CHEMISTRY OF CRYOPEGS ON BARTER ISLAND, NORTH SLOPE ALASKA

On Barter Island, NE Alaska, a large basal cryopeg extending from the coast to at least 300 m inland was recently discovered. The cryopeg was detected from surface penetrating multichannel electrical resistivity tomography (ERT) that imaged and characterized shallow subsurface permafrost. The ERT surveys revealed extensive subsurface regions of low resistivity at 5 meter depths suggesting the presence of hypersaline water underlying a much more resistive layer above. Borehole temperature data from a nearby inland location was -8° C at 10 m depth, too cold to for liquid fresh water needed to explain a less resistive layer below the frozen layer.

The presence of the cryopeg was confirmed by coring as deep as 8 m in four locations. Extracted pore water from the cores were fresh in the upper 2 m, and showed increasing salinity with depth. Enhanced chlorine, bromine, and other anions and cations were found below 2.5 m. Chlorinity of up to 31 g/kg was extracted from stiff clay at 7 m depth. Pore water sulfate concentrations were depleted relative to seawater ratios of chloride to sulfate. X-ray diffraction analyses of the sediment revealed the presence of gypsum and anhydrite (CaSO4 minerals), minerals that removed sulfate from the pore water when formed. Pore water 87/86Sr isotopic measurements of 0.7111 - 0.7161, and water oxygen isotopic compositions of -20 to -26 permil strongly suggest that groundwater rather than seawater is the source of the pore water.

Currently it is unknown if basal cryopegs are a prominent feature of northern Alaska permafrost, however genesis from groundwater as evidenced by this new data, suggests they might be extensive. Erosion rates are accelerating along portions of Northern Alaska due to recent warming trends and associated ice-free days along the coastline. If non-icebonded permafrost cryopegs are commonly present at elevations near present-day sea level, then we might expect these to enhance erosion as they become exposed by chemical and mechanical weathering of the ground surface. Enhanced erosion rates will contribute to greater releases of permafrost constituents into the atmosphere and the ocean.