Dear Sonja,

the C isotope fractionation between carbonate ion (CO3--) and the rest of
DIC is still in debate, even for simple aqueous solutions. There are
several experimental studies dealing with the carbon isotope fractionation
between CO3-- and CO2-gas (e.g., Thode et al., 1965; Halas et al., 1997;
Lesniak & Sakai, 1989) and additionally Thode made some theoretcial
calculations.
The situation for fractionation between CO2(aq)-HCO3- and CO2 gas seems to
much more consistent and most studies report quite similar values. This is
actually expected , considering the typically small concentrations of CO32--

C isotope fractionation in the carbonate system of seawater has been
investigated by Zhang et al. (1995) in seawater. They have found some
differences between seawater matrix and simple electrolytic solutions,
probably dut to more important complex formation of carbonate species in
seawater (and associated different individual isotope fractionation
factors). Complex formation was also discussed in the fundamental paper by
Thode et al.


Good luck, Michael

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Dr. Michael E. Böttcher
Department of Biogeochemistry
Max-Planck-Institute for Marine Microbiology
Celsiusstr.1
D-28359 Bremen
Voice:  (+49)421-2028-632
Fax:    (+49)421-2028-580
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