Middle Eocene CO2 and climate reconstructed from the sediment fill of a subarctic kimberlite maar

Alexander P. Wolfe1, Alberto V. Reyes2*, Dana L. Royer3, David R. Greenwood4, Gabriela Doria3,5, Mary H. Gagen6, Peter A. Siver7, and John A. Westgate8 1Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada 2Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada 3Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459, USA 4Department of Biology, Brandon University, Brandon, Manitoba R7A 6A9, Canada 5Department of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, UK 6Department of Geography, Swansea University, Singleton Park, Swansea SA2 8PP, UK 7Department of Botany, Connecticut College, New London, Connecticut 06320, USA 8Department of Earth Sciences, University of Toronto, Toronto, Ontario M5S 3B1, Canada

ABSTRACT mismatch and frustrates efforts to understand the sensitivity of past equi- Eocene paleoclimate reconstructions are rarely accompanied librium climate response to greenhouse gas forcing. by parallel estimates of CO2 from the same locality, complicating Our objective is to assess climate and greenhouse-gas forcing for assessment of the equilibrium climate response to elevated CO2. We Northern Hemisphere subarctic latitudes during the latest middle Eocene reconstruct temperature, precipitation, and CO2 from latest middle by exploiting a remarkable terrestrial sedimentary archive. The Giraffe Eocene (ca. 38 Ma) terrestrial sediments in the posteruptive sediment kimberlite locality (paleolatitude ~63°N) comprises the posteruptive sedi- fill of the Giraffe kimberlite in subarctic Canada. Mutual climatic mentary fill of a maar formed when kimberlite intruded Precambrian cra- range and oxygen isotope analyses of botanical fossils reveal a humid- tonic rocks of the Slave Province at 47.8 ± 1.4 Ma (Creaser et al., 2004). temperate forest ecosystem with mean annual temperatures (MATs) Pollen, δ18O from wood cellulose, and foliar stomata from this locality more than 17 °C warmer than present and mean annual precipitation provide a comprehensive reconstruction of late middle Eocene climate

~4× present. Metasequoia stomatal indices and gas-exchange modeling and CO2 for the northern subarctic latitudes. produce median CO2 concentrations of ~630 and ~430 ppm, respec- tively, with a combined median estimate of ~490 ppm. Reconstructed RESULTS MATs are more than 6 °C warmer than those produced by Eocene Exploration drill core BHP 99-01 (see Appendix DR1 in the GSA Data 1 climate models forced at 560 ppm CO2. Estimates of regional climate Repository ) captures ≥50 vertical-equivalent meters of lacustrine sediment sensitivity, expressed as ∆MAT per CO2 doubling above preindustrial overlain by 32 m of peat (Fig. 1), together representing the progressive levels, converge on a value of ~13 °C, underscoring the capacity for exceptional polar amplification of warming and hydrological inten- Giraffe kimberlite A C 0 Lithology Interval sification under modest CO concentrations once both fast and slow (64˚48’N, 110˚06’W) considered 2 Quaternary in this study feedbacks become expressed. 40 30 Ve till rt

40 ical equi va Canada 60 DCFT and INTRODUCTION 50 IPFT dates (Myrs) Wood-rich Efforts to understand climate response to sustained greenhouse gas 80 60 39.37 ± 3.53

USA e at 47˚ (m) peat lent depth (m) 37.42 ± 3.55 forcing commonly focus on periods of peak Cenozoic warmth during the drill core 70 Tephra 47˚ 100 Paleocene–Eocene thermal maximum and early Eocene (e.g., Zachos et BHP 99-01 B 36.88 ± 3.28 80 37.08 ± 3.41 al., 2008; Lunt et al., 2012a). The subsequent cooling trend of the middle Till Depth in cor 120 90 and late Eocene (Pagani et al., 2005) is also relevant because atmospheric Peat Precambrian 100 country rock Tephra Lacustrine CO concentrations dovetail the range projected for the coming century Lake 140 mudstone 2 (granodiorite) 110 (Maxbauer et al., 2014; Jagniecki et al., 2015; Anagnostou et al., 2016; sediments Rb-Sr age Volcaniclastics 160 120 Steinthorsdottir et al., 2016), ultimately crossing the threshold necessary (Ma): 47.8±1.4 Kimberlite Granodiorite to maintain continental ice sheets (~500 ppm; Royer, 2006). Observations from the Arctic Ocean suggest that ice rafting may have been initiated by Figure 1. A: Location of the Giraffe kimberlite in the Slave Province, Northwest Territories, Canada. Gray star indicates Yellowknife, the the middle Eocene (e.g., Tripati et al., 2008), in apparent conflict with location of the nearest climate station. B: Schematic cross section the warmth implied by the terrestrial biota (e.g., Eberle and Greenwood, of posteruptive sedimentary fill and the position of BHP core 99-01 2012). Climate models struggle with these critical early Cenozoic inter- and key stratigraphic features (arrows). C: Core stratigraphy showing vals because unrealistically high CO forcing is required to produce the the investigated section directly above tephra horizons dated by iso­ 2 thermal plateau (IPFT) and diameter-corrected (DCFT) glass fission temperature responses implied by proxies, particularly for the sparse track analyses (1σ uncertainty). network of terrestrial high-latitude sites (Lunt et al., 2012a). Furthermore, paleoclimate and CO reconstructions are not commonly derived from the 2 1 GSA Data Repository item 2017202, detailed methods (Appendix DR1), same sedimentary archive; this complicates assessment of proxy-model data tables (Tables DR1–DR5) and supplementary Figures DR1–DR5, is avail- able online at http://www.geosociety.org/datarepository/2017 or on request from E-mail: [email protected] [email protected].

GEOLOGY, July 2017; v. 45; no. 7; p. 1–4 | Data Repository item 2017202 | doi:10.1130/G39002.1 | Published online XX Month 2017 ©GEOLOGY 2017 The Authors.| Volume Gold 45 |Open Number Access: 7 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 1 18 CMMTMCR (˚C) δ Ocell (‰ VSMOW) CO2 (ppm) and MAP 3–5 times higher than present. The North American location -4 -2 024 6 23 24 25 300 700 1100 with a modern climatology that most closely matches the reconstructed 48 paleoclimate for the Giraffe locality is 3500 km to the southeast at Nash- 52 ville (Tennessee, USA), with MAT of 15.2 °C, CMMT of 3.2 °C, WMMT A B C D E F 56 stomatal of 26.3 °C, and MAP of 1200 mm (1981–2010 climate normals; http:// alent depth (m) index ncdc.noaa.gov). 60 Non-permineralized wood in the posteruptive Giraffe sequence has 64 ical equiv gas exceptional preservation and can be assigned unambiguously to Meta- rt exchange

