Search Results (12)

Water-capped tailings technology (WCTT) is a strategy where oil sand tailings are sequestered within a mined-out pit and overlayed with a layer of water in order to sequester tailings with the aim that the resulting pit lake will support aquatic plants and organisms over time. The Base Mine Lake Demonstration (BML) is the first full-scale demonstration of a pit lake in the Athabasca Oil Sands Region (AOSR). In the BML, the release of methane from the fluid tailings influences several key processes, including the flux of greenhouse gases, microbial oxygen consumption in the water column, and ebullition-facilitated transport of organics from the fluid tailings to the lake surface. It is hypothesized that the residual low molecular weight hydrocarbons (LMWHCs) derived from diluent naphtha used during bitumen extraction processes are the carbon sources fueling ongoing microbial methanogenesis within the BML. The aims of this study were to identify the LMWHCs in the BML fluid tailings, to elucidate their sources, and to assess the extent of biogeochemical cycling affecting them. A headspace GC/MS analysis identified 84, 44, and 56 LMWHCs (C4–C10) present in naphtha, unprocessed bitumen ore, and fluid tailings, respectively. Equilibrium mass balance assessment indicated that the vast majority (>95%) of LMWHCs were absorbed within residual bitumen rather than dissolving into tailings pore water. Such absorbed compounds would not be readily available to in situ microbial communities but would represent a long-term source for methanogenesis. Chromatographic analysis revealed that most biodegradable compounds (n-alkanes and BTEX) were present in the naphtha but not in fluid tailings or bitumen ore, implying they are sourced from the naphtha and have been preferentially biodegraded after being deposited. Among the LMWHCs observed in bitumen ore, naphtha, and fluid tailings, C2-cyclohexanes had the highest relative abundance in tailings samples, implying their relatively high recalcitrance to in situ biodegradation. Full article

The greenhouse effect, which is also promoted by naturally occurring biogas ebullition fluxes (released via bubbles), generated by the decomposition of organic matter in carbonate-enriched and black silt sediments, has been analyzed. This study is based on results obtained using passive gas collectors at different parts of eutrophic Lake Juodis, located in a temperate climate zone in the vicinity of Vilnius (Lithuania). The measured annual biogas (containing about 60% of biomethane) ebullition fluxes from carbonate-enriched sediments and black silt sediments were 16.9–23.0 L/(m²∙y) and 38.5–43.2 L/(m²∙y), respectively. This indicates that the gas fluxes from carbonate sediments were almost twice as low as those from black silt sediments. Oxygen, produced by the photosynthetic activity of green algae in the near-surface water and sediments, helps to retain carbonates in the sediments by preventing their dissolution. In turn, the calcite coating on sediment particles partially preserves organic matter from decomposition, reducing the effective thickness of the sediment layer generating biogas. The characteristic vertical distribution profile of ¹³⁷Cs activity, with sharp peaks in sediments, suggests that generated biogas bubbles move to the surface of the sediments forming vertical channels by pushing sediment particles asides without noticeably mixing them vertically. This examination showed that factors such as abundance of carbonates in the sediments may result in a significant reduction in biogas generation and emissions from the lake sediments. Full article

Reservoirs are an important source of methane (CH₄) emissions, but the relative contribution of CH₄ ebullition and diffusion fluxes to total fluxes has received little attention in the past. In this study, we systematically monitored the CH₄ fluxes of nine cascade reservoirs (Dahejia, Jishixia, Huangfeng, Suzhi, Kangyang, Zhiganglaka, Lijiaxia, Nina, and Longyangxia) in the upper reaches of the Yellow River in the dry (May 2023) and wet seasons (August 2023) using the static chamber gas chromatography and headspace equilibrium methods. We also simultaneously measured environmental physicochemical properties as well as the abundance of methanogens and methanotrophs in sediments. The results showed the following: (1) All reservoirs were sources of CH₄ emissions, with an average diffusion flux of 0.08 ± 0.05 mg m⁻² h⁻¹ and ebullition flux of 0.38 ± 0.41 mg m⁻² h⁻¹. Ebullition flux accounted for 78.01 ± 7.85% of total flux. (2) Spatially, both CH₄ diffusion and ebullition fluxes increased from upstream to downstream. Temporally, CH₄ diffusion flux in the wet season (0.09 ± 0.06 mg m⁻² h⁻¹) was slightly higher than that in the dry season (0.08 ± 0.04 mg m⁻² h⁻¹), but CH₄ ebullition flux in the dry season (0.38 ± 0.48 mg m⁻² h⁻¹) was higher than that in the wet season (0.32 ± 0.2 mg m⁻² h⁻¹). (3) qPCR showed that methanogens (mcrA gene) were more abundant in the wet season (5.43 ± 3.94 × 10⁵ copies g⁻¹) than that in the dry season (3.74 ± 1.34 × 10⁵ copies g⁻¹). Methanotrophs (pmoA gene) also showed a similar trend with more abundance found in the wet season (7 ± 2.61 × 10⁵ copies g⁻¹) than in the dry season (1.47 ± 0.92 × 10⁵ copies g⁻¹. (4) Structural equation modeling revealed that the ratio of mcrA/pmoA genes, water N/P, and reservoir age were key factors affecting CH₄ ebullition flux. Variation partitioning further indicated that the ratio of mcrA/pmoA genes was the main factor causing the spatial variation in CH₄ ebullition flux, explaining 35.69% of its variation. This study not only reveals the characteristics and influencing factors of CH₄ emissions from cascade reservoirs on the Qinghai Plateau but also provides a scientific basis for calculating fluxes and developing global CH₄ reduction strategies for reservoirs. Full article

