Seasonal variability in Arctic temperatures during early Eocene time Jaelyn J. Eberle a, ⁎, Henry C. Fricke b , John D. Humphrey c , Logan Hackett c , Michael G. Newbrey d , J. Howard Hutchison e a University of Colorado Museum of Natural History and Department of Geological Sciences, University of Colorado, Boulder, USA b Department of Geology, Colorado College, Colorado Springs, USA c Department of Geology and Geological Engineering, Colorado School of Mines, Golden, USA d Department of Biological Sciences, University of Alberta, Edmonton, and Royal Tyrrell Museum of Palaeontology, Drumheller, Canada e University of California Berkeley Museum of Paleontology, 1101 Valley Life Sciences, Berkeley, USA abstract article info Article history: Received 16 October 2009 Received in revised form 2 June 2010 Accepted 4 June 2010 Available online xxxx Editor: M.L. Delaney Keywords: Eocene Arctic paleoclimate seasonality oxygen isotopes Coryphodon fossil turtles As a deep time analog for today's rapidly warming Arctic region, early Eocene (52–53 Ma) rock on Ellesmere Island in Canada's High Arctic (∼ 79°N.) preserves evidence of lush swamp forests inhabited by turtles, alligators, primates, tapirs, and hippo-like Coryphodon. Although the rich flora and fauna of the early Eocene Arctic imply warmer, wetter conditions than at present, the quantification of Eocene Arctic climate has been more elusive. By analyzing oxygen isotope ratios of biogenic phosphate from mammal, fish, and turtle fossils from a single locality on central Ellesmere Island, we infer early Eocene Arctic temperatures, including mean annual temperature (MAT) of ∼ 8 °C, mean annual range in temperature of ∼ 16.5–19 °C, warm month mean temperature of 19–20 °C, and cold month mean temperature of 0–3.5 °C. Our seasonal range in temperature is similar to the range in estimated MAT obtained using different proxies. In particular, relatively high estimates of early Eocene Arctic MAT and SST by others that are based upon the distribution of branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in terrestrial soil bacteria and isoprenoid tetraether lipids in marine Crenarchaeota fall close to our warm month temperature, suggesting a bias towards summer values. From a paleontologic perspective, our temperature estimates verify that alligators and tortoises, by way of nearest living relative-based climatic inference, are viable paleoclimate proxies for mild, above-freezing year-round temperatures. Although for both of these reptilian groups, past temperature tolerances probably were greater than in living descendants. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Today's Arctic region has undergone unprecedented warming, with temperatures rising at almost twice the rate as the rest of the world (ACIA, 2005). As warming is predicted to continue well into the future (Zhang and Walsh, 2006), there is critical need for climatic and ecologic analogs of a mild, ice-free Arctic. Among the best deep time analogs, early Eocene (∼ 52–53 Ma) sediments on central Ellesmere Island, Arctic Canada (∼ 76°N. Paleolat.; Fig. 1) preserve fossil evidence of lush swamp forests (McIver and Basinger, 1999) inhabited by alligators, giant tortoises, varanid lizards, and fish (Estes and Hutchison, 1980), as well as a diverse mammalian fauna that included tapirs, brontotheres, primates, and hippo-like Coryphodon (Dawson et al., 1993; Eberle, 2005, 2006). This fauna shares most of its genera with contemporaneous mid-latitude faunas (Estes and Hutchison, 1980; Eberle and McKenna, 2002), and some of the large mammals appear to have lived year-round in the High Arctic (Eberle et al., 2009). The existence of dense forests inhabited by a rich vertebrate fauna implies much warmer and wetter climate in the Arctic than at present. Quantitative estimates of early Eocene climatic conditions at high latitudes (Table 1), however, are rare and often contradictory. For example, estimates of mean annual temperature (MAT) in continental settings range from 4 to 20 °C (e.g., Basinger et al., 1994; Greenwood and Wing, 1995; Fricke and Wing, 2004; Weijers et al., 2007), which is equivalent to the range of MAT from present-day Canada to Florida. Further, very few estimates of other important climatic variables such as mean annual range in temperature (MART), cold monthly mean temperature (CMMT) and warm monthly mean temperature (WMMT) have been published (Basinger et al., 1994; Greenwood and Wing, 1995; Table 1). Information regarding these variables is particularly important when considering the early Eocene as an analog for the future because many aspects of biodiversity, such as patterns of species richness (Currie, 2001) and geographic ranges of species (Letcher and Harvey, 1994), are related less to mean annual temperatures than to seasonal differences and ranges. The two proxy- Earth and Planetary Science Letters xxx (2010) xxx–xxx ⁎ Corresponding author. University of Colorado Museum of Natural History, 265 UCB, Boulder, CO 80309, USA. Tel.: +1 303 492 8069; fax: +1 303 735 2347. E-mail addresses: Jaelyn.Eberle@Colorado.edu (J.J. Eberle), hfricke@ColoradoCollege.edu (H.C. Fricke), jhumphre@Mines.edu (J.D. Humphrey), michael.newbrey@ualberta.ca (M.G. Newbrey), hutch@color-country.net (J.H. Hutchison). EPSL-10426; No of Pages 6 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.06.005 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Please cite this article as: Eberle, J.J., et al., Seasonal variability in Arctic temperatures during early Eocene time, Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.06.005