Turns out, the Bonneville Salt Flats have nothing to do with ancient Lake Bonneville

The longstanding theory of how Utah’s famed Bonneville Salt Flats formed is quickly dissolving.

For decades, pretty much everyone assumed the photogenic, salty expanse west of the Great Salt Lake was the remnant of the ancient Lake Bonneville. That massive inland sea once covered 20,000 square miles but disappeared 13,000 years ago. As its remaining water dried and evaporated, it left the salt pan behind — at least, that’s what many believed.

But recent radiocarbon analysis shows the salt flats are much younger than their namesake sprawling Ice Age lake.

“The plot thickens,” said Brenda Bowen, a geology professor at the University of Utah who has studied the salt flats’ changing landscape since 2013. “Our new research shows, by looking at the sediments that are there, that there’s a different history than we expected.”

The findings could throw a wrench into how the flats are managed moving forward. Their rapid rate of decline has revved up calls to action from the racing community.

“This is a system that formed in probably a different climate ... [that was] cooler and moister,” said Jeremiah Bernau, a recent Ph.D. graduate from the U. who led the study. “And now we see our climate shifting to be [warmer] and more arid. That’s creating an uphill challenge in preserving this landscape.”

The Bonneville Salt Flats are home to yearly land speed events, where racers have broken the sound barrier multiple times. In the last 30 years, however, the salt flats have lost a third of their volume and shrunk from 50 square miles to 35 square miles. The U. scientists’ previous research found declining groundwater mostly caused by pumping and other human disturbances was to blame.

The aquifer is so over tapped that groundwater is flowing away from the flats, carrying the salt away with it.

And a decadeslong effort to flood the flats in an attempt to get the crust to regrow is probably doing more harm than good.

“You need to have a holistic view of the system,” Bernau said. “Even if you’re putting water on top of the surface, it’s not doing you much good if it’s flowing out of your system underground.”

The U. researchers’ latest study, published this month in the journal Quaternary Research, took salt cores drilled at the flats and carbon-dated pollen trapped inside them. The results show the crust began growing between 5,400 and 3,500 years ago — millennia after Lake Bonneville evaporated away.

“This landscape is dynamic and changing,” Bowen said. “It shows us the conditions that led to accumulation and preservation of salt on the surface are pretty special.”

Bowen and Bernau dated sediment beneath the flats as well, using small fossilized crustaceans contained in the cores. They found those layers predate Lake Bonneville by 20,000 years or more — which means the ancient lakebed dried up and blew away before the salt flats formed.

“When the surface was being eroded,” Bernau said, “the landscape was out of equilibrium with climate.”

The findings offer “parallels with what might happen to the Great Salt Lake,” he added, which also finds itself out of balance with climate change and human water demands.

So, if the Bonneville Lake theory doesn’t pencil out, where did the salt flats come from?

“Typically, for a saline pan to form, you need groundwater levels that are close to the surface,” Bernau said, “as well as water coming in and evaporating at the surface.”

In other words, conditions need to be wet enough so surface water flowing from the surrounding mountains can accumulate, and then evaporate away, leaving behind salts and minerals that become a crust over time.

That might run counterintuitive to residents of the arid West, who associate salt formation with a dry environment, Bowen said.

“Actually, you need water to pull it all together,” she said. “It required the climate to get a little bit wetter to have enough water to concentrate those salts and bring standing water that allows the salt to grow.”

It also indicates the Bonneville Salt Flats are an ephemeral feature of the Great Basin, she said. Their disappearance may be inevitable.

“It shows just how rare and precious this unique space is,” Bowen said. “[We] need to keep being curious, and probe into what’s happened in human timescales and longer geological timescales. There are new stories to tell and be discovered.”

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