Isotopic Study of Environmental Change from Disseminated Carbonate in Polygenetic Soils

Abstract

Abstract We evaluated the extent to which δ 13 C and δ 18 O 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 14 C dates and δ 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, 14 C ages and δ 13 C 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 δ 13 C values of coexisting SOM. Shrub expansion during the past century was recorded by changes in the δ 13 C 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 δ 18 O values. Our analyses indicate that disseminated carbonate can provide a meaningful environmental signal in polygenetic soils, when pedogenic contributions to isotopic variability are constrained.