Organic matter, ice content and structure determined in a permafrost peatland trought GPR profiling and computed tomography scanning, Salluit, Nunavik, Canada

A polygonal peatland was studied in Salluit, Nunavik, Canada, in the continuous permafrost zone. To characterize the cryostructure of the permafrost and the size and position of the ice wedges, three methods were used: (1) three GPR profiles at the 225 MHz frequency were surveyed (2) intact frozen core samples were obtained by drilling at two sites in polygon centers and (3) CT-scan analysis was performed on these recovered cores. In view of obtaining a precise topography along the GPR profiles, terrain elevation was measured accurately every 10 cm using a digital level (Ziplevel™). Radiocarbon dates were also obtained on basal peat samples at the depths of 1.87 m and 2.43 m. The ice-wedges stand out and are evidenced by parabolic reflectors. Thick (5 cm) ice layers and lenses in the peat that were not evident to the naked eye on cores due to the fibrous nature of peat but which were imaged by CT-Scan have been correlated with specific reflectors on the GPR profiles. Clasts of coarse sand size are also scattered throughout the peat column due to winter wind erosion from the till-covered bedrock surfaces that surround the peatland. The ice-rich layers in the stratigraphic column are interpreted as former ice concentrations at the base of the active layer as syngenetic permafrost grew following the accumulation of the organic sediments over the past 4615 years. Given the very high ice content of the peat in general (> 90%), the amount of carbon stored over the total peat thickness in this permafrost is in fact quite small. This finding suggests that more probing in frozen peatlands across the Arctic could be useful to improve the precision of assessments of stored carbon in the permafrost. The approach combining GPR, coring, CT-Scan and C-14 dating yields a good understanding of past permafrost dynamics, especially when applied in a geomorphological context already relatively well understood. CT-scan alone allowed us to observe and analyze permafrost composition and structural organization on numerous samples extracted from the field. This new technology offers great possibilities for imaging and characterizing permafrost composition and cryostructure.