Stable oxygen and carbon isotopes of pedogenic carbonate as indicators of Plio-Pleistocene paleoclimate in southern Rio Grande Rift, south-central New Mexico

Abstract

Stable oxygen and carbon isotopes of pedogenic carbonate provide a detailed record of paleoclimatic changes from late Pliocene through early Pleistocene in the Rio Grande rift of south-central New Mexico. 30 calcic paleosols were sampled at three stratigraphic sections of the fluvial lithofacies of the Camp Rice Formation, and one calcic paleosol was sampled from fluvial sediment inset against the Camp Rice Formation. Paleosols commonly consist of an argillic B horizon (Bt) overlying a calcic (Bk) or petrocalcic (Km) horizon. The majority of paleosols consist of state II morphology calcic horizons, although one stage V horizon and five stage III horizons were also sampled. Reversal magnetostratigraphy at all four sample sites bracket the age of the paleosol-bearing strata between 2.4 and 0.7 Ma and allow estimates of the absolute age of individual paleosols. Three paleoclimatic stages are indicated by the carbon and oxygen isotopic data from south-central New Mexico. The initial stage, from 3.1 to 2.5 Ma, was characterized by the overall lowest values of [partial derivative][sup 18]O and [partial derivative][sup 13]C and by an increase in values with decreasing age, suggesting high effective moisture and abundant winter precipitation, which decreased through time, and/or relatively low temperature, which increased throughmore » time. The second stage (2.5-1.4 Ma) displays an increase in [partial derivative][sup 18]O with decreasing age but no significant change in [partial derivative][sup 13]C with time, suggesting that the effective moisture was nearly constant, but that the temperature and/or summer precipitation may have increased through time. The final stage (1.4-0.7 Ma) shows an overall increase in both [partial derivative][sup 18]P and [partial derivative][sup 13]C with decreasing age corresponding to less effective moisture, higher temperature, and/or greater summer precipitation through time. 56 refs., 6 figs., 2 tabs.« less