Assuming Too Much?
Jason Barnes is a geologist at the University of Michigan in Ann Arbor who was not involved in this research but recently completed his doctoral thesis on the deformation and erosion history of the central Andes.
He said the Science study is based on solid data but paints a different picture than the one suggested by his own research and preliminary modeling studies by his colleagues.
The difference, Barnes explained, has to do with interpreting isotope data.
Isotope variations deposited in soil depend largely on regional weather patterns. The major assumption of the Science study is that the modern weather pattern is representative of the climate pattern millions of years ago.
(Related: "Andes' Height Due to Climate, Study Says" [October 22, 2003].)
But "the climate might have actually been significantly different from today," Barnes said.
His colleagues' preliminary modeling studies of the ancient climate suggest that precipitation at times reached the central Andes from the Pacific Ocean side of South America, not the Atlantic side as it does today.
Precipitation from the Pacific, Barnes noted, provides a different isotopic signature. When this difference is factored in, it could suggest "the rise of the Andes was slow and steady rather than rapid and fast."
While he doesn't dismiss Garzione's work, he said "the jury is still out."
Maria Teresa Ramírez-Herrera is a geologist and visiting scholar at the University of California, Berkeley, who is studying the recent rise of bedrock in a section of the southern Andes in Argentina.
(Her research is funded by a grant from the National Geographic Society, which owns National Geographic News.)
She said her data "is suggesting rapid uplift" but cautioned that the results are preliminary and need to be confirmed.
If correct, she added, then globs of dense rock at the base of this section of the Andes could be dropping deeper into the mantle now, allowing the mountains to spring up in the same manner suggested by Garzione.
Other mountain-building processes such as crust thickening, she said, are also likely at play.
Barnes, of the University of Michigan, noted that the delamination theory has gained traction among geologists and is believed to also be occurring in the southern section of California's Sierra Nevada range.
The trick now is to further develop techniques that directly measure mountain elevations at various points in time, he said.
"It's adding a really cool component to our efforts to reconstruct the birth, life span, and death of a mountain range."
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