Shifty baselines: The landscape factor in archaeological applications of stable carbon and nitrogen isotopes

The carbon (C) and nitrogen (N) stable isotope systems (expressed as δ 13 C and δ 15 N values) are used extensively in archaeology to study food chains, diet and subsistence practices. However, the variability of δ 13 C and δ 15 N values in different types of landscape is often underestimated leading to calls to consider these values in archaeological contexts as isoscapes. I investigate the relation between landscape characteristics and δ C 13 and δ N 15 values in plants and (human and animal) bones, using a dataset of ∼2500 published measurements from modern and past environments in the Netherlands. Modern sedimentary δ C 13 and δ N 15 values are determined by mixing of terrestrial, riverine, estuarine and marine sediments. Plant δ 15 N and δ 13 C values vary between different landscape zones depending on , from freshwater wetlands to fully marine settings, depending on factors like the origin of organic matter and soil hydrology. Archaeological datasets show similar landscape-related variations, with clear differences between plants wild animals and domesticated animals from drylands, freshwater wetlands and marine-influenced wetlands. Plant δ 15 N values (including cereals) can reach levels normally associated with manuring, while herbivore and omnivore δ 15 N values can reach levels normally associated with carnivores. Similarly, humans from different landscape zones have different δ 13 C and δ 15 N values as well. This study shows the importance of understanding local and regional food chains when studying subsistence or when making dietary reconstructions - especially in coastal wetlands. Future dietary studies should therefore not rely on standardized universal food chain δ 13 C and δ 15 N values for plant baselines, herbivores, carnivores and humans. Instead, for each landscape type the food chains and associated isotope values should be determined locally or regionally. • Stable C,N isotope baselines differ significantly between different terrestrial and wetland landscapes. • Freshwater environments have similar δ 15 N baselines, but lower δ 13 C than coastal and marine wetlands. • Wetland-grown cereals can reach δ 15 N values similar to terrestrial manured fields. • Animals and humans with wetland-derived plant-based diets can have collagen δ 15 N values similar to those who have high-protein diets. • Past diet research requires constructing local or regional food pyramids, taking mobility into account.