Ve sequoia on the basis of xylotomy (Fig. DR1). This wood yields pristine 68 tephra α-cellulose (Fig. DR3) amenable to measurements of stable oxygen iso- 72 18 10 14 18 22 24 26 12 14 16 18 500 900 1300 tope ratios (δ Ocell) by pyrolysis and continuous-flow isotope ratio mass MAT (˚C) WMMT (˚C) MAT (˚C) CO (ppm) MCR MCR cell 2 spectrometry, which in turn can provide independent support for palyno- 18 Figure 2. Stratigraphy of climate proxies from the Giraffe peat sec- logical estimates of MAT (Appendix DR1). The values of δ Ocell range tion. Gray horizontal lines indicate tephra beds. Shading denotes 16th from 23.4‰ to 24.9‰ VSMOW (Vienna standard mean ocean water) (Fig. th to 84 percentile ranges from Monte Carlo resampling (A, B, C, E, F) 2D; Table DR3). Using a Monte Carlo implementation of the Anderson and one standard deviation of duplicate isotope analyses (D). A: Mean 18 annual temperature inferred from mutual climatic range (MCR) anal- et al. (2002) leaf-water model, we estimated values of δ O for environ- mental waters ( 18O ) accessed by the trees, and then calculated MAT ysis of pollen types (MATMCR). B: Coldest month mean temperature δ water (CMMT ). C: Warmest month mean temperature (WMMT ). D: Wood 18 MCR MCR from these inferred δ Owater values using an empirical relation between 18 cellulose δ Ocell (analytical uncertainty is ~0.3‰; VSMOW—Vienna Eocene environmental waters and MAT that accounts for Eocene latitu- Standard Mean Ocean Water). E: Median MAT inferred from a leaf- cell dinal temperature gradients (Fricke and Wing, 2004). The δ18O results water model and Eocene δ18O-MAT transfer function (Appendix DR1 cell yield a MAT estimate of 15.6 ± 2.0 °C at 1σ (Fig. 2E), which overlaps the [see footnote 1]). F: Median CO2 concentrations derived from Meta- sequoia stomatal indices (red) and a gas-exchange model (green). pollen-based MCR estimate of 14.5 ± 1.3 °C MAT (Fig. DR4).

Atmospheric CO2 Reconstruction infilling of the maar basin. Both facies have remarkable preservation of Stomatal indices derived from Giraffe Metasequoia leaves (Doria aquatic and terrestrial plant fossils (Wolfe et al., 2006; Doria et al., 2011). et al., 2011) yield a combined median reconstructed atmospheric CO2 We analyzed a 21 m section (vertical equivalent depth) of peat in core BHP concentration for all stratigraphic levels of ~630 ppm (433–1124 ppm at

99-01, representing ~20 k.y. assuming reasonable accumulation rates and 68% confidence) (Figs. 2F and 3; Table DR4). Combined CO2 estimates only moderate compaction. The common sampling interval over which we from the Franks et al. (2014) gas-exchange model, applied to the same estimate the mean climate state and CO2 concentration includes multiple foliage (Appendix DR1), are somewhat lower, ranging from 353 to 561 samples from 7 m of vertical equivalent thickness (Fig. 2), or ~7 k.y. of ppm at 68% confidence with a median of ~430 ppm (Fig. 3A). Given continuous deposition. Two rhyolitic tephra beds are present in the core overlap between the two methods of CO2 reconstruction, and because the directly below the lacustrine-to-peat transition (Fig. 1C). Glass fission stomatal index proxy is unbounded at high CO2 concentrations (Doria track dating (Westgate et al., 2013) of both tephra beds gives a weighted et al., 2011), we resampled randomly from the combined stomatal index mean age (±1σ) of 37.84 ± 1.99 Ma (Table DR1 and Appendix DR1). and gas-exchange model reconstructions to yield a consensus median

CO2 concentration of ~490 ppm (378–778 ppm at 68% confidence). This Paleoclimate of the Latest Middle Eocene Subarctic approach reduces biases inherent to either technique.

Pollen assemblages from Giraffe sediments are well preserved, diverse, This CO2 reconstruction is lower than inferences of ~800–1000 ppm and include numerous extant North American taxa (Fig. DR1). The rela- from alkenone δ13C between 39 and 37 Ma (Zhang et al., 2013) and 650 tive abundance of angiosperm pollen (53%–74%) is higher than that of ± 110 ppm (at 68% confidence) at 40.3 Ma fromδ 11B of pristine forami- gymnosperms throughout the section (Fig. DR2). The former is strongly niferal calcite (Anagnostou et al., 2016), but in agreement with estimates dominated by fagalean types (Quercoidites, Castanea, and Corylus), of 385–467 ppm (at 68% confidence) from the stomatal distributions of with lesser contributions from Ulmipollenites, Ericalean taxa, and the Canadian High Arctic Metasequoia foliage dating broadly to between 47.9 Eocene indicators Platycarya swasticoides and Pistillopollenites mcgre- and 37.8 Ma (Maxbauer et al., 2014) and ~350–650 ppm from the stoma- gorii. Conifer pollen is strongly dominated by Cupressaceae, for which tal density of extinct fagalean foliage (Steinthorsdottir et al., 2016). The

Metasequoia is likely the dominant source given the presence of well- results from the Giraffe locality thus support lower CO2 concentrations than preserved foliage and wood of this taxon (Fig. DR1). Pollen of Pinus previously envisaged for greenhouse climate intervals (Franks et al., 2014). and Picea is distributed throughout the section, whereas that of Ginkgo, Sciadopitys, and Tsuga occurs in trace amounts. 5 10 Mutual climate range (MCR) analysis (e.g., Ballantyne et al., 2010; A B

) GE 4 ) 8 SI -3 Thompson et al., 2012; see the methods description in Appendix DR1) -2 of these pollen assemblages using nearest living relatives yields recon- 3 CR 6 CR structed mean annual temperatures (MATs) of 12.5–16.3 °C (mean ± 1σ 2 SI 4 Density (x10

Density (x10 GE = 14.5 ± 1.3 °C), coldest month mean temperatures (CMMTs) of 0.5–4.5 1 2 °C (3.4 ± 1.7 °C), and warmest month mean temperatures (WMMTs) of 0 0 23.5–25.0 °C (24.5 ± 0.8 °C) (Figs. 2A–2C; Table DR2). Reconstructed 200 400600 8001000120014001600 01020304050 Reconstructed CO (ppm) Regional climate sensitivity (°C / CO doubling) mean annual precipitation (MAP) ranges from 1257 to 1292 mm, with a 2 2 mean uncertainty of 310 mm. The present-day climate for Yellowknife Figure 3. A: Probability density functions (PDFs) for reconstructed CO2 (62.45°N, 114.40°W), the nearest long-term climate station situated 300 concentrations from Metasequoia stomatal index (SI), gas-exchange km southwest of the Giraffe locality, has a MAT of −4.3 °C, CMMT of modeling (GE), and random resampling of the combined stomatal index and gas-exchange model reconstructions (CR). B: PDFs for regional -25.6 °C, WMMT of 17.0 °C, and MAP of 289 mm (1981–2010 climate climate sensitivity at the Giraffe locality, subarctic Canada (Appendix normals; htpp://climate.weather.gc.ca). Thus, middle Eocene MATs were DR1 [see footnote 1]). In both panels, horizontal lines indicate the 17–20 °C warmer, CMMTs 27–31 °C warmer, WMMTs 7–8 °C warmer, 16th to 84th percentile range, with median values marked by squares.

