Postglacial climates inferred from a lake at treeline, southwest Yukon Territory, Canada Joan Bunbury * , Konrad Gajewski Laboratory for Paleoclimatology and Climatology, Department of Geography, University of Ottawa, 60 University Street, Ottawa ON K1N 6N5, Canada article info Article history: Received 18 July 2008 Received in revised form 9 October 2008 Accepted 11 October 2008 abstract Pollen, chironomid, and ostracode records from a lake located at alpine treeline provide regional pale- oclimate reconstructions from the southwest Yukon Territory, Canada. The pollen spectra indicate herbaceous tundra existed on the landscape from 13.6–11 ka followed by birch shrub tundra until 10 ka. Although Picea pollen dominated the assemblages after 10 ka, low pollen accumulation rates and Picea percentages indicate minimal treeline movement through the Holocene. Chironomid accumulation rates provide evidence of millennial-scale climate variability, and the chironomid community responded to rapid climate changes. Ostracodes were found in the late glacial and early Holocene, but disappeared due to chemical changes of the lake associated with changes in vegetation on the landscape. Inferred mean July air temperature, total annual precipitation, and water depth indicate a long-term cooling with increasing moisture from the late glacial through the Holocene. During the Younger Dryas (12.9–11.2ka), cold and dry conditions prevailed. The early and mid-Holocene were warm and dry, with cool, wet conditions after 4 ka, and warm, dry conditions since the end of the Little Ice Age. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The southwest Yukon has long been an important region in Quaternary studies. This mountainous area supports ice sheets and glaciers that have been studied to learn about glacial landforms (Russell 1898). The importance of the Bering Land Bridge as a connection between Asia and North America has led to numerous biogeographic (e.g. Hulte´ n, 1937) and paleoenvironmental studies (e.g. Hopkins, 1967; Kontramavichus, 1976; Elias, 2001a), where a major goal was to gain an understanding of the full glacial biotic communities. Classic studies of late glacial and Holocene glacier fluctuations (e.g. Denton and Stuiver, 1966, 1967) provided evidence that Holocene climate changes were significant enough to change alpine glacier mass balance, and led to attempts to study the global nature of glacier fluctuations (Denton and Karle´ n, 1973, 1977; Grove, 1979). More recently, work has focused on understanding the higher frequency climate variability of Beringia (Viau et al., accepted) using a variety of paleoclimate records. Paleoenvironmental evidence for the Younger Dryas cold interval (YD) in northwestern North America is found in lake sediment records from Alaska (e.g. Eng- strom et al., 1990; Peteet and Mann, 1994; Brubaker et al., 2001; Hu et al., 2002; Hu and Shemesh, 2003; Yu et al., 2008), interior and coastal British Columbia (B.C.; e.g. Mathewes, 1993; Lacourse, 2005; Chase et al., 2008), in marine sediments off of the B.C. coast (Mathewes et al., 1993), in beetle remains from Alaska (Elias, 2001b), and in glacial moraines from Alaska and B.C. (Briner et al., 2002; Lakeman et al., 2008). However, the regional climates during the Younger Dryas are not entirely understood (Hu et al., 2006). Oxygen isotope records from ice cores collected from Mt. Logan and nearby Eclipse Icefield document climatic variability during the late Holocene (Holdsworth et al., 1992; Wake et al., 2002; Fisher et al., 2004), and the past 20,000 years (Fisher et al., 2008), and are used to interpret changes in the atmospheric circulation (Fisher et al., 2004, 2008). Qualitative interpretations of late glacial and Holocene paleoenvironmental sequences as well as quantitative paleoclimate reconstructions from Yukon, Alaska, and British Columbia are in agreement that the region experienced a long-term cooling trend with increasing moisture through the late Holocene (e.g. Abbott et al., 2000; Pienitz et al., 2000; Anderson et al., 2001; Palmer et al., 2002; Rosenberg et al., 2004). The interior southwest Yukon is within the boreal forest biome and forest and alpine tundra vegetation can be studied as analogues of postglacial succession following ice sheet retreat (Wright, 1980). Paleoecological and paleolimnological investigations using pollen (Birks, 1980; Cwynar, 1988; Stuart et al., 1989; Wang and Geurts, 1991b; Keenan and Cwynar,1992; Cwynar and Spear,1995; Lacourse and Gajewski, 2000), chironomids (Barley, 2004), isotopes * Corresponding author. Tel.: þ1 613 562 5800x1734; fax: þ1 613 562 5145. E-mail address: jbunbury@uottawa.ca (J. Bunbury). Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev 0277-3791/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2008.10.007 Quaternary Science Reviews 28 (2009) 354–369