Morphology and isotope heterogeneity of Late Quaternary pedogenic carbonates: Implications for paleosol carbonates as paleoenvironmental proxies

Stable isotope values and radiocarbon ages were determined for pedogenic carbonate microsamples from relict soils and palaeosols from the Rio Grande Rift region of New Mexico. Carbonate nodules and clusters were sampled from fluvial and piedmont soils and palaeosols with widely varying exposure durations (7-900 ky). Pedogenic carbonates from individual soils exhibited irregular relationships between age and depth, ranging in radiocarbon age by less than or equal to 13 ky at individual depths. delta super(18)O values, and to a lesser extent delta super(13)C values, likewise had wide ranges within soils. However, comparing the stable isotope values of carbonates with similar radiocarbon ages, within and between soils, revealed consistent trends in stable isotope values over time. delta super(18)O values of pedogenic carbonates tended to decrease with decreasing age 25-17 radiocarbon ka, and increased with decreasing age