OPTICALLY STIMULATED LUMINESCENCE DATING (OSL) OF STABILIZED SAND DUNES IN NORTHEASTERN WISCONSIN: EVIDENCE FOR HOLOCENE CLIMATE CHANGE

The southern margin of the Laurentide Ice Sheet retreated to far northern and eastern Wisconsin by about 12,000 years ago. By this time glacial lakes in central Wisconsin (Glacial Lake Wisconsin) and eastern Wisconsin (Glacial Lake Oshkosh) had drained, leaving behind un-vegetated, sandy, lake bottoms in many areas. In northern Oconto County, a small dune field formed on the plain of Glacial Lake Oconto, the northernmost extension of Glacial Lake Oshkosh. We developed a preliminary timeline for this dune field using optically stimulated luminescence dating (OSL), determined paleo-wind direction based on dune types, evaluated evidence of post-settlement reactivation using a new database of aerial photos, and developed a conceptual model for the Holocene formation of the dune field.

OSL dating indicates that the cores of the dunes formed between about 8.44 and 10.1 ka, within a few thousand years after the lake drained. The dune types are predominantly parabolic and transverse. Measurement of the slip faces of 46 dunes gives an average orientation of 139 degrees, thus paleo-wind was from the northwest similar to the modern winds. Analysis of a new database of 1930’s and 40’s aerial photos from the field area shows mostly well-vegetated dunes with only small, isolated blowouts. Minor reactivation of sand appears to have been caused by farming or was associated with roadcuts. There is no evidence for regional reactivation of sand from the Oconto County dune field in post-settlement times. This interpretation is corroborated by an interview with a landowner who lived in the dune area in 1945.

Our preliminary work indicates that the dune field in northern Oconto County stabilized early after the draining of Glacial Lake Oconto without evidence of older or younger reactivations. Historic evidence suggests only limited reactivation during the early part of the 20-th Century (Dust Bowl). More OSL dating could be used, especially in the transitional zone between lake sediment and dune sand, as well as the dune crests, to evaluate initiation of dune formation after the drainage of Lake Oconto and evidence for or against widespread post-settlement dune reactivation in northeastern Wisconsin.