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Biogeochemistry: Nocturnal escape route for marsh gas
Abstract
A field study of methane emissions from wetlands reveals that more of the gas escapes through diffusive processes than was thought, mostly at night. Because methane is a greenhouse gas, the findings have implications for global warming.
... . Fitting c-x profiles (left plots) and evaluated Ds (right plots) using different traditional functions, compared with the pre-set concentration profiles of c2 and c3 and monotonic D2 (equation (2)) and D3 (equation (3)): (a) Nested-exponential function, (b) Pseudo-Fermi function, (c) superposed-Boltzmann function, (d) Superposed-normal distribution function. ...
... In order to test the calculation result of these traditional functions directly, following the idea from Kailasam 32 2 The unit of D is um /s 2 , c denotes concentration of solutes in atom percent, the diffusion time is 10000 seconds, the length of x is 1000 um. Initially, c equals to 0 where x 500 ≤ and equals to 1 where x 500 > . ...
... Actually, the idea of the superposing method is to add a normal PDF f x ( ) 2 which allows the introduction of a swell to a normal CDF F x ( ) 1 . If there are two swells on the composition profile, equation (20) can be used. ...
Diffusion couple technique in combination with the Boltzmann-Matano method is the widely used
approach to evaluate the interdiffusion coefficients in the target systems. However, the quality of the
evaluated interdiffusion coefficients due to the Boltzmann-Matano method strongly depends on the
fitting degree of the utilized continuous function to the discrete experimental composition profiles.
In this paper, the application of different types of distribution functions is proposed to solve this
problem. For the simple D-c relations, the normal, pseudo-normal, skew normal, pseudo-skew normal
distributions can be employed, while for the complex D-c relations, the superposed distributions should
be used. Even for the cases with uphill diffusion, the combined superposition of distributions may be
chosen. Through validation in several benchmarks and real alloy systems, accurate diffusion coefficients
are proved to be successfully obtained by using the distribution functions. It is anticipated that the
Boltzmann-Matano method together with the distribution functions may serve as the general solution
for determining the accurate interdiffusion coefficients in different materials.
... Fluxes from all 16 sites were measured during the daytime period between 10:00 and 16:00 throughout the May-October monitoring period. It is important to note that diffusion rates were possibly underestimated because of the assumption that diffusive rates between 10:00 and 16:00 were representative of the total diel period, which excludes the higher diffusion rates that can occur outside this time due to convective mixing (Anthony & MacIntyre, 2016). The CH 4 ebullition and diffusion data sets are available through the Environmental Data Initiative repository (McClure et al., 2019). ...
... Diffusion was also likely underestimated because we assumed that diffusive rates were the same over a diel period when scaled from the short duration of time that we measured efflux. Thus, increases in CH 4 diffusion rates that occur during the night because of convective mixing (e.g., Anthony & MacIntyre, 2016;Deshmukh et al., 2014) were not accounted for in our study. Finally, we were also unable to measure sediment properties such as carbon quality (Zhou et al., 2019) and deposition rates, which can also influence CH 4 ebullition (Sobek et al., 2012;Wik et al., 2018). ...
Reservoirs emit large amounts of methane (CH4) to the atmosphere relative to their small surface area globally. Among the different pathways of reservoir CH4 emissions, bubbling from the sediments (ebullition) and diffusion from the water surface are major contributors of CH4 efflux. The magnitude of ebullition and diffusion can vary substantially over space and time in large reservoirs. However, it is unclear how the drivers of ebullition and diffusion vary along a reservoir's longitudinal gradient, particularly in small reservoirs. We measured ebullition, diffusion, and eight environmental driver variables at four transects along a longitudinal gradient within a small, eutrophic reservoir. We used time series modeling to examine how the drivers of ebullition and diffusion varied among transects. Sediment‐water interface temperature, inflow discharge, and wind speed were the most important drivers of CH4 ebullition in upstream transects of the reservoir, while phytoplankton biomass was the most important driver of ebullition in the downstream transect closest to the dam. Strikingly, CH4 ebullition dynamics were extremely well captured by the time series models, as the modeled rates for the furthest upstream transect closely matched the observed rates throughout the monitoring period. In contrast, CH4 diffusion dynamics were harder to model, with phytoplankton biomass as the primary driver of diffusion across all transects. Our results indicate that multiple drivers affect CH4 emissions along a small reservoir's longitudinal gradient and should be considered when upscaling site measurements to reservoir‐wide CH4 emissions and ultimately regional or global estimates.
... Meanwhile, plant presence can directly or indirectly affect the transformation of pollutants and consequently impact the emission of gases in CWs. Except for O 2 and CO 2 , gases emitted from wetlands have been commonly assumed to be generated from the sediment or soil surface through the water column, escape to the atmosphere by bubbling or diffusing called ebullition, or transport through the vascular system of emergent plants [43,44]. The plant species and combination can affect the contamination balances, and then impact removal efficiency and gas emission [45]. ...
