CLIMATE VARIABILITY IN THE EARLY PLIOCENE CANADIAN HIGH ARCTIC: EVIDENCE FROM SUBSEASONAL d18O, dD, AND d13C VALUES OF FOSSIL WOOD

One of the primary problems encountered in the investigation of palaeoclimate has been the lack of suitable proxies for annual to subannual variability that quantify seasonality, particularly in terrestrial environments. δ¹⁸O, δD, and δ¹³C isotope values of tree rings have provided valuable palaeoclimatic information for the Holocene and Late Pleistocene, but poor preservation of ancient wood has tended to limit their application to the Quaternary. Strathcona Fiord, located in central Ellesmere Island (79°N), is the site of a rich Early Pliocene (4-5 Ma) mixed coniferous-deciduous forest with a diverse mammalian fauna, resembling the extant boreal forests of Northern Canada and Eurasia. The fossil remains occur within braided fluvial deposits that are part of a regionally discontinuous non-marine unit commonly referred to as the “Beaufort Formation”. A large fossil trunk recovered from this area is exceptionally well preserved, and presents a 203-year record of climate variability at sub annual resolution, representing the first high-resolution secular isotope study of Pliocene wood. Information obtained from the δ¹⁸O, δD, and δ¹³C isotope record indicates a highly variable climate with periods of cool/wet climate alternating with warm/dry periods. δ¹⁸O values suggest variability in the source of precipitation, with a shift from locally-sourced oceanic precipitation, with an average δ¹⁸O value of -5.5‰, to values more reflective of a continental source (-8.3‰), in the 37th year of the record. Time-series wavelet analysis of our data reveals a strong 20-year cyclicity early in the record and a strong 50-year cyclicity throughout the record. This record has provided insight into Pliocene climate variability in the High Arctic, revealing patterns unrecognizable in lower-resolution records.