Dear GJ,

You can have significant enrichment of 13C through kinetic effects
related to the rapid freezing of bicarbonate water. We measured values
up to +17 permil in white powders from caves in the Yukon, and through
experiments found a kinetic fractionation factor for CaCO3-CO2 of 1.032
(Clark and Lauriol, Chemical Geology/Isotope Geoscience Section, 1992,
v102: 217-228). However, this doesn't get you into the range that you
report. The other way is through a Rayleigh reaction, usually associated
with methanogenesis by CO2 reduction. This was suspected in some deep
groundwater samples we measured with 13C-DIC values of +30 to +32
permil. I'd like to learn more about the context for your samples.

Regards,

Ian Clark
Geology
University of Ottawa
613-562-5838
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