Stable isotope paleoclimatology of the earliest Eocene using kimberlite-hosted mummified wood from the Canadian Subarctic

摘要

The recent discovery of well-preserved mummified wood buried within asubarctic kimberlite diamond mine prompted a paleoclimatic study of theearly Eocene "hothouse" (ca. 53.3 Ma). At the time of kimberlite eruption,the Subarctic was warm and humid producing a temperate rainforest biome wellnorth of the Arctic Circle. Previous studies have estimated that mean annualtemperatures in this region were 4–20 °C in the early Eocene,using a variety of proxies including leaf margin analysis and stableisotopes (δ13C and δ18O) of fossil cellulose.Here, we examine stable isotopes of tree-ring cellulose at subannual- toannual-scale resolution, using the oldest viable cellulose found to date. Weuse mechanistic models and transfer functions to estimate earliest Eocenetemperatures using mummified cellulose, which was well preserved in thekimberlite. Multiple samples of Piceoxylon wood within the kimberlite were crossdatedby tree-ring width. Multiple proxies are used in combination to tease apartlikely environmental factors influencing the tree physiology and growth inthe unique extinct ecosystem of the Polar rainforest. Calculations ofinterannual variation in temperature over a multidecadal time-slice in theearly Eocene are presented, with a mean annual temperature (MAT) estimate of11.4 °C (1 σ = 1.8 °C) based on δ18O, which is 16 °C warmer than the current MAT of the area(?4.6 °C). Early Eocene atmospheric δ13C (δ13C_atm) estimates were ?5.5 (±0.7) ‰. Isotopic discrimination(Δ) and leaf intercellular pCO₂ ratio(c_i/c_a) were similar to modern values (Δ = 18.7 ± 0.8 ‰; c_i/c_a = 0.63 ± 0.03 %), butintrinsic water use efficiency (Early Eocene iWUE = 211 ± 20 μmol mol−1) was over twice the level found in modern high-latitudetrees. Dual-isotope spectral analysis suggests that multidecadal climatecycles somewhat similar to the modern Pacific Decadal Oscillation likelydrove temperature and cloudiness trends on 20–30-year timescales,influencing photosynthetic productivity and tree growth patterns.