AGE CONSTRAINTS FOR AN UPDATED LAKE AGASSIZ PALEOHYDROGRAPH

Meltwater releases from Lake Agassiz to the Mississippi, Mackenzie and Great Lakes waterways have been implicated as triggers of abrupt climate change events during the last deglacial cycle. To evaluate these assertions a paleohydrograph for Lake Agassiz is needed with robust chronologic controls. Shorelines represent static water planes and transitions between shoreline positions represent significant water-level fluctuations. However, geologic ages constraining the timing of shoreline development, and thereby water-level fluctuations for Lake Agassiz, have been difficult to obtain, and therefore, sparse. In recent years we have had success in applying OSL dating techniques to shoreline deposits of Lake Agassiz that are more easily mapped using LiDAR data. This report will summarize a growing chronologic data set that includes 35 independent OSL ages from five different study areas around the southern basin, but thus far mostly focused around the southern outlet. The cumulative OSL data set for Lake Agassiz’s southern basin provides robust age constraints for the Herman, Norcross and Campbell strandlines with averages and standard deviations of 14.1 ± 0.3 ka, 13.6 ± 0.2 ka, and 10.5 ± 0.3 ka, respectively. In addition, the new age constraints are used to develop an updated paleohydrograph for Lake Agassiz spanning the time period of approximately 14.5 to 10 ka which includes rapid climate changes at the end of the last ice age.