Introduction: Andes Mountain Range
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 question of how quickly the mountains attained such heights has been a contentious one in geological
circles, with some researchers claiming the central Andes rose abruptly to nearly their current height and
others maintaining the uplift was a more gradual process.
Ancient Climate Change and Oxygen Isotopes
New research by U-M paleoclimatologist Christopher Poulsen and colleagues suggests that the quick-rise
view is based on misinterpreted evidence. What some geologists interpret as signs of an abrupt rise are
actually indications of ancient climate change, the researchers say. Their findings were
published online April 1 in Science Express.
The confusion results when ratios of oxygen's two main isotopes, oxygen-18 and oxygen-16, are used to
estimate past elevation, said Poulsen, an associate professor with appointments in the departments of
Geological Sciences and Atmospheric, Oceanic, and Space Sciences.
Interpreting Oxygen Isotope Data
"In the modern climate, there is a well-known inverse relationship between oxygen isotopic values in rain
and elevation," Poulsen said. "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 said. 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.
Other Factors That Influence Oxygen Isotopes
But elevation isn't the only factor that affects oxygen isotope ratios in rain, Poulsen said. "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, he and his 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 said.
"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."
This conclusion is backed up by geochemical and sedimentological data, Poulsen said. "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."
Author Team and Funding
Poulsen's coauthors are Todd Ehlers of the University of Tuebingen in Germany and U-M graduate student Nadja Insel.
Funding was provided by the National Science Foundation, the U-M Graham Environmental Sustainability Institute and the
Alexander von Humboldt Foundation.
| Formed through the process of plate tectonics, the Andes Mountains are located along the western edge of the South American continent. They are a result of a collision between the South American Plate and the Nazca Plate. Some researchers claim that the central Andes rose abruptly to nearly their current height and others maintaining the uplift was a more gradual process. Map by MapResoures and Geology.com. More maps of South America
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