Inferring Sediment Transport and Subglacial Hydrology from Esker Deposits of the Chippewa Lobe of the Laurentide Ice Sheet

Eskers are sinuous landforms created at the base of glaciers by water moving through the ice. As the water flows in channelized conduits, it entrains and deposits sediment that comprise esker landforms that remain once the ice has retreated. Sediment in eskers (unlike till) can be relatively well sorted because flowing water sorts the sediment by size and density as a function of the flowing water's boundary shear stress. The boundary shear stress depends on density of the water, gravity, depth of the water, and hydropotential gradient. Eskers are an important indicator of paleo subglacial hydrologic conditions because they are one of the few landforms that record those processes. However, large discrepancies in their formation mechanisms result in uncertainty about how they relate to the subglacial hydrologic network. For example, traditional esker models evoke long channels that extend kilometers up ice of the glacier margin with deposition occurring over the length of the channel. In contrast, newer models of esker formation advocate for the deposition occurring only in a zone within 100s of meters of the glacier margin. To investigate subglacial hydrology, we examine the sediment sequence of a large esker (~20 m tall) that formed under MIS-2 ice derived from the Lake Superior Lobe of the Laurentide Ice Sheet. The grain size distribution of the esker was measured through a vertical section and used to estimate the temporal variability of the critical shear stress necessary to mobilize sediment. We find that the critical shear stress changed nonmonotonically throughout the formation of the esker. The water velocity or depth of the channel likely changed sporadically with time while the esker formed. However, water input fluctuations on short time scales can also modify water pressurization within the channel, which can have quite different effects on the boundary shear stress for confined vs. unconfined channels. To account for this complexity, we also relate shear stress variation to sediment transport for both confined and unconfined channels with our field observations to examine the validity of various esker formation models.