This project will investigate the timing, causes, and consequences of glaciation in an extremely arid subtropical landscape on the eastern boundary of the Atacama Desert. Results of a pilot study demonstrate the past existence of an ice cap covering over 200 square kilometers on the spectacular Chajnantor Plateau in the high, subtropical Andes of Northern Chile, where far too little snow falls to form glacial ice today. These new results raise several intriguing questions: How extensively glaciated was this arid part of the high Andes? How does the timing of the glaciation compare with other records of aridity and moistening from the region? What changes in precipitation and atmospheric circulation were required to develop and maintain these glaciers? What is the relative importance of glacial, periglacial, and glacially-related fluvial erosion processes in landscape evolution in arid settings? Addressing these questions is the goal of this project, which will proceed along three main fronts: 1) mapping and dating of glacial landforms, 2) numerical glacier modeling, and 3) climate modeling. The major component of the research is to establish a rigorous geomorphologic chronology for the Chajnantor Plateau and immediately adjacent areas of the Andes, Altiplano and Atacama Desert, specifically including the Tatio Geysers area about 70 km to the north. The brunt of the work will focus on glacial deposits and related features. The rocks of the area permit reliable measurement of in-situ produced cosmogenic isotopes such as 10Be, 26Al, and 36Cl. This allows dating of moraines, erratics, and glacially polished bedrock to describe not only periods of maximum ice expansion but also more detailed histories of glacial retreat. The primary purpose of the glacier modeling will be to estimate the history of equilibrium line altitude and glacier mass balance as a paleoclimate proxy. In particular, it will be used to evaluate how much more precipitation is needed to sustain large ice bodies in the field area. This, in turn, will guide atmospheric modeling efforts by establishing a criterion that must be met by any proposed climate change mechanism. Atmospheric model experiments and diagnostics will be used to test climatological hypotheses that could explain the required precipitation increases in the subtropical Chilean Andes during glacial times. High-resolution regional models will be run to encompass approximately 30 years in both a modern control run and in an LGM scenario. Broader significance.
The history of glaciation and the climate mechanisms that modulate it are relatively well documented at high latitudes, but tropical and subtropical glacial histories are less robust. The subtropical Andes between 18 degrees S and 27 degrees S are presently unglaciated, even on mountains higher than 6 km, but show evidence of past glacial activity. The mountains in this region are easily cold enough to sustain glaciers today, but simply receive far too little snowfall. This project will establish when glaciers were last present and the amount of additional snowfall needed at that time to sustain them. This information is an important clue to understanding what was different about atmospheric circulation, i.e., climate, at that time to bring more moisture to the region. Therefore, this study will provide an important constraint on models of wetting and drying in the subtropics as a result of specific changes to climate. In other words, understanding the history of climate in the dry parts of the Andes contributes to predicting the impacts of climate change on water resource variability in arid subtropical areas. Additionally, the history of human occupation of the study area and the adjacent Atacama Desert are tied to the history of water in the region, which this study will help to resolve. This project is both international and cross-disciplinary in scope. It will establish additional scientific uses of the Chilean "science preserve" of the Chajnantor Plateau, where many astronomical observatories operate. It will also establish a working relationship with geoscientists at Chilean universities. Moreover, the papers and presentations resulting from this project will create a formal link in the literature between atmospheric sciences and geomorphology.
This project is supported by the Geomorphology and Land Use Dynamics Program and the Office of International Science and Engineering.
