The Sierra Nevada Mountains in California have been extremely reticent about their age, fuelling an ongoing controversy among earth scientists. Now, clues from ancient raindrops are suggesting the range attained its grand stature more than 50 million years ago, far earlier than some researchers thought.
The age of the Sierra has been hotly debated for over a decade. One camp believes the mountains were lifted up between 60 million and 40 million years ago by an ocean plate that slid beneath the continent and pushed it up. But another group argues that the range grew up between 5 million and 3 million years ago after a large chunk of crust broke off the bottom of the continent. This would have allowed hot buoyant material to rise from the molten mantle and jack up the crust. To know for sure when the mountains were rising, geologists must be able to reconstruct the altitude of rocks at various times in their histories. But current methods--such as comparing fossil leaves with modern plants at particular altitudes--aren't reliable enough to settle the debate.
Now, geochemist Andreas Mulch and colleagues at Stanford University have tried a new approach that could prove to be far more accurate. The team analyzed raindrops that had become incorporated into clay minerals in prehistoric Sierra river sediments. The key here is that raindrops have different ratios of hydrogen isotopes depending on the altitude of the cloud when they fell. Drops containing heavier hydrogen isotopes such as deuterium tend to fall when a cloud first encounters a mountain range. As the cloud climbs the mountain, it continually drops its heaviest rain, so the ratio of deuterium to lighter isotopes gets smaller at higher elevations. The isotopes in 50-million-year-old clay revealed that the Sierra's elevation back then was similar to that of the range today, the team reports in the 7 July issue of Science.
The new technique is promising, says earth scientist Craig Jones of the University of Colorado at Boulder, but he still has some reservations. The lofty height of the ancient range doesn't fit with other clues in the sediments, Jones says. Tall mountains have fast-flowing rivers, but some of the old Sierran river deposits contain fine-grained sand, which is normally found in slow rivers moving through flatter terrain, he says. Stanford team-member Steve Graham counters that the sand was likely deposited where the rivers flattened out briefly by following the contour of the mountain before heading back down the steep slope. The team hopes to apply the same technique to other western ranges whose histories have been equally hard to unravel.
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