Factors influencing (super 14) C ages of the Pacific rat Rattus exulans.

Abstract

An isotopic database for the Pacific/Polynesian rat (Rattus exulans) and foods that it scavenges is used to examine diet-induced (super 14) C age variation in omnivores. We discuss a suite of 26 Delta (super 14) C determinations and (super 13) C and (super 15) N analysis for modern Pacific/Polynesian rat bone gelatin and available food items from Kapiti Island, New Zealand (40 degrees 51'S, 174 degrees 75'E). These analyses provide the first isotopic data for modern specimens of the species, collected as part of a larger project to determine potential sources of bias in unexpectedly old (super 14) C age measurements on subfossil specimens of R. exulans from New Zealand. Stable C, N and (super 14) C isotopic and trapping data are used to trace carbon intake via the diet of the rats in each habitat. Data from specimens linked to five specific habitats on the island indicate that modern populations of R. exulans are not in equilibrium with atmospheric values of Delta (super 14) C, being either enriched or depleted relative to the atmospheric curve in 1996/97, the period of collection. The Delta (super 14) C values recorded for R. exulans are associated with diet, and result from variation in Delta (super 14) C values found in animal-protein food items available to a scavenging omnivore. The titer of carbon deviating from atmospheric values is believed to be derived from the essential amino acids in the protein-rich foods of the rat diet. Present evidence suggests that the depletion required to affect (super 14) C ages limits the possibility that diet introduces dramatic offsets from true ages. Marine diets, for example, would have a variable effect on ages for terrestrial omnivores, contraindicating the application of a standard marine correction for such specimens. We suggest that to identify the extent to which diet may influence the (super 14) C age in a given specimen of terrestrial omnivore, the separation and dating of essential amino acids vs. a nonessential amino, such as glycine, be applied.