2 www.gsapubs.org | Volume 45 | Number 7 | GEOLOGY DISCUSSION modest CO2 forcing. This contention is supported by temperature recon- structions from Ellesmere Island and Siberia during the Pliocene (Bal-

High Polar Amplification under Modest CO2 Forcing lantyne et al., 2010; Brigham-Grette et al., 2013), for which independent

These data provide an integrated estimate of the mean climate state proxies (e.g., Zhang et al., 2013) indicate CO2 concentrations of ~400 ppm for the continental subarctic Giraffe locality over the multimillennial (Fig. 4). Even greater ∆MATs (~32 °C) are estimated from middle Eocene interval common to all proxies. The MCR-inferred paleotemperature and fossil floras of Axel Heiberg Island (Eberle and Greenwood, 2012), also reconstructed CO2 concentrations can be plotted along a range of esti- dominated by Metasequoia, when CO2 was possibly as low as ~420 ppm mates for the sensitivity of MAT with respect to atmospheric CO2 (Fig. 4). (Maxbauer et al., 2014). Pronounced middle Eocene polar amplification Present-day estimates of global Charney climate sensitivity (CS) include is likewise expressed in the Southern Hemisphere high latitudes, where a subset of fast feedbacks only, while Earth-system sensitivity (ESS) temperate rainforests dominated by Nothofagus and araucarian conifers includes most fast and slow feedbacks (Royer, 2016). CS and ESS are existed along the Wilkes Land margin of East Antarctica, implying MATs typically expressed as globally averaged approximations of the tempera- >10 °C and MAPs severalfold higher than present (Pross et al., 2012). ture response to incremental CO2 doublings, expressed as ∆MAT relative Early Eocene climate model simulations for the latitudes of subarctic to preindustrial conditions. However, recent studies have demonstrated North America (Lunt et al., 2012a; Carmichael et al., 2016) underesti- the utility of regional approximations for these parameters (Dyez and mate the multiple proxy constraints presented here. For example, at 560

Ravelo, 2013; Eagle et al., 2013). ppm CO2, the ensemble mean of three models (Lunt et al., 2012a) for the

The latest middle Eocene MAT and stomatal index CO2 reconstruc- Giraffe region underestimates reconstructed MAT by 15.5 °C, with a mini- tion from Giraffe, when compared to the present climate of Yellowknife mum underestimate of 6.4 °C (Fig. 4). At 1120 ppm CO2, more than twice using the approach of Royer et al. (2012), yield a mean regional climate the inferred CO2 from Metasequoia foliage, the ensemble mean MAT is sensitivity of 12.7 °C per CO2 doubling for the North American subarctic 11.3 °C lower than proxy MATs, with a minimum underestimate of 4.5 latitudes (8.3–21.2 °C at 68% confidence; Fig. 3; Appendix DR1), more °C (Fig. 4). The model results compiled by Lunt et al. (2012a) consistently than twice the estimated ESS of ~6 °C per CO2 doubling for the Pleisto- estimated colder-than-present preindustrial Yellowknife MATs in 280 ppm cene climate system (Hansen et al., 2008; Fig. 4). Use of CO estimates 2 CO2 control runs. However, even when this model-dependent artifact is from the gas-exchange model produces even higher regional climate sensi- taken into account by expressing ∆MAT relative to instrumental Yellow- tivity values (14.0–32.8 °C at 68% confidence, median = 20.1 °C; Fig. 3). knife MAT (Fig. DR5), model ∆MATs remain substantially lower than the These estimates of regional climate sensitivity, based on paleoclimate proxy-based ∆MATs presented here for the Giraffe region (Table DR5). and CO2 proxies analyzed in parallel from the same sediment archive, Many mechanisms have been explored to explain the amplified warmth highlight the exceptional magnitude of polar amplification under relatively of high latitudes during the Cretaceous and Paleogene, including state- dependent CS (Caballero and Huber, 2013), decreased atmospheric pres- sure (Poulsen et al., 2015), altered cloud physics (Kiehl and Shields, 2013), 35 biogenic aerosols (Beerling et al., 2011), and teleconnection dynamics 30 Eocene Axel Heiberg with tropical oceans (Korty et al., 2008). Changes in atmospheric circula- Island proxies tion such that low pressure centers and associated cyclogenesis became Northern Hemisphere quasi-permanent features over the polar regions in the absence of perennial 25 polar amplification sea ice cover were also probable. Such configurations appear necessary to 20 increase MAP by the amounts mandated by the proxy record at Giraffe and Giraffe (ºC) Pliocene proxies Giraffe elsewhere, whereas intensification of the hydrologic cycle also increases 15 Ellesmere EoMIP Island 1120 ppm poleward heat transfer by water vapor (Pagani et al., 2014; Carmichael ∆M AT proxies Giraffe et al., 2016). Despite obvious differences in boundary conditions with 10 EoMIP y Pliocene 560 ppm m sensitivit respect to the state of the cryosphere and biosphere, these configurations Siberia Earth syste proxies provide some degree of analogy with contemporary warming of the Arc- 5 y Charney climate sensitivit tic, where dramatic losses of Northern Hemisphere sea ice over the last decade have contributed to deepening lows over the Arctic Ocean, coupled 0 modern warming to enhanced cyclogenesis that in turn exerts a strong positive feedback 300 400 500 600700 800 1100 on remaining sea ice (Screen et al., 2011; Simmonds and Rudeva, 2012). Atmospheric CO (ppm) 2 Because future temperatures are unlikely to decline appreciably over the time scales required for most fast and slow feedbacks to become fully Figure 4. Climate responses to elevated atmospheric CO2 concen- trations. Global estimates of Charney climate sensitivity (CS) and expressed (centuries to millennia; Royer, 2016), even if all anthropogenic Earth-system sensitivity (ESS) are depicted as light and dark gray greenhouse gas emissions are eliminated (Archer and Brovkin, 2008), the envelopes, respectively, with change in mean annual temperature latest middle Eocene forest ecosystem preserved in the Giraffe kimberlite (∆MAT) plotted relative to preindustrial. CS is after Rohling et al. (2012); offers considerable insight toward understanding high-latitude climate upper and lower bounds of the ESS envelope represent ESS/CS ratios of 2.0 estimated for the Pleistocene (Hansen et al., 2008) and 1.65 for states under elevated, but not extreme, atmospheric CO2 concentrations. the Pliocene (Lunt et al., 2012b). Together with contemporary global ACKNOWLEDGMENTS MAT at 400 ppm CO2 (green square), these global estimates provide a context to evaluate polar amplification as recorded by proxy estimates We thank BHP Billiton Inc. and the Geological Survey of Canada for access to of ∆MAT, relative to present MAT at the nearest climate station, for the cores; the Natural Sciences and Engineering Research Council (Canada) (Green- Giraffe locality (subarctic Canada; blue circle; combined resampled wood, Reyes, Westgate, and Wolfe); the U.S. National Science Foundation (Siver); J. Basinger for discussions on Metasequoia; G. Braybrook for scanning elec- CO2 median) and other high-latitude sites (gray circles) for the Plio- cene (Ballantyne et al., 2010; Brigham-Grette et al., 2013) and Eocene tron microscopy; the Climate Change Consortium of Wales, D. McCarroll, and (Maxbauer et al., 2014). Errors are the 16th to 84th percentile range for N. Loader for assistance with stable isotopes; R. Zetter for advice on pollen mor- the Giraffe locality and 1σ confidence interval for other sites. Model phology; and G. Ludvigson, M. Steinthorsdottir, B. Jacobs, and several other ensemble mean ∆MATs for the Giraffe region (computed relative to anonymous reviewers. We dedicate this paper to our departed colleagues Leo present Yellowknife MAT) at 2× and 4× pre-industrial CO2 (EoMIP; Lunt Hickey (1940–2013) and Mark Pagani (1960–2016), who trailblazed much of our et al., 2012a) are marked by orange and red squares, respectively; bars thought concerning greenhouse worlds of the past, and Art Sweet (1942–2017), mark the range of modeled ∆MAT for each ensemble. pillar of Canadian palynology and the first to analyze Giraffe pipe pollen.

