Radiocarbon age variation in bone and tissue from variable dietary effects

Radiocarbon dates of >2000 yrs BP have been reported for bones of the Polynesian rat Rattus exulans in avian predator deposits in New Zealand. Because R. exulans is a human commensal introduced to New Zealand, the oldest dates for this species would be a proxy for earliest human contact with these islands, as well as for the timing of the introduction of mammalian predators. The reliability of the oldest ages for R. exulans bone is thus important in both archaeology and palaeobiology. Our research has shown that contamination of samples is not a likely cause of the dates returned for some R. exulans, and that this species do produce reliable 14C ages as shown by agreement of rat bone and associated materials in various sites. There is evidence, however, for diet-derived radiocarbon anomalies, as supported by work on modern populations and archaeological samples of R. exulans and other species, which can cause minimal to extreme variation in the radiocarbon isotopic content of bone and other tissue. As these anomalous radiocarbon ages can actually offer important information about palaeoenvironments such as inter-species relationships, we would caution those who would 'cleanse' chronologies or otherwise exclude such data from the general interpretation of sites. (author). 42 refs., 4 figs., 2 tabs