Constructed wetlands (CWs) are an eco-technology for wastewater treatment and are applied worldwide. Due to the regular influx of pollutants, CWs can release considerable quantities of greenhouse gases (GHGs), ammonia (NH3), and other atmospheric pollutants, such as volatile organic compounds (VOCs) and hydrogen sulfide (H2S), etc., which will aggravate global warming, degrade air quality and even threaten human health. However, there is a lack of systematic understanding of factors affecting the emission of these gases in CWs. In this study, we applied meta-analysis to quantitatively review the main influencing factors of GHG emission from CWs; meanwhile, the emissions of NH3, VOCs, and H2S were qualitatively assessed. Meta-analysis indicates that horizontal subsurface flow (HSSF) CWs emit less CH4 and N2O than free water surface flow (FWS) CWs. The addition of biochar can mitigate N2O emission compared to gravel-based CWs but has the risk of increasing CH4 emission. Polyculture CWs stimulate CH4 emission but pose no influence on N2O emission compared to monoculture CWs. The influent wastewater characteristics (e.g., C/N ratio, salinity) and environmental conditions (e.g., temperature) can also impact GHG emission. The NH3 volatilization from CWs is positively related to the influent nitrogen concentration and pH value. High plant species richness tends to reduce NH3 volatilization and plant composition showed greater effects than species richness. Though VOCs and H2S emissions from CWs do not always occur, it should be a concern when using CWs to treat wastewater containing hydrocarbon and acid. This study provides solid references for simultaneously achieving pollutant removal and reducing gaseous emission from CWs, which avoids the transformation of water pollution into air contamination.
... That is the condition that allows anaerobic digestion and fermentation of any plant or animal matter, which then produces methane. The trapped methane can escape through any of three main pathways: by the diffusion of methane molecules across an air-water interface, by bubbling out of water in a process known as ebullition, or through plant-mediated transport (Anthony and MacIntyre, 2016). ...
Gas geochemistry is a relatively new branch among the fields of geochemistry. Gas geochemistry has become an increasingly sophisticated tool for understanding the geological history of hydrocarbons from their generation in source rocks to their accumulations in reservoirs. Interpretation of gas compositions is also an important factor in drilling operations because it is the foremost information used in mud logging. Because the complex history of gases is often difficult to decipher with the few chemical indicators that are available in routine gas analysis, an effort has been made for a better understanding of the chemistry and physics inducing the variability of gas compositions. Moreover, gas geochemistry has been successfully applied in mineral exploration, geothermal prospecting and climate change assessment. In addition, gas geochemistry is very important in predicting earthquakes and volcanoes. The present book is divided into five parts: the first part is related to the petroleum and renewable topics, the second part is allied to mineral exploration, the third part is referred to geothermal energy exploration, the forth part is concerned with forecasting earthquakes and volcano eruptions, and the fifth part is devoted to climate change. This book will be invaluable to graduate students and researchers in petroleum geology and engineering, mineral exploration, geothermal geology and engineering, climatology, seismology and volcanology, who use gas geochemistry techniques.
... Carbon (C) and nitrogen (N) cycle patterns in these ecosystems, as well as the regulatory mechanisms of such cycles, have attracted the attention of a wide range of scholars Fan et al., 2020;Shi et al., 2020;Xie et al., 2020;Wallenius et al., 2021;Wang et al., 2021). The abundant C storage (450 Gt) of wetlands makes them an essential natural source of CH 4 emissions to the atmosphere (Anthony and MacIntyre, 2016). Furthermore, coastal wetlands are affected by periodic flooding and have favorable anaerobic conditions, while their abundance of organic matter and various electron acceptors provide favorable conditions for the occurrence of AOM from different reaction pathways (Page and Dalal, 2011;Poffenbarger et al., 2011). ...
Anaerobic oxidation of methane (AOM) in wetland soils is widely recognized as a key sink for the greenhouse gas methane (CH4). The occurrence of this reaction is influenced by several factors, but the exact process and related mechanism of this reaction remain unclear, due to the complex interactions between multiple influencing factors in nature. Therefore, we investigated how environmental and microbial factors affect AOM in wetlands using laboratory incubation methods combined with molecular biology techniques. The results showed that wetland AOM was associated with a variety of environmental factors and microbial factors. The environmental factors include such as vegetation, depth, hydrogen ion concentration (pH), oxidation-reduction potential (ORP), electrical conductivity (EC), total nitrogen (TN), nitrate (NO3-), sulfate (SO42-), and nitrous oxide (N2O) flux, among them, soil N substances (TN, NO3-, N2O) have essential regulatory roles in the AOM process, while NO3- and N2O may be the key electron acceptors driving the AOM process under the coexistence of multiple electron acceptors. Moreover, denitrification communities (narG, nirS, nirK, nosZI, nosZII) and anaerobic methanotrophic (ANME-2d) were identified as important functional microorganisms affecting the AOM process, which is largely regulated by the former. In the environmental context of growing global anthropogenic N inputs to wetlands, these findings imply that N cycle-mediated AOM processes are a more important CH4 sink for controlling global climate change. This studying contributes to the knowledge and prediction of wetland CH4 biogeochemical cycling and provides a microbial ecology viewpoint on the AOM response to global environmental change.
