Glacial Lake Agassiz: A 5000 yr history of change and its relationship to the d18O record of Greenland James T. Teller² Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada David W. Leverington³ Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, D.C. 20560-0315, USA ABSTRACT Dredge and Cowan (1989), Karrow and Oc- a result of the interaction among (1) location chietti (1989), Lewis et al. (1994), and others. of the ice margin, (2) topography of the newly Lake Agassiz was the largest lake in Several special volumes of papers (e.g., Teller deglaciated surface, (3) elevation of the active North America during the last period of de- and Clayton, 1983; Karrow and Calkin, 1985; outlet, and (4) differential isostatic rebound glaciation; the lake extended over a total of Teller and Kehew, 1994) and hundreds of (Teller, 1987, 2001). This interaction was 1.5 3 106 km2 before it drained at ca. 7.7 journal articles over the past century have pro- complex, partly because it involved glacier 14C ka (8.4 cal. [calendar] ka). New com- vided important insight into this extensive melting and ice-dam failure, as well as ice puter reconstructionsÐcontrolled by lake system. Closely linked to the history of readvances and surges into the lake basin. beaches, isostatic rebound data, the margin proglacial lakes is the chronology of the rout- Thus, over¯ow outlets were opened and oc- of the Laurentide Ice Sheet, outlet eleva- ing of North American glacial runoff, which casionally closed again by a ¯uctuating re- tions, and a digital elevation model (DEM) has been discussed by many for various re- treat. Outlet erosion also played a role. The of modern topographic dataÐshow how gions, and synthesized by several, including interrelationship between differential isostatic variable the size and depth of this lake were Teller (1987, 1990a, 1990b, 1995) and Lic- rebound and the geographic location of the during its 4000 14C yr (5000 cal. yr) history. ciardi et al. (1999), and tied to past global outlet was especially important in dictating the Abrupt reductions in lake level, ranging ocean circulation by Broecker et al. (1989), extent and depth of Lake Agassiz. As de- from 8 to 110 m, occurred on at least 18 Clark et al. (2001), Teller et al. (2002), and scribed by Teller (2001), whenever the outlet occasions when new outlets were opened, others. carrying over¯ow was at the southern end of reducing the extent of the lake and sending The largest of all proglacial lakes was Lake the basin, lake levels receded everywhere to large outbursts of water to the oceans. Agassiz, which developed and expanded the north of the isobase (i.e., the contour of Three of the largest outbursts correlate northward along the margin of the Laurentide equal isostatic rebound) through the outlet closely in time with the start of large d18O Ice Sheet as it retreated downslope into the (Fig. 2A). Whenever the outlet was not at the excursions in the isotopic records of the Hudson Bay and Arctic Ocean basins in Sas- southern end of the basin, transgression oc- Greenland ice cap, suggesting that those katchewan, Manitoba, Ontario, Quebec, and curred to the south of the outlet, while regres- freshwaters may have had an impact on Northwest Territories. During the history of sion occurred everywhere to the north of it thermohaline circulation and, in turn, on Lake Agassiz, over¯ow was carried from the (Figs. 2B and 2C). Isobases across the Agassiz- climate. lake to the oceans by way of four different Ojibway basin are shown in Figure 3, along routes (Fig. 1): (A) southward via the Min- with the locations of the main outlets. Keywords: Lake Agassiz, history, out- nesota and Mississippi River Valleys to the As a result of the abrupt opening and clos- bursts, Greenland isotopic record. Gulf of Mexico, (B) northwestward through ing of outlets, plus the interaction of differ- the Clearwater-Athabasca-Mackenzie River ential isostatic rebound and the use of many INTRODUCTION Valleys to the Arctic Ocean, (C) eastward different outlet channels, the level of Lake through a series of channels that led to the Agassiz was constantly changing, and its his- During the last period of global deglacia- Great Lakes (or to glacial Lake Ojibway) and tory is very complex. Overall, its history is tion, large lakes formed along the margins of continental ice sheets in North America, Eu- then to the St. Lawrence Valley and North At- one of northward expansion, punctuated by rope, and Asia. The complex history of North lantic Ocean, and, ®nally, when Lake Agassiz abrupt drops in lake level when lower outlet American proglacial lakes has been summa- completely drained, (D) northward and east- channels were deglaciated; each decline was rized by Prest (1970), Teller (1987, 2004), ward