[OLD] Carbon isotopes

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From: T.C. Onstott

Subject: carbon isotopes

I'm trying to model heterotrophic sulfate reduction and would also like to validate using the shifts in del13C in the inorganic carbon we have detected. How does one go about entering the del13C of acetate, as an example, to that during kinetic reaction it is converted to 2*HCO3- and its del13C incorported into the bulk 13C of the system? Also, how can I interrogate the GWB program to find out the equivalent del13C for the pCO2? Finally, I noticed in the GWB isotope database that no fractionation coefficients are included for CO2(aq) and CO3--? Is this because these equations are hardwired in the program? If not, do I just modify the data base?

 

From: Craig Bethke

Subject: Re: carbon isotopes

React doesn't allow for isotopic disequilibrium within the equilibrium system. It does, however, allow not only for disequilibrium between aqueous species and minerals, as we discussed, but disequilibrium between aqueous species and reactants. I suggest, therefore, that instead of building isotopic fractionation into your kinetic model, you set up a simple titration model in which you add acetate to a fluid containing an oxidant. You can set the 13C of the acetate and initial fluid separately, and the program will predict the isotopic effects. You can then link the isotopic calculation to the kinetic model in terms of the amount of acetate consumed. Remember that you can output data from Gtplot as input files for a spreadsheet program like Excel, using the “Save as ...� option, which lets combine results from two runs onto a single plot. In response to your other questions, you can find the isotopic composition of CO2(g) in equilibrium with the system in the React_isotope.txt output dataset, written in your working directory. The fractionation factor for CO2(aq) is zero because this is the reference species. And the fractionation factor for CO3-- in the dataset is the same as that for HCO3- (remember that a blank line in isotope.dat means that the data for the previous line is to be used). And, finally, yes, you can modify the isotope.dat database. But be sure to work on a copy of the file, not the original file itself.