... Also, at night (no photosynthetic uptake), 485 CO 2 levels increased rapidly, but this could also be linked to the upwelling of CO 2 -enriched 486 waters. Some recent work indicates that nocturnal mixing enhances GHG emissions from 487 shallow wetlands (Poindexter et al., 2016), a mechanism also suggested to operate in shallow 488 thermokarst lakes and flooded forests (Walter Anthony and MacIntyre, 2016). We observed 489 an increase in both CO 2 and CH 4 in PIN ponds upon nocturnal and morning mixing (Fig. 5). ...
Accurate estimations of gaseous emissions and carbon sequestration in wastewater processing are essential for the design, operation and planning of treatment infrastructure, particularly considering the greenhouse gases reduction targets. In this study, we look at the interplay between biological productivity, hydrodynamics and evasion of carbon-based greenhouse gases (GHG) through diffusion and ebullition in order to provide directions for more accurate assessments of their emissions from waste stabilization ponds (WSPs). The ponds stratified in the day and mixed at night. Buoyancy flux contributed between 40 and 75% to turbulence in the water column during nocturnal cooling events, and the associated mixing lead to increasing CO2 and CH4 concentrations by up to an order of magnitude in the surface. The onset of stratification and phytoplankton surface blooms, associated with high pH as well as low and variable CO2 partial pressure resulted in an overall reduction of CO2 efflux. Ebullition represented between 40 and 99% of the total CH4 efflux, and up to 95% of the integrated GHG release during wastewater treatment (in CO2 equivalents). Hydrodynamic conditions, diurnal variability and ebullition need to be accounted for reliable assessments of GHG emissions from WSPs. Our study is an important step towards gaining a deeper understanding in the functioning of these hot spots of carbon processing. The contribution of WSPs to atmospheric GHG budget is likely to increase with population growth unless their performance is improved in this regard.
... Such opposing cycles in CO 2 and DO may have resulted from (1) the balance between night-time respiration and day-time photosynthesis, and (2) a diurnal pattern in mixing regime. Surface waters cooling at night create small eddies, slowly mixing with the deeper, GHGenriched, water mass, thus creating a potentially important escape pathway (Walter Anthony and MacIntyre 2016). ...
Lakes and ponds can be hotspots for CO2 and CH4 emissions, but Arctic studies remain scarce. Here we present diffusive and ebullition fluxes collected over several years from 30 ponds and 4 lakes formed on an organic‐rich polygonal tundra landscape. Water body morphology strongly affects the mixing regime—and thus the seasonal patterns in gas emissions—with ice‐out and autumnal turnover periods identified as hot moments in most cases. The studied thermokarst lake maintained relatively high ebullition rates of millennia‐old CH4 (up to 3405 14C YBP). Larger and deeper kettle lakes maintained low fluxes of both gases (century to millennium‐old), slowly turning into a CO2 sink over the summer. During winter, lakes accumulated CO2, which was emitted during the ice‐out period. Coalescent polygonal ponds, influenced by photosynthesizing benthic mats, were continuous CO2 sinks, yet important CH4 emitters (modern carbon). The highest fluxes were recorded from ice‐wedge trough ponds (up to 96 mmol CO2 equivalent m−2 d−1). However, despite clear signs of permafrost carbon inputs via active shore erosion, these sheltered ponds emitted modern to century‐old greenhouse gases. As the ice‐free period lengthens, scenarios of warmer and wetter conditions could favor both the production of CO2 and CH4 from thawing permafrost carbon, and CH4 production from recently fixed carbon through an atmospheric CO2‐to‐CH4 shunt at sites in which primary production is stimulated. This must be carefully considered at the landscape scale, recognizing that older carbon stocks can be mineralized efficiently in specific locations, such as in thermokarst lakes.
... Nighttime surface water cooling may enhance formation of eddies of cooled higher density surface water, facilitating convective mixing, which, in turn, could increase CH 4 emissions during the night. Convection has been suggested as important for facilitating high nighttime fluxes (23,24) in wetlands and also implied as a potential mechanism for nighttime fluxes in a lake (8). However, the convective periods, generated by daytime heating of the water column followed by nighttime cooling do not appear to govern diel emission patterns in our study. ...
Significance
Methane (CH 4 ) emissions from lakes are significant, yet still highly uncertain and a key bottleneck for understanding the global methane budget. Current lake flux estimates do not account for diel variability of CH 4 flux. Here, we apply a high-resolution spatiotemporal measurement approach in multiple lakes and report extensive data on variability between day and night lake CH 4 emissions. Our results demonstrate a clear and consistent diel pattern with more than twofold higher daytime fluxes. We show that it is critical to include diel variability to correctly estimate and extrapolate lake CH 4 flux, and that present northern lake emissions may have been overestimated by 15%.
... Additionally, reeds are plants with large leaf surface areas and low aerodynamic resistance to transpiration (Crundwell, 1986). Regarding CH 4 fluxes, given the relevant contribution of plant-mediated gas transport to the total CH 4 emission from this type of wetlands, apart from ebullition and diffusion from water (Anthony & MacIntyre, 2016), previous studies suggest that the growth dynamics of reeds also drive marked seasonality and diurnal patterns of CH 4 fluxes (Kim et al., 1998;Van den Berg et al., 2016). ...
