The Great Lakes drainage basin

After 16.5 ka (19,700 cal. yr), the southern LIS margin receded into the basins of the southern Great Lakes (Erie Interstade) (Fig. 28.4a & b) (Barnett, 1992). For several hundred years the Ontario-Erie and Huron lobes of the LIS retreated enough to switch 0.038 Sv of drainage from the Gulf of Mexico to the North Atlantic Ocean via the Mohawk and Hudson valleys (Licciardi et al., 1999; Lewis et al., 1994) (Figs 28.2 & 28.4b, outlets C and D). (1 Sv = 106m3 s-1, or about the present combined flow of all of Earth's rivers.) Runoff from the central LIS, from Michigan basin (outlet A) west, continued south through the Mississippi Valley to the Gulf of Mexico (Figs 28.2, 28.3c & 28.4b, outlet A). The eastward drainage diversion was switched back after 15.5ka (18,500 cal. yr) to the Mississippi Valley and Gulf of Mexico when the ice margin readvanced (Port Bruce Stade) to cover all the Great Lakes basins again (Barnett, 1992; Lewis et al., 1994) (Fig. 28.4c).

After 14.5ka (17,400cal.yr), a second recession of the Ontario-Erie, Huron and Michigan ice lobes again brought the ice margin back into the basins ofthe southern Great Lakes where large proglacial bodies of impounded meltwater were established. These lakes first continued to drain south to the Mississippi Valley, then switched to the Hudson Valley and North Atlantic route again when ice recession during the Mackinaw Interstade opened the Mohawk Valley (Fig. 28.2, outlet C) about 13.5-13.1 ka (16,200-15,800cal.yr) (Barnett, 1992; Lewis et al., 1994; Licciardi et al., 1999). This opening would have reduced the level of glacial lakes in the Ontario, Erie and southern Huron basins. The draw down of these lakes may have resulted in outburst floods to the North Atlantic Ocean superposed on the 0.056 Sv increase in baseline runoff from the southern Great Lakes region east of the Michigan basin (Fig. 28.3b) (Teller, 1990a; Licciardi et al, 1999).

Readvance of the southern LIS to moraines of the Port Huron Stade about 13.0-12.8ka (15,600-15,400cal.yr) again closed the Mohawk Valley route to the Atlantic Ocean. Glacial runoff from the eastern Great Lakes basins was switched again back to the Gulf of Mexico, and Licciardi et al. (1999) calculated that this resulted in an increase in runoff through that route by 0.059Sv (Fig.

Conventional radiocarbon age BPx10 8 10 12 14

to te

Conventional radiocarbon age BPx103 8 10 12 14 16

Lake outburst flood

Precipitation runoff

Ice melt runoff

▲ Lake outburst flood

Total baseline runoff

10 12 14 16 18 Age (calendar years BPx103)

Conventional radiocarbon age BPx103

10 12 14 16 18 Age (calendar years BPx103)

Conventional radiocarbon age BPx103 8 10 12 14 16

a Lake outburst flood

10 12 14 16 18 Age (calendar years BPx103)

CO te

Total baseline runoff

Total baseline runoff

10 12 14 16 18 Age (calendar years BPx103)

Conventional radiocarbon age BPx103 8 10 12 14 16

▲ Lake outburst flood

Total baseline runoff

10 12 14 16 18 Age (calendar years BPx103)

Figure 28.3 History of Lake Agassiz baseline runoff and outburst floods to oceans from southern LIS after Licciardi et al. (1999) and Teller et al. (2002). Triangles illustrate Lake Agassiz outburst flood fluxes assuming each lake draw down was completed in 1yr after the opening of a lower outlet by ice retreat. Flood discharges are added to baseline runoff. (a) Total runoff showing ice melt and precipitation components plus outburst floods through all routes. (b) Total (ice melt + precipitation) runoff plus outburst floods (triangles) to the North Atlantic Ocean via first the Hudson River and after 11 ka (13,000 cal. yr) the St Lawrence Valley routes: YD and horizontal arrows indicate period of the Younger Dryas cold event. In an alternative scenario the large Agassiz flood shown at the onset of the Younger Dryas event would not have discharged by this route, and baseline runoff would have been reduced (thin grey line) (see also Fig 28.3d). (c) Total (ice melt + precipitation) runoff via the Mississippi Valley route to Gulf of Mexico. Note the strong antiphased relationship in discharge with that of the route via Hudson River-St Lawrence Valley. (d) Total (ice melt + precipitation) runoff to western Arctic Ocean. The two outburst floods (black triangles representing floods after 12,000 cal. yr) through Lake Agassiz's northwest outlet are thought to have triggered the Preboreal Oscillation (PBO) cool event. An alternative scenario, shown by the first outburst flood (grey triangle at 13,000 cal. yr) and enhanced baseline flow (thin grey line), illustrates flux for the first Agassiz diversion beginning about 11 ka (13,000 cal. yr) if it went via the Athabasca-Mackenzie Valley rather than the Great Lakes-North Atlantic route. (e) Total (ice melt + precipitation) runoff to Atlantic Ocean via Hudson Bay and Hudson Strait. The final demise of the LIS in Hudson Bay led to one huge outburst flood of 5.2 Sv, or two or more closely spaced floods of about 3.6 Sv and 1.6 Sv. Horizontal arrows indicate the period of the 8200 yr BP cold event as recorded in Greenland ice cores.

