Dates Too Old?: Mixed Carbon Reservoirs Integrate Carbon from Freshwater Reservoirs and the Atmosphere

Summary

Sources of carbon in wetlands and calcareous areas represent unique challenges for interpreting the archaeological radiocarbon record. Atmospheric carbon dioxide is assumed to be the only carbon source for photosynthesis. However, dating modern and historic reference fish and modern reference wild rice indicates the presence of ancient carbon in bones and plant material. Dating four historic reference fish obtained from the Mississippi River in 1939 in southeastern Minnesota yielded four distinctly different dates that do not overlap. Instead, they span 1223 to 307 BP, leading to the conclusion that dissolved inorganic carbon (DIC) and perhaps other ancient carbon contributes to a freshwater reservoir affect. Trophic level of the fish (and their diet) is implicated in this date range since they were caught in the same river, probably during the same fishing expedition. Adding additional reference fish, including northern pike, caught elsewhere in the state returned differential offsets implicating geology and DIC as sources of ancient carbon. Wild rice harvested in 2015 yielded similar age offsets to the highest trophic level fish, suggesting both high tropic level fish and wild rice take in carbon from both the freshwater reservoir and the atmosphere. Implications for dating archaeological materials are discussed.

This Resource is Part of the Following Collections

• Society for American Archaeology 83rd Annual Meeting, Washington, DC (2018)

Keywords

Record Identifiers

Abstract Id(s): 21135