The Lake Agassiz basin

During the first six millennia after the LGM, glacial meltwater and precipitation runoff west of the Michigan basin passed without interruption down the Mississippi River Valley, as the LIS margin retreated toward the northern limit of the Gulf of Mexico watershed. By 11.7ka (13,600cal.yr), the margin had retreated north of the continental divide, and had begun to impound water in proglacial lakes which drained south over the divide (Fig. 28.5a). Continued retreat of the ice margin expanded the area and depth of these lakes, particularly Lake Agassiz in the Red River basin (Clayton & Moran, 1982; Fenton et al., 1983; Teller, 1987; Kehew & Teller, 1994; Teller & Leverington, 2004), as well as that of glacial lakes Duluth and Algonquin in the Superior basin (Farrand & Drexler, 1985).

Shortly after 11ka (13,000cal.yr), an outlet opened about 110 m below the early Agassiz spillway that had carried overflow to the south. All published interpretations have concluded that this new outlet was east into the Great Lakes (Figs 28.2 & 28.5b, outlet G). An outburst flood of 9500km3 resulted, with a flux of about 0.3 Sv, assuming the drawdown of the lake occurred in 1 yr (Leverington et al., 2000; Teller et al., 2002). The new outlet also diverted baseline runoff estimated at 0.075 Sv from the Gulf of Mexico to the Great Lakes and the Atlantic Ocean (Fig. 28.3b) (Licciardi et al., 1999). Recent evaluation of deglaciation, however, suggests that the eastern outlet area may not have been open at 11 ka (13,000 cal. yr), and that the Agassiz outburst flood may have discharged via a northwestern route to the Arctic Ocean (Teller et al., 2005).

Once lower northern outlets were opened by ice retreat in the more deeply depressed parts of the basin, differential glacial rebound began to play an important role in controlling the relative elevations of outlets, and in transferring lake discharge from one outlet to another. The interaction of ice retreat and outlet elevation with differential rebound for the Agassiz basin is illustrated in Fig. 28.6. A similar situation applied during ice retreat through the Great Lakes basin. In the Lake Agassiz basin successively lower outlets were uncovered in the direction of ice retreat and more rapid rebound. When ice retreated (box 1) in the lake's northern sector, a lower outlet was commonly opened (box 2). The lake then abandoned its beach and was drawn down to a lower elevation with smaller surface area (box 3). Ongoing differential rebound raised this northern outlet relative to the lake's southern shore (box 4), causing Lake Agassiz to transgress its basin as it rose and expanded in surface area (box 5) (Teller, 2001). Whether the rise of the lake reached the elevation of a previous outlet farther south, or not, determined the routing of lake drainage (circle 6). While the water level remained below the previously higher outlet, Lake Agassiz continued to overflow through its more northern outlet, i.e. by either the eastern or northwestern routings to the Atlantic or Arctic oceans, respectively (box 7). If the transgression of the lake level reached the previously higher outlet elevation, overflow was diverted, either to the Mississippi River valley from the eastern or northwestern outlets, or to the Great Lakes and Atlantic Ocean from the northwestern outlet (box 8).

The diverted baseline flow and initial outburst flood at about 10.9ka (13,000cal.yr) may have gone eastward via the Great Lakes to the North Atlantic Ocean (Teller, 1987, 1990a). Alternatively, the 11-10ka outflow (approximately 13,000-11,400 cal.yr ago) may have escaped via the northwestern Agassiz outlet to the Arctic Ocean (Figs 28.2 & 28.5c, outlet H). Teller & Leverington (2004) and Teller et al. (2005) describe a possible scenario where all overflow from Lake Agassiz between about 10.9 and 10.1 ka (12,900-11,700 cal. yr) may have been entirely through the northwestern outlet into the Athabasca-Mackenzie River Valley and to the Arctic Ocean. This routing difference (into the Arctic Ocean versus into the North Atlantic Ocean) is significant because of the potential role that Agassiz freshwater overflow played in changing ocean circulation at this time (see later discussion). Glacial read-vance around 10ka (11,400cal.yr) would have closed the northwestern outlet and forced Lake Agassiz to overflow briefly to the Mississippi Valley before being rerouted to the northwestern outlet for the next 500-600 yr. Floods associated with draw downs at these times of overflow diversion are estimated at 0.3 and 0.19 Sv, and were superposed on baseline runoff of 0.047 to 0.034 Sv (Teller et al., 2002).

