Ground Ice in the New Crrel Permafrost Tunnel
Abstract:Permafrost in the new CRREL tunnel was described based on mapping of massive-ice bodies and other permafrost features exposed in the tunnel as well as studying frozen sediment cores from numerous boreholes drilled around and above the tunnel and from its floor. The mapping covers 48-m long and 4-m high walls and the ceiling of the tunnel. A general cryostratigraphic map of the entire tunnel and more detailed maps of several parts of the tunnel were compiled, and ground-ice content was estimated. The tunnel presents features typical of ice-rich syngenetic permafrost (yedoma): ice wedges with a vertical extent of more than 10 m, the prevalence of micro-cryostructures, and the occurrence of large amount of almost undecomposed organic matter. The isotope composition of the Late Pleistocene ice wedges ranges between -28.7‰ and -25.5‰ for δ18O (mean value -27.3‰, n=18), while for the Holocene wedges the range is between -26.5‰ and -21.9‰ (mean value -24.3‰, n=26). Numerous bodies of thermokarst-cave ice and several layers of segregated ice (both of the Late Pleistocene age) were also sampled: δ18O values for thermokarst-cave ice vary from -25.5‰ to -19.3‰ (mean value -23.0‰, n=23), and for layers of segregated ice (ice “belts”) – from -23.4‰ to -21.0‰ (mean value -21.7‰, n=9).
Five main cryostratigraphic units include (described from the top): (1) active layer, 0.6 to 0.8-m thick; (2) intermediate layer (ice-rich silt, up to 0.6-m thick); (3) yedoma silts reworked by thermokarst and thermal erosion during the Holocene, 3 to 5-m thick, generally ice-poor, with thin ice wedges; (4) Late Pleistocene ice-rich yedoma silts with large ice wedges, 10 to 14-m thick; (5) ice-poor alluvial gravels. Occurrence of the ice-poor deposits in the upper permafrost protects ice-rich yedoma deposits from thermokarst development. These studies revealed significant differences in the cryostratigraphy of the new and old CRREL tunnels: permafrost in the new tunnel has been less affected by erosional events.