Paleoclimatic implications of stable oxygen and carbon isotopes of lacustrine and pedogenic calcite from the Gila Conglomerate (Plio-Pleistocene), Mangas Basin, west-central New Mexico

A BSTRACT — Stable carbon and oxygen isotopes of lacustrine (ancient lake Buckhorn) and pedogenic calcite of the Gila Formation (Plio-Pleistocene) in the Mangas Basin, west-central New Mexico, provide information about paleoclimate when compared to similar data from coeval strata in the southern Rio Grande rift near Las Cruces and in the Basin and Range of southeastern Arizona, as well as to modern calcite predicted to precipitate at the sites of the ancient strata. The pedogenic and lacustrine δ 18 O values from the ancient strata are more positive than those calculated for corresponding modern sites, implying the existence of air masses whose precipitation was enriched in 18 O and/or greater evaporation of lake and soil water during Plio-Pleistocene time compared to today. Although δ 18 O values of pedogenic calcite are similar among the ancient strata, the δ 13 C values of the Gila Formation are more negative, suggesting a higher proportion of C 3 plants in the Mangas Basin commen-surate with cooler and wetter conditions at higher elevation. Differences in δ 13 C values of pedogenic calcite within the Mangas basin may have resulted from higher water table and concomitant higher percentage of C 3 plants on the river floodplain than on the alluvial fans. The δ 18 O and δ 13 C values of lake-center ooid limestones of the Gila Formation are more positive than those of the other ancient strata, which could have occurred if lake Buckhorn received precipitation enriched in 18 O, had higher rates of evaporation, and/or precipitated limestones during times of diatom and/or algal blooms. In contrast, lake-margin ooid and ostracode limestones of the Gila Formation have significantly lower values of δ 18 O and δ 13 C compared to the other ancient sites and to the lake-center limestones of the Gila Formation. The low δ 18 O values imply that lake-margin sites may have been influ- enced by water from high-elevation catchments and/or by geothermal waters entering lake Buckhorn via a buried fault, while precipitation of lacustrine calcite during periods of low biological productivity could have produced the low δ 13 C values.