Quaternary Science Reviews 75 (2013) 100e113 Contents lists available at SciVerse ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev An assessment of the minimum timing of ice free conditions of the western Laurentide Ice Sheet Evan J. Gowan Research School of Earth Sciences, The Australian National University, Acton 2601, ACT, Australia article info abstract Article history: This study presents an investigation of the minimum timing of retreat of the western Laurentide Ice Received 22 November 2012 Sheet, which covered parts of Canada and the northern United States during the Wisconsin glaciation. Received in revised form The retreat of the ice sheet is poorly constrained due to low spatial resolution of chronological data 27 May 2013 that indicate ice free conditions. A limitation of radiocarbon and luminescence dating methods is that Accepted 3 June 2013 it is only possible to determine the time when it is certain that ice is absent from a region, which may Available online differ substantially from the true date of ice margin retreat. The minimum timing of ice free conditions is determined using a model of ice sheet retreat, inferred from landforms that formed during the late Keywords: Laurentide Ice Sheet stages of glaciation, and extrapolating the calibrated probability distribution of chronological data in Ice sheet retreat the opposite direction. The analysis excludes radiocarbon samples from materials that have been Lake Agassiz shown to give artificially old ages, such as bulk sediments. The results of the analysis identify 55 dates Radiocarbon dating that have the most impact on the model. The retreat of the western Laurentide Ice Sheet commenced Cosmogenic dating by at least 15 000 cal yr BP, while the minimum timing of ice free conditions between the Laurentide Luminescence dating Ice Sheet and the Cordillera is after 11 000 cal yr BP. A lack of data in the Keewatin sector prevents the Ice sheet modelling determination of a precise timing of ice free conditions in the last vestiges of the Laurentide Ice Sheet. Younger Dryas The model provides a guide to where additional samples could improve chronological control on ice Deglaciation margin location. Reservoir corrections Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction chronological data are extrapolated in the direction opposite of retreat to determine the minimum timing of retreat. These results The Laurentide Ice Sheet covered most of Canada and parts of can be used as constraints for ice sheet models, by determining the the northern United States, reaching a maximum extent at the Last region certain to be ice free for any time period. The model also Glacial Maximum, between 26.5 and 19 ka (Clark et al., 2009). The serves as a basis for determining where more data would best ice sheet retreated over a period of 14 000e16 000 years, affecting improve temporal controls of ice margin reconstructions. global climate and sea level. Dyke and Prest (1987) and Dyke (2004) The western Laurentide Ice Sheet includes the region of ice that presented reconstructions of the ice sheet margin during the retreat flowed from the Keewatin sector (Fig. 1). It converged with the of the Laurentide Ice Sheet, based on median ages of uncalibrated Cordillera Ice Sheet to the west, and extended into northern mid- radiocarbon dates (Dyke et al., 2003), glacial geomorphological western United States (Dyke, 2004). The easternmost boundary of features (i.e. moraines, ice contact deltas, meltwater channels), and this sector lay in northwestern Ontario and Manitoba, where ice glacial flow features that indicate the most likely direction of ice from northeastern sectors also influenced the direction of flow. The retreat. Uncertainties in margin position are one of the largest po- retreat of ice in this region has implications for major climatic tential sources of error in ice sheet reconstructions and models (i.e. events (i.e. Tarasov and Peltier, 2005; Broecker, 2006; Gregoire Tarasov et al., 2012). In this paper, I create a model of the minimum et al., 2012), the drainage of glacial lakes (i.e. Fisher et al., 2002; timing of ice free conditions in the western Laurentide Ice Sheet. Teller et al., 2002; Teller and Leverington, 2004; Broecker, 2006; The model is developed using techniques similar to those applied Fisher et al., 2009; Fisher and Lowell, 2012), and the migration of by past workers such as Dyke and Prest (1987) and Dyke (2004) mammals in deglacial and post-glacial North America (i.e. Meltzer, though, instead of determining a model of margin positions, 2003; Burns, 2010). It is important to recognise that radiocarbon dates only provide a minimum timing of retreat. There