RELATION BETWEEN SEDIMENT TRANSPORT AND SUBGLACIAL HYDROLOGY DURING ESKER FORMATION

Eskers are sinuous landforms formed in subglacial channels by flowing water capable of entraining and depositing sediment. The subglacial drainage architecture normally changes from a distributed network to a more localized channel system when approaching the margin of the glacier and as it channelizes its ability to mobilize and deposit sediment increases, thus creating conditions to form eskers that remain once the ice has retreated. Moreover, water can flow both up and downhill as it moves towards the margin as it follows the hydraulic potential gradient, which is primarily determined by the glacier’s surface slope. Thus, eskers are an important indicator of paleo subglacial hydrologic conditions as they are one of the few landforms that record subglacial hydrology. However, large discrepancies in their formation mechanisms result in uncertainty about how they relate to the subglacial hydrologic network. For example, it is often assumed that their formation results from constant water velocity and fluvial conditions, despite modern observations that routinely show water flow varies greatly both spatially and temporally.

The primary aim is to investigate the formation process of eskers and understand the relation between subglacial hydrology and sediment transport in R - channels. To account for this complexity, we analyze the sediment sequence of a large esker that formed under MIS-2 ice derived from the Lake Superior Lobe of the Laurentide Ice Sheet. We measure the grain size distribution along the vertical section of the esker to estimate the 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, which means that the water velocity or depth of the channel likely changed sporadically with time while the esker formed. A possible explanation is related to changes in water pressurization causing variations in sediment transport. To assess this hypothesis, we build an experiment that simulates water flowing through channels with different water pressure conditions and observe the respective differences.