North American Late-Quaternary Meltwater and Floods to the Oceans: Evidence and Impact — Introduction
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Palaeogeography, Palaeoclimatology, Palaeoecology 246 (2007) 1–7 www.elsevier.com/locate/palaeo Editorial North American late-Quaternary meltwater and floods to the oceans: Evidence and impact — Introduction Terrestrial sedimentary deposits in North America Agassiz began forming after 12 14C ka (13.9 cal ka) BP associated with glacial meltwater have been studied for as ice retreated north of the Mississippi Valley–Hudson more than a century, and early geological surveys Bay drainage divide (Fig. 1). This lake expanded focused on the mapping and study of fluvial channels, northward with ice retreat over five millennia, final- lacustrine sediments and shorelines associated with the ly joining with Lake Ojibway south of Hudson Bay to retreating Laurentide Ice Sheet (LIS) (e.g. summaries by become a giant water body of some 841,000 km2 Leverett and Taylor, 1915; Coleman, 1922; Leverett, (Teller and Leverington, 2004). Discharges from Lake 1929, and by many others). Attention was mainly di- Agassiz, some of them catastrophic, were at different rected to regions around and within the Great Lakes times to the south (Fig. 1, F), to the northwest (Fig. 1, basins (Fig. 1), which had, at different times, confined H), to the east (Fig. 1, G and I), then to the south- much larger (and smaller) lakes, their size and volume east (Fig. 1, J), before its final outburst flood drain- controlled by the retreating ice and by differential iso- age to Hudson Bay about 7.7 14C ka (8.4 cal ka) BP static rebound. Once the southern, international fringe of (Fig. 1, K). lakes and rivers had been studied, the record of more The legacy of glacial lake data from the early northern proglacial rivers and lakes was examined, studies, which was largely motivated by the challenge made possible by new access roads and technology such of correlating moraines and deciphering retreat of the as airplanes and aerial photography. LIS, has found new value through an evolving interest From the record of continental glacial geology, it is in system analysis on a continental scale and known that at the last glacial maximum, about 21 14Cka integration of North American events with global (25 cal ka) BP, the ice cover over northern North America change research. These latter interests have stimulated extended southward into the northern part of the still more studies with a focus on linking the North Mississippi Valley watershed (Fig. 1). At that time, American late-glacial meltwater system to ocean meltwater from the southern ice margin between the circulation events (Teller, 1987; Lewis and Teller, Rocky and Appalachian mountains drained via the 2006). For example, drainage connections from Lake Mississippi Valley to the Gulf of Mexico (Fig. 1). As Agassiz and its large watershed to the Great Lakes the southern margin retreated northward over the con- system (Clayton, 1983; Teller and Thorleifson, 1983; tinental divide after 16 14C ka (19 cal ka) BP, proglacial Farrand and Drexler, 1985; Teller, 1985, 1988; Lewis lakes formed in the southern Great Lakes basins with and Anderson, 1989),andwithmarineeventsbeyond overflow to the Gulf of Mexico. Farther retreat opened continental boundaries (Broecker et al., 1988), were lower outlets from the Great Lakes basins to the Atlantic discussed in the 1980s. Although many others have Ocean, first via the Hudson River valley (Fig. 1, D), described and commented on meltwater linkages, then through the St. Lawrence Valley and Gulf of St. integrative earth science (land, ice, ocean, and Lawrence. Some of the proglacial lakes discharged atmosphere) accelerated in the late 20th century with catastrophically when lower outlets were opened by the realization that a better understanding of past ice retreat, and these outburst floods were superposed on climate change processes and their impact would baseline precipitation and meltwater runoff. Lake contribute to the assessment of future global climate. 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.10.023 2 Editorial Fig. 1. Map of North America showing ice cover at the last glacial maximum (grey), and the four main present-day watersheds that affected glacial runoff to the oceans as discussed in this volume. These watersheds are the Mississippi Valley catchment draining to the Gulf of Mexico, the Great Lakes–St. Lawrence Valley catchment discharging to North Atlantic Ocean, the Athabasca–Mackenzie Valley catchment draining to western Arctic Ocean, and the Hudson Bay catchment discharging via Hudson Strait to northwestern Atlantic Ocean (Labrador Sea). The Laurentide Ice Sheet (LIS) covered the three northern watersheds and extended short distances into the northern part of the Mississippi Valley watershed. Areas that were significant outlet controls on runoff routing, or were significant sites of outflow include the Mississippi–Gulf of Mexico area (A), Chicago (B) from southern Lake Michigan basin, Wabash Valley (C) from western Lake Erie basin, Hudson