Changes in northeast Pacific marine ecosystems over the last 4500 years: evidence from stable isotope analysis of bone collagen from archeological middens

Abstract

Changes in food web dynamics and ocean productivity over the past 4500 years are investigated using stable isotope analysis of nitrogen and carbon in collagen from animal bones preserved in coastal archeological middens on Sanak Island, along the eastern edge of the Aleutian archipelgo. Samples included Steller sea lions, Harbor seals, Northern fur seals, sea otters, Pacific cod and sockeye salmon. Sea otters had the highest δ¹³C (−11.9 ± 0.7‰) and lowest δ¹⁵N values (14.5 ± 1.4‰), Northern fur seals had the lowest δ¹³C values (−13.6 ± 1.4‰), and Steller sea lions had the highest δ¹⁵N values (18.4 ± 1.4‰) of the marine mammals. Cod isotope values were consistent with those of demersal organisms from near shore habitats (−12.5 ± 0.9‰ δ¹³C, 16.1 ± 1.4‰ δ¹⁵N), while salmon values were consistent with those of organisms existing in an open ocean habitat and at a lower trophic level (−15.2 ± 1.4‰ δ¹³C, 11.5 ± 1.7‰ δ¹⁵N). When comparing six different prehistoric time periods, two time periods had significantly different δ ¹³C for salmon. Otters had significantly different δ¹⁵N values in two out of the six prehistoric time periods but no differences in δ¹³C. The mean δ¹³C, corrected for the oceanic Suess Effect, of modern specimens of all species (except Northern fur seals) were significantly lower than prehistoric animals. Several hypotheses are explored to explain these differences including a reduction in productivity during the twentieth century in this region of the Gulf of Alaska. If true, this suggests that North Pacific climate regimes experienced during the twentieth century may not be good analogs of North Pacific marine ecosystems during the late Holocene.