Carbon and Nitrogen Isotopic Investigations of the Late Pleistocene Paleoecology of Eastern Beringia, Yukon Territory, Using Soils, Plants and Rodent Bones

During the late Pleistocene (130-12 ka), Beringia, a largely ice-free land located in the Mammoth Steppe Ecosystem, was home to a large grazing community of megafauna. Many of these animals, including the woolly mammoth, became extinct at the terminal Pleistocene. Assessment of the paleoenvironment, nutrient cycling and foraging ecology in Beringia should help to understand the role of climate change in their extirpation. Such information might also help to explain the curiously higher δ15N of woolly mammoths relative to other coeval herbivores.\nThis study assessed eastern Beringian paleoecology using stable nitrogen (N) and carbon (C) isotopic analyses of late Pleistocene fossil and modern soils, plants and rodent bones from Yukon Territory. A principal goal was to determine whether a modern isotopic baseline for the grassland food web should be used to interpret the isotopic data for Beringian megafauna. The average δ13C of fossil plants from fossil arctic ground squirrel nests is 1.6 ‰ higher than modern equivalents, mostly because of the Suess effect. Experiments using modern grasses suggest that the fossil plants were enriched in 15N by microbially mediated decomposition. The pre-decomposition δ15N of the fossil plants was determined by measuring the N isotopic spacing between modern arctic ground squirrel collagen and their diet, and applying this spacing to the fossil bones. The results show that the δ15N of fossil plants was originally ~2.5 ‰ higher than their modern equivalents. This difference suggests a shift in N cycling between the late Pleistocene and present time. A controlled-growth study of the effects of progressive CO2 enrichment – which occurred after the terminal Pleistocene – on subarctic nutrient cycling, demonstrates a pattern of decreasing δ13C for mature grasses, although the change was not significant. The δ15N response was nonlinear and not significant.\nThe collective results indicate that modern plant isotopic compositions for Yukon Territory are not appropriate baselines for interpreting the δ13C and δ15N of Pleistocene megafauna from eastern Beringia. In addition, decomposed plants, which are typically enriched in 15N, may have been important foods that were preferentially available to the woolly mammoth during cold seasons, and thus biased their δ15N to higher values.

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