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US deserts wet until 8,200 years ago

Research suggests that around 8,200 years ago, the climate of the American West began transitioning from a lush landscape to the desert terrain we know today.

Death Valley. Near Lone Pine, California. Photo credit: Loïc Lagarde

Death Valley. Near Lone Pine, California. Photo credit: Loïc Lagarde

New research suggests that a desert region in the western U.S. – including Nevada, Utah, Oregon, and parts of California — was a rather damp setting until approximately 8,200 years ago, when the region began to dry out, eventually assuming the arid environments we see today. The study was published in the journal Quaternary Science Reviews in June, 2015.

All around the deserts of Utah, Nevada, southern Oregon, and eastern California, ancient shorelines line the hillsides above dry valley floors, like bathtub rings, remnants of the lakes once found throughout the region. Even as the ice sheets retreated at the end of the last ice age, 12,000 years ago, the region remained much wetter than it is today. The earliest settlers of the region are likely to have encountered a verdant landscape of springs and wetlands.

When and why did today’s desert West dry out?

The research team identified the climatic turning point after analyzing stalagmites from a cave in Great Basin National Park in Nevada. Stalagmites are pillars of deposited cave drippings that form over hundreds of thousands of years, as water slowly seeps down through the ground, and into caves. A stalagmite’s layers are essentially a record of a region’s moisture over time.

Graduate student Elena Steponaitis takes notes while collecting stalagmite and drip water samples in Lehman Caves, Nevada. Photo credit: Christine Y. Chen

Graduate student Elena Steponaitis takes notes while collecting stalagmite and drip water samples in Lehman Caves, Nevada. Photo credit: Christine Y. Chen

The researchers used a dating technique to determine the ages of certain layers within two stalagmites, then analyzed these layers for chemical signatures of moisture. They dated stalagmite layers ranging from 4,000 to 16,000 years old, observing that moisture content appears to drop dramatically in samples that are less than 8,200 years old.

Researcher David McGee, Assistant Professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says the results suggest that around 8,200 years ago, the climate of the American West began transitioning from a lush landscape to the desert terrain that we know today.

On a geological timescale, McGee says the region’s moisture content appears to have dropped rather suddenly — “like falling off a shelf,” he says. This steep drop likely had a dramatic impact on humans living in the region. McGee said:

Based upon these data, I would hypothesize that you should see some pretty big changes in how people were living just before and right after 8,000 years ago. What sort of game were they hunting, what plants were they eating, and where were they choosing to live? Montana’s going to start looking pretty good if the Great Basin is drying out.

What that significant drying event might be is up for debate, although McGee hazards a guess.

One of the big things that was happening at this time worldwide was the collapse of the last vestiges of this big ice sheet in Canada. An ice sheet is thought to have important effects on where the jet stream goes. By having this ice sheet here, it made it so the jet stream was more likely to bring storms into the American West, and when it collapsed, the region became more like it is today.

The team found that lake records from Nevada, Utah, Oregon, and eastern California suggest a similar drying-out period. McGee said:

Further work will help us figure out exactly what that fingerprint is.

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Bottom line: New researcher, published in Quaternary Science Reviews in June, 2015, suggests that a desert region in the western U.S. – including Nevada, Utah, Oregon, and parts of California — was a rather damp setting until approximately 8,200 years ago, when the region began to dry out, eventually assuming the arid environments we see today.

Read more about the study from MIT

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