Modeling Sediment Transport in Ice-Walled Subglacial Channels and Its Implications for Esker Formation and Proglacial Sediment Yields
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| Authors: | Beaud, F; Flowers, GE; Venditti, JG |
| Year: | 2018 |
| Journal: | J. Geophys. Res.-Earth Surf. 123: 3206-3227 Article Link (DOI) |
| Title: | Modeling Sediment Transport in Ice-Walled Subglacial Channels and Its Implications for Esker Formation and Proglacial Sediment Yields |
| Abstract: | Sediment yields from glacierized basins are used to quantify erosion rates on seasonal to decadal timescales as well as conditions at the glacier bed, and eskers hold valuable information about past subglacial hydraulic conditions in their spatial organization, geometry, and sedimentary structures. Ultimately, eskers are a record of past glacio-fluvial sediment transport, but there is currently no physical model for this process. We develop a 1-D model of morphodynamics in semicircular bedrock-floored subglacial channels. We adapt a sediment conservation law developed for mixed alluvial-bedrock conditions to subglacial channels. Channel evolution is a function of melt opening by viscous heat dissipation from flowing water and creep closure of the overlying ice, to which we add the closure or enlargement due to sediment deposition or removal, respectively. We apply the model to an idealized land-terminating glacier and find that temporary sediment accumulation in the vicinity of the terminus, or the formation of an incipient esker, is inherent to the dynamics of the channelized water flow. The alluviation of the bed combined with the pressurized channel flow produces unexpected patterns of sediment evacuation: We show that the direction of hysteresis between sediment and water discharge is not necessarily linked to a supply- or transport-limited system, as has been hypothesized for proglacial sediment yields. We also find that the deposition of an incipient esker is a function of a compromise between water discharge and sediment supply, but perhaps more importantly, ice-surface slope and the temporal pattern of water delivery to the bed. Plain Language Summary Glaciers and ice sheets are changing rapidly, impacting sea levels, landscapes, and ecosystems. These changes are tightly linked to the meltwater routing through glaciers' plumbing systems. If this plumbing is pressurized by water flowing into crevasses and moulin (which act like water wells), the ice base can move faster downstream, possibly leading to enhanced ice loss, or vice versa. As glaciers retreated at the end of the last glaciation, they left clues of their passage, including sediment casts of their plumbing system: eskers. Eskers are elongated ridges that snake across the landscape and can be hundreds of kilometers long. Although understanding their deposition can help us understand contemporary ice sheet plumbing systems, their origin has been puzzling for several decades. We build a numerical model tracking sediment as they move with the water under ice. Glaciers naturally produce a sediment bottleneck and tend to form such eskers, producing the first process-based model for their deposition. We identify ice geometry and temporal patterns of water input into the plumbing system as critical factors, when combined with sufficient sediment and water supplies. This model helps to reconcile contemporary glacier processes and sediment records, which is key to better understand glaciers' plumbing system. |
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