DATING THE LATEST TINTAH SHORELINE OF LAKE AGASSIZ IN NORTHERN MINNESOTA: ADDING TO THE GEOCHRONOLOGIC PUZZLE

The chronology of Lake Agassiz’s lake levels are important to decoding the climate changes at the end of the Pleistocene and early Holocene. A leading hypothesis suggests that mass drainage of Lake Agassiz may have initiated the Younger Dryas cooling event. Analysis of Greenland ice core records has put the onset of the Younger Dryas climate anomaly at 12.9 ka. Understanding the ages of lake levels in the Agassiz basin is needed to help correlate a potential drawdown of Lake Agassiz with the onset of the Younger Dryas. There are four major age groupings for Lake Agassiz stable shorelines that span the time period relevant to the Younger Dryas; these ages are the Herman, Norcross, Tintah and Campbell. The Herman, Norcross and Campbell have all been successfully and reproducibly dated using OSL with ages of 14.1 ± 0.3 ka, 13.6 ± 0.2 ka and 10.5 ± 0.3 ka, respectively. Thus far, the Tintah strandline has been difficult to date either due to being poorly expressed on the landscape or being composed of course-grained lag deposits unsuitable for OSL dating; leaving the age of the Tintah lake level unresolved at the present time. In this work new age results are reported for Tintah shoreline deposits sampled in Marshall and Roseau counties of northern Minnesota. In this region shoreline deposits have high sand content and strandlines have identifiable geomorphic expression. The team used digitized strandline data, soil surveys, and satellite imagery to remotely choose optimum sites for exploration. Samples were processed and dated using OSL SAR techniques on quartz sand. Preliminary age results do not appear to provide clarity to the age dilemma for the Tintah lake level and potentially raise difficult questions about our current geospatial understanding of Tintah strandlines.