Dietary freshwater reservoir effects and the radiocarbon ages of prehistoric human bones from Zvejnieki, Latvia

Aquatic food resources (fish and molluscs) were exploited intensively at Riņņukalns, a Neolithic freshwater shell midden at the outlet of Lake Burtnieks, north-eastern Latvia. Stable isotope data (δ 15 N and δ 13 C) from a rich fishbone assemblage and a wide range of terrestrial species complement published results on faunal samples from the famous prehistoric cemetery and settlement at Zvejnieki, on the same lake. Stable isotope data show that freshwater food resources made substantial but varying contributions to human diets at Zvejnieki and Riņņukalns throughout the Mesolithic and Neolithic. Our research has also shown significant radiocarbon freshwater reservoir effects (FRE) in aquatic species from Lake Burtnieks, which would presumably have affected the radiocarbon ages of prehistoric human remains, and may explain some apparent anomalies in published dates from Zvejnieki burials. We present new radiocarbon and stable isotope results from a multiple burial at Zvejnieki, of five contemporaneous individuals, whose remains are being re-examined to assess evidence of interpersonal violence. Contrary to earlier interpretations, our data show that differences between individuals in the amount of fish consumed would account for the 300-year spread in their radiocarbon ages, given the isotope data and our estimate of the effective FRE in local fish. Our FRE-corrected calibrated dates for these five individuals are therefore compatible with a single burial date. We propose that these models can be used to correct the calibrated radiocarbon ages of other Zvejnieki burials for dietary FRE, provided that the individuals concerned consumed mainly local resources. The same methodology can also be applied elsewhere, as long as local isotopic and FRE values are well understood, and values for terrestrial and aquatic species are sufficiently different.