Sulfur isotope analysis of collagen: Quality controls and proboscidean wetland habitats

Sulfur isotopes (δ³⁴S values) have significant potential for addressing archaeological, paleontological, and paleoecological research questions. Studies of ancient materials rely on the assumption that δ³⁴S values have been minimally altered by diagenesis, yet meaningful analyses of sulfur isotope preservation/alteration are rare. This paper has 3 objectives: (1) to review and revise previous approaches to evaluating sulfur isotope alteration of collagen, (2) to evaluate sulfur isotope preservation in Great Lakes mammoth (Mammuthus spp.) and mastodon (Mammut americanum) bone, tooth, and tusk collagen, and (3) to make inferences about proboscidean diets and habitat preferences based on δ³⁴S values. To evaluate sulfur isotope preservation in collagen we recommend 3 approaches. First, researchers should examine collagen %C, %N, and atomic C:N values, and exclude samples whose values fall outside the expected ranges (defined according to context-specific considerations). Second, researchers should examine collagen %S, C:S, and N:S values, and exclude samples that fall outside the ranges for modern taxa. These ranges are subject to revision, but this study provides a new compilation of modern mammalian collagen with %S = 0.14–0.63, C:S = 185–873, and N:S = 55–266 (n=119). Third, researchers should check for correlations between collagen δ³⁴S and %S, C:S, or N:S values, which could suggest systematic alteration of sulfur isotope values due to sulfur contamination or amino acid loss. For our Great Lakes proboscideans, the first approach was insufficient to identify sulfur isotope alteration, but the second and third approaches led to the exclusion of 4 samples with probable alteration. Great Lakes proboscideans had lower δ³⁴S values than recent taxa from the same region, and the δ³⁴S of mastodons tended to be lower than those of mammoths. These results suggest that mammoths and (to a greater extent) mastodons consumed an abundance of plants rooted in anoxic freshwater wetland sediments.