Climate change in response to orographic barrier uplift: Paleosol and stable isotope evidence from the late Neogene Santa Maria basin, northwestern Argentina

The late Cenozoic history of the Santa Maria basin in the northeastern Sier ras Pampeanas records the far-reaching climatic consequences of the develop ment of orographic barriers to Atlantic-derived moisture. The evolution of climate and ecosystems is coupled with the uplift of basement blocks west a nd east of the basin to their present elevations of 4000-5500 m, The climat ic and ecological history of the basin for the past 12 m.y. was reconstruct ed from paleosol characteristics and carbon and oxygen isotopes of pedogeni c material. A fluvial environment succeeded the final marine regression aft er ca, 12 Ma. Paleosols with illuvial clay and calcic nodules indicate a se asonally dry climate. Between ca, 9 and 7 Ma, paleosols formed under condit ions of episodic waterlogging, as shown by organic matter preservation and variegated colors, After ca, 7 Ma, pedogenesis occurred under enhanced drai nage due to tectonic steepening of the eastern basin margin. This can be re lated to westward thrusting of the eastern basin-bounding Sierra Aconquija and Cumbres Calchaquies. Calcic and silicic rhizoliths and authigenic clay point to persistent wet-dry seasonality, Carbon isotope values document the spread of C-4 plants. Between ca, 5 and 3 Ma, carbon isotopes show an incr ease in C-3 plants, This is likely a consequence of uplift of the Sierra de Quilmes to the west, resulting in a transitory episode of orographic preci pitation within the Santa Maria basin. Aridification of the valley occurred between ca, 3 and 2.5 Ma and was related to creation of high topography to the east. Development of thick calcretes suggests that the present semiari d renditions were established by the end of Pliocene time. Carbon isotopes show that the Quaternary vegetation was C-4-plant dominated, and oxygen iso topes attest to intense soil-water evaporation.