Holocene environmental history of thermokarst lakes on Richards Island, Northwest Territories, Canada: Thecamoebians as paleolimnological indicators

Richards Island, Northwest Territories, Canada, is characterized by thermokarst lakes which record Holocene limnological change. This study is the first report of thecamoebian assemblages and continuous annual lake water temperatures from these Arctic lakes. Ecological environments on Richards Island are influenced by a climatic gradient resulting from the contrasting influences of the cold Beaufort Sea to the north and the warm waters of the Mackenzie Delta to the east and west. This climatic gradient in turn influences modern thecamoebian assemblages, and is an indication of the complexity involved in interpreting past conditions from core material in this area. Population abundance and species diversity of thecamoebian assemblages on Richards Island are not significantly different from those reported from temperate and semi-tropical latitudes. However, certain assemblage characteristics, such as large and coarse agglutinated tests, dominance of assemblages by one or two species and low morphological variation are interpreted to be diagnostic of Arctic conditions. Thecamoebian assemblages in core material from the area indicate that the local paleolimnological conditions may have changed within the last 3 ka, and this is unrecorded in previously reported pollen data. Paleoenvironmental interpretations in a permafrost landscape have to take into account morphological instability of thermokarst lakes, which can be the cause of paleolimnological and consequently faunal change. In this area ecosystem development is clearly related to geomorphology and local climatic effects and is not exclusively controlled by regional climate change

Experimental and numerical studies on geomechanical behavior of various gas hydrate-bearing sediments in China

1st International Conference on Energy Geotechnics, ICEGT 2016, Germany, 29-31 August 2016Gas hydrates exist in pores as a solid, bonding surrounding soil grains together and also densifying the host sediments. As a result, hydrate-bearing sediments exhibit stiffer, stronger and more dilatant behavior than hydrate-free sediments. This paper presents experimental and numerical studies to capture these features of the geomechanical behavior of hydrate-bearing sediments. The experimental data includes triaxial tests on reconstituted soils of South China Sea and synthetic samples of carbon dioxide hydrate-bearing soils, while the critical state-based soil constitutive model is calibrated using an optimization-based technique. The results show that the critical state-based model is capable of predicting shearing response of the hydrate-bearing sediments under different confining stresses, drainage conditions and degrees of hydrate saturation. It is also found that the influence of gas hydrates manifests itself mainly through enlargement of the initial yield surface. The corresponding model parameters are presented for the host sediments in China, which can be adopted for the simulation of future gas exploration and gas production in China.Department of Civil and Environmental Engineerin

A 100-km wide slump along the upper slope of the Canadian Arctic was likely preconditioned for failure by brackish pore water flushing

Highlights • Multiple submarine landslide scars occur on the Beaufort Continental Slope. • Scars are left by large-scale Late Holocene retrogressive slope failures. • Scars coalesce downslope to form basin wide feature at ≥1200 m depths. • Failure planes are at 30–75 depths within rapidly deposited glaciomarine sediments. • Widespread brackish water infusion into failure zone preconditions slope for failure. Exploration of the continental slope of the Canadian Beaufort Sea has revealed a remarkable coalescence of slide scars with headwalls between 130 and 1100 m water depth (mwd). With increased depth, the scars widen and merge into one gigantic regional slide scar that is more than 100 km wide below ~1100 mwd. To understand the development of these features, five sites were investigated with an Autonomous Underwater Vehicle, which provided 1-m bathymetric grids and Chirp profiles, and surveyed with a Remotely Operated Vehicle. The morphologies are consistent with retrograde failures that occurred on failure planes located between 30 and 75 m below the modern seafloor. At issue is whether the continental slope in this area is preconditioned for failure. While rapid sedimentation during glacial periods, and the presence of shallow gas cannot be ruled out, given the geological environment, it is unclear that they are primary preconditioning factors. Evidence of widespread flushing of the slope with brackish waters, and observed flows of brackish water within slide scars, suggest fluid venting and overpressure may play a role in the development of the extensive slope failures seen along this margin. The impact of pore water salinity changes at the depth of the failure plane on slope stability has not been considered in marine settings previously