EVIDENCE OF ACTIVE PERMAFROST IN MINNESOTA FOLLOWING THE WISCONSIN GLACIATION

The presence of ice wedge polygons suggests that permafrost conditions occurred in Minnesota following the Wisconsin glaciation.

Permafrost conditions following the Wisconsin glaciation have been inferred in Minnesota based on tundra pollen assemblages and the scarcity of wood debris (French and Millar, 2014; Johnson, 1990). High resolution Google aerial imagery collected during the summer and fall 2013 drought reveal polygonal patterns in Wisconsinan sand and gravel (outwash plains), clay (ice-walled lake plains), and diamicton (drumlins and hummocky terrain) that are otherwise undetectable unless identified in cross-section at exposures. Features are primarily identified in agricultural fields and are often obscured by surrounding forests and urban land. Nonsorted polygons are ~15-60 m in diameter with ~1-3 m borders. These polygonal patterns are interpreted as networks of ice wedge casts, relicts of ice wedges which occur in polygonal patterns in the active layer of modern permafrost and are formed by annual thawing and contraction. Ice wedge polygons indicate sustained conditions with an annual mean air temperature of -5°C or below (Baulin et al, 1978). Cold and dry conditions continued long enough to develop widespread permafrost in Minnesota. As the regional climate warmed after the Wisconsin glaciation, permafrost thawed and a mix of surrounding material and finer eolian sediments filled in the former ice wedges. Since these ice wedge casts generally contain finer material than the surrounding glacial deposits, they retain soil moisture better during a drought, permitting remote identification (Clayton et al., 2001).

Although ice wedge polygons have been reported in Wisconsin (Clayton et al., 2001) and in southeastern Minnesota (Zanner, 1999), this is the first time that they have been reported in central and east-central Minnesota. These features must postdate deposition of the host materials; however, future studies can help to constrain the timing of active permafrost.