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4 www.gsapubs.org | Volume 45 | Number 7 | GEOLOGY GSA Data Repository 2017202

Middle Eocene CO2 and climate reconstructed from the sediment fill of a subarctic kimberlite maar, DOI:10.1130/G39002.1

A.P. Wolfe, A.V. Reyes, D.L. Royer, D.R. Greenwood, G. Doria, M. Gagen, P.A. Siver, J.A. Westgate

APPENDIX DR1 - METHODS Geochronology Exploration drill core BHP 99-01 was drilled by BHP Billiton Inc. in 1999 and archived at the Geological Survey of Canada (Calgary). Drilled at a 47° angle, the core captures ~50 vertical- equivalent meters of lacustrine sediment overlain by 32 m of peat (Fig. 1), together representing the progressive infilling of the maar basin. Two rhyolitic tephra at 70.88 and 73.41 m vertical-equivalent core depth were dated by isothermal plateau (ITPFT) and diameter-corrected (DCFT) glass fission-track techniques, which both correct for post-depositional partial annealing of fission tracks in volcanic glass. With ITPFT, correction for partial track fading is made by heating the glass shards for 30 days at 150 °C, while DCFT uses a correction factor based on a comparison of the average diameters of the induced and spontaneous fission tracks (Sandhu and Westgate, 1995; Westgate, 1989; Westgate et al., 2013). Two DCFT dates and one ITPFT date for these tephra beds were reported by Doria et al. (2011), but analytical details were not provided. Here we provide one additional ITPFT date and full analytical details and results for the two tephra beds (Table DR1), yielding ages ranging from 36.88 ± 3.28 Ma to 39.37 ± 3.53 Ma at 1σ. Given the <10% errors associated with each age estimate, the glass fission-track dates suggest an age of ~38 Ma for the transition between lacustrine and peat facies. Slightly more precise constraining ages for the uppermost lacustrine mudstone facies can be obtained by calculating the weighted mean and error of the two corrected ages for each of the upper tephra (38.40 ± 2.50 Ma, 1σ) and lower tephra (36.98 ± 2.36 Ma, 1σ). These tephra beds are conformable and undeformed; the apparent age reversal cannot be explained by deformation, but rather is due to error at the 1σ level of ~10% typical of glass fission-track dating methods. The glass fission-track age of the internal standard (Huckleberry Ridge Tuff) is very close to its consensus 40Ar/39Ar age (Table DR1), providing strong support for the accuracy of the Giraffe tephra glass fission-track ages. We analyzed in detail a 21 m section (vertical equivalent depth) of peat in core BHP 99-01 that captures the terrestrialization of the lake system. This interval contains nonpermineralized wood of Metasequoia occidentalis (Coniferae: Cupressaceae) identified by xylotemy (Visscher and Jagels, 2003), and unambiguous foliage belonging to the same taxon (Fig. DR1). On the basis of tephra glass fission-track geochronology, the maximum age of the analyzed sequence lies between 36-40 Ma. The duration of the full 21 m of analyzed section can be tentatively estimated at ~20,000 years, accepting accumulation rates for middle Eocene lignites in the order of 1 mm yr-1 and only moderate compaction (Kojima et al., 1998). The interval over which we estimate the mean climate state and attendant CO2 concentrations spans 7 m of vertical equivalent thickness, or ~7,000 years of continuous deposition.