is an unknown period of time E-mail address: [email protected]. between when ice retreated from a region and when organisms 0277-3791/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2013.06.001 E.J. Gowan / Quaternary Science Reviews 75 (2013) 100e113 101 70˚N 2. Dating methods MD 2.1. Radiocarbon dating GBL LaLake McConnell NU in Temporal control on the retreat of the western Laurentide Ice MackenzieM k t e McConn ack LLake a 60˚N ake w e NT Sheet comes primarily from radiocarbon dates. An updated version nz e Hudson YT ie e KeKeewatin Bay of the compilation by Dyke et al. (2003) (A.S. Dyke, pers. comm. l GSL l CordiCordillera Ice Sheet 2010) was the source of the majority of radiocarbon dates used in MB LakeLa Agassiz this study. These are supplemented by samples from recent studies k llera Ice CL e Ag FM ass that have focused on the chronology of strandlines and drainage 50˚N iz ON BC AB routes of glacial Lake Agassiz (e.g. Lowell et al., 2005, 2009; Teller S he SK et al., 2005; Yansa and Ashworth, 2005; Boyd, 2007; Fisher, 2007; e LS t Lepper et al., 2007; Fisher et al., 2009; Anderson, 2012; Fisher ND 500 km MN and Lowell, 2012), other studies focussing on ice sheet retreat, MT glacial geology and other glacial lakes (e.g. Fulton et al., 2004; JL SD DL IA Dredge and McMartin, 2005; Bateman and Murton, 2006; Little, 140˚W 90˚W 2006; Couch and Eyles, 2008; Huntley et al., 2008; Yu et al., 130˚W 2010; Breckenridge et al., 2012) plus other studies that did not 100˚W 120˚W 110˚W have a particular focus on the retreat of the Laurentide Ice Sheet but nonetheless provide evidence of ice free conditions (a compilation Fig. 1. Map of the western Laurentide Ice Sheet and adjacent areas. The maximum of samples used in this study is available in the Supplementary extent of late Wisconsin ice is shown by the blue line, based on maps by Dyke (2004), and the “Illinoian” maximum extent by Fullerton et al. (1995, 2004, 2007), Hallberg material). et al. (1994) and Swinehart et al. (1994), which is now suspected to date to the late Radiocarbon dates need to be calibrated to account for Wisconsinan (Jackson et al., 2011). The brown outline is the area investigated in this changes in the concentration of 14C in the atmosphere. OxCal 4.1 study, which is bounded on the west by the convergence zone of the Laurentide and (Bronk Ramsey, 2009) is used to calibrate dates in this study. Cordillera ice sheets. The area covered by glacial lakes Mackenzie (Smith, 1992), McConnell (Smith, 1994) and Agassiz (Leverington and Teller, 2003) are shown in light Terrestrial samples were calibrated using the IntCal09 curve, blue. The Upper Campbell Beach, the most extensive beach of Lake Agassiz, is shown in while marine samples used the Marine09 curve (Reimer et al., orange (Fisher and Lowell, 2012). The location of the Cree Lake Moraine (CL) is shown 2009). Calibrated radiocarbon dates are denoted as “cal yr BP”, with a red line (Fulton, 1995). Locations mentioned in this paper: MT e Montana; ND e while uncalibrated dates are denoted as “14CyrBP”. Ages re- e e e e North Dakota; SD South Dakota; MN Minnesota; IA Iowa; BC British Columbia; ported in this paper are at the 95% confidence interval, rounded AB e Alberta; SK e Saskatchewan; MB e Manitoba; ON e Ontario; YT e Yukon; NT e Northwest Territories; NU e Nunavut; JL e James Lobe; DL e Des Moines Lobe; GSL e to the nearest hundred years, though the full calibrated distri- Great Slave Lake; GBL e Great Bear Lake; LS e Lake Superior; MD e Mackenzie Delta; bution is used in the modelling. FM e Fort McMurray region. (For interpretation of the references to colour in this Organisms that grew in marine environments have measured fi gure legend, the reader is referred to the web version of this article.) ages that are older than their true age due to the delay in incor- poration of atmospheric 14C into the water, the upwelling of 14C deficient waters by currents, and the influx of glacial meltwater into the ocean (Hutchinson et al., 2004; Coulthard et al., 2010; Vickers reoccupied it. The full suite of available radiocarbon dates also et al., 2010). An estimation of the deficiency of 14C in a modern represent results from a wide variety of studies, many that were not reservoir is made by finding the difference between the true and attempting to constrain the precise timing of retreat. These data apparent ages of pre-bomb molluscs of known age (i.e. McNeely nevertheless provide estimates of the minimum timing of ice free et al., 2006). Subtracting that value from the apparent age of the conditions. During the past 20 years, researchers have also started samples gives an improved estimate of its true age.