ISOTOPIC STUDY OF ENVIRONMENTAL-CHANGE FROM DISSEMINATED CARBONATE INPOLYGENETIC SOILS

We evaluated the extent to which delta(13)C and delta(18)O values of d isseminated carbonate preserve a signal of Holocene and modern vegetat ion changes in polygenetic soils of the Jornada Basin, New Mexico. Fac tors and processes influencing carbonate chemistry were constrained in a relict grassland community and recent mesquite [Prosopis glandulosa var. torreyana (L. Benson) M. Johnston] dunes, using C-14 dates and d elta(13)C data of soil organic matter (SOM) as independent measures of site history. Total-profile carbonate contents ranged from 39 to 451 kg m(-2) due to local patterns of soil water infiltration and erosion. In grassland soils, C-14 ages and delta(13)C values of disseminated c arbonate in A and B horizons were influenced by the presence of older detrital carbonate, reworked from surrounding surfaces. As a result, c arbonate isotopes did not record mid-Holocene climate and vegetation c hanges, which were inferred from delta(13)C values of coexisting SOM. Shrub expansion during the past century was recorded by changes in the delta(13)C values of disseminated carbonate and SOM from mesquite dun es. Carbon-14 data and mass balance estimates indicated that at least 80% of the dune carbonate (in C horizons) has been isotopically altere d by dissolution and recrystallization processes in <100 yr. The moder n carbonate is apparently forming from soil waters composed of nearly equal amounts of summer and winter rainfall, as evidenced by carbonate delta(18)O values. Our analyses indicate that disseminated carbonate can provide a meaningful environmental signal in polygenetic soils, wh en pedogenic contributions to isotopic variability are constrained.