Erosion Beneath the Laurentide Ice Sheet, and its Role in Pleistocene Ice Age Dynamics

University Of Washington, Seattle WA

Investigators

Abstract

EROSION BENEATH THE LAURENTIDE ICE SHEET, AND ITS ROLE IN PLEISTOCENE ICE AGE DYNAMICS Subglacial erosion and sediment dynamics influence the size, stability, and climatic sensitivity of large ice sheets. These processes may regulate large-scale surging behavior, initiating rapid shifts in climate and sea level (MacAyeal, 1993a,b), and perhaps dictate the periodicity of the Quaternary ice ages (Clark and Pollard, 1998). We intuitivly associate the scoured landscapes of the northern continents with subglacial erosion, yet estimates of the rates, timing and spatial pattern of erosion by the Pleistocene ice sheets are conflicting or ambiguous. Subglacial erosion is difficult to study because: (i) The processes involved take place beneath large ice sheets. (ii) As in all eroding landscapes, the record of change is continually effaced as the surface is removed, and (iii) Although thick deposits of Pleistocene glacial sediment, which contain information about erosional conditions, survive around the margins of former ice sheets, they are patchily preserved and difficult to date. We propose to study the history of erosion by the Laurentide ice sheet, using the cosmogenic isotopes 10 Be and 26 Al. First, we will date glacial sedimentary sequences at the southern margin of the former ice sheet using the technique of "burial dating" (Granger et al., 1997). Cosmogenic 10 Be and 26 Al are produced within the crystal structure of sedimentary quartz grains while the sediment is exposed to cosmic radiation near the Earth's surface. Once the sediment is buried, these isotopes decay at different rates and their ratio provides a measure of the burial time. We have used this technique in trial measurements on glaciofluvial sands interbedded with till, providing stratigraphically consistent mid-Pleistocene bracketing ages on previously undated glacial deposits. Second, we will use atmospherically-produced 10 Be, which accumulates in soils and is highly concentrated near the surface of deeply weathered terrains, to investigate the contribution of pre-glacial regolith to Laurentide tills. We presume the deep regolith that covered the Canadian Shield prior to the Quaternary must have contained an enormous inventory of 10 Be, far exceeding what could have accumulated on the craton during Quaternary interglacial periods. By measuring the 10 Be content of sediments eroded and deposited by the Laurentide ice sheet throughout the Pleistocene, we will track the removal of pre-glacial regolith and evaluate the ice sheet's response to the onset of hard-bed conditions. Our initial measurements show that mid-Pleistocene tills contain enormous concentrations of 'meteoric' 10 Be. Concentrations in Wisconsin till are two orders of magnitude lower. It appears that the Laurentide ice sheet was 'mining' a cover of heavily-weathered surficial material in the middle Pleistocene, but that this source has now been largely exhausted. We will apply these methods to well-studied sequences of till and interbedded glaciofluvial sediments in southwestern Minnesota, and early Pleistocene tills bracketed by well-dated ash layers in drill cores from South Dakota and Nebraska. This combination will provide samples of material eroded by the Laurentide Ice Sheet throughout the Pleistocene. The field area in southwestern Minnesota provides access to at least nine separate till units in outcrop and drill core. Extensive prior work in the area, and easy access to both surface and subsurface samples make it ideal for this study.