PRECIPITATION ISOTOPE-BASED RECONSTRUCTION OF PLIOCENE TEMPERATURES IN ALASKA AND YUKON FROM FOSSIL PLANT N-ALKANOIC ACIDS

Anthropogenic CO₂ forcing and positive feedbacks have shifted Earth’s mean climate state, with contemporary rates of warming being especially pronounced at higher latitudes. Uncertainty in future climate projections has led researchers to examine the Pliocene, a period of comparable boundary conditions and pCO₂ (~260-450 ppm), and a possible analogue for near future climate equilibria.

The purpose of this study is to characterize long-chain fossil plant wax hydrogen isotope ratios (δD_wax) from Pliocene deposits, a proxy for precipitation δD (δD_precip) and paleotemperature by extension. We focus on 5 sites in two regions – east Alaska Range (n = 3) and eastern Beringia straddling the Alaska-Yukon border (n = 2). ⁴⁰Ar/³⁹Ar ages for the Alaska Range (AR) sites are 6.2 ± 0.075, 5.1 ± 0.056 and 3.8 ± 0.054 Ma, and glass shard fission track ages for the Beringian sites are 3.2 ± 0.4 and 2.9 ± 0.4 Ma. We analyzed δD_wax from n-C₂₆ alkanoic acids as this compound is abundant across all sites and linked to terrestrial plants. Modern δD_precip was estimated for the AR sites using an empirical formula by Lachniet et al. (2016), and the Beringian sites using local GNIP data. We assume a net fractionation (e_wax/precip) of -104 ± 11‰ (Yukon-Alaska modern soil calibration dataset) to estimate δD_precip from δD_wax. Paleotemperature was estimated based on ∆δD_precip and a temperature-δD_precip slope of 3.48 ± 0.05 ‰·°C^-1 defined by empirical data from the six nearest continental GNIP stations.

Paleo δD_precip at the AR sites was 18-23‰ (± 7‰) lower than modern, which can be interpreted as a ΔT of 5-7°C colder than modern (compound σ = ± 4°C). δD_precip estimates for the Beringian sites are 10-21‰ (± 3‰) lower than modern, or a ΔT of 3-6°C colder than modern (c.σ = ± 3°C). Our mean estimates indicate a cooler Pliocene climate, and overlap with other reconstructions in E. Beringia from fossil beetles that suggest slightly cooler to slightly warmer conditions. Ice-wedge casts in E. Beringian Pliocene deposits are direct evidence of permafrost, which requires a similar to modern or colder climate. This research provides novel constraints on the regional paleoclimate. As all proxies are subject to uncertainties, continued diversification of the proxy network will help to improve paleoclimate estimates during this important interval.