Complex distribution of ancient carbon between tributaries of the Upper Lena seen in radiocarbon dates of modern fish

The results of radiocarbon dating of modern fish bone collagen from the Upper Lena River have identified a large and highly variable freshwater reservoir effect, with a maximum offset age of 2389 ± 19 14C yr BP. Variable contributions from both dissolved bedrock carbonate and permafrost soil melt were identified as the drivers for these reservoir offsets using a combination of bulk and amino acid specific stable carbon and nitrogen isotopes, as well as the novel application of strontium isotope ratios (87Sr/86Sr) to the study of reservoir effects. Strontium isotope ratios were used as a means of identifying the contributions of dissolved limestone and dolostone carbonate to the fish tissues in comparison to their associated radiocarbon (14C) offset ages. An updated model for δ13C and 14C age offsets in fish bone collagen was produced to interpret the potential combined contributions of old carbon from both permafrost soil melt and carbonates from bedrock erosion. The wide range of 14C offset ages identified in this study poses challenges to producing robust corrections for both modern and archaeological reservoir effects in human populations. Specifically, there is a need to address the potentially wide range of 14C ages which fish could represent, rather than using reservoir effect correction models which function best with uniform endmember contributions of old carbon.