Thanks Brian,
the method you suggested works, but I encountered another problem:
GSS allows to me add carbonate to the system only as CO32- (or C). I do not have carbonate (or TIC) data, but I do have CO2 fugacity data (I conduct experiments under very specific P(CO2)).
GSS seems to be able to calculate fugacities based on dissolved species, but not the other way around. Without calculated carbonate I cannot proceed to calculate SIs for calcite, dolomite etc.
I suppose I could run separate simulations in SpecE8 with only carbonate (substituted for CO2), Ca and pH, and then plug in the aqueous CO3 results back to GSS... Yet that is not the most efficient way.
Cheers,
Assaf