U. MICHIGAN (US)—What geologists have long viewed as proof of an abrupt rise of the Andes mountain range is actually an indication of ancient climate change, according to a new study.
Trailing like a serpent’s spine along the western coast of South America, the Andes are the world’s longest continental mountain range and the highest range outside Asia, with an average elevation of 13,000 feet.
The quick-rise view is based on misinterpreted evidence, says Christopher Poulsen, associate professor in the departments of Geological Sciences and Atmospheric, Oceanic, and Space Sciences at the University of Michigan.
What some geologists interpret as signs of an abrupt rise are actually indications of ancient climate change, he says.
The confusion results when ratios of oxygen’s two main isotopes, oxygen-18 and oxygen-16, are used to estimate past elevation.
Details of the study were published online April 1 in Science Express.
“In the modern climate, there is a well-known inverse relationship between oxygen isotopic values in rain and elevation,” Poulsen explains.
“As a rain cloud ascends a mountain range, it begins to precipitate. Because oxygen-18 is more massive than oxygen-16, it is preferentially rained out. Thus, as you go up the mountain, the precipitation becomes more and more depleted in oxygen-18, and the ratio of oxygen-18 to oxygen-16 decreases.”
Geologists use the ratio of these isotopes, preserved in rock, to infer past elevations.
“If the ratio decreases with time, as the samples get younger, the interpretation would typically be that there has been an increase in elevation at that location,” Poulsen says.
In fact, that’s exactly the conclusion of a series of papers on the uplift history of the Andes published over the past four years.
Using oxygen isotopes in carbonate rocks, the authors posited that the central Andes rose about 8,200 to 11,500 feet in three million years, rather than gaining height over tens of millions of years, as other geologists believe.
But elevation isn’t the only factor that affects oxygen isotope ratios in rain, Poulsen says. “It can also be affected by where the vapor came from and how much it rained—more intense rainfall also causes oxygen-18 to be preferentially rained out.”
Skeptical of the rapid-rise scenario, Poulsen and colleagues performed climate modeling experiments to address the issue.
“The key result in our modeling study is that we identified an elevation threshold for rainfall,” Poulsen says.
“Once the Andes reached an elevation greater than 70 percent of the current elevation, the precipitation rate abruptly increased. In our model, the increased precipitation also caused the ratio of oxygen-18 to oxygen-16 to significantly decrease.
“Our conclusion, then, is that geologists have misinterpreted the isotopic records in the central Andes. The decrease in the ratio is not recording an abrupt increase in elevation; it is recording an abrupt increase in rainfall.”
Poulsen says the conclusion is backed up by geochemical and sedimentological data. “There is evidence that the central Andes became less arid at the same time that the isotope records show a decrease in the ratio of oxygen-18 to oxygen-16.”
Researchers at the University of Tuebingen in Germany contributed to the study, which was funded by the National Science Foundation, the University of Michigan Graham Environmental Sustainability Institute, and the Alexander von Humboldt Foundation.
University of Michigan news: www.ns.umich.edu/