Isotopic Study of Environmental Change from Disseminated Carbonate in Polygenetic Soils

Abstract

AbstractWe evaluated the extent to which δ13C and δ18O values of disseminated carbonate preserve a signal of Holocene and modern vegetation changes in polygenetic soils of the Jornada Basin, New Mexico. Factors 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 14C dates and δ13C 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, 14C ages and δ13C values of disseminated carbonate in A and B horizons were influenced by the presence of older detrital carbonate, reworked from surrounding surfaces. As a result, carbonate isotopes did not record mid‐Holocene climate and vegetation changes, which were inferred from δ13C values of coexisting SOM. Shrub expansion during the past century was recorded by changes in the δ13C values of disseminated carbonate and SOM from mesquite dunes. Carbon‐14 data and mass balance estimates indicated that at least 80% of the dune carbonate (in C horizons) has been isotopically altered by dissolution and recrystallization processes in <100 yr. The modern carbonate is apparently forming from soil waters composed of nearly equal amounts of summer and winter rainfall, as evidenced by carbonate δ18O values. Our analyses indicate that disseminated carbonate can provide a meaningful environmental signal in polygenetic soils, when pedogenic contributions to isotopic variability are constrained.