Stable carbon- and oxygen-isotope variations in Holocene soil carbonates that formed in the unsaturated zone were examined along several elevation transects in the southern Great Basin, United States, a region with a semi-arid climate. Our intent was to study the relationship between the stable isotopic composition of pedogenic carbonates and climate, ecological variations, differences in parent material, and soil depth.
δ13C of pedogenic carbonate in three different soil profiles from different elevations decreases with soil depth, indicating a decrease in the ratio of atmospheric to plantderived CO2 downprofile. Pedogenic carbonate at the soil-air interface approaches a δ13C value in equilibrium with atmospheric CO2 in all three soils. Observed δ13C profiles for pedogenic carbonate can be described using a one-dimensional model for 12CO2 and 13CO2, assuming isotopic equilibrium between soil CO2 and soil carbonate. The modeled best fit to observed isotopic profiles suggests that the profile differences in part result from differing soil-respiration rates at each site.
δ13C in deep pedogenic carbonate (>50 cm) varies by about 12 per mil over a 2,440-m elevation range, being enriched in 13C at the lowest elevations. The slope of δ13C for these carbonates versus elevation is very similar for soils developed on carbonate and on noncarbonate parent materials: depletion by 4.6 to 4.7 per mil per 1,000 m increase in altitude between 300 to 2,740 m above mean sea level for the localities studied. This concordance makes it likely that there has been complete isotopic exchange between HCO3- in solution and soil CO2 prior to carbonate precipitation.
Soil CO2 and soil-respiration rates increase systematically with elevation. The plantderived component of soil CO2 indicates that C3 plants dominate the biomass at most measured sites, in agreement with plant surveys. Calculated equilibrium fractionation factors between soil CO2 and soil carbonate are very similar to those observed, again indicating complete isotopic exchange between carbon species. In all, the soil CO2 and soil-carbonate data suggest that the δ13C variation with elevation observed in the soil carbonates results from differing soil-respiration rates at each site, as well as from variations in the proportion of C3 to C4 and CAM plants in each site's surface biomass.
δ18O values in pedogenic carbonates are higher at lower elevations, due in part to the more positive δ18O values for meteoric waters at lower elevations. The average δ18O value of deep (>50 cm) pedogenic carbonate at all sites, however, is enriched 2.4 to 3.7 per mil with respect to values we predict should be in equilibrium with the isotopic composition of local meteoric waters. This suggests that evaporative isotopic enrichment of soil waters may have occurred at all elevations prior to precipitation of carbonate, or that seasonal differences in the isotopic composition of meteoric waters coupled with differential infiltration may be taking place. One or both of these processes also may explain the δ18O decrease in soil carbonate with depth observed in two of three soil profiles.