10, p. 1887-1893Article in journal (Refereed)
    Abstract [en]

    In response to a joint request from the World Climate Research Program's Climate and Cryosphere Project, the International Arctic Science Committee, and the Arctic Council's Arctic Monitoring and Assessment Program an updated scientific assessment has been conducted of the Arctic Freshwater System (AFS), entitled the Arctic Freshwater Synthesis (AFS(sigma)). The major reason behind the joint request was an increasing concern that changes to the AFS have produced, and could produce even greater, changes to biogeophysical and socioeconomic systems of special importance to northern residents and also produce extra-Arctic climatic effects that will have global consequences. The AFS(sigma) was structured around six key thematic areas: atmosphere, oceans, terrestrial hydrology, terrestrial ecology, resources, and modeling, the review of each coauthored by an international group of scientists and published as separate manuscripts in this special issue of Journal of Geophysical Research-Biogeosciences. This AFS(sigma) summary manuscript reviews key issues that emerged during the conduct of the synthesis, especially those that are cross-thematic in nature, and identifies future research required to address such issues.

  • 15. Tang, J.
    et al.
    Valolahti, H.
    Kivimäenpää, M.
    Michelsen, A.
    Rinnan, R.
    Acclimation of biogenic volatile organic compound emission from subarctic heath under long-term moderate warming2017In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961Article in journal (Refereed)
    Abstract [en]

    Biogenic volatile organic compound (BVOC) emissions from subarctic ecosystems have shown to increase drastically in response to a long-term temperature increase of only 2 °C. We assessed whether this increase takes place already after three years of warming, and how the increase changes over time. To test this, we measured BVOC emissions and CO2 fluxes in a field experiment on a subarctic wet heath, where ecosystem plots were subjected to passive warming by open top chambers for three (OTC3) or 13 years (OTC13), or were kept as un-manipulated controls. Already after three years of moderate temperature increase of 1-2 °C, warming increased the emissions of isoprene (5-6-fold) and monoterpenes (3-4-fold), from the subarctic heath. The several-fold higher BVOC emissions in the warmed plots are likely a result of increased vegetation biomass and altered vegetation composition as a shift in the species coverage was observed already after three years of warming. Warming also increased gross ecosystem production and ecosystem respiration, but the increases were much lower than those for BVOCs. Our results demonstrate that the strong BVOC responses to warming already appeared after three years, and the BVOC and CO2 fluxes had acclimated to this warming after three years, showing no differences with another 10 years of warming. This finding has important implications for predicting CO2 and BVOC fluxes in subarctic ecosystems.

  • 16.
    Tesi, Tommaso
    et al.
    Stockholms universitet, Institutionen för miljövetenskap och analytisk kemi.
    Semiletov, Igor
    Dudarev, Oleg
    Andersson, August
    Stockholms universitet, Institutionen för miljövetenskap och analytisk kemi.
    Gustafsson, Örjan
    Stockholms universitet, Institutionen för miljövetenskap och analytisk kemi.
    Matrix association effects on hydrodynamic sorting and degradation of terrestrial organic matter during cross-shelf transport in the Laptev and East Siberian shelf seas2016In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 121, no 3, p. 731-752Article in journal (Refereed)
    Abstract [en]

    This study seeks an improved understanding of how matrix association affects the redistribution and degradation of terrigenous organic carbon (TerrOC) during cross-shelf transport in the Siberian margin. Sediments were collected at increasing distance from two river outlets (Lena and Kolyma Rivers) and one coastal region affected by erosion. Samples were fractionated according to density, size, and settling velocity. The chemical composition in each fraction was characterized using elemental analyses and terrigenous biomarkers. In addition, a dual-carbon-isotope mixing model (C-13 and C-14) was used to quantify the relative TerrOC contributions from active layer (Topsoil) and Pleistocene Ice Complex Deposits (ICD). Results indicate that physical properties of particles exert first-order control on the redistribution of different TerrOC pools. Because of its coarse nature, plant debris is hydraulically retained in the coastal region. With increasing distance from the coast, the OC is mainly associated with fine/ultrafine mineral particles. Furthermore, biomarkers indicate that the selective transport of fine-grained sediment results in mobilizing high-molecular weight (HMW) lipid-rich, diagenetically altered TerrOC while lignin-rich, less degraded TerrOC is retained near the coast. The loading (mu g/m(2)) of lignin and HMW wax lipids on the fine/ultrafine fraction drastically decreases with increasing distance from the coast (98% and 90%, respectively), which indicates extensive degradation during cross-shelf transport. Topsoil-C degrades more readily (903.5%) compared to the ICD-C (6011%) during transport. Altogether, our results indicate that TerrOC is highly reactive and its accelerated remobilization from thawing permafrost followed by cross-shelf transport will likely represent a positive feedback to climate warming.

