Nitrogen isotopes suggest a change in nitrogen dynamics between the Late Pleistocene and modern time in Yukon, Canada

RESEARCH ARTICLE Nitrogen isotopes suggest a change in nitrogen dynamics between the Late Pleistocene and modern time in Yukon, Canada Farnoush Tahmasebi1*, Fred J. Longstaffe1*, Grant Zazula2 a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 1 Department of Earth Sciences, The University of Western Ontario, London, Ontario, Canada, 2 Yukon Palaeontology Program, Department of Tourism & Culture, Government of Yukon, Whitehorse, Yukon Territory, Canada * (FT); (FJL) Abstract OPEN ACCESS Citation: Tahmasebi F, Longstaffe FJ, Zazula G (2018) Nitrogen isotopes suggest a change in nitrogen dynamics between the Late Pleistocene and modern time in Yukon, Canada. PLoS ONE 13 (2): e0192713. https://doi.org/10.1371/journal. pone.0192713 Editor: Anthony Fiorillo, Perot Museum of Nature and Science, UNITED STATES Received: October 17, 2017 Accepted: January 29, 2018 Published: February 15, 2018 Copyright: © 2018 Tahmasebi et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: Funding was provided by a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (FJL), an Ontario Graduate Scholarship (FT), the Canada Foundation for Innovation (FJL) and the Ontario Research Fund (FJL). Additional research time was provided through the Canada Research Chairs program (FJL). A magnificent repository of Late Pleistocene terrestrial megafauna fossils is contained in icerich loess deposits of Alaska and Yukon, collectively eastern Beringia. The stable carbon (δ13C) and nitrogen (δ15N) isotope compositions of bone collagen from these fossils are routinely used to determine paleodiet and reconstruct the paleoecosystem. This approach requires consideration of changes in C- and N-isotope dynamics over time and their effects on the terrestrial vegetation isotopic baseline. To test for such changes between the Late Pleistocene and modern time, we compared δ13C and δ15N for vegetation and bone collagen and structural carbonate of some modern, Yukon, arctic ground squirrels with vegetation and bones from Late Pleistocene fossil arctic ground squirrel nests preserved in Yukon loess deposits. The isotopic discrimination between arctic ground squirrel bone collagen and their diet was measured using modern samples, as were isotopic changes during plant decomposition; Over-wintering decomposition of typical vegetation following senescence resulted in a minor change (~0–1 ‰) in δ13C of modern Yukon grasses. A major change (~2–10 ‰) in δ15N was measured for decomposing Yukon grasses thinly covered by loess. As expected, the collagen-diet C-isotope discrimination measured for modern samples confirms that modern vegetation δ13C is a suitable proxy for the Late Pleistocene vegetation in Yukon Territory, after correction for the Suess effect. The N-isotope composition of vegetation from the fossil arctic ground squirrel nests, however, is determined to be ~2.8 ‰ higher than modern grasslands in the region, after correction for decomposition effects. This result suggests a change in N dynamics in this region between the Late Pleistocene and modern time. 1 Introduction The terminal Pleistocene ~13,000 years ago was a time of dynamic changes in large mammal communities [1], climate [2], ice sheet extent [3], and range and composition of vegetation [4], all of which was accompanied by a large global rise in atmospheric CO2 concentration (pCO2) PLOS ONE | https://doi.org/10.1371/journal.pone.0192713 February 15, 2018 1 / 31 Change in Yukon 15N vegetation baseline between Late Pleistocene and modern time Competing interests: The authors have declared that no competing interests exist. [5]. Given the strong feedback mechanisms among herbivores, plant nutrient contents and ecosystem nutrient cycling [6], a comparable shift in nutrient dynamics likely accompanied such major environmental changes. Faith [7] suggested that a mode transition in N cycling was the main cause of megafauna extinction in North America after the terminal Pleistocene, driven mainly by a change in the N content of plants. He argued that environmental changes including rising atmospheric CO2 concentrations and possibly elevated temperature and precipitation amounts shifted the nutrient cycle from an accelerating to a decelerating mode. In the accelerating mode, abundant, excess plant N was returned to the soil by herbivores in readily bioavailable forms. By comparison, the decelerating mode was associated with lower plant N contents–a feedback reflecting lower soil N contents arising in large part by reduced returns of easily bioavailable nitrogen to the soil via herbivore excreta. Consequences of this shift included lower plant and soil N contents, reduced rates of nutrient cycling through the food web, reduced forage production, a lower biomass carrying capacity of the ecosystem, and ultimately collapse of megafauna populations. A study of lake sediment δ15N from a wide range of ecosystems also reported a gradual decrease in N availability of terrestrial ecosystems between ~15,000 to 7,000 years ago, which suggests a shift in the nature of terrestrial N cycling [8]. A change in N dynamics and availability should be traceable using the δ15N of plants [9] and animals. Higher plant δ15N generally reflects higher N availability and a more open N cycle [10]. This higher δ15N is passed on to the second trophic level (consumers) through the food chain [11]. Several studies have reported significantly different δ15N for herbivores over different Quaternary time periods (pre-, full- and post-Last Glacial Maximum (LGM)) in Alaska [12] and Eurasia [13–18], and some related those differences to a possible shift in the δ15N of herbivore diet in response to climate change. Considering these studies and empirical evidence for the influence of environmental factors on terrestrial N dynamics [19–21], some changes in N-isotope dynamics should be expected between the Late Pleistocene and modern time. If this prediction is accurate, then a suitably calibrated N-isotope baseline for vegetation should be utilized for Late Pleistocene ecosystems when comparing the δ15N of fossil bone collagen with modern counterparts [22]. In this study, we combine the stable carbon- and nitrogen-isotope compositions of modern [9] and fossil plants and animals to test for changes in N-isotope dynamics in Yukon Territory, northwest North America, between the Late Pleistocene and present time. Our study was focused in the Yukon, including portions that were not glaciated during the Pleistocene. This unglaciated region, known as Beringia, was an important terrestrial biotic refugium. It was home to a large community of flora and megafauna, (...truncated)