Late Pleistocene Air Temperatures in the Bighorn Basin, Wyoming: Inferences from Periglacial Ice-Wedge Casts and the Numerical Modeling of Near-Surface Ground Temperatures

Abstract Ice-wedge casts and nonsorted polygons in the Bighorn Basin, Wyoming demonstrate the existence of Late Pleistocene permafrost. Previous estimates based on the presence of ice-wedge casts and other evidence suggest that mean annual air temperature was from 10 to 16°C colder in the Bighorn Basin during the Late Pleistocene. The structure of ice-wedge casts is indicative of the depth of the permafrost table during their formation. An estimate of the depth to the Late Pleistocene permafrost table on Polecat Bench in the northern Bighorn Basin was obtained from the ice-wedge sites. A one-dimensional finite difference model was used to simulate ground temperatures on the Polecat Bench resulting from air temperature functions considered possible for Late Pleistocene climate. First, mean monthly air temperature data were used to define an annual air temperature function and calibrate the numerical model by matching present-day ground temperature data recorded in a well located on Polecat Bench. The calibrated model was then used to simulate the effect of colder air temperatures on the subsurface thermal regime. Late Pleistocene air temperature was considered to differ from the present in two ways: Lower mean annual air temperatures (mean-shift), and possibly a greater range of annual air temperatures (amplitude-shift). For each air-temperature function the modeled depth to the permafrost table was analyzed with regard to the estimate obtained from the ice-wedge sites. Results show that Late Pleistocene mean annual air temperatures in the northern Bighorn Basin were 15.9 to 16.7°C colder than the present, and that an annual temperature range greater than the present would have corresponded with an even lower mean.