"The plateau's Zhada basin is really high right now but we think it was one km or more higher just three-four million years ago," said Katharine Huntington, associate professor of earth and space sciences at the University of Washington.
The Zhada basin has rugged terrain with exposed deposits of ancient lake and river sediments that make fossil shells of gastropods such as snails easily accessible, and determining their age is relatively straightforward.
To understand this, researchers studied shells dating from millions of years ago and from a variety of aquatic environments.
They also collected modern shell and water samples from a variety of environments for comparison.
"The work confirms results of a previous study that examined the ratio of heavy isotope oxygen-18 to light isotope oxygen-16 in ancient snail shells from the Zhada basin," Huntington said.
Oxygen-18 levels decrease in precipitation at higher elevations in comparison with oxygen-16, so shells formed in lakes and rivers that collect precipitation at higher elevations should have a lower heavy-to-light oxygen ratio.
The team also employed a technique called clumped isotope thermometry to determine the temperature of shell growth and get an independent estimate of elevation change in the basin.
They found the ratios were very low, which suggested the basin had a higher elevation in the past.
The paper was published online in the journal Geological Society of America Bulletin.