Estimates of Arctic land surface temperatures during the early Pliocene from two novel proxies

During the Pliocene (2.6 to 5 Ma ago) atmospheric CO2 levels have been estimated as similar to or slightly above present levels (Tripati et al., 2009; Pagani et al., 2010), and yet Earth's climate was considerably different. Recent evidence suggests that although global temperatures were 2–3 °C warmer than pre-industrial, Arctic warming may have been amplified during the Pliocene. Thus precise temperature records of this interval are required to assess the sensitivity of Earth's climate to persistent levels of CO2 between 365 and 415 ppm.We present records of two independent proxies for terrestrial growing-season temperatures at the Early Pliocene Beaver Pond site on Ellesmere Island. δ18O values of cellulose from well-preserved peat constrain the δ18O values of meteoric water to − 20.7 ± 0.3‰, which we combined with δ18Ovalues of aragonitic freshwater molluscs found within the peat in order to calculate mollusc growth temperatures. This approach results in an average growing-season temperature of 14.2 ± 1.3 °C. Temperatures were independently derived by applying carbonate ‘clumped isotope’ thermometry to mollusc shells from the same site, indicating an average growing-season temperature of 10.2 ± 1.4 °C. A one-way ANOVA indicates that the differences between the two techniques are not significant as the difference in mean temperatures between both methods is no different than the difference between individual shells using a single technique. Both techniques indicate temperatures ~ 11–16 °C warmer than present (May–Sept temperature = − 1.6 ± 1.3 °C) and represent the first thermodynamic proxy results for Early Pliocene Ellesmere Island.

Research Highlights
► δ18O values of meteoric water are 3–5‰ higher then modern values on Ellesmere Island.
► Temperatures estimated from stable isotopes average 14.2 ± 1.4 °C.
► Temperatures estimated from ‘clumped isotopes average 10.2 ± 1.4 °C
► Both temperature estimates are statistically indistinguishable according to a one way ANOVA.
► Temperature estimates indicate temperatures on the order of 11–16 °C warmer than present.