Published in Meteoritics, 30, pp. 580-581.
We report here the stable isotope composition of carbonate measured from a suite of desert soils from the Dry Valleys of Antarctica  to determine the 13C enrichments attributed to cryogenic freezing in terrestrial environments. These data are then used to gauge whether cryogenic freezing is a viable aqueous process that can produce extreme 13C enrichments observed in martian carbonates (e.g., ALH 84001 ).
Analyses of ALH 84001 have shown that the delta13C of carbonate is the most positive yet recorded for an SNC meteorite (~42) . The source of C is thought to be martian atmospheric CO2 that has been recycled through an aqueous medium into the solid phase. The delta13C of the carbonate is consistent with a precipitation temperature below ~300°C , assuming the delta13C of martian CO2 lies somewhere between 26 and 46 [4,5]. An equilibrium temperature of formation near 0°C is difficult to reconcile if the atmospheric source of C is <26, despite the fact that equilbrium isotope enrichments are large at this temperature (12-14) [6-8]. Low delta13C for atmospheric CO2 is only compatible with high delta13C for carbonate when nonequilibrium processes are the primary mechanism of isotopic fractionation. An inorganic surficial process known to enrich carbonate by >15 over ambient atmospheric CO2 is cryogenic freezing . Carbonate-bearing soils from Wright Valley, Antarctica, were studied as a terrestrial analog to the carbonates in ALH 84001 to characterize isotopic fingerprints associated with cryogenic freezing.
Prospect Mesa Soil Pit delta13C and delta18O carbonate values range from 0.89 to -20.46 (PDB) within the permanently frozen zone (below 0.4 m), and 4.20 to -11.87 at the surface. The most enriched 13C and 18O tend to occur at the surface, where seasonal variations in temperature or precipitation have imposed cyclical precipitation/dissolution of calcite. This observation is consistent, albeit less pronounced, with isotope enrichments in salts deposited in sediments from Lake Vanda, Antarctica .
Cryogenic freezing provides a possible explanation for the extreme enrichments observed in carbonate from Mars (e.g., ALH 84001, EET 79001, Nakhla). During freezing of subsurface fluids, dissolved ions become concentrated in the residual liquid phase, to the point where dissolved inorganic C can no longer be held in solution due to the reduced activity of water, and CO2 exsolves. If differences in diffusivity alone control isotopic fractionation, then the escaping CO2 gas should be enriched in 12C due to kinetic effects, with the resultant 13C enrichment in the fluid being passed on to any carbonate that may precipitate. Enrichments in 13C of up to 20 above atmospheric CO2 are observed for carbonates from Arctic cave deposits  and lake beds of the Dry Valleys in Antarctica . Using an enrichment of this magnitude in a fractionation model, carbonate with a delta13C of 42 can be generated from martian atmospheric CO2, as long as the CO2 source has a delta13C greater than about 20. This model is compatible with less extreme enrichments if the delta13C of atmospheric CO2 is increased in a complementary fashion.
Cryogenic freezing is a weathering process that is shown to enrich the carbonates in 13C above that which can be obtained by isotope effect at low temperature. Those processes operating in the cold desert environment of the Dry Valleys of Antarctica may be the ideal terrestrial analog to processes acting at or near the martian surface.
References:  Gibson E. K. et al. (1983) Proc . LPS 13th, in JGR, 88, A912.  Romanek C. S. et al. (1994) Nature, 372, 655.  Romanek C. S. et al. (1994) GSA Abstr. with Progr., 26, A-424.  Carr R. H. et al. (1985) Nature, 314, 248.  Nier A. O. and McElroy M. B. (1977) JGR, 82, 4341.  Bottinga Y. (1968) J. Phys. Chem., 72, 800.  Chacko T. et al. (1991) GCA, 55, 2867.  Romanek C. S. et al. (1992) GCA, 56, 419.  Clark I. D. and Lauriol B. (1992) Chem. Geol., 102, 217.  Nakai N. et al. (1975) Geochem. J., 9, 7.