[1] There is growing concern about the transfer of methane originating from water bodies to the atmosphere. Methane from sediments can reach the atmosphere directly via bubbles or indirectly via vertical turbulent transport. This work quantifies methane gas bubble dissolution using a combination of bubble modeling and acoustic observations of rising bubbles to determine what fraction of the methane transported by bubbles will reach the atmosphere. The bubble model predicts the evolving bubble size, gas composition, and rise distance and is suitable for almost all aquatic environments. The model was validated using methane and argon bubble dissolution measurements obtained from the literature for deep, oxic, saline water with excellent results. Methane bubbles from within the hydrate stability zone (typically below $500 m water depth in the ocean) are believed to form an outer hydrate rim. To explain the subsequent slow dissolution, a model calibration was performed using bubble dissolution data from the literature measured within the hydrate stability zone. The calibrated model explains the impressively tall flares (>1300 m) observed in the hydrate stability zone of the Black Sea. This study suggests that only a small amount of methane reaches the surface at active seep sites in the Black Sea, and this only from very shallow water areas (<100 m). Clearly, the Black Sea and the ocean are rather effective barriers against the transfer of bubble methane to the atmosphere, although substantial amounts of methane may reach the surface in shallow lakes and reservoirs.

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Recent research into C02 geological storage has shown that it has potential to be a safe and effective way to rapidly decrease short-term anthropogenic C02 emissions. Despite this progress, stakeholders must be convinced that the scientific community has studied all possible scenarios, including a potential leak into the biosphere. To better understand the potential impact of such an event, a detailed geochemical and biological study was conducted during two different seasons on a naturally occurring gas vent located within a Mediterranean pasture ecosystem (Latera geothermal field, central Italy). Results from botanical, soil gas, and gas flux surveys, and from chemical and biological analyses of shallow soil samples (0-20 cm depth), show that a significant impact is only observed in the 6 In wide centre of the vent, where C02 flux rates exceed 2000-3000 g M-2 d-1. In this "vent core" there is no vegetation, pH is low (minimum 3.5), and small changes are observed in mineralogy and bulk chemistry. in addition, microbial activities and populations are regulated in this interval by near-anoxic conditions, and by elevated soil gas C02 (>95%) and trace reduced gases (CH4, H2S, and 142). An approximately 20 In wide halo surrounding the core forms a transition zone, over which there is a gradual decrease in C02 concentrations, a rapid decrease in C02 fluxes, and the absence of reactive gas species. In this transition zone grasses dominate near the vent core, but these are progressively replaced by clover and a greater plant diversity moving away from the vent centre. Physical parameters (e.g. pH, bulk chemistry, mineralogy) and microbial systems also gradually return to background values across this transition zone. Results indicate that, even at this anomalous high-flux site, the effects ofthe gas vent are spatially limited and that the ecosystem appears to have adapted to the different conditions through species substitution or adaptation. (c) 2008 Elsevier Ltd. All rights reserved

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This paper reviews research into the potential environmental impacts of leakage from geological storage of CO 2 since the publication of the IPCC Special Report on Carbon Dioxide Capture and Storage in 2005. Possible impacts are considered on onshore (including drinking water aquifers) and offshore ecosystems. The review does not consider direct impacts on man or other land animals from elevated atmospheric CO 2 levels. Improvements in our understanding of the potential impacts have come directly from CO 2 storage research but have also benefitted from studies of ocean acidification and other impacts on aquifers and onshore near surface ecosystems. Research has included observations at natural CO 2 sites, laboratory and field experiments and modelling. Studies to date suggest that the impacts from many lower level fault-or well-related leakage scenarios are likely to be limited spatially and temporarily and recovery may be rapid. The effects are often ameliorated by mixing and dispersion of the leakage and by buffering and other reactions; potentially harmful elements have rarely breached drinking water guidelines. Larger releases, with potentially higher impact, would be possible from open wells or major pipeline leaks but these are of lower probability and should be easier and quicker to detect and remediate.

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The relation between acoustic seafloor backscatter and seep distribution is examined by integrating multibeam backscatter data and seep locations detected by single-beam echosounder. This study is further supported by side scan sonar recordings, high-resolution 5 kHz seismic data, pore-water analysis, grain-size analysis and visual seafloor observations. The datasets were acquired during the 2003 and 2004 expeditions of the EC-funded CRIMEA project in the Dnepr paleo-delta area, northwestern Black Sea.More than 600 active methane seeps were hydro-acoustically detected within a small (3.96 km by 3.72 km) area on the continental shelf of the Dnepr paleo-delta in water depths ranging from -72 m to -156 m. Multibeam and side scan sonar recordings show backscatter patterns that are clearly associated with seepage or with a present dune area. Seeps generally occur within medium-to highbackscatter areas which often coincide with pockmarks.High-resolution seismic data reveal the presence of an undulating gas front, i.e. the top of the free gas in the subsurface, which domes up towards and intersects the seafloor at locations where gas seeps and medium-to high-backscatter values are detected. Pore-water analysis of 4 multi-cores, taken at different backscatter intensity sites, shows a clear correlation between backscatter intensity and dissolved methane fluxes. All analyzed chemical species indicate increasing anaerobic oxidation of methane (AOM) from medium-to high-backscatter locations. This is confirmed by visual seafloor observations, showing bacterial mats and authigenic carbonates formed by AOM. Grain-size analysis of the 4 multi-cores only reveals negligible variations between the different backscatter sites.Integration of all datasets leads to the conclusion that the observed backscatter patterns are the result of ongoing methane seepage and the precipitation of methane-derived authigenic carbonates (MDACs) caused by AOM. The carbonate formation also appears to lead to a gradual (self-)sealing of the seeps by cementing fluid pathways/horizons followed by a relocation of the bubble-releasing locations. KeywordsMethane seeps; acoustic seafloor backscatter; anaerobic oxidation of methane; bacterial mats; pockmarks; methane-derived authigenic carbonates