Mountain ranges are important contributors to the global climate because they influence weather patterns and the flora and wildlife that live on their slopes and in the valleys below. Moisture condenses into rain and snow as warm air rises up windward hills and cools. On the leeward side, it’s quite the opposite.
Deserts prevail, a phenomenon known as rain shadow. Therefore, scientists who research and predict previous climates are quite interested in how mountain ranges arise.
With the publication of a new research in the journal Nature Geoscience, that discussion will soon get even hotter. In order to demonstrate that one of the world’s most well-known mountain ranges, the Himalayas, did not form as experts have long assumed, a team of researchers at the Stanford Doerr School of Sustainability modified a technique used to analyse meteorites to determine ancient elevations in sedimentary rocks.
“The controversy rests mainly in what existed ‘before’ the Himalayas were there,” explains Page Chamberlain, professor of Earth and planetary sciences and of Earth system science at the Doerr School of Sustainability, and senior author of the study. “Our study shows for the first time that the edges of the two tectonic plates were already quite high prior to the collision that created the Himalayas about 3.5 kilometers on average.”
“That’s more than 60% of their present height,” added Daniel Ibarra, Ph.D., a postdoctoral researcher from Chamberlain’s lab, first author of the paper, and now an assistant professor at Brown University. “That’s a lot higher than many thought and this new understanding could reshape theories about past climate and biodiversity.”
Experts have long thought that it takes a massive tectonic collision, on the order of continent-to-continent scale, to produce the sort of uplift required to produce Himalaya-scale elevations. This study disproves that and sends the field in some interesting new directions.
Daniel Ibarra
The discoveries will probably result in new paleoclimatic hypotheses concerning the Gangdese Arc region of Southern Tibet’s Himalayas, at the very least requiring the calibration of old climate models. It could also beget closer scrutiny of other key mountain ranges, such as the Andes and the Sierra Nevada.
Old technique, new insight
It has a lot to do with the difficulties of estimating the topographic elevations of the past, a topic known as paleoaltimetry, which is why this long-running argument is suddenly heated. It is extremely challenging work, the researchers say.
There are not many proxies for altitude in the geologic record, but the Stanford team found one in collaboration with study authors from China University of Geosciences (Beijing).
Rainfall is increased on windward slopes, and as the air rises toward the summits, the chemical makeup of the precipitation also changes. Lighter isotopes tend to be closer to the peaks and heavier isotopes to drop out sooner. As a result, by examining the isotopic composition of the rocks, specialists can identify the telltale marks of the altitude at which they were formed.
In the sedimentary record, oxygen exists in three stable isotopes: oxygen 16, 17, 18. Dauntingly, the key isotope, oxygen 17, is extremely rare. It comprises just 0.04% of the oxygen on Earth. That means, in a sample containing a million atoms of oxygen, just four atoms are oxygen 17.
“There are maybe eight labs in the world that can do this analysis,” said Chamberlain, who helped process samples at the Terrestrial Paleoclimate lab at Stanford. “Still, it took us three years to get numbers that made some sense and that were working every day.”
Tectonic shifts
That explains why triple oxygen analysis had been disregarded as a substitute for ancient altitude, or perhaps it had been dismissed too quickly. But Chamberlain and his colleagues saw an opportunity.
Using a grant from the Heising-Simons Foundation, the team adapted the technique to paleoaltimetry and used the mountains of Sun Valley, Idaho, for a proof-of-concept paper in 2020. With the science established, they then turned their sights higher to the Himalayas.
The scientists demonstrated that the Gangdese Arc’s foundations were far higher than expected, long before any tectonic collision happened, by sampling quartz veins from lower altitudes in southern Tibet and employing triple oxygen analysis.
“Experts have long thought that it takes a massive tectonic collision, on the order of continent-to-continent scale, to produce the sort of uplift required to produce Himalaya-scale elevations,” Ibarra said. “This study disproves that and sends the field in some interesting new directions.”
Contributing authors include Yuan Gao, Jingen Dai, and Chengshan Wang at China University of Geosciences (Beijing). Chamberlain is also a member of Bio-X and an affiliate with the Stanford Woods Institute for the Environment.
