Soil Development as an Indicator of Relative Pingo Age, Northern Alaska, U.S.A.

Soils of pingos in the Prudhoe Bay region of northern Alaska (70 degrees N, 148 degrees W) were examined to determine if their development could be used to support or nullify a hypothesis of differential age between two morphologically distinct groups of pingos. The two morphological types, one with steep side slopes and small basal diameters ("steep-sided") and one with gentle side slopes and large basal diameters ("broad-based"), have been proposed to represent two different age groups. Steep-sided types are found on landscape surfaces of all ages, always within recognizable thaw-lake basins, and are all presumed to have formed within the last 10 kyr. Broad-based types are found only on pre-Holocene surfaces and never within recognizable thaw-lake basins, leading to speculation that they formed during a previous thaw-take cycle on the older surfaces. Soils were described and collected from nine localities on each of three steep-sided and two broad-based pingos. Profile development was quantified using an index that compares soil morphology with parent material characteristics. A second index quantified profile anisotropy of various soil properties. Multiple lines of evidence from the pingo soils supports the differential age hypothesis. Broad-based pingos have greater concentrations of clay and iron, lower pH, more developed color, structure, and consistence, and greater profile anisotropy. Absolute pingo ages were estimated by developing regression equations for profile development of temperate soils from the literature and applying these to the pingo soils. The regression resulted in estimated ages of the steep-sided pingos of approximately 5 ky and of broad-based pingos of 14-22 kyr, which are thought to be minimum age estimates.