Wetlands are crucial ecosystems modulating climate change due to their great potential to capture carbon dioxide (CO2), emit methane (CH4), and regulate local climate through evapotranspiration (ET). Common reed wetlands are particularly interesting given their high productivity, abundance, and highly efficient internal gas‐transport mechanism. However, little is known about the interannual behavior and dominant controlling factors of Mediterranean reed wetlands, characterized by seasonal flooding and remarkable weather variability. After six years of ecosystem carbon and ET flux measurements by eddy covariance (three years for CH4 fluxes), this study shows the functional vulnerability of such wetlands to climate variability, switching between carbon (CO2 + CH4) sink (660 g CO2‐eq m⁻² yr⁻¹, in 2014) and source (360 g CO2‐eq m⁻² yr⁻¹, in 2016) in short periods of time. According to our analyses, the great interannual variability appeared to mainly depend on the behavior of reed growth dynamics during the transition to senescence period, what is confirmed through the enhanced vegetation index as a proxy of photosynthetic activity. Additionally, a similar behavior of seasonal and daily patterns of carbon fluxes and ET was found compared with other wetlands under different climates.
... Measurements similar to these and combined with chamber studies in other thaw ponds, some of which are sheltered from wind and some which have a long fetch, will lead to accurate predic- tions of mixing events which bring dissolved gases to the air-water interface and gas transfer coefficients for these abundant water bodies with high concentrations of CO 2 and CH 4 . Besides applying to larger lakes on which the equations for dissipation rates were developed ( Tedford et al. 2014), the similarity scaling is applicable to other small water bod- ies and wetlands with low wind speeds (Walter Anthony and MacIntyre 2016). Thus, these analyses of the extent of turbu- lence during heating and cooling provide a basis for improved computation of gas fluxes from pond and lake sites. ...
Small ponds, numerous throughout the Arctic, are often supersaturated with climate‐forcing trace gases. Improving estimates of emissions requires quantifying (1) their mixing dynamics and (2) near‐surface turbulence which would enable emissions. To this end, we instrumented an arctic pond (510 m2, 1 m deep) with a meteorological station, a thermistor array, and a vertically oriented acoustic Doppler velocimeter. We contrasted measured turbulence, as the rate of dissipation of turbulent kinetic energy, ε, with values predicted from Monin–Obukhov similarity theory (MOST) based on wind shear as u*w, the water friction velocity, and buoyancy flux, β, under cooling. Stratification varied over diel cycles; the thermocline upwelled as winds changed allowing ventilation of near‐bottom water. Near‐surface temperature stratification was up to 7°C per meter. With respect to predictions from MOST: (1) With positive β under heating and strong near‐surface stratification, turbulence was suppressed; (2) under heating with moderate stratification and under cooling with light to moderate winds, measured ε was in agreement with MOST; (3) under cooling with no wind and when surface currents had ceased, as occurred 20% of the time, turbulence was measurable and predicted from β. Near‐surface turbulence was enhanced under cooling and light winds relative to that under a neutral atmosphere due to higher values of drag coefficients under unstable atmospheres. Small ponds are dynamic systems with wind‐induced thermocline tilting enabling vertical exchanges. Near‐surface turbulence, similar to that in larger systems, can be computed from surface meteorology enabling accurate estimates of gas transfer coefficients and emissions.
... In lakes, reservoirs and ponds, emissions of dissolved CH 4 can also be driven by hydrodynamic transport, whereby temperature gradients (primarily at night time) drive thermal convection causing relatively rapid upwelling of CH 4 from deeper water layers by both diffusion and advection [Poindexter et al., 2016]. This process varies on a seasonal basis and could potentially be important on a global scale [Walter Anthony and MacIntyre, 2016]. ...
Methane (CH4) is produced in many natural systems that are vulnerable to change under a warming climate, yet current CH4 budgets, as well as future shifts in CH4 emissions, have high uncertainties. Climate change has the potential to increase CH4 emissions from critical systems such as wetlands, marine and freshwater systems, permafrost, and methane hydrates, through shifts in temperature, hydrology, vegetation, landscape disturbance and sea level rise. Increased CH4 emissions from these systems would in turn induce further climate change, resulting in a positive climate feedback. Here we synthesize biological, geochemical and physically focused CH4 climate feedback literature, bringing together the key findings of these disciplines. We discuss environment-specific feedback processes, including the microbial, physical and geochemical inter-linkages and the timescales on which they operate, and present the current state of knowledge of CH4 climate feedbacks in the immediate and distant future. The important linkages between microbial activity and climate warming are discussed with the aim to better constrain the sensitivity of the CH4 cycle to future climate predictions. We determine that wetlands will form the majority of the CH4 climate feedback up to 2100. Beyond this timescale, CH4 emissions from marine and freshwater systems and permafrost environments could become more important. Significant CH4 emissions to the atmosphere from the dissociation of methane hydrates are not expected in the near future. Our key findings highlight the importance of quantifying whether CH4 consumption can counter balance CH4 production under future climate scenarios.
