DERIVATION OF PALEOCLIMATE AND PALEOECOSYSTEM DYNAMICS FROM PEDOGENIC CARBONATES : THE EFFECT OF OVERPRINTING ON CARBONATE STABLE ISOTOPE COMPOSITIONS

The stable-isotope composition of pedogenic carbonates has been increasingly applied to reconstructions of regional paleoprecipitation and paleoatmospheric circulation patterns, spatial and temporal evolution of paleofloral communities, and paleoatmospheric pCO₂ and CO₂ d¹³C values. Despite the potential of pedogenic carbonates as quantitative paleoenvironmental and paleoclimate proxies, the mechanism and timing of their formation, and the influence of these on their stable-isotope compositions, remain poorly documented. Petrographic and stable (d¹³C and d¹⁸0) and radiogenic (¹⁴C) isotope study of pedogenic carbonates from late Quaternary soils in the Rio Grande Rift region of New Mexico documents that carbonate formation in Aridisols occurs principally by overprinting and can result in a complex isotopic record that "time averages" changing climatic, paleoenvironmental, and paleoecologic conditions. With increasing duration of exposure (< 7 to 900 ky) of profiles, pedogenic carbonates exhibit increasing intraprofile, intermorphologic and intramorphologic d¹³C and d¹⁸O heterogeneity, commonly in samples distributed laterally within narrow depth intervals. Interpretation of the carbonate d¹³C and d¹⁸O values within a ¹⁴C chronologic framework provides a record of shifts in C₃:C₄ plant ratios and regional atmospheric circulation in the Rio Grande Rift region during the latest Pleistocene through Holocene that compares well with independently derived climate records. Shifts in paleoclimatic conditions, however, may be recorded by the stable-isotope compositions of carbonates distributed within narrow depth intervals of a given soil or even from within individual carbonate samples. Other implications of this study for the use of pedogenic carbonate isotope records as paleoclimate proxies are that carbonates may record only a fraction of the duration of development of their host profile, and considerable uncertainty exists with using the sequence of morphologic development of carbonate as a function of time or maturity in soils.