T9 Miocene–Pliocene Terrestrial Ecosystem Response to the Climate System I

Sunday, 22 October 2017
8:00 AM - 12:00 PM
Room 609

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

Paper number: T9-13
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

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 (δDwax) from Pliocene deposits, a proxy for precipitation δD (δDprecip) 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 δDwax from n-C26 alkanoic acids as this compound is abundant across all sites and linked to terrestrial plants. Modern δDprecip 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 (ewax/precip) of -104 ± 11‰ (Yukon-Alaska modern soil calibration dataset) to estimate δDprecip from δDwax. Paleotemperature was estimated based on ∆δDprecip and a temperature-δDprecip slope of 3.48 ± 0.05 ‰·°C-1 defined by empirical data from the six nearest continental GNIP stations.

Paleo δDprecip 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). δDprecip 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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-296789

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