This study will examine the Miocene climate and elevation history of the Andean plateau by combining a new carbonate environmental geothermometer, based on clumping of carbon-12 and oxygen-18 into bonds with each other in the carbonate mineral lattice, with conventional analyses of the stable isotope compositions of soil carbonates, lake carbonates, and fossil mammal teeth. Previous studies in the Andean plateau have estimated paleoelevations based on paleobotanical data, the delta oxygen-18 of sedimentary carbonates, and the new clumped isotope method. However, these studies estimate elevation by assuming a base level climate similar to today, with today's relationships among altitude, mean annual surface temperature, and the delta oxygen-18 of meteoric waters. This assumption is a source of uncertainty unless the temperature or starting composition of the vapor source can be constrained. Part of this study will establish a low elevation climate record from the Subandes against which the paleoelevation records from the Altiplano can be compared. By this approach, a more accurate record of elevation will be extracted by calibrating against secular variations in climate observed in the Subandes. Importantly, this new approach will allow study of the entire plateau, including its arid southern half, which has resisted attempts to determine paleoelevation based on delta oxygen-18 of surface carbonates alone.
With a better resolution of the spatial and temporal variation in the magnitude of surface uplift of the Andes, the broader significance of this study will be an understanding of processes that lead to surface uplift of mountain belts, including thickening of crustal and mantle lithosphere and removal of dense, thickened lower lithosphere. Recent studies of the elevation history of the northern Altiplano suggest that rapid surface uplift ( approximately 3 km in 3 million years) occurred in the late Miocene, postdating major crustal thickening by upper crustal folding and faulting. There is evidence of similar-age surface uplift across the width of the Andes for over greater 5 degrees of latitude, implicating the removal of mantle lithosphere as an important mechanism for surface uplift. However, independent data have only predicted mantle lithosphere removal in the southern Altiplano and Puna. Documentation of the timing and magnitude of surface uplift along strike in the Andes will shed light on the combined roles of these processes as well their regional extent.
Additional broader impacts of this work are to examine the effects of mountain building on regional climate. This project will examine the spatial variations in climate from middle Miocene to recent over different latitudes in the Altiplano, as well as from low to high elevation along the eastern flank of the Andes, to evaluate the potential influence of surface uplift on local climate. A pilot study of the tooth enamel of late Miocene to modern large, water-dependent grazers shows that these animals were ingesting surface water similar in composition to modern rainfall. These data will be extended back to the middle Miocene to evaluate whether the Late Miocene to modern pattern of rainfall resulted from regional surface uplift. In addition, temporal changes in low elevation and high elevation precipitation amount will be evaluated to test the hypothesis that late Miocene surface uplift of the Altiplano forced climate change that was manifested by an increase in low elevation rainfall amount and a simultaneous decrease in high elevation rainfall amount. This study will provide a model for a new and widely applicable approach to reconstructing past climates and paleoelevation through the clumped carbon-12 and oxygen-18 paleothermometry method to reconstruct past temperatures and conventional stable isotope studies to reconstruct patterns of precipitation.
