Nitrogen Isotope Composition of Peat Samples as a Proxy for Determining Human Colonization of Islands

The Aleutian Islands comprise a unique ecosystem, providing nesting grounds and habitat for more than ten million seabirds. No doubt their numbers were even larger prior to the introduction of foxes and rats, invasive species that have shaped the current population sizes and distributions of bird-breeding colonies. Here we present evidence that the peopling of the Aleutian Islands had the same dramatic effect on seabird populations. Analyses of nitrogen stable isotopes in Shemya Island peat deposits revealed significant ¹⁵N enrichment of layers formed cal. 4,700–2,800 yrs BP suggesting high seabird guano input at that time. Following this period there was a sharp decline in δ¹⁵N that coincides with the first appearance of human settlement and reflects drastic reduction in seabird abundance. The isotope signature of marine-derived nitrogen vectored by seabirds is specific only for coastal peat deposits; neither ¹⁵N enrichment nor decline was detected in inland peat deposits.