... CH 4 transportation is driven by three major mechanisms, namely, molecular diffusion, bubble ebullition, and plant-mediated transportation (Bridgham et al., 2013;Chen et al., 2013;Zhu et al., 2016). These mechanisms are affected by water stratification and seasonal overturns of the water mass, which are determined by temperature (Palma-Silva et al., 2013;Rõõm et al., 2014), wind-forced mixing (Wanninkhof, 1992;Palma-Silva et al., 2013), water depth , boundary layer dynamics (Poindexter et al., 2016;Anthony and Macintyre, 2016), hydrostatic pressure (Chanton, 1989), and different vascular plants (Juutinen et al., 2009;Zhu et al., 2016). Most studies examined CH 4 emissions and their influencing factors in small lakes because of their large contribution to the global CH 4 budget (Bastviken et al., 2004;Downing, 2010;Bartosiewicz et al., 2015;Holgerson and Raymond, 2016). ...
Lakes are an important natural source of CH4 to the atmosphere. However, the multi-seasonal CH4 efflux from lakes has been rarely studied. In this study, the CH4 efflux from Poyang Hu, the largest freshwater lake in China, was measured monthly over a 4-year period by using the floating-chamber technique. The mean annual CH4 efflux throughout the 4 years was 0.54 mmol m⁻² day⁻¹, ranging from 0.47 to 0.60 mmol m⁻² day⁻¹. The CH4 efflux had a high seasonal variation with an average summer (June to August) efflux of 1.34 mmol m⁻² day⁻¹ and winter (December to February) efflux of merely 0.18 mmol m⁻² day⁻¹. The efflux showed no apparent diel pattern, although most of the peak effluxes appeared in the late morning, from 10:00 to 12:00 CST (GMT + 8). Multivariate stepwise regression on a seasonal scale showed that environmental factors, such as sediment temperature, sediment total nitrogen content, dissolved oxygen, and total phosphorus content in the water, mainly regulated the CH4 efflux. However, the CH4 efflux only showed a strong positive linear correlation with wind speed within 1 day on a bihourly scale in the multivariate regression analyses but almost no correlation with wind speed on diurnal and seasonal scales.
... More recently, AOM has also been linked to the microbial reduction of nitrate (3,4) and nitrite (5), as well as Fe(III) and Mn(IV) oxides (6)(7)(8), in freshwater and marine environments. Wetlands are the largest natural source of CH 4 (9), contributing to about one-third of global emissions (10), but key drivers, such as electron acceptors fueling methanotrophic activities in these habitats, are poorly understood. CH 4 emissions from wetlands have been strongly responsive to climate in the past and will likely continue to be responsive to anthropogenic-driven climate change in the future, predicting a large impact on global atmospheric CH 4 concentration (10). ...
Wetlands constitute the main natural source of methane on Earth due to their high content of natural organic matter (NOM), but key drivers, such as electron acceptors, supporting methanotrophic activities in these habitats are poorly understood. We performed anoxic incubations using freshly collected sediment, along with water samples harvested from a tropical wetland, amended with ¹³C-methane (0.67 atm) to test the capacity of its microbial community to perform anaerobic oxidation of methane (AOM) linked to the reduction of the humic fraction of its NOM. Collected evidence demonstrates that electron-accepting functional groups (e.g., quinones) present in NOM fueled AOM by serving as a terminal electron acceptor. Indeed, while sulfate reduction was the predominant process, accounting for up to 42.5% of the AOM activities, the microbial reduction of NOM concomitantly occurred. Furthermore, enrichment of wetland sediment with external NOM provided a complementary electron-accepting capacity, of which reduction accounted for ~100 nmol ¹³CH4 oxidized · cm⁻³ · day⁻¹. Spectroscopic evidence showed that quinone moieties were heterogeneously distributed in the wetland sediment, and their reduction occurred during the course of AOM. Moreover, an enrichment derived from wetland sediments performing AOM linked to NOM reduction stoichiometrically oxidized methane coupled to the reduction of the humic analogue anthraquinone-2,6- disulfonate. Microbial populations potentially involved in AOM coupled to microbial reduction of NOM were dominated by divergent biota from putative AOMassociated archaea. We estimate that this microbial process potentially contributes to the suppression of up to 114 teragrams (Tg) of CH4 · year⁻¹ in coastal wetlands and more than 1,300 Tg · year⁻¹, considering the global wetland area.
Inland waters are a significant source of atmospheric greenhouse gas (GHG) emissions. The thin boundary layer (TBL) model is often employed as a means of estimating GHG diffusion in inland waters based on gas transfer velocity (k) at the air-water interface, with k being subject to regulation by near-surface turbulence that is primarily driven by wind speed in many cases. This wind speed-based estimation of k (wind-k), however, can introduce substantial uncertainty for turbulent waterways where wind speed does not accurately represent overall turbulence. In this study, GHG diffusion in the Beijing-Hangzhou Grand Canal (China), the first and longest man-made canal in the world, was estimated using the TBL model, revealing that this model substantially underestimated GHG diffusion when relying on wind-k. Strikingly, the carbon dioxide, methane, and nitrous oxide diffusions were respectively underestimated by 159%, 162%, and 124% when using this model. These findings are significant for developing more reliable approaches to evaluate GHG emissions from inland waterways.