The mechanistic model LAKE2.3 was tested for its capability to predict of methane (CH₄) emissions from reservoirs. Estimates of CH₄ emissions from the Mozhaysk reservoir (Moscow region) provided by the model showed good agreement with instrumental in situ observations for several parameters of the water ecosystem. The average CH₄ flux calculated by the model is 37.7 mgC-CH₄ m⁻² day⁻¹, while according to observations, it is 34.4 mgC-CH₄ m⁻² day⁻¹. Ebullition makes the largest contribution to the emissions from reservoirs (up to 95%) due to low methane solubility in water and the high oxidation rate of diffusive methane flux. During the heating period, an increase in methane emission is observed both in the model and empirical data, with a maximum before the onset of the autumn overturn. An effective parameter for calibrating the diffusive methane flux in the model is the potential rate of methane oxidation. For ebullition flux, it is the parameter q10 (an empirical parameter determining the relationship between methane generation and temperature) because methane production in bottom sediments is the most important. The results of this research can be used to develop mechanistic models and provide a necessary step toward regional and global simulations of lacustrine methane emission using LAKE2.3. Full article

This paper presents the results of an acoustic survey carried out from the fast ice in the shallow waters of the East Siberian Arctic Shelf (ESAS) using a single beam echosounder. The aim of this paper is to demonstrate an improved approach to study seafloor seepages in the Arctic coastal zone with an echosounder calibrated on site. During wintertime field observations of natural rising gas bubbles, we recorded three periods of their increased activity with a total of 63 short-term ejections of bubbles from the seabed. This study presents quantitative estimates of the methane (CH₄) flux obtained in wintertime at two levels of the water column: (1) at the bottom/water interface and (2) at the water/sea ice interface. In winter, the flux of CH₄ transported by rising bubbles to the bottom water in the shallow part of the ESAS was estimated at ~19 g·m⁻² per day, while the flux reaching the water/sea ice interface was calculated as ~15 g·m⁻² per day taking into account the diffusion of CH₄ in the surrounding water and the enrichment of rising bubbles with nitrogen and oxygen. We suggest that this bubble-transported CH₄ flux reaching the water /sea ice interface can be emitted into the atmosphere through numerous ice trenches, leads, and polynyas. This CH₄ ebullition value detected at the water/sea ice interface is in the mid high range of CH₄ ebullition value estimated for the entire ESAS, and two orders higher than the upper range of CH₄ ebullition from the northern thermocarst lakes, which are considered as a significant source to the atmospheric methane budget. Full article

Freshwater reservoirs are widely recognized as methane (CH₄) emission hotspots. Existing research has shown that temperature and hydrological conditions significantly affect wetland CH₄ cycling processes. However, the feedback of the CH₄ cycle to climate warming remains unclear for deep reservoirs where seasonal water thermal stratification exists. This study combined a reservoir CH₄ cycling model and a Statistical DownScaling Model (SDSM) to evaluate reservoir CH₄ cycling feedbacks under multiple climate change scenarios while accounting for hydrological uncertainty. Daily air temperatures in 2100 were predicted by the combination of the CanESM5 model and a SDSM. To address hydrological uncertainty, we selected three representative hydrological years (i.e., wet, normal, and dry) to create hydrological scenarios. Results showed that annual sediment CH₄ production increased with warming, ranging 323.1–413.7 × 10³ t C year⁻¹ among multiple scenarios. Meanwhile, the CH₄ oxidation percentage decreased with warming, which meant warming promoted sediment CH₄ release non-linearly; 67.8–84.6% of sediment ebullient flux was ultimately emitted to the atmosphere (51.3–137.7 × 10³ t C year⁻¹), which showed ebullition was the dominant emission pathway. Higher air temperatures and drier conditions generally promote reservoir emissions. This study is helpful for predicting reservoir emissions while directing decision-making for reservoir sustainability. Full article

