T209 Advances in Terrestrial Paleoclimate Reconstructions Using Paleosols

Tuesday, 24 October 2017
8:00 AM - 12:00 PM
Skagit 3

APPARENT FRACTIONATION OF HYDROGEN ISOTOPES IN N-ALKANOIC ACIDS IN A SOIL TRANSECT FROM BOREAL FORESTS IN EASTERN BERINGIA

Paper number: T209-14
Program: Technical Sessions
Time: 11:25 AM – 11:40 AM

Authors

Abstract

Fossil plant waxes are well preserved in sedimentary archives (e.g., lake sediments, paleosols), and stable hydrogen isotope ratios (δD) of these waxes can provide constraints on past climates. Studies have shown that modern plant wax δD (δDwax) reflects the δD of local precipitation (δDprecip), which correlates with air temperature in high latitude regions. However, δDwax is depleted relative to δDprecip by the net (or apparent) fractionation (εwax/precip), the sum of environmental and biosynthetic fractionations. εwax/precip must be accounted for when using fossil plant waxes to estimate past δDprecip, but may vary regionally with climate and vegetation type and, thus, a local εwax/precip value may help to improve paleoclimate interpretations. Little is known about the εwax/precip in northern boreal regions which limits our ability to interpret fossil δDwax archived in terrestrial Quaternary deposits in eastern Beringia (northwestern North America).

This study constrained the modern εwax/precip for high latitude boreal forest sites in 25 topsoil samples from a network of 12 sites spanning 58-68°N from Southern Yukon to the Mackenzie Delta region, and modern precipitation isotope data from the Online Isotopes in Precipitation Calculator. We measured the δD values of n-C24, -C26 and C28 alkanoic acids from topsoil depths 0-20 cm, which represent an accumulation of Holocene plant lipids from all vegetation types at the site. The modal chain-length (Cmax) was n-C24, which indicates major contributions from Sphagnum (the primary ground vegetation) and secondary contributions from conifers and deciduous shrubs. Modern δDprecip ranges from -153 to -176‰ across the study region. δD values of the n-C28 acid average -251‰ displaying variability within and between sites (1σ = 8‰, n = 25). Calculated εwax/precip for discrete soil samples across the transect display variability within and between sites. Values range from -80 to -136‰ (x̅ = -100‰, 1σ = 14‰, n = 25), which are within the range of published εwax/precip values for plants and sediments in the high latitudes and globally. The mean epsilon value, and the range of likely values from this large spatial survey, will assist future studies in using δDwax archives to develop quantitative estimates of past δDprecip in this high latitude boreal region.

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

doi: 10.1130/abs/2017AM-305424

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