Diet-derived variations in radiocarbon and stable isotopes: A case study from shag river mouth, New Zealand

Accelerator mass spectrometry (AMS) determinations of rat bones from natural and cultural sites in New Zealand have produced ages at odds with the accepted date for early human settlement by over 1000 yr. Since rats are a human commensal, this implies either an earlier visitation by people or problems with the reliability of the AMS determinations. One explanation for the extreme ages is dietary variation involving movement of depleted radiocarbon through dietary food chains to rats. To investigate this, we ¹⁴C dated fauna from the previously well-dated site of Shag River Mouth. The faunal remains were of species that consumed carbon derived from a variety of environments within the orbit of the site, including the estuary, river, land, and sea. The ¹⁴C results showed a wide range in age among estuarine and freshwater species. Terrestrial and marine organisms produced ages within expectations. We also found differences between bone dated using the Oxford ultra-filtration method and those treated using the filtered gelatin method. This implies that contamination could also be of greater importance than previously thought.