Comparative Characterization of a Thermokarst Pond in Nunavik and an Analog Circular Pond in France by Multi-Method Geophysical Approach

Abstract:

The Tasiapik valley (Nunavik, Canada, N56° W76°) and the Leyre watershed (France, N45° W0°) are both sedimentary basins filled with sandy to silty formations, from respectively glaciomarine and fluvio-aeolian origin, that host shallow circular depressions of 20-30 m in diameter. While depressions in the Tasiapik valley are undoubtedly thermokarst ponds due to the current thawing of ice-rich mounds within discontinuous permafrost, it is still unsure whether depressions in the Leyre watershed are relic traces of thermokarst ponds from the last glacial. If so, they could constitute a temporal analog to the Tasiapik site after the thawing of permafrost is complete. The temporal analogy would be relevant to anticipate future distribution of groundwater resource in Tasiapik valley and conversely to provide information about processes of groundwater recharge in the Leyre watershed when the environment transited from periglacial to temperate conditions.

In order to compare the origin of the depressions and to provide accurate groundwater recharge models, precise data about soil properties (CEC, water content, texture) and geometry of geological layers are required. Sample extraction and physico-chemical analysis have been combined with multiple 3D geophysical surveys to acquire data density relevant with the soil properties variability: Following geophysical methods had been carried out: Electrical Resistivity Tomography (ERT) and Spectral Induced Polarization Tomography (SIPT) in wenner and dipole-dipole array using 64 electrodes and 50 cm interspace distance, Ground Penetrating Radar (GPR) profiling with at least 100 MHz, or even 250 and 500 MHz when possible, and ElectroMagnetic Induction (EMI) mapping. Our working assumptions are as follows: (1) in a thermokarst pond, the resistivity contrast between the frost susceptible material (thin, if not organic) that constituted the ice-rich permafrost mound and the coarser host formation is sharp enough to be imaged by ERT; (2) radargrams mainly reflect lithology or moisture contrasts and provide thin and accurate z-location of such boundaries that help constraint ERT and SIPT; (3) being sensitive to grain surface properties, SIP response allows to identify the texture of the various units.

Combined results of the two study sites will be presented.