Pre-glacial and interglacial pollen records over the last 3 Ma from northwest Canada: Why do Holocene forests differ from those of previous interglaciations? Charles Schweger a, * , Duane Froese b , James M. White c , John A. Westgate d a Department of Anthropology, University of Alberta, Edmonton, Alberta, Canada T6G 2H4 b Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 c Natural Resources Canada, Geological Survey of Canada, Calgary, Alberta, Canada T2L 2A7 d Department of Geology, University of Toronto, Toronto, Ontario, Canada M5S 3B1 article info Article history: Received 17 September 2009 Received in revised form 11 June 2010 Accepted 22 January 2011 Available online 8 March 2011 Keywords: Yukon Interglacials Forest history Pollen analysis Anthropogenic fire Pliocene Pleistocene Holocene Pinus Abies abstract We synthesize pollen spectra from eleven dated stratigraphic sections from central and northern Yukon. Palaeomagnetic and tephra dating indicates the earliest assemblages, representing closed canopy Pinus and Picea forest, are middle-late Pliocene age. More open forest conditions, indicated by increased Poaceae and with evidence of permafrost, are dated at ca 3 Ma. While Pinus pollen is abundant at 3 Ma, it is reduced in records after 2.6 Ma, and subsequent Pleistocene interglacial forest records are repeatedly dominated by Picea, along with Alnus and small but significant amounts of Abies. Surface sample comparisons indicate that Abies was more widespread and abundant in past interglaciations than at present and that Middle-Pleistocene PiceaeAbies forest grew in the northern Yukon Porcupine Basin, 500 km beyond modern Abies limits. In contrast, Pinus, which occurs today in southern and central Yukon, was not a significant component of these Pleistocene interglacial forests. Late-Holocene pollen assemblages with rare Abies and high Pinus are the most distinct in the past 2.6 Ma. Possible factors driving Holocene difference are paleoclimate, paludification, changes in megafaunal herbivory and an unusual fire regime. Anthropogenic burning is a factor unique to the Holocene, and if it is shown to be important in this case, it would challenge our notion of what constitutes boreal wilderness. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction North American studies of Pliocene and interglacial chronology and paleoecology have lagged behind those for Northwestern Europe. The pre-glacial Pliocene is well known from North Sea marine cores and at least eight distinct interglaciations are recog- nized from classic study areas in the Rhine Valley and East Anglia (West, 1980; Gibbard et al., 1991). North American interglacial records are geographically concentrated in the midwestern United States and, temporally, on the last interglaciation (Heusser and King, 1988). The Yukon Territory, northwestern Canada, is an exception. Here, detailed stratigraphic research, application of palaeomagnetic chronologies, recognition and dating of distal tephras, pollen and plant macrofossil analysis of organic-rich sediment, vertebrate remains and ancient DNA have all contributed to a growing under- standing of Pliocene and Pleistocene paleoecology (e.g. Westgate et al., 2001; Zazula et al., 2007; Froese et al., 2009). Long term changes such as the tectonic history of the Alaska and St. Elias Ranges, and opening of the Bering Strait have had a great influence on regional climate and in turn on late-Neogene vegetation (White et al., 1997). In contrast, rapid oscillations between glacial and interglacial conditions saw dramatic vegetation changes from steppe-tundra to closed canopy boreal forest (Schweger, 1997). Here we present pollen records from dated Yukon sites and discuss their stratigraphic and paleoecological significance. The focus is mostly on forest tree and shrub taxa: Picea, Pinus, Abies and Alnus, in order to document the composition of Yukon forests over the past 3 Ma. These records fall within the pre-glacial Pliocene, spanning ca 5e2.6 Maealthough there is evidence of extensive Northern Hemi- spheric glaciation in the late Pliocene (>2.6 Ma), prior to the newly established Pliocene/Pleistocene boundary (Gibbard et al., 2008, 2010)eand within the Pleistocene, spanning ca 2.6e0.09 Ma. The Yukon is a terrain of mountain ranges and plateaus with numerous large river valleys and basins. Because of topographic variability the vegetation is a complex of alpine and coastal tundra, woodlands and forests (Scudder, 1997) with four tree species reaching their northern or western limits within the territory. Picea * Corresponding author. Tel.: þ1 780 492 3489. E-mail addresses: charles.schweger@ualberta.ca (C. Schweger), duane.froese@ ualberta.ca (D. Froese), james.white@NRCan-RNCan.gc.ca (J.M. White), westgate@ geology.utoronto.ca (J.A. Westgate). Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.01.020 Quaternary Science Reviews 30 (2011) 2124e2133