A SIMILAR-TO-MODERN PLIOCENE CLIMATE IN CONTINENTAL ALASKA AND YUKON RECORDED BY MULTIPLE WATER ISOTOPE PROXIES: PLANT LIPIDS, WOOD CELLULOSE AND HYDRATED GLASS SHARDS

The Pliocene is regarded as a future climate analogue due to similar ocean-continent configurations and CO₂ levels. The eastern Canadian Arctic has yielded evidence for a dramatically warmer Pliocene compared to modern, +11-16°C in some cases, but there are few records from other Arctic regions for comparison. Here we infer Pliocene temperatures in continental Alaska and Yukon from three independent water isotope proxies – leaf wax fatty acids, wood ɑ-cellulose and hydrated glass shards. In contrast to the eastern Arctic, all lines of evidence point to a similar-to-modern Pliocene climate. A mid-Pliocene (4.3-4.6 Ma) lacustrine sequence from Fort Yukon, Alaska, yields well-preserved long-chain fatty acids (C28-C30) with δD values integrating thousands of years of annual precipitation. Likewise, the δD of hydrated glass shards of three tephra beds in the core integrate annual precipitation over millennia. Meteoric δD estimates for coeval fatty-acids and glass shards are -180‰ (± 2‰) and -178‰ (± 4‰), respectively, which equate to mean annual temperature estimates of -8°C (± 7°C) and -7°C (± 8°C) using a transfer function calibrated with continental North American GNIP data > 50°N; reconstruction errors do not include lipid or glass fractionation uncertainties. The modern Fort Yukon mean annual temperature is -5.2°C. Wood cellulose δ¹⁸O also reflects source water δ¹⁸O and climate. Ch’ijee’s Bluff in northern Yukon, 250 km NE from Fort Yukon, provides Pinaceae wood from Late Pliocene (ca. 3 Ma) and MIS 5e deposits with ɑ-cellulose mean δ¹⁸O values of 18.1‰ and 18.8‰ (± 0.3‰), respectively, indicating comparable climates. Modern ɑ-cellulose at the site, however, is relatively enriched with a mean of 20.5‰ (± 0.3‰). If the δ¹⁸O offset (ca. 2‰) between modern and Pliocene/MIS 5e is source water-related, modern climate can be interpreted as ca. +4°C warmer, but within confidence intervals of indifference. Overall, these independent proxies argue for a regional Pliocene climate that is not appreciably different from today, consistent with sedimentological evidence for Pliocene permafrost and a predominantly boreal paleoflora in the pollen record. In turn, this suggests that the strong continentality of the region, a result of the recent uplift of the St. Elias Mountains and Alaska Range, was present by at least this time.