River valley (D) draining to Long Island Sound and Atlantic Ocean, Hudson Shelf Valley (E), southern Lake Agassiz outlet (F), eastern Agassiz outlets (G and I), northwestern Agassiz outlet (H), Kinojevis outlet (J) to Ottawa and St. Lawrence River valleys, and (K) drainage to Hudson Bay. Adapted from Lewis and Teller (2006). Editorial 3 Our understanding of the role of freshwater additions within the same basin or region are organized generally in to ocean circulation and, in turn, on climate change, is still chronological order (Table 1). in its infancy. In the past decade many have modeled the Proglacial Lake Agassiz was the major source for impact of freshwater inputs and their influx sites to the meltwater in north-central North America for 5000 yr North Atlantic Ocean, and the varying controls of flux (see Teller and Leverington, 2004), and its outburst and timing in relation to ocean circulation (see Teller floods have been implicated in suppressing THC during et al., 2002; Alley and Ágústsdóttir, 2005; Lewis and the Younger Dryas, Preboreal Oscillation, and 8.2 cal ka Teller, 2006, for summaries). Debate over the role of BP cold climate events (e.g. Broecker et al., 1989; freshwater additions during the last retreat of the LIS Barber et al., 1999; Fisher et al., 2002; Teller and continues, especially focusing on Lake Agassiz outbursts Leverington, 2004). The paper by Birks et al. (2007-this near the start of the Younger Dryas cooling (e.g. Broecker volume) on the isotopic evolution of Lake Agassiz et al., 1989; Broecker, 2006) and the 8.2 cal ka cold event evaluates the δ18O signature in its lacustrine sediments. (Alley et al., 1997; Barber et al., 1999; Rohling and Two different isotopic archives are studied – one from Pälike, 2005; Alley and Ágústsdóttir, 2005), whose cellulose in the sediment, the other from sediment effects are recorded in sediment records in the circum- porewater – and these provide a conceptual model that Atlantic region and far beyond. shows that hypolimnion (bottom) waters were supplied North American runoff varied dramatically during primarily from LIS meltwater, but that surface waters late-glacial time, as the LIS margin fluctuated and new were seasonally enriched by evaporation of runoff from routes were abruptly opened and closed (Teller, 1990; deglaciated terrain and from the lake surface itself. Fisher and Smith, 1994; Licciardi et al., 1999; Teller, These findings indicate that outflowing waters from 2004; Lewis and Teller, 2006). These abrupt re-routings Lake Agassiz may have had higher δ18O values than (Broecker et al., 1989; Clark et al., 2001), occasionally previously assumed, supporting the proposal by Lewis initiated with a rapid release of stored ice-marginal lake et al. (1994) that the record of Agassiz overflow into the waters (e.g. Teller et al., 2002), have been identified as Great Lakes reflects evaporative enrichment in central likely forcing agents in altering North Atlantic thermo- Canada. haline circulation (THC) and in inducing abrupt climate Two papers investigate the region of the Lake change (Clark et al., 2002). In cases where freshwater Agassiz basin where overflow was routed to the Arctic inputs have been linked to changes in ocean circulation, Ocean. Rayburn and Teller (2007-this volume), using the routing of the new influx has been shown to be new GPS elevations and a new correlation for Upper and critical in triggering the change (e.g. Fanning and Lower Campbell beaches in the northwestern part of the Weaver, 1997). Proglacial lake systems, and surface and Lake Agassiz basin, revise the trend of isobases to a subglacial runoff were important in shaping not only the west–east orientation from the N 56° W orientation continent and continental margin but also in influencing projected by Teller and Thorleifson (1983). This means oceanic sedimentation and circulation events — thus it that there was greater isostatic depression in this region becomes essential to understand the paleohydrology of that, unlike previous isobase reconstructions, allows North America during retreat of the LIS. overflow through the Wycherley Lake outlet channels This special issue of Palaeogeography, Palaeoclima- identified by Fisher and Souch (1998) and Fisher (2007- tology, Palaeoecology originated following a session this volume) as carrying overflow into the Clearwater– entitled “Tracing North American meltwater and floods Athabasca River system about 9.9–9.4 14C ka 11.3– to the North Atlantic”, organized by Mike Lewis, Jim 10.6 cal ka BP. The paper by Fisher (2007-this volume) Teller, and Bob Mott, held at a Canadian Quaternary describes the complex channel morphology in the north- Association (CANQUA) meeting in Halifax in 2003. western outlet region in the headwaters of the Clear- The goal of this special issue is to present new data on water–Athabasca River system. He integrates this some important new research, designed to better under- morphology with an assessment of the stratigraphic stand late-glacial runoff from North America, and to record of lakes and radiocarbon dates in that region.