through Hudson Bay and Hudson Strait then followed by transgressive deepening of Dyke and Prest (1987), Klassen (1989), to the North Atlantic Ocean, between the Kee- the lake in the basin south of the isobase watin and Labrador ice centers. through the outlet and regression north of that ²E-mail: [email protected]. The extent, con®guration, and depth of isobase due to differential rebound. ³E-mail: [email protected]. Lake Agassiz, as with all proglacial lakes, was In this paper, a series of maps are presented GSA Bulletin; May/June 2004; v. 116; no. 5/6; p. 729±742; doi: 10.1130/B25316.1; 5 ®gures; 1 table; Data Repository item 2004079. For permission to copy, contact [email protected] q 2004 Geological Society of America 729 TELLER and LEVERINGTON bound curve, because isostatic rebound varied through time; older beaches have slightly steeper curves, whereas younger ones have gentler curves (see Teller and Thorleifson, 1983, Fig. 2). The trend of the isobases over the life of the lake are assumed to be the same, although we recognize that changes in relative thickness of the Laurentide Ice Sheet during deglaciation are likely to have led to some changes in isobase orientation and con®guration. The isostatically deformed rebound curves were used to generate a paleo±water surface of Lake Agassiz associated with each beach (Mann et al., 1999; Leverington et al., 2002b). Each rebound surface was computationally projected to intersect modern topography (de- ®ned by a digital elevation model [DEM] with individual grid dimensions of ;1 3 1 km), generating an outline of the lake south of the Laurentide Ice Sheet (Leverington et al., 2000, 2002a, 2002b). The GLOBE (Globe Task Team, 1999) and ETOPO 5 (NGDC, 1988) da- Figure 1. Routing of runoff to the oceans from Lake Agassiz (Teller et al., 2002). Ice tabases were used as the sources for the mod- 14 margin at ca. 9 C ka shown by dashed line. Out¯ow paths A±D described in text. ern DEM. Because control points for isostatic rebound are nearly all at the margin of the that outline the extent of Lake Agassiz at 18 eastern extension, as described by Vincent and lake, the simple curvilinear isobases repre- different transgressive maximums and 15 in- Hardy (1979) and Veillette (1994), is included senting this rebound (Fig. 3) may be more tervening minimums. These span the history in our reconstructions. Elson (1967), Prest complex, as suggested by Dredge and Cowan of the lake from ca. 10.9 14C ka (thousand ra- (1970), and Dyke and Prest (1987) provided a (1989) and Rayburn and Teller (1999). How- diocarbon years B.P.; 12.94 thousand calendar number of snapshots of the lake through its ever, lake bathymetry and total lake volume years B.P.), until its ®nal stage at ca. 7.7 14C history, and some, such as Clayton and Moran would have been in¯uenced only slightly by ka (8.45 cal. ka), after it had amalgamated (1982), Teller (1985), Klassen (1989), Dredge such isostatic variability, and the coincidence with glacial Lake Ojibway. In addition, we and Cowan (1989), and Thorleifson (1996), of actual beaches and wave-trimmed cliffs discuss some of the relationships of these out- presented maps and integrated its history with with the topographically modeled shorelines bursts to the d18O curves in the Greenland deglaciation across central North America. indicates that the outlines of various lake stag- GISP2 and GRIP ice cores. Lake reconstructions in various regions and at es are accurately depicted, except perhaps in various times were also published by research- far northern regions where rebound curves LAKE RECONSTRUCTION ers in the volume Glacial Lake Agassiz (Teller were projected and are not controlled by strandline data. and Clayton, 1983), among them Brophy and Maps depicting various stages in the history The ice margin across the Lake Agassiz ba- Bluemle (1983), Dredge (1983), Klassen of Lake Agassiz have been published for more sin through time is controlled in only a few (1983a, 1983b), Fenton et al. (1983), and than a century, beginning with those in U.S. places, mainly by scattered and dated end mo- Schreiner (1983). Smith and Fisher (1993) and Geological Survey Monograph 25 by Upham raines and by some dated lithostratigraphic re- Fisher and Souch (1998) expanded the lake (1895). Johnston's (1946) ®eld work on the lationships. Additional ``local'' control for northwestward to the Clearwater-Mackenzie beaches provided important new data on placement of the ice margin is based on the outlet. shorelines in the Canadian part of the lake, linkage of lake levels (beach elevations) to north of where Upham had worked. Modern Reconstructions in this paper expand on speci®c outlets carrying over¯ow at a given reconstructions have been guided by the those in Leverington et al.