Quantifying the Cenozoic Oxygen Isotopic Variability of Precipitation on the Andes: A Test of Stable Isotope Paleoaltimetry and Plateau Uplift

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

The formation of the Andean Plateau is not well understood partly because the timing and rates of plateau uplift are poorly constrained. Estimates based on paleoclimate and thermochronometer data are consistent with formation of a plateau of modern width by about 13 million years ago. In contrast estimates based on stable isotope paleoaltimetry, a relatively new technique that employs the oxygen isotope concentration (delta-18O) of ancient soil carbonates to infer past elevation change, suggest a rapid uplift about 10 to 7 million years ago. Isotope paleoaltimetry is a promising new tool for quantifying elevation change, but several outstanding issues may complicate its interpretation. In particular, changes over the last 40 million years in surface temperature, atmospheric circulation, precipitation rate, and vapor source may affect the oxygen isotopic concentration of ancient soil carbonates. If substantial, these effects could compromise paleoelevation inferences based on the isotopes The objective of this project is to evaluate the processes that control delta-18O of precipitation in the Andes and to test the hypothesis that Andean Plateau uplift was in fact steady over the last 20 million years. The apparent rapid rise of the plateau in the late Miocene may be an artifact of changes in Cenozoic climate and atmospheric circulation that caused depletion of Andean precipitation 18O. Global and regional climate models with the capability to predict oxygen and deuterium isotope transport and fractionation provide a tool for quantifying these effects over the Andean Plateau. However, before these models can be used with any confidence, they must be validated against modern observations of oxygen isotopes in precipitation, which are currently sparsely available in the Andes. In this project, a multi-year sampling campaign across central Peru and southern Bolivia measures monthly variations in the oxygen isotope composition of precipitation. These data, when integrated with existing measurements, meteorological data, and backtracking analyses will provide critical data for assessing climate model predictions of precipitation oxygen isotopic composition and evaluate whether a Rayleigh law adequately predicts isotope fractionation on the Andes. The validated regional and global climate models are used to predict past changes in Andean delta-18O composition and to assess the influence of past climate changes on precipitation delta-18O used in soil carbonates for paleoaltimetry studies. The Andean Plateau is one of the most dramatic topographic features on Earth. Despite its impressive nature, the mechanisms and rates of its formation remain poorly understood. A range of competing geodynamic models have been proposed to explain the formation of plateaus. To distinguish between these models it is necessary to understand the deformation and erosion history associated with the plateau and its marginal thrust belts, the present day crustal and lithospheric structure, and the paleoelevation history of the plateau. Of these, current understanding of Andean paleoelevation is arguably the most uncertain, and yet offers great insights into the formation of the plateau. The paleoelevation history can be used to make inferences about the rate and timing of mountain building, and ultimately the geodynamic processes that govern mountain building. The results of this project will provide an important evaluation of the isotope paleoaltimetry technique and provide improved calculations of the surface uplift history of the Andean Plateau.