Pollen and mutual climate range analyses For pollen analysis, 12 g of sediment was crushed and successively digested in 25% HCl, 70% HF, Schulze’s solution (KClO3 + HNO3), and NaClO in a hot water bath. Neutralized slurries were floated in heavy liquid (ZnBr at 2.0 g cm-3), sieved to isolate the 10-150 µm fraction, and stained with Safranin-O. Mounts for light microscopy were prepared from the 10-150 µm, 45-150 µm, 20-45 µm, and 10-20 µm fractions; at least 200 higher plant pollen grains were identified for each sampled core interval (Fig. DR2; Sweet, 2000). The mutual climate range (MCR) method is based on the principle that all organisms have physiological climate tolerances that limit their natural distribution to areas where reproductive success is possible (Ballantyne et al., 2010; Thompson et al., 2012). All taxa present in a single plant 1 community therefore coexist within the common overlap of their range of tolerance. The MCR approach benefits from the redundancy of incorporating climate tolerances of multiple taxa, and is not biased by the relative frequency of occurrence of individual floristic elements. We used climate range data from the natural distributions of modern North American trees and shrubs identified as nearest living relatives (NLRs) at the rank of genus, supplemented with data from other regions for taxa no longer native in North America (Table DR2; Fang et al., 2011; Thompson et al., 1999, 2015). MCR analysis identifies the range of a given climatic parameter for the set of identified NLRs of a fossil assemblage where they can coexist today; e.g., the minimum MATmax and maximum MATmin where all NLRs overlap. Many studies using MCR have used the midpoint of this range across all identified NLRs as the estimate of the likely past climatic condition associated with the assemblage (Thompson et al., 2012). The estimate is often given, however, with the range also expressed as a measure of the uncertainty of the estimate (e.g., Eldrett et al., 2009, 2014; Pross et al., 2012). The list of NLRs from the Giraffe assemblages (pollen for most taxa; foliage and wood for Metasequoia) used for MCR is given in the footnotes of Table DR2. We used the modern climate envelope for natural (i.e. not cultivated) populations of Metasequoia glyptostroboides, while recognizing that this may be narrower than actual tolerances of early Cenozoic congeners (Liu et al., 2007). Removing M. glyptostroboides from our analyses lowers the estimates of MAT, CMMT and WMMT by only 0.0 - 1.3 °C and by only 5 cm for MAP. We excluded Ginkgo on the grounds that natural populations of living Ginkgo potentially reflect a small fraction of its potential climate range (Tang et al., 2012). We do not adjust our NLR climate tolerance ranges to account for the possible acquisition of increased cold tolerance during progressive Cenozoic cooling, suggesting that reconstructed temperature parameters are likely conservative with respect to the magnitude of inferred warming. In common with other analyses of pollen NLR records (e.g., Eldrett et al., 2009, 2014; Kotthoff et al., 2014), the bisaccates Picea and Pinus were not included in the MCR analysis. These pollen types were present at <15% abundance for 15 of 16 samples, consistent with these grains being transported great distances and so potentially not reflecting local vegetation (Davis and Webb, 1975). Were megafossils present, these taxa would have been included (e.g., Ballantyne et al., 2010). Thompson et al. (2012) demonstrated for modern North American pollen spectra that the MCR method yielded temperature estimates that matched actual site values with small anomalies (e.g., anomalies for MAT and CMMT ± 0.8–2.2 °C, WMMT ± 0.3–1.8 °C) for broadleaf deciduous forest and mesic conifer forest of the west and east coasts of North America. However, boreal forest and tundra vegetation yielded poorer correspondences between observed and MCR estimated temperatures (MAT ± 2.7 & 4.7 °C respectively). In the test of MCR by Thompson et al. (2012), MAP estimates showed anomalies ± 108–285 mm versus observed values. As the Giraffe site paleovegetation is reconstructed as a mixed deciduous broadleaf-conifer forest, the results from the test of MCR by Thompson et al. (2012) lends confidence to our estimates. The MCR technique has been rigorously tested for North American Neogene and extant samples for both accuracy and precision and is appropriate for Eocene pollen assemblages, yielding temperature estimates congruent with those from independent geochemical proxies (Eldrett et al., 2009, 2014; Pross et al., 2012). Because plant taxa can persist at their climatic extremes, with the potential of distorting the objectivity of reconstructions, Thompson et al. (2012) recommended an approach where equally ‘weighted’ values are applied from the 10th to 90th percentiles, and lower weights applied outside that range. However, we applied a bootstrap re-sampling technique to the pollen-based climate estimates in order to identify the highest probability estimates (Table DR2) for each stratigraphic level, consistent with recommendations to assess uncertainty for NLR-method proxies (Harbert and Nixon, 2015). All samples used in our analysis had at least 13 NLRs with climate range data (minimum: 13; maximum: 19; mean: 15 NLRs), in keeping with the recommendations of Thompson et al. (2012). Overall Giraffe locality paleoclimate estimates in the main text are calculated as the mean and standard deviation of the results from each stratigraphic level. 2 Wood cellulose and δ18O analyses We used a Wiley mill to grind pieces of Giraffe wood to <420 µm. α-cellulose was extracted from the ground samples using a modified Jayme-Wise method (Gaudinski et al., 2006), whereby samples were refluxed in 2:1 toluene:ethanol in a Soxhlet apparatus to remove resins and waxes, boiled in deionized water, delignified in NaClO2 with glacial acetic acid, and washed in 17% NaOH to remove hemicellulose. We also leached Fe and Mn oxyhydroxides using hydroxylamine hydrochloride (H3NO•HCl). The purity of α-cellulose was confirmed by electron microscopy and x-ray diffraction (Fig. DR3). Cellulose extracts were homogenized by sonication and freeze-dried prior to analysis for stable oxygen isotope ratios at Swansea University. Homogenized α-cellulose was weighed (~300 µg) into silver capsules and pyrolized in a Europa ANCA GSL elemental analyser interfaced by continuous flow to a Europa 20/20 gas-source isotope ratio mass spectrometer. Instrumental drift was monitored using multiple internal laboratory cellulose standards. All unknown samples were analyzed in duplicate (Fig. 2D), and reported values are the arithmetic mean of these analyses (Table DR3). α-cellulose oxygen isotope data are presented in standard δ-notation with respect to Vienna Standard Mean Ocean Water (VSMOW). Repeat analyses of laboratory standards indicate analytical precision of ~0.3‰. 18 We estimated the corresponding values of δ O for environmental waters accessed by the trees 18 (δ Owater) using the leaf-water model of Anderson et al. (2002): 18 18 δ Owater ≈ δ Ocellulose – (1 – f) * (1 – h) * (εe + εk) - εbiochem where f is the fraction of leaf water not subject to evaporation; εe is the liquid-vapor equilibrium fractionation factor (9.8‰ at 20 °C; Manjoube, 1971); εk is the liquid-vapor kinetic fractionation (28‰); h is the relative humidity; and εbiochem is the biologic fractionation for sugar converting to cellulose (27 ‰). For each stratigraphic level, we solved the leaf-water model 1000 time, with fixed variables for εe, εk, and εbiochem, and random resamples of normal distributions for h (0.7±0.05), f 18 (0.38±0.05), and δ Ocellulose (mean of duplicate isotope analyses ± 0.3‰ analytical precision). The distributions for f and h capture ranges determined from, respectively, a transfer function based on contemporary observations of temperature and relative humidity (Anderson et al., 2002) and independent studies of Eocene climates in the Canadian Arctic (Eberle and Greenwood, 2012; Eldrett et al., 2009; Jahren, 2007). MAT was then calculated for the 1000 leaf-water model solutions for each stratigraphic level using the empirical relation between Eocene river water and MAT (Fricke and Wing, 2004), in order to capture the different latitudinal temperature gradient during the epoch: 18 2 δ Owater = -0.01T + T – 22.91 where T is MAT. Median and 16th and 84th percentile values were computed from the range of solutions for reconstructed MAT at each stratigraphic level (Fig. 2E). We determined an overall cellulose-derived MAT estimate for the Giraffe site by combining the distributions of MAT estimates from all seven stratigraphic levels and calculating a median MAT and associated 16.5% to 83.5% quantile range of 15.6 ± 2.0 °C (Fig. DR4).

CO2 reconstructions Stomatal index-based CO2 estimates for the Giraffe peaty sediments are described fully in Doria et al. (2011). In brief, the stomatal index (SI; stomatal density/[stomatal density+epidermal cell density]×100) of many plant species responds inversely to CO2 partial pressure (Royer, 2001). The response is underpinned by clear genetic, functional, and signaling pathways and is furthermore not strongly affected by environmental gradients in water availability and irradiance (Gray et al., 2000; Salisbury, 1927; Woodward and Bazzaz, 1988). We measured fossil Metasequoia leaf populations from ten discrete stratigraphic levels that preserved sufficient intact foliage. For each leaf, a leaf-mean SI was computed from five fields of view, and for each level a mean SI was computed from five leaf means. Corresponding atmospheric CO2 concentrations were then calculated using the extensive calibration between CO2 and SI in extant Metasequoia (Beerling et al., 2009), coupled to Monte Carlo

3 simulations to generate robust uncertainties (Table DR4) propagated from both fossil and modern calibration SI (Doria et al., 2011). We also estimated CO2 with a plant gas-exchange model that uses stomatal density, stomatal size, and leaf δ13C as inputs (Table DR4; Franks et al., 2014; Maxbauer et al., 2014). In contrast to stomatal indices, the gas-exchange method does not require extant calibrations and can produce well- bounded CO2 estimates even at high CO2 (Table DR4). However, the stomatal density measurements for Giraffe material are based on stomata-bearing portions of the leaf only, rather than mean stomatal density across the leaf as required by the gas exchange model. We used a scalar developed for Axel Heiberg Island Metasequoia (Maxbauer et al., 2014) to correct the measured stomatal density by a factor of 0.29±0.0005 at 1σ, the fractional leaf surface area composed of stomatal zones. Photosynthetic rate and the ratio of operational to maximum conductance, which are also needed for the model, came from measurements on extant Metasequoia occidentalis (Maxbauer et al., 2014). Uncertainties in all model input terms are provided in Table DR4; uncertainties were fully propagated by Monte Carlo simulations (10,000 random samples of all input terms). For both the SI and gas-exchange CO2 reconstructions at the ten stratigraphic levels (Fig. 2F), we combined the estimates from Monte Carlo simulations into a site-level estimate (“SI” and “GE” in Fig. 3A). We did this for each proxy by randomly drawing 10,000 resamples from the associated level×reconstruction matrix. The sharp drop in probability above 1500 ppm for the SI-based estimates reflects the upper bound of the extant calibration (Doria et al., 2011), meaning that the probability of >1500 ppm CO2 is low but not zero. Many of the individual-level estimates of CO2 quantitatively exclude this possibility (Fig. 2F; Table DR4). To calculate a single, multi-proxy CO2 estimate for the site (“CR” in Fig. 3A), we randomly drew 5000 resamples from each of the “SI” and “GE” distributions. For each of these combined estimates of CO2 concentration, reported uncertainties around the median are given as the 16th to 84th percentile of the distribution, similar to a 68% or 1σ confidence interval.

