From Phe to physiology: disentangling complex amino acid isotope compositions in Late Holocene pinnipeds from the Aleutian Islands, Alaska

Understanding human impacts on marine ecosystems is increasingly urgent as climate change and other anthropogenic pressures intensify, particularly at high latitudes. Long-term records are essential for distinguishing recent impacts from natural variability, yet most ecological observations span only the recent past. Using bulk and compound-specific stable carbon and nitrogen isotope analysis of amino acids in bone collagen from Steller sea lions ( Eumetopias jubatus ) and northern fur seals ( Callorhinus ursinus ) recovered from ancestral Unangax̂ midden sites in the Aleutian Islands, we were able to extend the ecological perspective across the past 5,500 years. Despite climatic variability across the Late Holocene, isotopic baselines and trophic structure remained broadly stable through time, suggesting long-term resilience in the Aleutian marine system. However, trophic position estimates derived from amino acids varied depending on the calculation approach, highlighting persistent challenges in reconstructing trophic ecology in high-trophic marine mammals. Elevated and variable phenylalanine δ 15 N values, commonly assumed to reflect baseline signals with minimal enrichment, suggest that this amino acid can also respond to metabolic strain or physiological stress. Patterns observed in glycine, serine, and threonine further indicate that amino acid isotope values reflect not only trophic transfer and baseline variability but also metabolic routing and physiological state. Because these amino acids constitute a major fraction of the bulk collagen this complicates not only amino acid-specific trophic estimates but also has major implications for bulk collagen studies.