Understanding the mechanism of bubble growth is crucial to modeling boiling heat transfer and enabling the development of technological applications, such as energy systems and thermal management processes, which rely on boiling to achieve the high heat fluxes required for their operation. This paper presents analyses of the evaporation of “microlayers”, i.e., ultra-thin layers of liquid present beneath steam bubbles growing at the heated surface in the atmospheric pressure nucleate of boiling water. Evaporation of the microlayer is believed to be a major contributor to the phase change heat transfer, but its evolution, spatio-temporal stability, and impact on macroscale bubble dynamics are still poorly understood. Mass, momentum, and energy transfer in the microlayer are modeled with a lubrication theory approach that accounts for capillary and intermolecular forces and interfacial mass transfer. The model is embodied in a third-order nonlinear film evolution equation, which is solved numerically. Variable wall-temperature boundary conditions are applied at the solid–liquid interface to account for conjugate heat transfer due to evaporative heat loss at the liquid–vapor interface. Predictions obtained with the current approach compare favorably with experimental measurements of microlayer evaporation. By comparing film profiles at a sequence of times into the ebullition cycle of a single bubble, likely values of evaporative heat transfer coefficients were inferred and found to fall within the range of previously reported estimates. The result suggests that the coefficients may not be a constant, as previously assumed, but instead something that varies with time during the ebullition cycle. Full article

Reservoirs are an integral part of the global carbon cycle and generally considered to be methane (CH₄) emission hot spots. Although remarkable research achievements have been made concerning CH₄ ebullition from inland waters, such as rivers, lakes, and ponds, few have been devoted to CH₄ ebullition from plateau reservoirs. The present study focused on CH₄ ebullition from the Dahejia Reservoir located in the upper reaches of the Yellow River. We analyzed the spatial and temporal characteristics of CH₄ ebullition flux across the water-atmosphere interface between July and August 2021. We also evaluated the influence of microbes on CH₄ ebullition flux. The results showed that (1) CH₄ ebullition was the dominant mode of CH₄ emissions in the study site, which contributed to 78.85 ± 20% of total CH₄ flux. (2) The mean CH₄ ebullition flux in the nighttime (0.34 ± 0.21 mg m⁻² h⁻¹) was significantly higher than that in the daytime (0.19 ± 0.21 mg m⁻² h⁻¹). The mean CH₄ ebullition flux first decreased and then increased from the upstream (0.52 ± 0.57 mg m⁻² h⁻¹) to the downstream (0.43 ± 0.3 mg m⁻² h⁻¹) of the Yellow River. (3) Sediment microbes affected the CH₄ ebullition flux primarily by changing the microbial community abundance. The regression analysis showed that CH₄ ebullition flux had a significantly linear negative correlation with microbial abundance in sediments. The redundancy analysis further showed CH₄ ebullition flux was significantly positively correlated with the abundances of Firmicutes and Actinobacteria, and negatively with that of Proteobacteria and Chloroflexi. Among abiotic variables, CH₄ ebullition flux was closely related to total phosphorus, total organic carbon, pH and nitrate nitrogen. Full article

Ponds are abundant in the boreal peatland landscape, which are potential hotspots for greenhouse gas (GHG) emissions. However, compared to large lakes, ponds are difficult to identify by satellite, and they have not been adequately studied. Here, we observed methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) fluxes in the growing season at three sites along the water table gradient from the pelagic zone, littoral zone and bog across a shallow pond in a boreal peatland landscape in Northeastern China. The results showed that the littoral zone, dominated by herb Carex, was the hotspot for CH₄ emissions. CH₄ fluxes in littoral zone averaged 78.98 ± 19.00 mg m⁻² h⁻¹. The adjacent bog was a weak source of CH₄ emissions, with an average flux of 0.07 ± 0.05 mg m⁻² h⁻¹. Within the pond, CH₄ was mainly emitted through ebullition, accounting for 88.56% of the total CH₄ fluxes, and the ebullition fluxes were negatively correlated with dissolved oxygen (DO). CO₂ fluxes were highest in the pelagic zone, with an average of 419.76 ± 47.25 mg m⁻² h⁻¹. Wind and strong sediment respiration were key factors that led to the high fluxes. The observed three sites were all atmospheric N₂O sinks ranging from −0.92 to −10.90 μg m⁻² h⁻¹. This study highlights the spatial variation in greenhouse gas fluxes from the pond and its adjacent bog, ignoring the ecotone area may underestimate CH₄ fluxes. Although ponds are a hotspot for CH₄ and CO₂ emissions, they can also be a sink for N₂O, which provides a reference for the quantification of global pond GHG fluxes. Therefore, finer-scale in situ observations are necessary to better understand the feedback of permafrost peatland ponds to global warming. Full article

