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.
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.
Estimates on when the Sierra Nevada began its major uplift vary widely. Some
volcanic and structural studies place the date between 8 and 5 million years
ago, while cave and other geomorphologic studies support very high elevations
for the Sierra Nevada (currently over 14,000 feet at its highest) millions of
years before then.
The latest study takes advantage of what's 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. One way to track this effect millions of years later
is through isotope ratios. 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 teeth enamel, which scientists can then link to mountain elevations.
Thus, graduate student Brooke Crowley of the University of California at Santa
Cruz sampled the tooth enamel — which is less likely to alter than bone,
she says — of fossil mammals that lived in the Sierra Nevada mountain range,
as well as to the east and west of the mountains. She eventually focused on
fossil horse specimens in the University of California at Berkeley collection
because horses drink water regularly and were more common in the fossil record
than some other large mammals.
Crowley 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
(and its one-sided concentration of heavy isotopes) by at least 18 million years
ago, Crowley told an audience at the Society of Vertebrate Paleontology annual
meeting in Denver, Colo., last 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." The added benefit of using
horses, MacFadden says, is that they have evolved into several different forms
over time, and thus their fossils provide benchmarks for stratigraphy. The results,
he says, rely on the assumption "that weather systems are similar to today."
Indeed, the Nevada region may have had different water sources in the past,
Crowley says, and several other variables also could throw off her results,
including glacial cycles and past migration patterns. She hopes to test teeth
from horses living today for comparison, and will conduct other analyses of
her data before she publishes her results.
Naomi Lubick
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