Inland waters are important emitters of the greenhouse gasses (GHGs) carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) to the atmosphere. In the framework of the 2nd phase of the REgional Carbon Cycle Assessment and Processes (RECCAP‐2) initiative, we review the state of the art in estimating inland water GHG budgets at global scale, which has substantially advanced since the first phase of RECCAP nearly 10 years ago. The development of increasingly sophisticated upscaling techniques, including statistical prediction and process‐based models, allows for spatially explicit estimates that are needed for regionalized assessments of continental GHG budgets such as those established for RECCAP. A few recent estimates also resolve the seasonal and/or interannual variability in inland water GHG emissions. Nonetheless, the global‐scale assessment of inland water emissions remains challenging because of limited spatial and temporal coverage of observations and persisting uncertainties in the abundance and distribution of inland water surface areas. To decrease these uncertainties, more empirical work on the contributions of hot‐spots and hot‐moments to overall inland water GHG emissions is particularly needed.
Thermokarst lakes are known to emit methane (CH4) and carbon dioxide (CO2), but little attention has been given to those formed from the thawing and collapse of lithalsas, ice-rich mineral soil mounds that occur in permafrost landscapes. The present study was undertaken to assess greenhouse gas stocks and fluxes in eight lithalsa lakes across a 200 km gradient of permafrost degradation in subarctic Québec. The northernmost lakes varied in their surface-water CO2 content from below to above saturation, but the southern lakes in this gradient had much higher surface concentrations that were well above air-equilibrium. Surface-water CH4 concentrations were at least an order of magnitude above air-equilibrium values at all sites, and the diffusive fluxes of both gases increased from north to south. Methane oxidation in the surface waters from a northern lake was only 10% of the emission rate, but at the southern end it was around 60% of the efflux to the atmosphere, indicating that methanotrophy can play a substantive role in reducing net emissions. Overall, our observations show that lithalsa lakes can begin emitting CH4 and CO2 soon after they form, with effluxes of both gases that persist and increase as the permafrost continues to warm and erode.
Terrestrial net CH<sub>4</sub> surface fluxes often represent the difference between much larger gross production and consumption fluxes and depend on multiple physical, biological, and chemical mechanisms that are poorly understood and represented in regional- and global-scale biogeochemical models. To characterize uncertainties, study feedbacks between CH<sub>4</sub> fluxes and climate, and to guide future model development and experimentation, we developed and tested a new CH<sub>4</sub> biogeochemistry model (CLM4Me) integrated in the land component (Community Land Model; CLM4) of the Community Earth System Model (CESM1). CLM4Me includes representations of CH<sub>4</sub> production, oxidation, aerenchyma transport, ebullition, aqueous and gaseous diffusion, and fractional inundation. As with most global models, CLM4 lacks important features for predicting current and future CH<sub>4</sub> fluxes, including: vertical representation of soil organic matter, accurate subgrid scale hydrology, realistic representation of inundated system vegetation, anaerobic decomposition, thermokarst dynamics, and aqueous chemistry. We compared the seasonality and magnitude of predicted CH<sub>4</sub> emissions to observations from 18 sites and three global atmospheric inversions. Simulated net CH<sub>4</sub> emissions using our baseline parameter set were 270, 160, 50, and 70 Tg CH<sub>4</sub> yr<sup>−1</sup> globally, in the tropics, in the temperate zone, and north of 45° N, respectively; these values are within the range of previous estimates. We then used the model to characterize the sensitivity of regional and global CH<sub>4</sub> emission estimates to uncertainties in model parameterizations. Of the parameters we tested, the temperature sensitivity of CH<sub>4</sub> production, oxidation parameters, and aerenchyma properties had the largest impacts on net CH<sub>4</sub> emissions, up to a factor of 4 and 10 at the regional and gridcell scales, respectively. In spite of these uncertainties, we were able to demonstrate that emissions from dissolved CH<sub>4</sub> in the transpiration stream are small (<1 Tg CH<sub>4</sub> yr<sup>−1</sup>) and that uncertainty in CH<sub>4</sub> emissions from anoxic microsite production is significant. In a 21st century scenario, we found that predicted declines in high-latitude inundation may limit increases in high-latitude CH<sub>4</sub> emissions. Due to the high level of remaining uncertainty, we outline observations and experiments that would facilitate improvement of regional and global CH<sub>4</sub> biogeochemical models.
Gas fluxes from lakes and other stratified water bodies, computed using conservative values of the gas transfer coefficient k600, have been shown to be a significant component of the carbon cycle. We present a mechanistic analysis of the dominant physical processes modifying k600 in a stratified lake and resulting new models of k600 whose use will enable improved computation of carbon fluxes. Using eddy covariance results, we demonstrate that i) higher values of k600 occur during low to moderate winds with surface cooling than with surface heating; ii) under overnight low wind conditions k600 depends on buoyancy flux β rather than wind speed; iii) the meteorological conditions at the time of measurement and the inertia within the lake determine k600; and iv) eddy covariance estimates of k600 compare well with predictions of k600 using a surface renewal model based on wind speed and β.
