Abnormally high acoustic sea-floor backscatter patterns in active methane venting areas, Dnepr paleo-delta, northwestern Black Sea

During the 58th and 60th cruise of R.V. Vodyanitskiy, conducted in the framework of the EU-funded CRIMEA project, almost 3000 active bubble-releasing seeps were detected with an adapted split-beam echosounder within the 1540 km2 of the studied Dnepr paleo-delta area. The distribution of these active seeps is not random, but is controlled by morphology, by underlying stratigraphy and sediment properties, and by the presence of gas hydrates acting as a seal and preventing upward migrating gas to be released as bubbles in the water column (Naudts et al., 2006).Here we present the relation between acoustic sea-floor backscatter and the distribution of more than 600 active methane seeps detected within a small area on the continental shelf. This study is further sustained by visual sea-floor observations, highresolution seismic data, pore-water data and grain-size analysis.The backscatter data indicate that seeps are generally not located within highbackscatter areas, but rather surround them. Most seeps are located within shallow pockmarks which are characterized by medium-backscatter values, whereas deeper pockmarks have high-backscatter values with much lower seep densities. The seismic data show the presence of a distinct gas front (free gas); shallow gas fronts correspond to high- and medium-backscatter areas, which are associated with gas seeps, whereas deep gas fronts correspond to low-backscatter areas without seeps. The presence of shallow gas is also confirmed by the pore-water data, showing higher amounts of dissolved-methane concentrations for areas with medium- to high-backscatter values.Visual observations showed that the high-backscatter areas correspond to white Beggiatoa mats. These thiotrophic bacterial mats are indicators for the anaerobic oxidation of methane (AOM) which results in the formation of methane-derived carbonates (MDAC’s). AOM was also confirmed by the pore-water data. No clear correlation with grain-size distribution could be established.Based on the integration of all datasets, we conclude that the observed highbackscatter anomalies are a result of methane-derived authigenic carbonates (MDAC’s). The carbonate formation appears to lead to a gradual (self)-sealing of the seeps (Hovland, 2002), followed by a relocation of the bubble-releasing holes. Furthermore, the degree of MDAC-formation is directly linked to the backscatter intensity and seep activity which makes it possible to use the backscatter strength as a proxy for the seep activity and distribution

Anomalous seafloor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea

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 high backscatter 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

Potential Environmental Impacts of CO2 Leakage from the Study of Natural Analogue Sites in Europe

AbstractSites of natural CO2 leakage provide opportunities to study the potential environmental impacts of such leakage on near-surface ecosystems. As part of the FP7 RISCS (Research into Impacts and Safety in CO2 Storage) project a geochemical, botanical and microbiological study have been conducted on a natural CO2 vent in Florina, Greece and the findings are compared with the results drawn from Latera, Italy and Laacher See, Germany. Plant and microbial communities appear to have adapted to long-term CO2 exposure. Therefore the findings may not be representative of the effects of potential leakage from man made storage sites

Comparison of the impacts of elevated CO₂ soil gas concentrations on selected European terrestrial environments

Selected European studies have illustrated the impacts of elevated CO₂ concentrations in shallow soils on pasture. For the first time, general unified conclusions can be made, providing CO₂ thresholds where effects on plants and soil microbiology are observed and making recommendations on how this information can be used when planning projects for CO₂ storage. The sites include those where CO₂ is being naturally released to the atmosphere from deep geological formations; and a non-adapted site, with no previous history of CO₂ seepage, where CO₂ has been injected into the unsaturated soil horizon. Whilst soil gas concentrations will be influenced by flux rates and other factors, the results suggest that a concentration of between 10% and 15% CO₂ soil gas at 20 cm depth, which is within the root zone, is an important threshold level for observing changes in plant coverage. Site-specific plant ‘indicators’ are also observed for CO₂ concentrations at ≥35%. Microbiological changes are seen where CO₂ soil gas concentrations are between 15% and 40%. As part of site characterisation, an evaluation of the risks of leakage and their potential environmental impacts should be undertaken

Radon studies for investigation of nuclear waste deposits and natural emissions

In underground repositories for radioactive waste, significant quantities of gases may be generated due to several processes (i.e., corrosion of metals in waste and packaging, radiolysis of water, microbial degradation of various organic waste, etc). These gases may migrate through the engineered barrier system and the natural geological barrier. It is therefore recommended that the potential impact of gas accumulation and migration on the performance of the various barriers should be addressed and assessed in the development of safety cases for radioactive waste repositories

Chemistry of chromium in lakes

The inputs of industrial wastes have changed the levels and forms of chromium in many lakes, and there is growing concern about the accumulation of Cr(VI), a known carcinogen, in drinking water supplies. This report provides a critical review of the effects of speciation, transformations, and solubility on the cycling of chromium in lacustrine environments. Unique features of chromium dynamics associated with the decoupling of redox-driven chromium cycle from those of iron and manganese at the sediment–water and air–water interfaces are highlighted. Reductive precipitation is important in the immobilization of pollutant chromium in sediments. Photooxidative dissolution of Cr(III) especially at the air–water interface is identified as a key area for further research. The available database is inadequate for establishing any linkages between the chromium and biological cycles in lakes