The emissions of greenhouse gases (GHGs) from inland waters are an important component of the global carbon (C) cycle. However, the current understanding of GHGs emissions from arid river systems remains largely unknown. To shed light on GHGs emissions from inland waters in arid regions, high-resolution carbon dioxide (CO₂) and methane (CH₄) emission measurements were carried out in the arid Kuye River Basin (KRB) on the Chinese Loess Plateau to examine their spatio-temporal variability. Our results show that all streams and rivers were net C sources, but some of the reservoirs in the KRB became carbon sinks at certain times. The CO₂ flux (FCO₂) recorded in the rivers (91.0 mmol m⁻² d⁻¹) was higher than that of the reservoirs (10.0 mmol m⁻² d⁻¹), while CH₄ flux (FCH₄) in rivers (0.35 mmol m⁻² d⁻¹) was lower than that of the reservoirs (0.78 mmol m⁻² d⁻¹). The best model developed from a number of environmental parameters was able to explain almost 40% of the variability in partial pressure of CO₂ (pCO₂) for rivers and reservoirs, respectively. For CH₄ emissions, at least 70% of the flux occurred in the form of ebullition. The emissions of CH₄ in summer were more than threefold higher than in spring and autumn, with water temperature being the key environmental variable affecting emission rates. Since the construction of reservoirs can alter the morphology of existing fluvial systems and consequently the characteristics of CO₂ and CH₄ emissions, we conclude that future sampling efforts conducted at the basin scale need to cover both rivers and reservoirs concurrently. Full article

Seeps found offshore in the East Siberian Arctic Shelf may mark zones of degrading subsea permafrost and related destabilization of gas hydrates. Sonar surveys provide an effective tool for mapping seabed methane fluxes and monitoring subsea Arctic permafrost seepage. The paper presents an overview of existing approaches to sonar estimation of methane bubble flux from the sea floor to the water column and a new method for quantifying CH₄ ebullition. In the suggested method, the flux of methane bubbles is estimated from its response to insonification using the backscattering cross section. The method has demonstrated its efficiency in the case study of single- and multi-beam acoustic surveys of a large seep field on the Laptev Sea shelf. Full article

Thermokarst lakes in the Arctic and Subarctic release carbon from thawing permafrost in the form of methane and carbon dioxide with important implications for regional and global carbon cycles. Lake ice impedes the release of gas during the winter. For instance, bubbles released from lake sediments become trapped in downward growing lake ice, resulting in vertically-oriented bubble columns in the ice that are visible on the lake surface. We here describe a classification technique using an object-based image analysis (OBIA) framework to successfully map ebullition bubbles in airborne imagery of early winter ice on an interior Alaska thermokarst lake. Ebullition bubbles appear as white patches in high-resolution optical remote sensing images of snow-free lake ice acquired in early winter and, thus, can be mapped across whole lake areas. We used high-resolution (9–11 cm) aerial images acquired two and four days following freeze-up in the years 2011 and 2012, respectively. The design of multiresolution segmentation and region-specific classification rulesets allowed the identification of bubble features and separation from other confounding factors such as snow, submerged and floating vegetation, shadows, and open water. The OBIA technique had an accuracy of >95% for mapping ebullition bubble patches in early winter lake ice. Overall, we mapped 1195 and 1860 ebullition bubble patches in the 2011 and 2012 images, respectively. The percent surface area of lake ice covered with ebullition bubble patches for 2011 was 2.14% and for 2012 was 2.67%, representing a conservative whole lake estimate of bubble patches compared to ground surveys usually conducted on thicker ice 10 or more days after freeze-up. Our findings suggest that the information derived from high-resolution optical images of lake ice can supplement spatially limited field sampling methods to better estimate methane flux from individual lakes. The method can also be used to improve estimates of methane ebullition from numerous lakes within larger regions. Full article