Methane emissions from natural wetlands constitute the largest methane source at present and depend highly on the climate. In order to investigate the response of methane emissions from natural wetlands to climate variations, a one-dimensional process-based climate-sensitive model to derive methane emissions from natural wetlands is developed. In the model the processes leading to methane emission are simulated within a one-dimensional soil column and the three different transport mechanisms, diffusion, plant-mediated transport, and ebullition, are modeled explicitly. The model forcing consists of daily values of soil temperature, water table, and net primary productivity, and at permafrost sites the thaw depth is included. The methane model is tested using observational data obtained at five wetland sites located in North America, Europe, and Central America, representing a large variety of environmental conditions. It can be shown that in most cases seasonal variations in methane emissions can be explained by the combined effect of changes in soil temperature and the position of the water table. Our results also show that a process-based approach is needed because there is no simple relationship between these controlling factors and methane emissions that applies to a variety of wetland sites. The sensitivity of the model to the choice of key model parameters is tested and further sensitivity tests are performed to demonstrate how methane emissions from wetlands respond to longer-term climate variations.
Inland waters (lakes, reservoirs, streams, and rivers) are often substantial methane (CH4) sources in the terrestrial landscape. They are, however, not yet well integrated in global greenhouse gas (GHG) budgets.
Data from 474 freshwater ecosystems and the most recent global water area estimates indicate that freshwaters emit at least
103 teragrams of CH4 year−1, corresponding to 0.65 petagrams of C as carbon dioxide (CO2) equivalents year−1, offsetting 25% of the estimated land carbon sink. Thus, the continental GHG sink may be considerably overestimated, and
freshwaters need to be recognized as important in the global carbon cycle.
Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 34 (2007): L10601, doi:10.1029/2006GL028790. Air-water gas transfer influences CO2 and other climatically important trace gas fluxes on regional and global scales, yet the magnitude of the transfer is not well known. Widely used models of gas exchange rates are based on empirical relationships linked to wind speed, even though physical processes other than wind are known to play important roles. Here the first field investigations are described supporting a new mechanistic model based on surface water turbulence that predicts gas exchange for a range of aquatic and marine processes. Findings indicate that the gas transfer rate varies linearly with the turbulent dissipation rate to the inline equation power in a range of systems with different types of forcing - in the coastal ocean, in a macro-tidal river estuary, in a large tidal freshwater river, and in a model (i.e., artificial) ocean. These results have important implications for understanding carbon cycling. This research was performed and the manuscript prepared with support from: the National Science Foundation (OCE-03-27256, OCE-05-26677, ATM 01-20569, and DEB-05-32075), the Office of Naval Research Young Investigator Program (N00014-04-1-0621), the Hudson River Foundation (010/02A), NOAA (NA03OAR4320179), the Marie Curie Training Site Fellowship (HPMFCT- 2002-01865), the NERC (NER/B/S/2003/00844), the David and Lucille Packard Foundation, and the LDEO Climate Center.
The sources of methane and its flux to the troposphere from the Amazonian floodplain were investigated during the dry season of July and August 1985, using measurements of methane concentration gradients obtained aboard a houseboat laboratory anchored in Lago Calado, a stratified dendritic lake of about 6-sq km area located near the center of the Amazon Basin. Methane concentrations in the mixed layer of the lake were found to vary from 0.0001 to 0.0055 mM, with no consistent temporal trend. The measured methane flux from the surface of the open lake to the atmosphere averaged 27 mg CH4/sq m per day, consistent with the buildup in ambient methane in the nocturnal surface mixed layer of the troposphere. Ebullition contributed 70 percent to the average total flux. The source of methane to the lake and, ultimately, to the troposphere is the benthic sediments.
Methane flux was linearly correlated with plant biomass (r = 0.97, n = 6 and r = 0.95, n = 8) at two locations in a Florida Everglades Cladium marsh. One location, which had burned 4 months previously, exhibited a greater increase in methane flux as a function of biomass relative to sites at an unburned location. However, methane flux data from both sites fit a single regression (r = 0.94, n = 14) when plotted against net CO2 exchange suggesting that either methanogenesis in Everglades marl sediments is fueled by root exudation below ground, or that factors which enhance photosynthetic production and plant growth are also correlated with methane production and flux in this oligotrophic environment. The data presented are the first to show a direct relationship between spatial variability in plant biomass, net ecosystem production, and methane emission in a natural wetland.
