Warm Winters and Cool Summers in the Arctic During the Early Eocene Determined From Isotopes in Cellulose Extracted from Fossil Wood

The sediments of northern Canada preserve the remains of lush temperate forests that thrived above the Arctic Circle for millions of years during the Eocene Epoch (55.8-33.9 Ma). Geochemical and paleobotanical analyses across multiple sites have consistently indicated mean annual temperatures ~25 to 30 oC warmer than present day, but with highly divergent estimates for seasonal temperature (e.g., cold-month mean temperatures ranging from -7.9 to +11.2 oC, i.e., equivalent to the temperature difference between Wisconsin and Louisiana, USA). Here we report new estimates of cold-month and warm-month mean temperature in the Arctic during the early Eocene based on 80 high-resolution intra-ring oxygen isotope measurements of cellulose (δ18Ocell) extracted from fossil tree-rings from Banks Island, Canada. Using published models that quantitatively relate changes in δ18Ocell to changes in seasonal temperature, we calculated very low temperature seasonality, with warm winters and cool summers across the region. Specifically, we found cold month mean temperature = 1.9 to 11.9 °C and warm month mean temperature = 11.6 to 18.0 °C. Potential mechanisms for this reduced seasonality will be discussed, including the effects of high summer rainfall on regulating summertime temperatures. These results indicate the Arctic experienced a greater increase in winter temperatures than summer temperatures in response to past increases in CO2 level.