Using a synthetic dataset, we show how to best determine the mean source signature, δ_S, at high temporal resolution using continuous CO₂ and δ¹³C(CO₂) data. We apply this method to measured data from Heidelberg and find a distinct seasonal cycle of δ_S. Disentangling this record into its source components requires the isotopic end members of CO₂ from the biosphere and those from the fuel mix. They can be estimated from the δ_S record, but only when their relative share is close to 100 %.