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 O-18 values, and to a lesser extent delta C-13 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 O-18 values of pedogenic carbonates tended to decrease with decreasing age 25-17 radiocarbon ka, and increased with decreasing age <15 radiocarbon ka, with a marked shift to more positive values at 9-6 ka. Pedogenic carbonates from the piedmont soils showed minimal variation in C-13 values with either depth or age, although the oldest palaeosol piedmont carbonates (radiocarbon ages of 24.5-19.2 ka) are greater than or equal to1 parts per thousand more negative than the younger piedmont palaeosol carbonates. Pedogenic carbonates in the relict fluvial soil, which has by far the longest exposure period, showed the greatest range in delta C-13 values. Comparing the delta C-13 values of samples of similar radiocarbon age from the relict fluvial soil indicates a transition from a mixed C-3:C-4 ecosystem at similar to 11 ka to an effectively pure C-3 ecosystem at similar to9 ka, followed by a return to mixed vegetation by similar to5 ka, Thus, although C-4 vegetation persisted in southern New Mexico through the last glaciation, drought-resistant C-3 vegetation became dominant at similar to9 ka, prior to significant strengthening of the summer monsoon. This study illustrates that the stable isotope values of carbonate nodules and clusters from relict soils and/or slowly buried palaeosols can be used for palaeoenvironmental reconstructions in the context of a detailed chronologic framework. (C) 2001 Elsevier Science B.V. All rights reserved.