CO2 control on carbon-isotope fractionation during aqueous photosynthesis: A paleo-pCO2 barometer

A carbon-isotope study of the various carbon phases during an annual productivity cycle in eutrophic Lake Greifen, Switzerland, indicates that values for the photosynthetic isotopic fractionation (ϵ_p) between dissolved CO₂ and particulate organic carbon (POC) and δ¹³C_POC increase in response to decreasing concentrations of dissolved carbon dioxide ([CO₂(aq)]) due to increased carbon demand during photosynthesis. Seasonal changes in the dominant biological assemblage may also be related to [CO₂(aq)] and may influence the fractionation as a consequence of changes in the dominant photosynthetic mechanism (CO₂ vs. HCO₃^- Utilization). A logarithmic relation between ϵ_p and [CO₂(aq)] may be useful for evaluating partial pressures of paleoatmospheric CO₂.