Rapid differential rebound then raised the northwestern outlet, and by about 9.4ka (10,600 cal.yr) the lake had transgressed to the level of an eastern col on the divide between Lake Agassiz and the Great Lakes watershed. Overflow from the 2 million km2 Agassiz watershed was then diverted into the Nipigon basin northwest of Lake Superior, where overflow was then into the Great Lakes-North Atlantic system, with an initial outburst of 7000 km3 (0.22 Sv, if it occurred in 1yr) superposed on a 0.034 Sv baseline flow (Fig. 28.2, outlet I) (Licciardi et al., 1999; Teller et al., 2002). Only briefly did overflow return to the southern outlet, before returning to the eastern outlets for the next 1400yr. A succession of outbursts from Lake Agassiz punctuated the retreat of the LIS, as progressively lower spillways were deglaciated (Teller & Thorleifson, 1983; Leverington & Teller, 2003). The lake phases associated with these discharges are recorded in the Agassiz basin by a series of beaches, each representing the transgressive maximum of the lake (caused by differential rebound) reached before a lower outlet was opened (e.g. Teller, 2001). These draw downs ranged from 8 to 58 m and generated flood fluxes of about 0.06 to 0.26 Sv from flood volumes of 2000 km3 to 8100 km3

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Legend

Ice cover

Lakes

Drainage

Figure 28.5 Ice-marginal positions during the first changes of Agassiz outlets, from southern to eastern to northwestern outlets. (a) Just prior to 11 ka (13,000 cal.yr), Lake Agassiz drained south at (F) to the Mississippi Valley and Gulf of Mexico. (b) Ice retreat about 11 ka (13,000cal.yr) uncovered first eastern (lower) outlet (G) to Superior basin in the Great Lakes, thereby initiating a large outburst flood as the lake was drawn down to the outlet sill. The flood discharge and Agassiz baseline runoff were diverted to the North Atlantic Ocean via the Great Lakes-St Lawrence route. Alternatively, if ice retreat was sufficient, Agassiz discharge may have switched first to the northwestern outlet as in Fig. 28.5c. (c) By 10ka (11,400 cal.yr), advance of the Superior ice lobe (Marquette advance, MA) had closed the first eastern Agassiz outlet and diverted the Agassiz baseline runoff briefly back to the Mississippi Valley route to Gulf of Mexico. Then, with opening of the northwestern outlet (H) to the Athabasca-Mackenzie Valley as ice retreated along the southwestern margin of LIS, runoff and a draw-down outburst flood were directed to the western Arctic Ocean. (See www.blackwellpublishing.com/knight for colour version.)

(Leverington & Teller, 2003), superposed on baseline runoff flows ranging 0.034-0.050Sv (Teller et al., 2002). Following overflow via the Nipigon basin to the Great Lakes, ice retreat farther east may have briefly opened outlets directly to Lake Superior (Thorleifson, 1996).

About 8ka (8900cal.yr), Lake Agassiz merged with glacial Lake Ojibway to the east, forming a huge lake of >800,000km2 (Leverington et al., 2002), more than three times the area of the modern Great Lakes, which bordered the entire southern LIS

south of Hudson Bay. Drainage from these combined glacial lakes bypassed the Great Lakes basin down the Ottawa and St Lawrence valleys to the North Atlantic Ocean (Fig. 28.2, outlet J).

In addition,beginning about 10 ka (11,400 cal.yr),another great proglacial lake developed along the northwestern part of the LIS in the Mackenzie River system, Lake McConnell, which overflowed to the western Arctic Ocean. This lake eventually covered an area of 215,000km2 in the isostatically depressed region on its eastern shore adjacent to the retreating LIS. This lake changed depth and

Figure 28.6 Interaction of ice retreat, opening of lower outlets, and differential rebound resulting in the switching of Lake Agassiz discharge from one outlet to another (after Teller et al., 2001). See text for explanation.

configuration in response to ice-marginal retreat and isostatic rebound, while it expanded downslope until it drained by 8.3ka (9300cal.yr) (Lemmon et al., 1994; Smith, 1994).Lake Agassiz discharged to Lake McConnell when it overflowed through the northwestern outlet into the Athabasca River valley. As noted before, this may have been only from around 10ka until 9.4ka (11,400 until 10,600 cal.yr) (Smith, 1994),or could have spanned a longer interval between 10.9 and 9.4ka (12,900 and 10,600cal.yr).

Downwasting and thinning of the residual ice sheet in Hudson Bay significantly reduced its weight relative to buoyancy forces of the sea entering Hudson Strait to the north and those of the deep (>400m) proglacial lakes to the south (Leverington et al., 2002). By 7.7ka (8450cal.yr), the merged Lake Agassiz-Ojibway breached the LIS in the Hudson Bay basin (Barber et al., 1999) and drained catastrophically, initially passing beneath the remaining ice in Hudson Bay, and then through Hudson Strait into the North Atlantic Ocean. Teller et al. (2002) calculated that this event released 163,000km3 of water from the lake (5.2 Sv, if released in 1yr). Alternatively, the final release may have occurred in two steps, with an initial flood of 113,100km3 or 3.6Sv, and a subsequent flood of 49,900km3 or 1.6Sv a short time later (Teller et al., 2002). Clarke et al. (2003, 2004) modelled this final drainage of the world's largest lake, concluding that, once initiated, complete drainage would have occurred in only 6 months. This terminated overflow of the Hudson Bay drainage basin through the St Lawrence system, which had been occurring since 9.4ka (10,600 cal. yr), and established modern runoff patterns on the continent.

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