Timing and duration of North American glacial lake discharges and the Younger Dryas climate reversal John A. Rayburn a, ⁎, Thomas M. Cronin b , David A. Franzi c , Peter L.K. Knuepfer d , Debra A. Willard b a Department of Geological Sciences, SUNY New Paltz, New Paltz, NY 12561, USA b 926A U.S. Geological Survey, Reston, VA 20192, USA c Center for Earth and Environmental Sciences, SUNY Plattsburgh, Plattsburgh, NY, 12901, USA d Department of Geological Sciences and Environmental Studies, Binghamton University, Binghamton, NY 13902, USA abstract article info Article history: Received 26 August 2010 Available online 31 March 2011 Keywords: Glacial lake Champlain sea Discharge Younger dryas Foraminifers Radiocarbon-dated sediment cores from the Champlain Valley (northeastern USA) contain stratigraphic and micropaleontologic evidence for multiple, high-magnitude, freshwater discharges from North American proglacial lakes to the North Atlantic. Of particular interest are two large, closely spaced outflows that entered the North Atlantic Ocean via the St. Lawrence estuary about 13,200–12,900 cal yr BP, near the beginning of the Younger Dryas cold event. We estimate from varve chronology, sedimentation rates and proglacial lake volumes that the duration of the first outflow was less than 1 yr and its discharge was approximately 0.1 Sv (1 Sverdrup = 10 6 m 3 s -1 ). The second outflow lasted about a century with a sustained discharge sufficient to keep the Champlain Sea relatively fresh for its duration. According to climate models, both outflows may have had sufficient discharge, duration and timing to affect meridional ocean circulation and climate. In this report we compare the proglacial lake discharge record in the Champlain and St. Lawrence valleys to paleoclimate records from Greenland Ice cores and Cariaco Basin and discuss the two-step nature of the inception of the Younger Dryas. © 2010 University of Washington. Published by Elsevier Inc. All rights reserved. Introduction Climate-model simulations suggest that the ocean's meridional overturning circulation (MOC), a significant component of the global climate system, is sensitive to small volumes of fresh-water input. Model simulations also show that the oceanic and climate response to freshwater forcing is highly dependent on the background climate state (Ganopolski and Rahmstorf, 2001; LeGrande et al., 2006; LeGrande and Schmidt, 2008), the volume of freshwater (Rahmstorf, 1995; Stouffer et al., 2006), the location (Manabe and Stouffer, 1997; Tarasov and Peltier, 2005), the duration of the discharge event (Rensson et al., 2001; Wiersma and Renssen, 2006), and complex surface and deep-ocean dynamics in the North Atlantic (Born and Levermann, 2010). Concern about freshwater forcing of climate change, possibly influenced by anthropogenic greenhouse gas emissions, has also grown in part due to recent trends in the high-latitude Northern Hemisphere's hydrologic budget (Peterson et al., 2002, 2006) that include diminishing sea-ice (Serreze et al., 2007), decreasing North Atlantic salinity (Dickson et al., 2002; Antonov et al., 2002; Curry and Mauritzen, 2005) and slowing MOC (Häkkinen and Rhines, 2004; Bryden et al., 2005). However, variability in ocean salinity and circulation are also associated with decadal and lower frequency modes of climate variability, such as the North Atlantic Oscillation (Hurrell and Dickson, 2004; Delworth and Mann, 2000), the Arctic Oscillation (Thompson and Wallace, 1998; Rigor and Wallace, 2004), and the Atlantic Multidecadal Oscillation (Enfield et al., 2001; Trenberth and Shea, 2006). Consequently, detecting human and natural unforced MOC variability remains an elusive goal (Dickson et al., 2003). Large proglacial lake outflows during the last deglacial interval offer opportunities to examine the role of freshwater forcing of ocean circulation and climate. Broecker et al. (1989) first proposed that the Younger Dryas climate reversal 12,900–11,500 calibrated years before present (cal yr BP) was caused by glacial Lake Agassiz discharge through the St. Lawrence Valley. Prior to the beginning of the Younger Dryas Lake Agassiz was at its high-water Lockhart Phase which drained southward through the Mississippi to the Gulf of Mexico. Work on cores from the Gulf of Mexico using oxygen isotopes from planktonic foraminifera and percentage of reworked calcareous nanofossils indicate that Agassiz discharge ceased to flow southward around the beginning of the Younger Dryas (Broecker et al., 1989; Marchitto and Wei, 1995). The Lockhart Phase of Lake Agassiz was followed by the significantly lower Moorhead Phase, bringing the level of Lake Agassiz below the elevation of the southern outlet indicating a change in outlet and a large loss of volume at this time (Fisher, 2005). Teller et al. (2002) estimate that the level of Lake Quaternary Research 75 (2011) 541–551 ⁎ Corresponding author. Fax: +1 845 257 3755. E-mail address: rayburnj@newpaltz.edu (J.A. Rayburn). 0033-5894/$ – see front matter © 2010 University of Washington. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.yqres.2011.02.004 Contents lists available at ScienceDirect Quaternary Research journal homepage: www.elsevier.com/locate/yqres