In the later part of deglaciation of middle North America, after 11.7ka (13,600cal.yr), the retreat of ice under the influence of relatively high summer insolation led to the formation of proglacial Lake Agassiz. The lake, which was impounded by the retreating ice margin on its northern side, initially overflowed the continental divide to the south. During its lifespan, Lake Agassiz enlarged significantly and varied its overflow among southern, eastern and northwestern outlets, as described above. The variations in outlet use are understood to have resulted from a combination of oscillations in ice-margin positions and changes in relative outlet elevation by differential glacio-isostatic uplift. Three possible explanations or hypotheses for ice-margin and outlet changes that may have influenced Agassiz overflow routings are described below in terms of external insolation forcing, regional climatic feed-backs, and differential rebound.
1 Numerical modelling studies by Hostetler et al. (2000) addressed the early history of Lake Agassiz (11-9.5ka; 13,000-10,700cal.yr) when southern overflow was first diverted eastward through a lower outlet opened by ice retreat. The overflow was subsequently diverted away from the eastward outlet by the Marquette ice advance across the Lake Superior basin (Fig. 28.5c). Comparisons of the lake-atmosphere-ice system in this region at about 11ka (13,000cal.yr), with and without Lake Agassiz, showed that the lake induced a zone of reduced precipitation along its adjacent ice margin, including the ice that dammed the eastern outlets of the lake. When waters of Lake Agassiz were present, the lake-ice-atmosphere interactions may have set up oscillations in the ice margin which induced rerouting of overflow and outburst floods when the lake fell to the level of a new lower outlet. The climatic feed-back effects of Hostetler et al. (2000) are outlined in Fig. 28.8a. Overall, the deglaciation is assumed to have been driven by high summer insolation (Kutzbach et al., 1998) which caused long-term, progressive ice retreat and growth of the impounded lake surface (boxes 1 and 2). Based on the numerical experiments of Hostetler et al. (2000), low evaporation from the cold lake surface, combined with anticyclonic air flow, blocked atmospheric moisture from reaching the lake and adjacent LIS (box 3). This feed-back would have reduced mass balance of the adjacent LIS (box 3), leading to ice retreat (box 4). The retreat would have increased the lake area (return to box 2), and/or may have opened a lower outlet, in this case to the east (box 5). A rapid draw down of the lake to the sill elevation of the lower outlet would have produced a reduction in lake size (box 6). Crustal rebound (box 7) would have enlarged the lake through time. Simultaneously, according to the Hostetler et al. (2000) model, climatic feed-back from the reduced lake surface area would have enhanced atmospheric moisture and glacial mass balance (box 8), leading to advance of the ice margin (box 9). If the advance overrode an outlet (box 10), the lake would refill to the next higher outlet level. The larger lake (box 2), arrived at by either an ice advance, or differential uplift, or both, would start the cycle again.
2 In contrast, Teller (1987, fig. 24) suggested that an increase in lake size would lead to more evaporation and growth of the ice sheet. The presence of a large ice-marginal lake along the LIS would also encourage brief rapid expansions of the ice margin by surging, although subsequent rapid calving of the ice margin would occur, and that would expedite the demise of the ice sheet (Teller, 1987). In this conceptual model (Fig. 28.8b), external forcing by relatively high summer insolation (Kutzbach et al., 1998) drove general ice retreat as before (box
I). The ice retreat could result in a larger lake surface (box 2). Alternatively, if retreat opened a lower outlet (box 3), a smaller lake (box 4) would result after a draw down and outburst flood through the new lower outlet. Considering the first scenario, the resulting larger lake (box 2) would have led to more atmospheric water vapour by evaporation from the enhanced water surface (box 5). In turn, more vapour would have increased precipitation on the adjacent ice surface, and enhanced glacial mass balance (box 6). This increased nourishment of the ice sheet would have led to an advance of the ice margin (box 7), and a consequent reduction in lake surface area (box 4). This smaller lake would have induced climatic effects (boxes 8-10) which, along with differential uplift (box
II), would have resulted in enlargement ofthe proglacial lake. Continued ice retreat as a result of high summer insolation (box 1) could have reinforced the lake enlargement (box 2), or have deglaciated a new lower outlet (box 3), starting the cycle again.
3 Krinner et al. (2004) modelled the lake-ice-atmosphere interaction in relation to the big Eurasian glacial lake complex, concluding differently to Hostetler et al. (2000). Specifically, they found that, although a large ice-marginal lake would bring about cooling and less snowfall, there was a significant reduction in summer melting of the ice sheet that resulted in readvance (or slowed retreat) of the ice margin. The application of this alternative interpretation to Lake Agassiz is shown in Fig. 28.8c. As before, the general deglaciation and ice retreat is assumed to follow from an increase in insolation (box 1), which would enlarge the lake surface area (box 2), or open a new lower outlet (box 3). A lower outlet would reduce the lake area after a draw down and outburst flood (box 4). The larger lake would induce cooling (box 5), reduce summer ice melt (box 6) and thereby increase glacial mass balance (box 7). Either this ice advance or a draw down through a lower outlet (box 3) could lead to a smaller lake (box 4). As before, the climatic effects of the reduced water surface (boxes 8-10) and the parallel effect of differential uplift (box 11) would tend to increase the lake area (box 2) and start the cycle again.
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