High-resolution seismic-reflection investigation of the northern Gulf of Mexico gas-hydrate-stability zone

We recorded high-resolution seismic-reflection data in the northern Gulf of Mexico to study gas and gas-hydrate distribution and their relation to seafloor slides. Gas hydrate is widely reported near the seafloor, but is described at only one deep drill site. Our data show high-reflectivity zones (HRZs) near faults, diapirs, and gas vents and interbedded within sedimentary sections at shallow depth (<1 km). The HRZs lie below the gas-hydrate-stability zone (GHSZ) as well as within the zone (less common), and they coincide with zones of shallow water-flows. Bottom simulating reflections are rare in the Gulf, and not documented in our data.We infer HRZs result largely from free gas in sandy beds, with gas hydrate within the GHSZ. Our estimates for the base BHSZ correlate reasonably with the top of HRZs in some thick well-layered basin sections, but poorly where shallow sediments are thin and strongly deformed. The equivocal correlation results from large natural variability of parameters that are used to calculate the base of the GHSZ. The HRZs may, however, be potential indicators of nearby gas hydrate. The HRZs also lie at the base of at least two large seafloor slides (e.g. up to 250 km²) that may be actively moving along decollement faults that sole within the GHSZ or close to the estimated base of the GHSZ. We suspect that water/gas flow along these and other faults such as ‘chimney’ features provide gas to permit crystallization of gas hydrate in the GHSZ. Such flows weaken sediment that slide down salt-oversteepened slopes when triggered by earthquakes.     

Burial dolomitization driven by modified seawater and basal aquifer‐sourced brines: Insights from the Middle and Upper Devonian of the Western Canadian Sedimentary Basin

Dolomitization in the Western Canadian Sedimentary Basin has been extensively researched, producing vast geochemical datasets. This provides a unique opportunity to assess the regional sources and flux of dolomitizing fluids on a larger scale than previous studies. A meta‐analysis was conducted on stable isotope, strontium isotope (⁸⁷Sr/⁸⁶Sr), fluid inclusion and lithium‐rich formation water data published over 30 years, with new petrographic, X‐ray diffraction, stable isotope and rare‐earth element (REE+Y) data. The Middle to Upper Devonian Swan Hills Formation, Leduc Formation and Wabamun Group contain replacement dolomite (RD) cross‐cut by stylolites, suggesting replacement dolomitization occurred during shallow burial. Stable isotope, REE+Y and ⁸⁷Sr/⁸⁶Sr data indicate RD formed from Devonian seawater, then recrystallized during burial. Apart from the Wabamun Group of the Peace River Arch (PRA), saddle dolomite cement (SDC) is more δ¹⁸O_(PDB) depleted than RD, and cross‐cuts stylolites, suggesting precipitation during deep burial. SDC ⁸⁷Sr/⁸⁶Sr data indicate contributions from ⁸⁷Sr‐rich basinal brines in the West Shale Basin (WSB) and PRA, and authigenic quartz/albite suggests basinal brines interacted with underlying clastic aquifers before ascending faults into carbonate strata. The absence of quartz/albite within dolomites of the East Shale Basin (ESB) suggests dolomitizing fluids only interacted with carbonate strata. We conclude that replacement dolomitization resulted from connate Devonian seawater circulating through aquifers and faults during shallow burial. SDC precipitated during deep burial from basinal brines sourced from basal carbonates (ESB) and clastic aquifers (WSB, PRA). Lithium‐rich formation waters suggest basinal brines originated as residual evapo‐concentrated Middle Devonian seawater that interacted with basal aquifers and ascended faults during the Antler and Laramide Orogenies. These results corroborate those of previous studies but are verified by new integrated analysis of multiple datasets. New insights emphasize the importance of basal aquifers and residual evapo‐concentrated seawater in dolomitization, which is potentially applicable to other regionally dolomitized basins.     

Identification, prediction, and mitigation of sinkhole hazards in evaporite karst areas