One of the defining properties of electrons is their mutual Coulomb repulsion. However, in solids this basic property may change; for example, in superconductors, the coupling of electrons to lattice vibrations makes the electrons attract one another, leading to the formation of bound pairs. Fifty years ago it was proposed that electrons can be made attractive even when all of the degrees of freedom in the solid are electronic, by exploiting their repulsion from other electrons. This attraction mechanism, termed 'excitonic', promised to achieve stronger and more exotic superconductivity. Yet, despite an extensive search, experimental evidence for excitonic attraction has yet to be found. Here we demonstrate this attraction by constructing, from the bottom up, the fundamental building block of the excitonic mechanism. Our experiments are based on quantum devices made from pristine carbon nanotubes, combined with cryogenic precision manipulation. Using this platform, we demonstrate that two electrons can be made to attract each other using an independent electronic system as the 'glue' that mediates attraction. Owing to its tunability, our system offers insights into the underlying physics, such as the dependence of the emergent attraction on the underlying repulsion, and the origin of the pairing energy. We also demonstrate transport signatures of excitonic pairing. This experimental demonstration of excitonic pairing paves the way for the design of exotic states of matter.
Key Points: We compare hydrodynamic methane transport and total methane flux at a temperate, freshwater marsh Approximately one third of the total annual methane flux occurs via hydrodynamic transport Hydrodynamic transport is largest at night because of thermal convection Abstract: Wetland methane transport processes affect what portion of methane produced in wetlands reaches the atmosphere. We model what has been perceived to be the least important of these transport processes: hydrodynamic transport of methane through wetland surface water, and show that its contribution to total methane emissions from a temperate freshwater marsh is surprisingly large. In our 1-year study, hydrodynamic transport comprised more than half of nighttime methane fluxes, and was driven primarily by water column thermal convection occurring overnight as the water surface cooled. Overall, hydrodynamic transport was responsible for 32% of annual methane emissions. Many methane models have overlooked this process, but our results show wetland methane fluxes cannot always be accurately described using only other transport processes (plant-mediated transport and ebullition). Modifying models to include hydrodynamic transport and the mechanisms that drive it, particularly convection, could help improve predictions of future wetland methane emissions.
Recent studies suggested that under low to moderate wind conditions without bubble entraining wave breaking, the air-water gas transfer velocity k+ can be mechanistically parameterized by the near-surface turbulence, following the small eddy model (SEM). Field measurements have supported this model in a variety of environmental forcing systems. Alternatively, surface divergence model (SDM) has also been shown to predict the gas transfer velocity across the air-water interface in laboratory settings. However, the empirically determined model coefficients (α in SEM and c1 in SDM) scattered over a wide range. Here we present the first field measurement of the near-surface turbulence with a novel floating PIV system on Lake Michigan, which allows us to evaluate the SEM and SDM in situ in the natural environment. k+ was derived from the CO2 flux that was measured simultaneously with a floating gas chamber. Measured results indicate that α and c1 are not universal constants. Regression analysis showed that α ~ log(ε) while the near surface turbulence dissipation rate ε is approximately greater than 10–6 m2s–3 according to data measured for this study as well as from other published results measured in similar environments or in laboratory settings. It also showed that α scales linearly with the turbulent Reynolds number. Similarly, coefficient c1 in the SDM was found to linearly scale with the Reynolds number. These findings suggest that larger eddies are also important parameters, and the dissipation rate in the SEM or the surface divergence β’ in the SDM alone may not be adequate to determine k+ completely. This article is protected by copyright. All rights reserved.
This book covers information on the study of biosphere-atmosphere trace gas exchange including the following subject areas: currently available methods and approaches used to measure trace gases exchanges; the methods that will be used in the future; and the studies, mathematical models, and extrapolation approaches used to describe the fluxes over a range of spatial and temporal scales.
Most earth system models that simulate wetland methane emissions ignore spatial variability of water table depth. We compared methane emissions estimated using two commonly-used water table approximations (wet-dry and uniform) to a scheme that accounts for spatial variability, for several combinations of methane model parameter values and a range of water table depth spatial variability (sigmazwt). The approximations' biases were consistent across methane model parameter sets. The wet-dry scheme's biases were smallest in absolute value (+/-5-10%) when sigmazwt was large (38 cm) but increased to between -85% and -95% as sigmazwt approached 0. The uniform scheme's biases were largest (+/-100% or more) when sigmazwt was large and vanished as sigmazwt approached 0. For sigmazwt values typical of boreal wetlands, these schemes' previous predictions that global wetland methane emissions will double by 2100 are in error by factors ranging from 0.7 to 1.3. Similar biases may affect paleoclimate studies.
The findings of an extensive midsummer survey of CH4 emissions measurements representing the Alaska Arctic tundra are presented. Variability in rates of emissions was similar in magnitude on local and regional scales, ranging from 0 to 286.5 mg/sq m/d overall and often varying across two orders of magnitude within 0.5 m distances. Primary control on rates of emission was determined by the substrate and position of the water table relative to the surface. Emission rates in the Arctic Foothills ranged from 0.2 mg/sq m/d for tussock tundra to 55.53 mg/sq m/d over wet meadows. Plant-mediated release of CH4 to the atmosphere was directly proportional to green leaf area and represented 92-98 percent of the total emission rates over vegetated sites. The results suggest the current published emission rates may have overestimated the contribution of boreal ecosystems to the global CH4 budget by several fold.
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