MELT-OUT TIME LAG, AND ITS IMPACT ON ASSESSING AGES OF QUATERNARY EVENTS

The radiocarbon dating of basal sediments from kettle lakes is a common method used for assigning ages to deglacial events. Early studies suggested that these basins experience a 1,000 – 5,000 year lag to allow for pedogenesis, plant succession, and permafrost thaw before organic accumulation can begin. Recent studies suggest lags may vary locally due to differences in the extent of permafrost, water drainage, and burial conditions. For example, vegetation studies in the Yukon report that transitions from tundra to spruce forest result in a 120 year lag (Birks, 1980). Additionally, kettles in northern Michigan related to the Marquette readvance record a ~ 500 year lag (Derouin et al., 2007). In Poland, kettles along the same ice margin recorded lag times between a few hundred, to thousands of years (Blaszkiewicz, 2011). Recently in Michigan and Indiana, Horton (2015) proposed a younger date for the Saginaw Lobe’s Sturgis Moraine by 2,000 years. An oldest age of 13.7 ± 80 ka ¹⁴C yr BP came from the basal sediments (pebbly sand) of three scour lakes within tunnel channels that extend past the moraine and are overlaid by outwash fans. These lakes were not expected to have experienced any lag time as they are interpreted to have formed from glaciofluvial scour, rather than melt-out of a buried ice block. Fullerton (1980) had previously assigned an age to the Sturgis Moraine between 16.1 – 15.5 ka ¹⁴C yr BP from radiocarbon ages similar to those generated by Horton, but he assumed a lag of 2,000 years. If Fullerton’s age is correct, these scour lakes must have experienced a lag. To test this, Meteer Lake kettle, 6 km in front of the Sturgis Moraine near Howe, IN, was cored. Geomorphically, the kettle lies within a collapsed tunnel channel system younger than adjacent Saginaw and Huron-Erie Lobe outwash flow channels. Wood from basal sands in Meteer Lake gave a radiocarbon age of 13.4 ± 75 ka ¹⁴C BP. This one date suggests an even greater lag than Horton’s ages. Due to evidence of variable lag times along one ice margin in Poland, the single age of Meteer Lake and the uncertain origin of the tunnel channels, additional and different types of lakes associated with the Sturgis Moraine are being cored. Along with OSL dating of kettle and outwash sediments, more dates from a variety of depositional environments should allow for better understanding of this time lag.