Quaternary Science Reviews 21 (2002) 873–878 Preboreal oscillation caused by a glacial Lake Agassiz flood Timothy G. Fisher a, *, Derald G. Smith b , John T. Andrews c a Department of Geosciences, Indiana University-Northwest, 3400 Broadway, Gary, IN 46408 USA b Department of Geography, University of Calgary, Calgary, AB, Canada T2N 1N4 c INSTARR and Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA Abstract The Preboreal oscillation (PBO) has been attributed to increased meltwater, but the source of the meltwater and causative mechanism of the PBO has remained elusive. Here we attribute the source to a massive meltwater discharge event from an abrupt drainage of glacial Lake Agassiz, Canada, via the Mackenzie River into the Arctic Ocean. A maximum volume of 21,000 km 3 was discharged over a 1.5–3 yr period with a peak discharge of 0.500 Sverdrups (Sv), equivalent to a 6m rise in the Arctic Ocean (or 0.062m rise in global sea level). The flood occurred at about 11,335calyrBP, and was followed by a B0.042Sv flow until 10,750cal yrBP when the southern outlet of Lake Agassiz reopened and diverted drainage to the Mississippi River system. We estimate that only 2–4% of the flood water would have frozen into sea ice within the Beaufort region, but coupled with increased river ice production during winter, and thicker pack ice growth throughout the Arctic Ocean, a thicker, longer lasting and more extensive pack ice may have been flushed through Fram Strait. The thicker and more extensive pack ice, and freshened sea surface, may have triggered the PBO by increasing albedo, and generating a low salinity anomaly upon melting in the North Atlantic, thus decreasing the formation of North Atlantic Deep Water. r 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction The Preboreal oscillation (PBO), a brief (150–250yr) cooling event that began at 11,300calyrBP, is widely recognized in high-resolution climate records from the North Atlantic region (Johnson et al., 1992; O’Brian et al., 1995; Bj . orck et al., 1996, 1997; Hald and Hagen, 1998). An increase in freshwater to the North Atlantic causing a reduction in the thermohaline circulation, has been proposed as the cause of the PBO (Bj . orck et al., 1996; Hald and Hagen, 1998), but the source of that freshwater remains uncertain. Multiple, high-magnitude discharges of freshwater from the Baltic Sea basin predate the PBO (Bj . orck, pers. comm., 1995; Boden et al., 1997). Hald and Hagen (1998) concluded that the PBO was caused by an increased flux of meltwater to the Nordic Seas from adjacent ice sheets in response to post- Younger Dryas warming. This hypothesis, however, does not explain why the event was short-lived, nor why subsequent similar events did not occur while ice sheets remained in the region. Here we argue that the origin of the PBO was associated with a large, but short-lived, increase in freshwater discharge to the Arctic Ocean resulting from an abrupt drainage and lowering of Lake Agassiz when deglaciation uncovered the northwest outlet to the Mackenzie drainage basin. We propose that the increased discharge and increased production of river ice and sea ice in the Arctic Ocean was then exported to the North Atlantic causing the PBO. 2. The Lake Agassiz connection Glacial Lake Agassiz, which formed along the south- western margin of the retreating Laurentide Ice Sheet (Teller et al., 1983) discharged meltwater either south to the Gulf of Mexico, east to the North Atlantic Ocean, or north to the Arctic Ocean (Fig. 1). Flow from the northwest Agassiz outlet to the Arctic via the Mackenzie River is our hypothesized meltwater source that indirectly initiated PBO cooling. Investigations of the northwest outlet (Smith and Fisher, 1993; Fisher and Smith, 1994; Fisher and Souch, 1998) suggest that it opened at 11,250calyrBP (9900 14 C BP) shortly after closure of the eastern outlets, which was at 11,450 calyrBP (10,025 14 CyrBP; Lowell et al., 1999). *Corresponding author. Tel.: +1-219-980-7122; fax: +1-219-980- 6673. E-mail address: tgfisher@iun.edu (T.G. Fisher). 0277-3791/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0277-3791(01)00148-2