Systematic variations in the carbon and oxygen isotopic composition of pedogenic carbonate along elevation transects in the southern Great Basin, United States
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. δ 13 C 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 CO 2 downprofile. Pedogenic carbonate at the soil-air interface approaches a δ 13 C value in equilibrium with atmospheric CO 2 in all three soils. Observed δ 13 C profiles for pedogenic carbonate can be described using a one-dimensional model for 12 CO 2 and 13 CO 2 , assuming isotopic equilibrium between soil CO 2 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. δ 13 C in deep pedogenic carbonate (>50 cm) varies by about 12 per mil over a 2,440-m elevation range, being enriched in 13 C at the lowest elevations. The slope of δ 13 C 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 HCO 3 - in solution and soil CO 2 prior to carbonate precipitation. Soil CO 2 and soil-respiration rates increase systematically with elevation. The plantderived component of soil CO 2 indicates that C 3 plants dominate the biomass at most measured sites, in agreement with plant surveys. Calculated equilibrium fractionation factors between soil CO 2 and soil carbonate are very similar to those observed, again indicating complete isotopic exchange between carbon species. In all, the soil CO 2 and soil-carbonate data suggest that the δ 13 C 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 C 3 to C 4 and CAM plants in each site9s surface biomass. δ 18 O values in pedogenic carbonates are higher at lower elevations, due in part to the more positive δ 18 O values for meteoric waters at lower elevations. The average δ 18 O 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 δ 18 O decrease in soil carbonate with depth observed in two of three soil profiles.