Middle Eocene CO and Climate Reconstructed from the Sediment Fill
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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 Canada 60 DCFT and INTRODUCTION 50 IPFT dates (Myrs) Wood-rich Efforts to understand climate response to sustained greenhouse gas 80 60 va 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).