Zinc isotopes from archaeological bones provide reliable tropic level information for marine mammals

Abstract

In marine ecology, dietary interpretations of faunal assemblages often rely on nitrogen isotopes as the main or only applicable trophic level tracer. We investigate the geographic variability and trophic level isotopic discrimination factors of bone zinc ⁶⁶Zn/⁶⁴Zn ratios (δ⁶⁶Zn value) and compared it to collagen nitrogen and carbon stable isotope (δ¹⁵N and δ¹³C) values. Focusing on ringed seals (Pusa hispida) and polar bears (Ursus maritimus) from multiple Arctic archaeological sites, we investigate trophic interactions between predator and prey over a broad geographic area. All proxies show variability among sites, influenced by the regional food web baselines. However, δ⁶⁶Zn shows a significantly higher homogeneity among different sites. We observe a clear trophic spacing for δ¹⁵N and δ⁶⁶Zn values in all locations, yet δ⁶⁶Zn analysis allows a more direct dietary comparability between spatially and temporally distinct locations than what is possible by δ¹⁵N and δ¹³C analysis alone. When combining all three proxies, a more detailed and refined dietary analysis is possible.

Fig. 1. Isotopic composition of P. hispida and U. maritimus bone samples from Arctic archaeological sites.

Pusa hispida (squares, n = 104) and U. maritimus (dots, n = 47) bone samples are colour coded as geographic groups. a Schematic map indicating the archaeological sites analysed and geographic colour coding: Light green for the Bering Strait; dark green for the Amundsen and Coronation Gulf; blue for the CAA; orange for the Hudson Bay; purple for the North Water Polynya; and red for sites influenced by the Labrador Sea in the Hudson Strait and Frobisher Bay. b δ¹⁵N versus δ¹³C plot for P. hispida samples (p-value < 0.05; R² = 0.21; n = 104). c) δ¹⁵N versus δ⁶⁶Zn plot for P. hispida samples (p-value < 0.05; R² = 0.08; n = 104). d δ¹⁵N versus δ¹³C plot for U. maritimus samples (no correlation, p-value > 0.05; n = 47). e δ¹⁵N versus δ⁶⁶Zn plot for U. maritimus samples (p-value < 0.05; R² = 0.42; n = 47). We included already published δ¹⁵N and δ¹³C values^,,, and already published δ⁶⁶Zn values from QjJx-1. The map is redrawn and modified using Adobe Illustrator CS6 after www.google.com/maps. Error bars represent the measurement uncertainty.

Fig. 2. Range of δ¹³C, δ¹⁵N and δ⁶⁶Zn values for P. hispida and U. maritimus bones.

Range of δ¹³C (a), δ¹⁵N (b) and δ⁶⁶Zn (c) values for P. hispida and U. maritimus bones for all locations. Site names are colour coded following Fig. 1. Dashed lines represent mean values when including all sites. We included already published δ¹⁵N and δ¹³C values from sites RbJu-1, PaJs-13, QkHn-13, KTZ and QjJx-1 sites^,,, and already published δ⁶⁶Zn values from QjJx-1. Error bars represent the measurement uncertainty.

Fig. 3. Isotope values for the SfFk-4 and KkDo-1 & JfEl-4 sites.

Isotopic composition (δ¹⁵N, δ¹³C versus δ⁶⁶Zn) of U. maritimus (magenta circles), P. hispida (brown squares), P. groenlandicus (green triangle) and D. leucas (blue stars) bones for the SfFk-4 (a, c) and combined KkDo-1 and JfEl-4 sites (b, d). For (a) and (c) we present n = 5 U. maritimus, n = 8 P. hispida and n = 4 P. groenlandicus bone samples and for (b) and (d) n = 7 U. maritimus, n = 18 P. hispida, n = 7 P. groenlandicus and n = 2 D. leucas bone samples. Error bars represent the measurement uncertainty.