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. 2019 Nov 12;9:16637. doi: 10.1038/s41598-019-52873-4

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Estimated trendline of isotopic values over time for (a) carbon and (b) nitrogen. Smoothed functions were determined using generalized additive models with shaded areas representing 95% confidence intervals and points representing the data points. Dashed lines separate sub-epoch boundaries; Late Pleistocene – Early Holocene (11,700 ya), Early Holocene – Middle Holocene (8,200 ya), and Middle Holocene – Late Holocene (4,200 ya). Stable isotope measurements were determined on amino acid hydrolysate samples.