STABLE OXYGEN AND CARBON ISOTOPES OF PEDOGENIC CARBONATE AS INDICATORS OF PLIOPLEISTOCENE PALEOCLIMATE IN THE SOUTHERN RIO-GRANDE RIFT, SOUTH-CENTRAL NEW-MEXICO

Stable oxygen and carbon isotopes of pedogenic carbonate provide a det ailed record of paleoclimatic changes from late Pliocene through early Pleistocene in the Rio Grande rift of south-central New Mexico. A tot al of 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 Ric e Formation. Paleosols commonly consist of an argillic B horizon (Bt) overlying a calcic (Bk) or petrocalcic (Km) horizon. The majority of p aleosols consist of stage II morphology calcic horizons, although one stage V horizon and five stage III horizons were also sampled. Reversa l magnetostratigraphy at all four sample sites bracket the age of the paleosol-bearing strata between 3.4 and 0.7 Ma and allow estimates of the absolute age of individual paleosols. Three paleoclimatic stages a re 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 O-18 and partial de rivative C-13 and by an increase in values with decreasing age, sugges ting high effective moisture and abundant winter precipitation, which decreased through time, and/or relatively low temperature, which incre ased through time. The second stage (2.5-1.4 Ma) displays an increase in partial derivative O-18 with decreasing age but no significant chan ge in partial derivative C-13 with time, suggesting that the effective moisture was nearly constant, but that temperature and/or summer prec ipitation may have increased through time. The final stage (1.4-0.7 Ma ) shows an overall increase in both partial derivative O-18 and partia l derivative C-13 with decreasing age corresponding to less effective moisture, higher temperature, and/or greater summer precipitation thro ugh time. Plio-Pleistocene paleoclimatic changes in south-central New Mexico correlate well with paleoclimatic data elsewhere in the southwe stern United States and adjacent Great Plains and may have been influe nced by the onset of Northern Hemisphere continental glaciation, rise of the Sierra Nevada and Transverse Ranges, and/or broad regional upli ft of the western United States.