  • 17.
    Wik, Martin
    et al.
    Stockholms universitet, Institutionen för geologiska vetenskaper.
    Crill, Patrick M.
    Stockholms universitet, Institutionen för geologiska vetenskaper.
    Varner, Ruth K.
    Bastviken, David
    Stockholms universitet, Institutionen för geologiska vetenskaper.
    Multiyear measurements of ebullitive methane flux from three subarctic lakes2013In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 118, no 3, p. 1307-1321Article in journal (Refereed)
    Abstract [en]

    Ebullition (bubbling) from small lakes and ponds at high latitudes is an important yet unconstrained source of atmospheric methane (CH4). Small water bodies are most abundant in permanently frozen peatlands, and it is speculated that their emissions will increase as the permafrost thaws. We made 6806 measurements of CH4 ebullition during four consecutive summers using a total of 40 bubble traps that were systematically distributed across the depth zones of three lakes in a sporadic permafrost landscape in northernmost Sweden. We identified significant spatial and temporal variations in ebullition and observed a large spread in the bubbles' CH4 concentration, ranging from 0.04% to 98.6%. Ebullition followed lake temperatures, and releases were significantly larger during periods with decreasing atmospheric pressure. Although shallow zone ebullition dominated the seasonal bubble CH4 flux, we found a shift in the depth dependency towards higher fluxes from intermediate and deep zones in early fall. The average daily flux of 13.4mg CH4 m(-2) was lower than those measured in most other high-latitude lakes. Locally, however, our study lakes are a substantial CH4 source; we estimate that 350kg of CH4 is released via ebullition during summer (June-September), which is approximately 40% of total whole year emissions from the nearby peatland. In order to capture the large variability and to accurately scale lake CH4 ebullition temporally and spatially, frequent measurements over long time periods are critical.

  • 18. Wik, Martin
    et al.
    Johnson, Joel E.
    Crill, Patrick M.
    DeStasio, Joel P.
    Erickson, Lance
    Halloran, Madison J.
    Fahnestock, M. Florencia
    Crawford, Maurice K.
    Phillips, Stephen C.
    Varner, Ruth K.
    Sediment Characteristics and Methane Ebullition in Three Subarctic Lakes2018In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 123, no 8, p. 2399-2411Article in journal (Refereed)
    Abstract [en]

    Ebullition (bubbling) from climate-sensitive northern lakes remains an unconstrained source of atmospheric methane (CH4). Although the focus of many recent studies, ebullition is rarely linked to the physical characteristics of lakes. In this study we analyze the sediments of subarctic postglacial lakes and investigate how sediment properties relate to the large spatial variation in CH4 bubble flux, quantified over multiple years using bubble traps. The results show that the sediments from our lakes are rich in total organic carbon, containing 37 kg/m3 on average. This number is roughly 40% higher than the average for yedoma deposits, which have been identified as high CH4 emitters. However, the quantity of total organic carbon is not a useful indicator of high emissions from the study lakes. Neither is the amount of CH4 in the sediment a reliable measure of ebullition potential. Instead, our data point to coarse detritus, partly from buried submerged aquatic vegetation and redeposited peat as spatial controls on fluxes, often in combination with previously established effects of incoming solar radiation and water depth. The results once again highlight the climate sensitivity of northern lakes, indicating that biological responses to warmer waters and increased energy input and heating of organic sediments during longer ice-free seasons can substantially alter future CH4 emissions.

  • 19.
    Wik, Martin
    et al.
    Stockholms universitet, Institutionen för geologiska vetenskaper.
    Thornton, Brett F.
    Stockholms universitet, Institutionen för geologiska vetenskaper.
    Varner, Ruth K.
    McCalley, Carmody
    Crill, Patrick M.
    Stockholms universitet, Institutionen för geologiska vetenskaper.
    Stable Methane Isotopologues From Northern Lakes Suggest That Ebullition Is Dominated by Sub-Lake Scale Processes2020In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 125, no 10, article id e2019JG005601Article in journal (Refereed)
    Abstract [en]

    Stable isotopes have emerged as popular study targets when investigating emission of methane (CH4) from lakes. Yet little is known on how isotopic patterns conform to variations in emission magnitudes—a highly relevant question. Here, we present a large multiyear data set on stable isotopes of CH4 ebullition (bubbling) from three small adjacent subarctic lakes. The δ13C‐CH4 and δD‐CH4 range from −78.4‰ to −53.1‰ and from −369.8‰ to −218.8‰, respectively, and vary greatly among the lakes. The signatures suggest dominant hydrogenotrophic methanogenesis, particularly in the deep zones, but there are also signals of seemingly acetoclastic production in some high fluxing shallow areas, possibly fueled by in situ vegetation, but in‐sediment anaerobic CH4 oxidation cannot be ruled out as an alternative cause. The observed patterns, however, are not consistent across the lakes. Neither do they correspond to the spatiotemporal variations in the measured bubble CH4 fluxes. Patterns of acetoclastic and hydrogenotrophic production plus oxidation demonstrate that gains and losses of sediment CH4 are dominated by sub‐lake scale processes. The δD‐CH4 in the bubbles was significantly different depending on measurement month, likely due to evaporation effects. On a larger scale, our isotopic data, combined with those from other lakes, show a significant difference in bubble δD‐CH4 between postglacial and thermokarst lakes, an important result for emission inventories. Although this characteristic theoretically assists in source partitioning studies, most hypothetical future shifts in δD‐CH4 due to high‐latitude lake area or production pathway are too small to lead to atmospheric changes detectable with current technology.

  • 20. Wilson, Rachel M.
    et al.
    Tfaily, Malak M.
    Advanced Molecular Techniques Provide New Rigorous Tools for Characterizing Organic Matter Quality in Complex Systems2018In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 123, no 6, p. 1790-1795Article in journal (Refereed)
    Abstract [en]

    Carbon flux rates are widely understood to be substrate controlled; however, characterizing substrate quality continues to be a challenge. We suggest that, while optical measurements have their place, they are not the only, or the best, tool for characterizing organic matter quality. Nominal oxidation state of the carbon provides a thermodynamically relevant measure, which could be used as a metric of organic matter quality. Calculating nominal oxidation state of the carbon requires a suite of advanced complementary analysis but is then trivial to calculate from the resulting data sets.

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