Millions of years ago, the Sierra Nevada, the mountain range that forms the
eastern boundary of California, spiked from a moderate bump to an extreme crest,
now at over 14,000 feet in height. While geologists agree that the Sierra Nevada's
mountain-building events have been going on for long time, the exact timing
of its growth spurt remains controversial. In an effort to pinpoint the escalation,
some geoscientists are going straight to the horse's mouth, literally
using horse teeth to date the mountains' uplift.
The latest study takes advantage of what is known as a "rain shadow": As a cloud moves over a high mountain, it dumps its water as it cools, lifting it up the leeward slope. The first water molecules to drop tend to contain more heavier isotopes, in particular, oxygen-18. Any animals that live on either side of the mountain and drink water will trap the oxygen isotope ratios in their bones and tooth enamel, which scientists can then link to mountain elevations.
Looking for such signatures, graduate student Brooke Crowley of the University of California at Santa Cruz sampled the tooth enamel of fossil horse specimens in the University of California at Berkeleys collection. She found "a gradient between the isotopes from the west and east; they get very light going east and stay light in the east," she says, a pattern that held for the oldest specimens of horses in the collection, which are 18 million years old. Because storms move from west to east across the Sierra Nevada, that means that enough height had been gained to create a rain shadow by at least 18 million years ago, Crowley told an audience at the Society of Vertebrate Paleontology annual meeting in Denver, Colo., in November.
Bruce MacFadden, a paleontologist at the Florida Museum of Natural History in Gainesville, says Crowley used "a very innovative technique that's catching on" and "has a lot of potential."
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