Rhizobitoxine Pathway

Figure 28.4 Ice-marginal positions during the first oscillation of runoff between Mississippi and Hudson River valleys, after Licciardi et al. (1999) and Dyke et al. (2003). Base image from Figs 28.1 and 28.2. (a) The ice margin at the LGM (about 1821 ka; 21,000-25,000 cal. yr) covered the Great Lakes basins (named) and Hudson River Valley. Runoff west of the Appalachian Mountains (dotted) drained to Gulf of Mexico, and runoff to the east of the mountains went to the North Atlantic Ocean. (b) From about 16.5-15.2ka (19,700-18,200 cal.yr) ice had retreated enough to switch runoff generated in the eastern Great Lakes region from Gulf of Mexico to the Atlantic Ocean via the Mohawk (C) and Hudson (D) valleys. Runoff from southern Lake Michigan basin continued to drain via the Chicago outlet (A) to Gulf of Mexico. (c) By 15ka (17,900cal.yr) ice advance had diverted the Mohawk-Hudson drainage back to the Mississippi River and Gulf of Mexico. (See www.blackwellpublishing.com/ knight for colour version.)

Figure 28.4 Ice-marginal positions during the first oscillation of runoff between Mississippi and Hudson River valleys, after Licciardi et al. (1999) and Dyke et al. (2003). Base image from Figs 28.1 and 28.2. (a) The ice margin at the LGM (about 1821 ka; 21,000-25,000 cal. yr) covered the Great Lakes basins (named) and Hudson River Valley. Runoff west of the Appalachian Mountains (dotted) drained to Gulf of Mexico, and runoff to the east of the mountains went to the North Atlantic Ocean. (b) From about 16.5-15.2ka (19,700-18,200 cal.yr) ice had retreated enough to switch runoff generated in the eastern Great Lakes region from Gulf of Mexico to the Atlantic Ocean via the Mohawk (C) and Hudson (D) valleys. Runoff from southern Lake Michigan basin continued to drain via the Chicago outlet (A) to Gulf of Mexico. (c) By 15ka (17,900cal.yr) ice advance had diverted the Mohawk-Hudson drainage back to the Mississippi River and Gulf of Mexico. (See www.blackwellpublishing.com/ knight for colour version.)

28.3c). This runoff was discharged by overflow from glacial Lake Whittlesey in the southern Huron and Erie basins to glacial Lake Glenwood II in southern Michigan basin, and thence to the Mississippi Valley (Barnett, 1992; Lewis et al., 1994).

Following the Port Huron Stade, ice margins were in a sensitive position relative to outlets in the southern Great Lakes region; their variations caused lake basin drainage to oscillate between the Mohawk-Atlantic and Mississippi-Gulf-of-Mexico routes. From about 12.8-12.6ka (15,400-14,600cal.yr) Erie basin overflow (Lake Wayne) was briefly routed east to the Atlantic Ocean via the Mohawk-Hudson Valley, then switched to a westward route into the Michigan basin, which overflowed into the Mississippi River system (Lake Warren II). About 12.5ka (14,500cal.yr), overflow returned to the Mohawk-Hudson route during the Lake Grassmere to Early Lake Erie stages (Calkin & Feenstra, 1985). By 12.2-12.0ka (14,200-14,000cal.yr), with recession ofice from the Michigan basin during the Two Creeks Interstade, waters of the Michigan, Huron and Ontario basins were all draining to the North Atlantic via the Mohawk and Hudson valley routes (Fig. 28.3b) (Hansel et al., 1985; Calkin & Feenstra, 1985; Lewis et al.,

1994). With the Greatlakean (formerly Two Rivers) advance about 11.8ka (13,600cal.yr) into the Michigan basin, overflow there was diverted south into the Mississippi Valley and Gulf of Mexico (Calumet Lake phase) (Hansel et al., 1985), while the eastern basins continued draining to the Atlantic (Lewis et al., 1994). Ice retreat from the northern flank of the highland east of the Ontario basin about 11.3ka (13,200 cal. yr) (Fig. 28.4b) released glacial Lake Iroquois down the Hudson River Valley into the Atlantic Ocean where it is thought to have suppressed oceanic thermohaline circulation and induced the Intra-Allerod cold period (Donelly et al., 2005). Continued ice retreat switched discharge routes and released a glacial lake outburst flood and subsequent drainage down the St Lawrence Valley to the Atlantic Ocean about 11.1 ka (13,010 cal. yr) (Richard & Occhietti, 2005) close to the initiation of the Younger Dryas cold event. After recession of Greatlakean ice about 11.2 ka (13,100cal.yr), waters from the Michigan-Huron and eastern basins returned to their eastward routing (Hansel et al., 1985; Barnett, 1992; Lewis et al., 1994). At this time, the switching of overflow out of the Great Lakes, back and forth from east to south, between the North

Atlantic and Gulf of Mexico, ended. Additional details and insight of the above switches in continental runoff have been obtained through studies of proxies of enhanced freshwater inflow in marine sediments of the Gulf of Mexico (Brown & Kennett, 1998; Brown et al., 1999; Aharon, 2003).

After about 10.2ka (11,900cal.yr), water in the Michigan and Huron basins went to the St Lawrence Valley when ice retreat opened a lower overflow route via the Ottawa Valley (Fig. 28.2). This overflow continued until about 5.5 ka (6300cal.yr) when differential rebound raised the Ottawa Valley outlet region to the elevation of the southern rim of the basin. Southward overflow at Chicago (Fig. 28.4b, outlet A) from the Michigan basin and from the southern end of the Huron basin at Port Huron (Fig. 28.4b) continued until about 4.7ka (5400cal.yr) when the northern outlet to Ottawa Valley emerged above lake level owing to its faster uplift (Lewis, 1969). By about 3.4ka (3600cal.yr) outlet erosion lowered the Port Huron channel. This lower channel captured all overflow from the upper Great Lakes, directing it through the Erie and Ontario basins (Fig. 28.4b) to the St Lawrence River (Fig. 28.2), a routing that continues today (Larsen, 1985; Blasco, 2001).

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