| Summary: | The influence of geothermally derived carbon on the radiocarbon dating of human bone from archaeological sites is poorly understood and has rarely been rigorously examined. This study identifies a previously unknown reservoir effect at the archaeological site of Klin-Yar in the Russian North Caucasus. AMS-dated human bones yielded results that were older than expected when compared with dates of coins found in the same grave contexts. We investigated the reasons for this offset by AMS dating modern plant, fish, and water samples to examine the source of the old carbon. We identified a potential source in geothermally derived riverine and spring water, with an apparent age of several thousand years, and hypothesize that carbon from here is being transferred through the food chain to humans. If humans consume resources from the local rivers, such as fish, then they ought to be affected by this reservoir offset. An extensive analysis of carbon and nitrogen isotopes of human and animal bone showed evidence for a mixed diet that may be masking the amount of freshwater-derived protein being consumed. Due to the highly variable nature of the 14C offset (0 to ~350 yr), no suitable average correction factor is applicable to correct for the human dates at the site. A 14C chronology based on dates obtained from terrestrial ungulate bones, which we subsequently obtained, is instead a more reliable indicator of age.
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