indicated by multiple independent proxies Significantly warmer Arctic surface temperatures during the Pliocene

Temperatures in the Arctic have increased by an astounding 1 °C in response to anthropogenic forcing over the past 20 years and are expected to rise further in the coming decades. The Pliocene (2.6–5.3 Ma) is of particular interest as an analog for future warming because global temperatures were signifi cantly warmer than today for a sustained period of time, with continental confi gurations similar to present. Here, we estimate mean annual temperature (MAT) based upon three independent proxies from an early Pliocene peat deposit in the Canadian High Arctic. Our proxies, including oxygen isotopes and annual ring widths (MAT = –0.5 ± 1.9 °C), coexistence of paleovegetation (MAT = –0.4 ± 4.1 °C), and bacterial tetraether composition in paleosols (MAT = –0.6 ± 5.0 °C), yield estimates that are statistically indistinguishable. The consensus among these proxies suggests that Arctic temperatures were ~19 °C warmer during the Pliocene than at present, while atmospheric CO2 concentrations were ~390 ppmv. These elevated Arctic Pliocene temperatures result in a greatly reduced and asymmetrical latitudinal temperature gradient that is probably the result of increased poleward heat transport and decreased albedo. These results indicate that Arctic temperatures may be exceedingly sensitive to anthropogenic CO2 emissions. 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, Boulder, CO 80301, USA. TABLE 1. PALEOTEMPERATURE PROXY ESTIMATES FOR THE ARCTIC DURING THE PLIOCENE AND THEIR ASSOCIATED STATISTICS Temperature proxies Temperature transfer function MAT (°C) ΔMAT (°C) ±SE Tetraethers MAT = (MBT – 0.122 – 0.187 × CBT)/0.020 –0.6†,§ 19.1 5.0 Tree ring isotopes MAT = 17.5 + 0.98 × δOpre – 2.71 × RW –0.5*,† 19.2 1.9 * 4 . 0 – T S M I L C n o i t a t e g e v o e l a P ,§ 19.3 4.1 4 . 0 3 . 9 1 4 . 0 – . A . N e t i s o p m o C Note: Estimates of mean annual temperature (MAT), difference from modern temperature (ΔMAT), 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 from paleovegetation. Estimates of MAT from the three independent proxies were statistically indistinguishable. The composite estimate is based upon the joint distribution of temperature estimates 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. *No statistical difference between distributions of estimates (p-value = 0.79). †No statistical difference between distributions of estimates (p-value = 0.77). §No statistical difference between distributions of estimates (p-value = 0.72). on April 12, 2013 geology.gsapubs.org Downloaded from