Paleoclimate inferences The proxy-based estimate of Giraffe MAT was compared to results from an ensemble of Eocene climate simulations (Lunt et al., 2012) by calculating Giraffe-region MATs from the array of model grid cells spanning 65 °N, 108.75 °W to 60 °N, 78.75 °W (Table DR5), in order to incorporate paleogeographic uncertainties. Regional climate sensitivity specific to the Giraffe locality was calculated following Equation 3 of Royer et al. (2012): climate sensitivity = ΔMAT / (CO2;double + ΔS +ΔTgeo) where ΔMAT is relative to present-day Yellowknife, CO2;double is reconstructed CO2 expressed as units of CO2 doubling relative to preindustrial, ΔS is a correction for lower Eocene solar luminosity, and ΔTgeo is a paleogeography correction factor. In a Monte Carlo simulation, we resampled 10,000 times from each of the SI, gas-exchange, and combined CO2 distributions and the pollen MCR MAT normal distribution of 14.5±1.3 °C. These resamples were recast, respectively, to units of CO2 doubling relative to pre-industrial and ΔMAT relative to present-day Yellowknife, where a doubling of CO2 has a forcing of +3.7 W/m2 (Myhre et al., 1998). This calculation was corrected for the radiative effects of 2 both slightly lower middle Eocene solar luminosity (-0.215 units of CO2 doubling, or -0.79 W/m at 38 Ma relative to the present-day; Gough, 1981) and Eocene paleogeography (see below), and is equivalent to a regional S[CO2,GEO] following the syntax of Rohling et al. (2012). We made these corrections because the radiative forcings related to changes in luminosity and paleogeography are unrelated to the CO2 radiative response; for our calculations, we assume that all other changes in radiative forcing are a feedback response to the change in CO2 (e.g., other well-mixed greenhouse gases such as methane).

4 There is currently insufficient knowledge of regional paleogeography and climate boundary conditions to apply a precise paleogeography correction. Instead, we parameterize this effect with 10,000 resamples drawn from a normal distribution of +0.50 ± 0.25 units of CO2 doubling relative to pre-industrial (+1.85 ± 0.93 W/m2). In this strategy, the generated paleogeographic effect is higher than most estimates of the mean global effect (Royer et al., 2012), which has the net effect of producing conservative, even underestimated, values of ESS. When the paleogeography correction is removed, regional climate sensitivity for the Giraffe locality increases by 2.4 °C per CO2 doubling. Given age model limitations, it is not possible to apply an orbital forcing correction factor for MAT; the estimated duration of deposition for the full 21 m sampled interval spans most of a precessional cycle and half an obliquity cycle. Probability density functions for plotting the resampled CO2 reconstructions and regional climate sensitivity were generated using the R software package density routine for kernel density estimation, with a Gaussian kernel and default smoothing bandwidth.

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6 Thompson, R.S., Anderson, K.H., Pelltier, R.T., Strickland, L.E., Bartlein, P.J., and Shafer, S.L., 2012, Quantitative estimation of climatic parameters from vegetation data in North America by the mutual climatic range technique: Quaternary Science Reviews, v. 51, p. 18-39. Thompson, R.S., Anderson, K.H., Pelltier, R.T., Strickland, L.E., Shafer, S.L., Bartlein, P.J., and McFadden, A.K., 2015, Atlas of relations between climatic parameters and distributions of important trees and shrubs in North America - Revisions for all taxa from the United States and Canada and new taxa from the western United States: U.S. Geological Survey Professional Paper 1650-G. Tipple, B.J., Meyers, S.R., and Pagani, M., 2010, Carbon isotope ratio of Cenozoic CO2: a comparative evaluation of available geochemical proxies: Paleoceanography, v. 25, PA3202. Visscher, G., and Jagels, R., 2003, Separation of Metasequoia and Glyptostrobus (Cupressaceae) based on wood anatomy: IAWA Journal, v. 24, p. 439-450. Westgate, J.A., 1989, Isothermal plateau fission-track ages of hydrated glass shards from silicic tephra beds: Earth and Planetary Science Letters, v. 95, p. 226-234. Westgate, J.A., Naeser, N.D., and Alloway, B., 2013, Fission-track dating, in Elias, S.A., and Mock, C.J., eds., Encyclopedia of Quaternary Science (2nd edition): Amsterdam, Elsevier, p. 643-662. Woodward, F.I., and Bazzaz, F.A., 1988, The responses of stomatal density to CO2 partial pressure: Journal of Experimental Botany, v. 39, p. 1771-1781.

7

Figure DR1. Extant and fossil Metasequoia and assorted images demonstrating the quality of botanical preservation from the Giraffe kimberlite locality. (a, b) Metasequoia glyptostroboides from the Berlin arboretum. Specimen in (a) is ~20 m tall. (c) Litter-rich horizon in the Giraffe core, strongly dominated by Metasequoia foliage. (d) Wood fragment from the Giraffe core, showing preservation of shaggy cupressoid bark on the specimen surface. (e) Scanning electron microscopy (SEM) of unpermineralized wood in transverse section, revealing largely intact cell structure with only slight compression and the preservation of annual growth rings with diagnostic abrupt early- to late-wood transitions. (f) Bordered tracheid pits and rays in radial section under SEM. (g) Close-up of tracheid pits in radial section and (h), of uniseriate rays in tangential section. (i) SEM of individual leaf (abaxial surface). The white box is enlarged in (j), showing stomata (arrows) arranged in longitudinal rows. (k) Individual stoma under high-magnification SEM. (l) Pinus pollen grain. (m) Metasequoia pollen grain. (n) Fraxinus pollen grain. (o-r) Diverse fagalean palynomorphs. Scale bars are 1 cm (c, d); 1 mm (i); 100 µm (e, f, j); and 10 µm (g, h, k-r).

8 Angiosperms Conifers

Assemblage composition 48 Quercoidites CastaneaUlmipollenitesCorylus AlnipollenitesEricales Cupressaceae Pinaceae

52

56 Angiosperms 60

64

68

Gymnosperms tephra Vertical equivalent depth (m) Vertical 72 0 50 100 0 30 60 0 20 0 2 0 3 6 0 4 0 4 8 0 25 50 0 3 6 Relative frequencies (% of total pollen sum excluding spores; scales vary)

Figure DR2. Summary pollen diagram from the Giraffe peat section (data from Sweet, 2000).

