Revisiting the timing of Saginaw lobe ice retreat and implications for drainage adjustments across southern Michigan, USA

Abstract

Understanding the behavior of paleo-ice sheets in response to past climate change and land surface processes can provide insights into the interplay between the atmosphere, ocean surface, and the cryosphere, which helps update current climate models and understand landscape evolution. The timing of the advancement and retreat of ice lobes along Quaternary ice sheet margins and their link to ocean surface temperature fluctuation are of particular interest. The Saginaw lobe along the southern margin of the Laurentide Ice Sheet (LIS) was bound by the Lake Michigan and Huron-Erie lobes. It flowed out of Saginaw Bay, across Lower Michigan, and into northern Indiana. All three of these lobes retreated across the region asynchronously, with the Saginaw lobe retreating first. The asynchronous behavior of the lobes allowed the Lake Michigan and Huron-Erie lobes to overprint and alter landscapes once occupied by Saginaw lobe ice.Lacustrine sediments within the Sturgis Moraine, deposited during the Saginaw lobe retreat in extreme southwestern Michigan, yielded optically stimulated luminescence (OSL) ages of 30.1 ± 1.8, 28.7 ± 1.5 and 25.2 ± 1.9 ka. An additional radiocarbon date (22.22 ± 0.8 cal ka) was measured from bryophytes sampled near the base of an ice-walled lake plain atop the moraine. These dates indicate that deglaciation of the Saginaw lobe could have been underway in southwestern Michigan between ∼ 30 and 22 ka, while ice margins of the Huron-Erie and Lake Michigan lobes were further south in Indiana and Illinois. The new dates indicate that southwestern Michigan may have witnessed ice-free conditions four thousand years earlier than previously thought. Moreover, the paleo-hydrological analyses of southwestern Michigan using detailed surficial geology mapping, Lidar-based longitudinal and terrace profiles along meltwater channels, and OSL data indicate that meltwater originating from the Saginaw—Huron-Erie interlobate area drained westward across the Saginaw lobe terrain between ∼ 18.0 to 14.6 Ka ago. The findings call for revisiting the current understanding of the timing and behavior of the southern margins of the LIS and the Holocene landscape evolution in Michigan.