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Significantly Warmer Arctic Surface Temperatures During the Pliocene

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Significantly Warmer Arctic Surface Temperatures During the Pliocene Signifi cantly warmer Arctic surface temperatures during the Pliocene indicated by multiple independent proxies A.P. Ballantyne1*, D.R. Greenwood2, J.S. Sinninghe Damsté3, A.Z. Csank4, J.J. Eberle1, and N. Rybczynski5 1Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA 2Department of Biology, Brandon University, Brandon, Manitoba R7A 6A9, Canada 3Department of Marine Organic Biogeochemistry, Royal Netherlands Institute for Sea Research (NIOZ), Texel 1797 SZ, Netherlands 4Department of Geosciences and Laboratory of Tree-Ring Research, University of Arizona, Tucson, Arizona 85721, USA 5Canadian Museum of Nature, Ottawa, Ontario K1P 6P4, Canada ABSTRACT making fossil tetraether lipids an excellent Temperatures in the Arctic have increased by an astounding 1 °C in response to anthropo- proxy for reconstructing TSTs (Weijers et al., genic forcing over the past 20 years and are expected to rise further in the coming decades. 2007) (see Appendix). The analysis of bacterial The Pliocene (2.6–5.3 Ma) is of particular interest as an analog for future warming because tetraether composition in peat collected from global temperatures were signifi cantly warmer than today for a sustained period of time, the Beaver Pond site indicates a MAT of –0.6 with continental confi gurations similar to present. Here, we estimate mean annual tempera- ± 5.0 °C that suggests Pliocene temperatures in ture (MAT) based upon three independent proxies from an early Pliocene peat deposit in the Arctic were considerably warmer than the the Canadian High Arctic. Our proxies, including oxygen isotopes and annual ring widths modern MAT of –19.7 °C at Eureka, Nunavut (MAT = –0.5 ± 1.9 °C), coexistence of paleovegetation (MAT = –0.4 ± 4.1 °C), and bacterial (Environment Canada, 2009) (79°N, 85°W), tetraether composition in paleosols (MAT = –0.6 ± 5.0 °C), yield estimates that are statistically yielding a temperature difference (ΔMAT) of indistinguishable. The consensus among these proxies suggests that Arctic temperatures were ~19 °C (Table 1). ° ~19 C warmer during the Pliocene than at present, while atmospheric CO2 concentrations Refi ned paleotemperature estimates from the were ~390 ppmv. These elevated Arctic Pliocene temperatures result in a greatly reduced and annual growth rings and δ18O of cellulose in asymmetrical latitudinal temperature gradient that is probably the result of increased pole- fossil wood also showed considerably warmer ward heat transport and decreased albedo. These results indicate that Arctic temperatures TSTs in the Arctic during the Pliocene, yielding Δ may be exceedingly sensitive to anthropogenic CO2 emissions. a MAT of –0.5 ± 1.9 °C and a MAT of ~19 °C (Table 1). Although a previous estimate based INTRODUCTION GSA Data Repository1) to derive independent on this approach yielded a MAT of approxi- Throughout most of the Cenozoic era paleotemperature estimates. mately –5 °C, additional information from oxy- (0–65.5 Ma), temperatures at Earth’s surface gen isotopes in mosses at the site allowed us to have exceeded modern temperatures (Zachos RESULTS AND DISCUSSION calculate isotopic enrichment in the cellulose of et al., 2008). This is especially true for the The bacterial tetraether membrane composi- fossil trees, thereby reducing assumptions and Arctic (Dowsett, 2007; Robinson, 2009). The tion in soils has been shown to be very sensi- increasing the precision of our MAT estimates most recent interval in which sustained global tive to environmental conditions (Weijers et al., (see the Data Repository). temperatures exceeded those of today was dur- 2007). In particular, the degree of cyclization It is conceivable that the isotope and tetraether ing the Pliocene epoch (2.6–5.3 Ma), when and methylation among branched tetraethers paleotemperature proxies are biased toward the global surface temperatures were between 2 is highly dependent upon pH and temperature, warm season because they are effectively the and 3 °C warmer than present (Dowsett, 2007). Although tropical surface temperatures dur- ing the Pliocene were only slightly warmer TABLE 1. PALEOTEMPERATURE PROXY ESTIMATES FOR THE ARCTIC DURING THE PLIOCENE than present (Dowsett, 2007), Arctic tempera- AND THEIR ASSOCIATED STATISTICS tures were probably much warmer (Robinson, Temperature proxies Temperature transfer function MAT ΔMAT ±SE 2009). However, the magnitude of Arctic Plio- (°C) (°C) cene warming remains poorly constrained, pri- Tetraethers MAT = (MBT – 0.122 – 0.187 × CBT)/0.020 –0.6†,§ 19.1 5.0 marily because ice core records do not extend δ18 ,† Tree ring isotopes MAT = 17.5 + 0.98 × Opre – 2.71 × RW –0.5* 19.2 1.9 this far back. Thus, researchers have had to rely Paleovegetation CLIMST –0.4*,§ 19.3 4.1 on other proxy records to reconstruct Arctic Composite N.A. –0.4 19.3 0.4 temperatures during the Pliocene. Note: Estimates of mean annual temperature (MAT), difference from modern temperature (ΔMAT), Here we use three independent proxies to and standard error (SE) for each temperature proxy are reported. We used the CLIMST algorithm based on the coexistence approach (Mosbrugger and Utescher, 1997) to estimate temperature better constrain terrestrial surface temperatures from paleovegetation. Estimates of MAT from the three independent proxies were statistically (TSTs) for the High Arctic during the Pliocene. indistinguishable. The composite estimate is based upon the joint distribution of temperature estimates We use fossil tetraether lipids derived from soil resampled using a bootstrap technique (Efron and Tibshirani, 1997). CBT—Cyclisation of branched tetraethers; MBT—Methylation of branched tetraethers; pre—precipitation; RW—annual ring-width. 18 bacteria, oxygen isotope ratios (δ O) and annual *No statistical difference between distributions of estimates (p-value = 0.79). ring widths in fossil wood, as well as the com- †No statistical difference between distributions of estimates (p-value = 0.77). § position of paleovegetation (see the Appendix) No statistical difference between distributions of estimates (p-value = 0.72). from the Beaver Pond site on Ellesmere Island (78°N, 82°W, and 378 masl; see Fig. DR1 in the 1GSA Data Repository item 2010165, supplemental information, is available online at www.geosociety.org/ pubs/ft2010.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, *E-mail: [email protected]. Boulder, CO 80301, USA. © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, July July 2010; 2010 v. 38; no. 7; p. 603–606; doi: 10.1130/G30815.1; 3 fi gures; 1 table; Data Repository item 2010165. 603 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/38/7/603/3538666/603.pdf by guest on 25 September 2021 result of summer productivity. Temperature esti- CMMT MAT WMMT CONCLUSIONS mates derived from tetraether composition may The three independent temperature proxies be more representative of summer processes Alnus incana* measured from the same peat deposit converge when temperatures are higher and soils have Saxifraga oppostifola on an Arctic MAT for the Pliocene of ~0 °C that Myrica gale greater water content, promoting the facultative Empetrum nigrum corresponds with a ΔMAT of ~19 °C (Table 1). anaerobic bacteria that are hypothesized to syn- Andromeda polifola Because these independent proxy estimates of Carex diandra thesize tetraether lipids (Weijers et al., 2007). Potentilla norvegica MAT are statistically indistinguishable from Menyanthes trifoliata However, temperature estimates derived from Vaccinium vitis-idae each other (Table 1), we combined all of the tetraethers in modern soils from Svalbard, Nor- Betula nana* estimates into a joint distribution that was then Genera Betula papyrifera* way (MAT ≈ –4 °C) were within 2 °C of instru- Thuja occidentalis resampled using a bootstrap technique (see ≈ Salix spp. mental temperature records (MAT –6 °C), Picea mariana Appendix). This approach yielded a composite suggesting that tetraethers are an effective proxy Pinus albicaulis* MAT estimate of –0.4 ± 0.4 °C, and a ΔMAT Larix laricina* for reconstructing temperatures from paleosols of 19.3 °C (Table 1). This more robust tempera- at high latitudes (Peterse et al., 2009). It is also ture estimate suggests that Arctic temperatures possible that MAT estimates from isotopes −20 −10 0 10 20 were remarkably warmer during the Pliocene and annual ring widths may be biased toward Temperature (°C) (Fig. 2). In fact, these estimates are 5–10 °C summer months because this is when trees are warmer than previous proxy estimates (Bal- accruing biomass and synthesizing cellulose Figure 1. Paleotemperatures for the Pliocene lantyne et al., 2006; Elias and Matthews, 2002). from the available water. Although it is unclear Beaver Pond site inferred from paleovegeta- These temperature estimates are also consider- tion. Plotted are the inferred temperature es- how much of the water pool available for photo- timates based on the coexistence approach ably warmer than model simulations at high synthesis is derived from winter versus summer (Mosbrugger and Utescher, 1997). Black latitudes (Haywood et al., 2009). However, cli- precipitation, research on oxygen isotopes in boxes represent mean annual temperature mates at high latitude are known to be very sen- modern larch suggests that larch rely on spring (MAT) estimates for each genus identifi ed sitive to orbital parameters affecting insolation
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