Figure DR3. Cellulose extracted from Metasequoia wood in the Giraffe kimberlite. a, Scanning electron micrograph of extracted cellulose showing the purity of the material. b, X-ray diffractogram of the same material, revealing the strong peaks at 16.5 and 22.8 degrees 2-θ that are associated with crystallinity of α-cellulose.

9

Figure DR4. Probability density functions for key climate parameters. a, coldest-month mean temperature (CMMT), mean annual temperature (MAT) and warmest-month mean temperature (WMMT) derived from pollen by the nearest living relative method, mutual climate range analysis (MCR). b, MAT derived from wood cellulose δ18O, with the 16th to 84th percentile indicated by the horizontal grey bar.

Figure DR5. Comparison of reconstructed (this study) and model results from various EoMIP simulations (Lunt et al., 2012) for MAT in the Giraffe region, relative to MAT for Yellowknife from 1981-2010 climate normals and the 280 ppm model control runs of Lunt et al. (2012). The corresponding ΔMAT for these models run at 560 and 1120 ppm CO2 are plotted in Fig. 4 and tabulated in Table DR5. 10 Table DR1. Glass fission-track ages of tephra beds in the Giraffe kimberlite maar sediments.

Depth in Vertical Method Spontaneous Corrected Induced Track density Etching Ds (µm) Di (µm) Ds/Di Age core (m) equivalent track density spontaneous track on muscovite conditions or (Ma) 2 2 5 o depth (m) (10 t/cm ) track density density (10 dosimeter glass (HF %: C: Di/Ds# (102 t/cm2) t/cm2) (105 t/cm2) seconds)

UT2114 – Giraffe pipe tephra 98.92 73.41 uncorrected 533.00 ± 6.13 2.28 ± 0.01 3.76 ± 0.04 24: 24: 120 5.03 ± 0.06 6.56 ± 0.08 0.77 ± 26.39 ± (7557) (33653) (9567) [397] [596] 0.01 2.34

DCFT 693.00 ± 7.97 2.12 ± 0.01 3.76 ± 0.04 24: 24: 120 5.03 ± 0.06 6.56 ± 0.08 1.30 ± 36.88 ± (7557) (33653) (9567) [397] [596] 0.02# 3.28**

uncorrected* 409.00 ± 7.26 1.73 ± 0.01 3.75 ± 0.01 24: 20.5: 5.03 ± 0.06 6.56 ± 0.08 0.77 ± 26.52 ± (3171) (61838) (14393) 130 [397] [596] 0.01 2.44

DCFT* 531.18 ± 9.43 1.61 ± 0.01 3.75 ± 0.01 24: 20.5: 5.03 ± 0.06 6.56 ± 0.08 1.30 ± 37.08 ± (3171) (61838) (14393) 130 [397] [596] 0.02# 3.41

UT2115 – Giraffe pipe tephra 96.39 70.88 uncorrected 502.00 ± 6.97 2.04 ± 0.01 3.81± 0.04 24: 24: 125 5.62 ± 0.09 7.02 ± 0.10 0.8 ± 28.14 ± (5184) (32988) (9567) [199] [398] 0.02 2.67

DCFT 627.00 ± 8.71 1.91± 0.01 3.81± 0.04 24: 24: 125 5.62 ± 0.09 7.02 ± 0.10 1.25 ± 37.42 ± (5184) (2988) (9567) [199] [398] 0.03# 3.55**

ITPFT 455.00 ± 6.07 1.32 ± 0.01 3.81± 0.04 24: 24: 180 6.47 ± 0.12 6.65 ± 0.08 0.97 ± 39.37 ± (5612) (23890) (9567) [199] [398] 0.02 3.53**

UT1366 – Huckleberry Ridge tephra (internal standard) uncorrected 47.68 ± 4.06 3.19 ± 0.05 3.67 ± 0.04 24: 23: 120 5.77 ± 0.05 7.20 ± 0.08 0.80 ± 1.65 ± (138) (4660) (9567) [611] [474] 0.01 0.14

DCFT 59.60 ± 5.07 3.18 ± 0.05 3.67 ± 0.04 24: 23: 120 5.77 ± 0.05 7.20 ± 0.08 1.25 ± 2.07 ± (138) (4660) (9567) [611] [474] 0.02# 0.18

Notes: t = tracks; HF = hydrofluoric acid; Ds = mean spontaneous track diameter; Di = mean induced track diameter. Induced track density is calculated using the population-subtraction method (Laurenzi et al., 2007). Ages calculated using the zeta approach and λD =1.551 x 10-10y-1. Zeta value is 301 ± 3 based on six irradiations at the McMaster Nuclear Reactor, Hamilton, Ontario, using the NIST SRM 612 glass dosimeter and the Moldavite tektite glass (Lhenice locality) with an 40Ar/39Ar plateau age of 14.34 ± 0.08 Ma (Laurenzi et al., 2003, 2007). Ages in bold font are those corrected for partial track fading, achieved by the diameter-corrected procedure (DCFT, Sandhu and Westgate, 1995) and the isothermal plateau method (ITPFT, Westgate, 1989). Area estimated using the point-counting method for UT 2114 and UT2115 and an eyepiece graticule for UT1366. Number of tracks counted is given in parentheses; number of tracks measured is given in brackets. The weighted mean ages and errors of corrected ages for UT2114 and UT2115 are 38.40 ± 2.50 Ma and 36.98 ± 2.36 Ma, respectively. The single-crystal (sanidine) laser-fusion 40Ar/39Ar age and 2σ error of Huckleberry Ridge tephra, the internal standard, is 2.003 ± 0.014 Ma (Gansecki et al., 1998). Note that UT2114* was irradiated at a different time to the other samples and the track-size data of UT2114 were used to calculate the age corrected for partial track fading. ** Indicates age determination first reported by Doria et al. (2011).

! 11! Table DR2. Palynological estimates of MAT, CMMT, and WMMT using the Mutual Climate Range technique (Thompson et al., 2012), and list of nearest living relative (NLR) taxa used in the analysis

Vertical equivalent MAT MAT MAT error CMMT CMMT error WMMT WMMT error MAP MAP error depth (m) NLRs (˚C) (˚C) (˚C) (˚C) (˚C) (˚C) (mm yr-1) (mm yr-1) 50.13 14 12.5 3.2 0.5 3.5 23.5 1.8 1257 304 56.50 17 14.9 0.7 4.5 0.5 25.0 0.3 1257 304 59.35 14 14.9 0.7 4.5 0.5 25.0 0.3 1257 304 63.36 15 14.9 0.7 4.5 0.5 25.0 0.3 1257 304 66.37 13 16.3 0.7 4.5 0.5 25.0 0.3 1292 339 70.94 19 13.8 3.2 2.1 1.9 23.5 1.8 1257 304 Mean 15 14.5 1.5 3.4 1.2 24.5 0.8 1263 310 SD 1.3 1.7 0.8 14 Note: MAT = mean annual temperature; NLR = nearest living relative; CMMT = coldest month mean temperature; WMMT = warmest month mean temperature; MAP = mean annual precipitation

NLRs used, all identified to modern genus from Giraffe pollen except where indicated; where only one species was available with climate range data, this is listed: Metasequoia glyptostroboides (from megafossils; the fossil taxon M. occidentalis is an anatomical match for the extant species M. glyptostroboides; modern genus is monotypic), Thuja spp., Sciadopitys verticillata (modern genus is monotypic), Tsuga spp., Alnus spp., Castanea spp., Corylus spp., Quercus spp., Ulmus spp., Acer spp., Betula spp., Carya spp., Celtis spp., Cercidiphyllum japonica (1 of 2 species with data), Engelhardia spp., Eucommia ulmoides (modern genus is monotypic), Fagus spp., Fraxinus spp., Ilex spp., Juglans spp., Liquidambar styraciflua, Nyssa spp., Platycarya strobilacea (modern genus is monotypic), Pterocarya spp., Rhododendron spp. (best match for pollen), Rhus spp., Symplocos tinctoria (one North American species with data), Tilia spp.

