Quality control for modern bone collagen stable carbon and nitrogen isotope measurements

(1) Isotopic analyses of collagen, the main protein preserved in sub
fossil bone and tooth, has long provided a powerful tool for the
reconstruction of ancient diets and environments. Although isotopic
studies of contemporary ecosystems have typically focused on more
accessible tissues (e.g., muscle, hair), there is growing interest in the
potential for analyses of collagen because it is often available in hard
tissue archives (e.g., scales, skin, bone, tooth), allowing for enhanced
long-term retrospective studies. The quality of measurements of the stable
carbon and nitrogen isotopic compositions of ancient samples are subject
to robust and well-established criteria for detection of contaminants and
digenesis. Among these quality control (QC) criteria, the most widely
utilized is the atomic C:N ratio (C:NAtomic), which for ancient samples
has an acceptable range between  2.9 and 3.6. While this QC
criterion was developed for ancient materials, it has increasingly being
applied to collagen from modern tissues. (2) Here we use a large survey of
published collagen amino acid compositions (n = 436) from 193 vertebrate
species as well as recent experimental isotopic evidence from 413 modern
collagen extracts to demonstrate that the C:NAtomic range used for ancient
samples is not suitable for assessing collagen quality of modern and
archived historical samples. (3) For modern tissues, collagen C:NAtomic
falling outside 3.00–3.30 for fish and 3.00–3.28 for mammals and birds can
produce systematically skewed isotopic compositions and may lead to
significant interpretative errors. These findings are followed by a review
of protocols for improving C:NAtomic criteria for modern collagen
extracts. (4) Given the tremendous conservation and environmental
policy-informing potential that retrospective isotopic analyses of
collagen from contemporary and archived vertebrate tissues have for
addressing pressing questions about long-term environmental conditions and
species behaviours, it is critical that QC criteria tailored to modern
tissues are established.