Timing and Spatial Variability of Neogene Tectonic, Climatic, and Ecologic Changes Along the Southern Puna Plateau, Northwestern Argentina

Tephrostratigraphic studies of Mio/Pliocene strata along the southern margin of the Puna plateau (NW Argentina) provide a framework capable of controlling the absolute timing of tectonic, climatic, and ecologic changes. 40Ar/39Ar ages and geochemical characterization of widespread volcanic ash beds indicate that the rich record of explosive volcanism changed dramatically at 5.5 Ma and maintained high frequency and large volume eruptions through 3.6 Ma. Thick conglomeratic deposits associated with basin margin uplift exhibit time-transgressive progradation beginning before 3.6 Ma. Thick Neogene strata in the area were not folded or uplifted until well after 3.6 Ma. These observations delineate a sequence of increased and sustained volcanism on the Puna, followed by plateau margin uplift, and eventual incorporation of sedimentary basins into growing topography along the plateau margin. Stable isotope values of carbonate from paleosols and fossil tooth enamel provide complementary proxies of climatic and ecologic change. Both fossil Notoungulate and Rodent enamels record the presence of C4 vegetation by 8 Ma, 1 Ma earlier than the paleosol record. At 5.6 Ma some Notoungulate taxa were consuming large proportions of C4 vegetation throughout the year, an observation consistent with the presence of grasslands. A dramatic positive shift in δ13C values of paleosol carbonate after 3.6 Ma is not accompanied by a similar shift in fossil tooth enamel, suggesting that the stratigraphic paleosol record may be recording a shift across ecotone boundaries associated with progradation of syntectonic conglomerates. The range in paleosol δ13C values indicates a mosaic of plant communities during the Pliocene. This is further supported by a bimodal distribution of δ13C values among Pliocene tooth enamel. Modern plants, soil carbonate, and tooth enamel indicate the predominance of water stressed C3 plants. The modern monsoon climate in the region is conducive to proliferation of C4 plants, but many of the study localities are near the upper elevation range of modern C4 plants in Argentina; consequently significant post-3.6 Ma surface uplift is inferred to be partially responsible for the ecological changes observed between the Pliocene and today. The average δ18O values of tooth enamel exhibit consistent enrichment relative to coeval soil carbonate. Synchronously, both proxies record a 1.5‰ Pliocene enrichment in δ18O values over the Miocene and a dramatic 3.5‰ enrichment of modern values over the Pliocene. These proxies track the δ18O value of environmental water and indicate a positive shift of 5‰ from 7 Ma to present. The δ18O values of modern tooth enamel also show a marked increase in the observed range of values (12‰) over the Miocene (8‰) and Pliocene (9‰). This is consistent with an increase in aridity and implies that the shift in δ18O values from the Miocene to present cannot be attributed wholly to a change in the isotopic composition of precipitation. These results are also consistent with the growth of complex topography and surface uplift since 3.6 Ma.