! 12! Table DR3. Oxygen stable isotopic results from cellulose extracted from Giraffe Metasequoia wood macrofossils.

18 Estimated δ Owater Estimated MAT (˚C) (‰ VSMOW) 18 Vertical equivalent δ Ocell. median 16th – 84th median 16th – 84th depth (m) (‰ VSMOW) percentile percentile 55.53 23.44 ± 0.07 -10.56 -11.9 - -9.3 14.4 12.6 - 16.2 56.28 24.28 ± 0.09 -9.68 -11.0 - -8.5 15.7 13.8 - 17.5 57.30 24.45 ± 0 -9.53 -10.9 - -8.3 15.9 13.9 - 17.8 57.69 24.7 ± 0.3 -9.26 -10.7 - -8.0 16.3 14.3 - 18.2 58.08 23.79 ± 0.09 -10.21 -11.5 - -9.0 14.9 13.1 - 16.7 59.68 24.46 ± 0.08 -9.50 -10.9 - -8.3 16.0 14.0 - 17.8 63.58 24.7 ± 0.04 -9.25 -10.7 - -8.1 16.3 14.3 - 18.1

18 18 Note: δ Ocell = δ O of α-cellulose, mean±stdev of duplicate isotope analyses; VSMOW = Vienna Standard Mean Ocean Water; MAT = 18 mean annual temperature. δ Owater and MAT estimates are calculated following Anderson et al. (2002) and Fricke and Wing (2004).

! 13! Table DR4. Stomatal index and gas-exchange estimates of CO2 from Metasequoia leaves from the Giraffe locality.

SI gas-exchange-model reconstruction Vertical stomatal median 16th 84th stomatal pore leaf median 16th 84th 13 equivalent index CO2 percentile percentile density length δ C CO2 percentile percentile depth (m) (%) (ppm) (ppm) (ppm) (mm-2) (mm) (‰) (ppm) (ppm) (ppm) 58.16 9.85 440 353 592 57.6 29.0 -24.88 341 302 387 (0.43) (4.5) (1.7) 58.50 9.30 502 392 663 58.1 25.8 -26.18 425 373 489 (0.29) (4.5) (2.0) 58.73 8.77 761 487 1236 62.7 24.5 -26.74 452 393 525 (0.56) (4.7) (2.5) 59.20 9.40 544 392 894 65.9 25.1 -26.69 433 378 499 (0.61) (4.8) (2.7) 59.24 7.73 1072 749 1365 50.3 24.3 -26.81 521 448 616 (0.73) (4.2) (2.7) 60.00 9.59 746 429 1246 59.7 24.1 (1.14) (3.1) (0.8) 62.06 8.59 860 509 1289 55.4 25.0 -26.82 483 422 561 (1.03) (4.4) (1.3) 64.46 8.97 673 446 1144 53.6 27.7 -26.42 440 383 510 (0.55) (4.3) (2.8) 65.69 8.92 778 472 1252 74.2 26.7 -28.47 522 447 615 (0.89) (5.1) (3.7) 65.70 8.95 799 476 1250 61.8 24.3 -24.58 352 303 417 (1.23) (4.6) (4.5) Overall 9.01 634 433 1124 60.0 25.8 -26.40 432 353 538 Giraffe (0.59) (7.2) (1.7) (1.15) locality Notes: Values in parentheses are the measured ±1 s.e.m. Only one leaf per horizon was measured for δ13C; for these inputs, a one standard deviation uncertainty of ±0.2‰ was assumed. The following values and uncertainties for the gas-exchange model were assumed constant across all horizons (see Maxbauer et al. [2014] and Franks et al. [2014] for justification and Franks et al. [2014] for scripts to make CO2 13 estimates): δ C of paleo-atmosphere: -6 ± 0.7 ‰ (based on the model of Tipple et al. [2010]); photosynthetic rate (A0) at a set CO2 value -2 -1 -2 -1 (CO2_0): 6.67 ± 0.39 mmol m s at 396 ppm CO2; leaf boundary layer conductance to CO2 (gcb): 2 ± 0.1 mol m s ; scaling from -1 photosynthetic rate (A) to leaf mesophyll conductance to CO2 (gm): 0.013 ± 0.00065 mol mmol ; scaling from pore length to single guard cell width: 0.70 ± 0.01; scaling from single guard cell width to stomatal depth: 1 ± 0.05; scaling from the area of a circle with the diameter of pore length to maximum stomatal pore area (amax): 0.5 ± 0.025; scaling from maximum leaf conductance to CO2 (gcmax) to operational leaf conductance to CO2 (gcop): 0.34 ± 0.038. Overall Giraffe locality values are arithmetic mean for stomatal index, stomatal density, pore 13 length, and leaf δ C, and calculated from Monte Carlo simulations for SI and gas-exchange CO2 reconstructions.

! 14!

Table DR5. Eocene Modelling Intercomparison Project (EoMIP; Lunt et al., 2012) estimates of MAT (ºC) for the grid cells of the Giraffe region and ΔMAT relative to modern and modeled Yellowknife MAT.

Giraffe 560 ppm 1120 ppm proxies

Model MATYellowknife_280ppm MATGiraffe_Eocene ΔMAT280ppm ΔMATmodern MATGiraffe_Eocene ΔMAT280ppm ΔMATmodern ΔMATmodern

HadCM3L -5.4 -3.8 ± 2.2 1.6 0.5 0.7 ± 2.3 6.1 5.0 -

ECHAM5 -6.2 6.6 ± 2.7 12.8 10.9 - - - -

GISS -5.0 - - - 8.5 ± 2.1 13.5 12.8 -

CCSM_W -9.7 - - - 6.4 ± 1.6 16.1 10.7 -

CCSM_H -9.7 -5.8 ± 4.23 3.9 -1.5 -2.6 ± 3.3 7.1 1.7 -

Giraffe proxies ------18.8 Notes: MATYellowknife_280ppm is the modelled MAT for the Yellowknife grid cell in control runs at 280 ppm CO2; MATGiraffe_Eocene is the modeled early Eocene MAT for the array of 27 grid cells corresponding to the Giraffe locality; ΔMATmodern and ΔMATEocene are expressed relative to Yellowknife 280 ppm CO2 control runs and modern instrumental MAT, respectively.

! 15!