Collaborative Research: Miocene Climate Change West of the Central Andes: Cause or Consequence of Uplift?

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The primary objective of this work is to document temporal and spatial variations of Miocene (23-5 Million years ago) precipitation along an extensive region (~1000 kilometers) of the western flank of the Central Andes that is today's hyperarid Atacama Desert. Whereas previous data reveal that there was major change in climate over that time interval, the data were insufficient to resolve the magnitude and timing of the regional scale first-order pattern of change. Recently, it was proposed that the dramatic rise of the Central Andes was the consequence of, rather than the cause for, the aridification of the western margin of South America. That hypothesis invokes a fundamental geodynamic relationship between climate and tectonics, coupled through the affect of sediment fill of the trench on the mechanical properties of the plate interface. But competing models that more comprehensively incorporate the dynamics of the plate boundary-Andes system conclude that climate-related phenomena, while intriguing, are not of first-order importance. This project develops a Miocene paleoclimate history that serves as a rigorous test of these competing hypotheses. Paleosols are used in this project as a paleoclimate proxy. Used to estimate paleo-precipitation are a variety of soil morphological and geochemical characteristics, including the chemical composition of saline soil cements (nitrate, sulfate, carbonate), chemical-weathering ratios of argillic horizons, and isotopic values of soil carbonate, all features that are highly correlated with precipitation between 0 and 400 millimeters/year in Chile today. This project builds from a pilot project in the center of the Atacama Desert to a regional-scale description of when precipitation change occurred, which allows one to discern whether the change was simultaneous or diachronous along the western margin of the Central Andes, whether it preceded or followed surface uplift, and whether climate fluctuations occurred multiple times during the Miocene. In addition to testing the geodynamic models, the new knowledge of spatial and temporal patterns of paleoclimate change allow comparison of the western Central Andes climate history to that of the eastern margin of the Central Andes, to test whether the aridification was caused by surface uplift or by global climate change.Does regional-scale climate change cause mountain belts to rise in elevation, or does the rise of mountain belts cause the climate of a continent to change? Resolution of this "chicken-and-egg" question motivates this study of the history of climate and mountain growth in the Atacama Desert of the Andes Mountains of northern Chile. Positive feedback relations between climate and construction of mountain ranges and entire mountain chains are current topics of research in geological sciences. The Atacama Desert affords an unparalleled opportunity to examine the feedback processes at the very dry end of Earth's climate spectrum. The database for ancient precipitation that will be extracted from ancient soil properties is also valuable in documentation of the long-term history of interior South America, inclusive of whether the Amazon basin, with its high degree of biodiversity, evolved in response to global climate change or to climate change induced by uplift of the Andes Mountains. The uncertainties are comparable to those in the debate about whether today's climate changes are caused by natural phenomena or anthropogenic activity. Through participation of U.S. undergraduate and graduate students, the project trains next-generation geologists to study interrelations between soils, vegetation, climate, and topography. Through participation of a Chilean professor and graduate student, the project provides models of and practice in respectful international collaboration for the students, and more efficiently fulfill the project objectives.