Sinkholes usually have a higher probability of occurrence and a greater genetic diversity in evaporite terrains than in carbonate karst areas. This is because evaporites have a higher solubility and, commonly, a lower mechanical strength. Subsidence damage resulting from evaporite dissolution generates substantial losses throughout the world, but the causes are only well understood in a few areas. To deal with these hazards, a phased approach is needed for sinkhole identification, investigation, prediction, and mitigation. Identification techniques include field surveys and geomorphological mapping combined with accounts from local people and historical sources. Detailed sinkhole maps can be constructed from sequential historical maps, recent topographical maps, and digital elevation models (DEMs) complemented with building-damage surveying, remote sensing, and high-resolution geodetic surveys. On a more detailed level, information from exposed paleosubsidence features (paleokarst), speleological explorations, geophysical investigations, trenching, dating techniques, and boreholes may help in investigating dissolution and subsidence features. Information on the hydrogeological pathways including caves, springs, and swallow holes are particularly important especially when corroborated by tracer tests. These diverse data sources make a valuable database—the karst inventory. From this dataset, sinkhole susceptibility zonations (relative probability) may be produced based on the spatial distribution of the features and good knowledge of the local geology. Sinkhole distribution can be investigated by spatial distribution analysis techniques including studies of preferential elongation, alignment, and nearest neighbor analysis. More objective susceptibility models may be obtained by analyzing the statistical relationships between the known sinkholes and the conditioning factors. Chronological information on sinkhole formation is required to estimate the probability of occurrence of sinkholes (number of sinkholes/km² year). Such spatial and temporal predictions, frequently derived from limited records and based on the assumption that past sinkhole activity may be extrapolated to the future, are non-corroborated hypotheses. Validation methods allow us to assess the predictive capability of the susceptibility maps and to transform them into probability maps. Avoiding the most hazardous areas by preventive planning is the safest strategy for development in sinkhole-prone areas. Corrective measures could be applied to reduce the dissolution activity and subsidence processes. A more practical solution for safe development is to reduce the vulnerability of the structures by using subsidence-proof designs.     

Place-Based Pathways to Proenvironmental Behavior: Empirical Evidence for a Conservation–Recreation Model

This study expands existing models of proenvironmental behavior (PEB) to examine the potentially important and interacting influences of nature-based recreation and sense of place on participation in conservation-oriented activities. We tested hypothesized relationships using a structural equation modeling approach that accounted for common behavioral antecedents within a sample of 1,124 nature-based recreationists and property owners in rural counties of New York, USA. We found that place attachment played an important role in strengthening connections between nature-based recreation and PEB. Birdwatching and hunting participation contributed independently and significantly to PEB, both directly and indirectly through effects on place meanings and place attachment. While birdwatching appeared to influence place attachment by impacting environmental place meanings, hunting exerted similar effects by affecting sociocultural place meanings. Ultimately, our synthesis results in the articulation of a comprehensive conservation–recreation model that could help researchers and practitioners identify and explore novel pathways to PEB.     

Modelling changes in suspended sediment from forest road surfaces in a coastal watershed of British Columbia

Erosion from logging road surfaces, cut slopes, banks, and ditches represents a chronic source of sediment input to streams that can degrade aquatic habitats. Road surface erosion is of particular concern because the magnitude of sediment generation when traffic levels are high can be large. Current models for predicting sediment production from roads require information on area‐specific sediment delivery, which is not often available. Here, we developed a model to quantify suspended sediment concentrations (SSC) generated by forest roads surfaces under different conditions of use and density. This model is designed for a typical medium‐size coastal watershed of British Columbia or the American Pacific Northwest, and was applied to the Chilliwack River watershed as a case study. The results illustrate that intensive use of forest roads combined with high road density can increase the number of extreme sedimentation events over a predetermined threshold. A comparison of the effects of road density and the level of road use suggests that the level of road use is more important than the road density for the generation of fine sediment from road surfaces. However, the model omits the impact of roads on mass movements in a watershed, which represent a major source of sediment in steep watersheds, so the effect of road density is likely more substantial than the model predicts. The model is an attempt to overcome field data limitations by using an empirical relation between SSC and traffic variables, and presents a starting point for more intensive field studies that could be used to validate it. Copyright © 2013 John Wiley & Sons, Ltd.     

Benchmarking sustainability in cities: The role of indicators and future scenarios

Scenarios are a useful tool to help think about and visualise the future and, as such, are utilised by many policymakers and practitioners. Future scenarios have not been used to explore the urban context in much depth, yet have the potential to provide valuable insights into the robustness of decisions being made today in the name of sustainability. As part of a major research project entitled Urban Futures, a toolkit has been developed in the UK to facilitate the use of scenarios in any urban context and at any scale relevant to that context. The toolkit comprises two key components, namely, (i) a series of indicators comprising both generic and topic area-specific indicators (e.g., air quality, biodiversity, density, water) that measure sustainability performance and (ii) a list of characteristics (i.e., 1–2-sentence statements about a feature, issue or small set of issues) that describe four future scenarios. In combination, these two components enable us to measure the performance of any given sustainability indicator, and establish the relative sensitivity or vulnerability of that indicator to the different future scenarios. An important aspect of the methodology underpinning the toolkit is that it is flexible enough to incorporate new scenarios, characteristics and indicators, thereby allowing the long-term performance of our urban environments to be considered in the broadest possible sense.