Using a synthetic dataset, we show how to best determine the mean source signature, δS
, at high temporal resolution using continuous CO2
) data. We apply this method to measured data from Heidelberg and find a distinct seasonal cycle of δS
. 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 δS
record, but only when their relative share is close to 100 %.