Utah's ancient Lake Bonneville holds clues to the West's changing climate

  • An ancient shoreline left by Lake Bonneville 16,000 years ago lies between the northwest slopes of the Silver Island Range, in the foreground, and Pilot Range.

    Douglas Fox
  • Grad student Tyler Huth inspects a sample with University of Texas archaeologist David Madsen.

    Ed Kosmicki
  • University of Minnesota geologist David McGee and University of Texas archaeologist David Madsen study a sample found in Cathedral Cave.

    Ed Kosmicki
  • Geologist Jay Quade in Utah's Silver Island Range. Behind him is the basin that held Lake Bonneville 14,000 years ago.

    Douglas Fox
  • The Bonneville Basin, viewed from the entrance of one of the caves geologists are studying.

    Kevin Jones
  • Mineral "bathtub rings" -- consisting of aragonite and calcite -- formed deep inside Cathedral Cave.

    Ed Kosmicki
  • David McGee, left, and Jay Quade, sitting near a shoreline around 600 feet above the valley floor, examine a mineral crust fragment left behind by Lake Bonneville on the slopes of the Silver Island Range.

    Douglas Fox
  • These renderings by L.A. Ramsey show Lake Bonneville as it might have looked thousands of years ago.

    Utah geological survey
  • Jay Quade jots down notes about silt layers buried in the floor of Cathedral Cave. The vials hold snail shells, fish bones, and other samples for radiocarbon dating.

    Ed Kosmicki
  • The University of Minnesota’s David McGee gazes toward the entrance of Cathedral Cave, which once sat as far as 750 feet below the surface of Lake Bonneville. Lightcolored tufa stone coats the inside of the cave. It was laid down thousands of years ago by algae that grew in sunlight filtering in through the then-submerged

    Ed Kosmicki
  • The entrance to a much smaller cave, dubbed the “doghouse” cave, that yielded some of the best rock samples of the trip.

    Douglas Fox
 

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Europeans probably noticed Bonneville's high-and-dry shores the first time they visited the Great Basin, but it has taken 200 years for people to accept the idea of an inland sea.

In 1853, Edward Griffin Beckwith, a U.S. Army lieutenant, led an expedition to Utah to map routes for the transcontinental railroad. The dry shoreline, he wrote, "attracted the observation of even the least informed teamsters of our party -- to whom it appeared artificial."

Beckwith theorized that an arm of the Pacific Ocean had once reached deep into North America to form those shores. It was a radical idea. The bottom of the lake sat 4,000 feet above sea level -- a fact that Beckwith was well aware of -- so the continent would have had to have risen nearly a mile to reach its present elevation. But to Beckwith that probably seemed less outlandish than the only other alternative -- the possibility that the Utah desert once received so much rain and snow that it drowned in 900 feet of water.

Grove Karl Gilbert, a scientist with the U.S. Geological Survey, dispelled the idea that the Pacific had invaded Utah. Gilbert spent most of the 1870s exploring the Great Basin, and his men mapped 500 miles of Bonneville's shores using surveying telescopes. They found multiple concentric rings of shorelines up and down the mountains, formed as the lake rose and fell. They showed that Bonneville was hemmed in by mountains on all sides -- except at one point. Thirty miles north of the Idaho border, they found a spot where Bonneville had poured through a mountain saddle into Idaho's Snake River Valley -- and ultimately into the Pacific. The lake overflowed at this point for around 500 years. But then a catastrophe occurred 18,000 years ago. The waters pouring through this mountain saddle eroded their way through rock into a soft layer of gravel below. Lake Bonneville burst its geologic waistband.

Within days, the water chewed a channel 350 feet deep in the soft, underlying strata -- unleashing a flood 40 times greater than the flow of the Mississippi River. Over 1,200 cubic miles of water gushed out over the next few weeks, overflowing the Snake River Canyon, blasting soil from bedrock and rolling boulders -- some the size of automobiles -- for 200 miles downstream. Bonneville's water level dropped by 350 feet, to the depth of the newly cut channel. The lake stayed at that level for another 2,000 or so years before slowly receding another 550 feet -- down to the level of the modern Great Salt Lake.

Jack Oviatt, a geologist at Kansas State University, has spent 35 years carbon-dating snail shells, driftwood and algal tufa rocks at the lake's various shorelines to assemble the timeline of these ups and downs. That history provides a good framework, but refining it further will help scientists do a better job of reconstructing the climate. "We know the general picture, but we don't know the rates of change between these lake stands," says David McGee, a geologist post-doc from the University of Minnesota who is studying Bonneville along with Quade and several others. "We don't know the responses of the lake to big, sudden climate shifts."

Even as the great ice sheets started retreating around 25,000 years ago, temperatures in the Northern Hemisphere went through a series of dramatic swings: rapid cooling during so-called Heinrich Events, when ice sheets dropped armadas of icebergs into the North Atlantic and caused it to cool, followed by heat waves when temperatures in Greenland soared by as much as 15 degrees Fahrenheit over just a few decades. Knowing how Bonneville and other ancient lakes responded to these swings would enable scientists to build climate models with a better chance of predicting what will happen to rain and snowfall in the West over the next 100 years.

Unfortunately, Bonneville's shorelines provide only fragmentary evidence of the lake's status at specific points in time. What Broecker has long sought is a continuous record of its changes. Tree rings provide this kind of smooth record, but they generally don't go back further than several thousand years -- whereas the most interesting parts of Bonneville's history happened in the more distant past. In 2007, Quade, a longtime friend of Broecker's, found the solution in an unlikely place.

That fall, Quade was busy moving his research lab from one building to another. As he sorted boxes, he came across a canvas sack inside a rat-gnawed wooden chest. Inside the sack, he found several hunks of dingy yellow-white layered crystalline stone in Ziploc bags. Quade had pried them from the walls of a cave in northern Utah in 1994. Back then, they were a mere curiosity -- the kind of pure carbonate crystals that only seem to grow in dark nooks and crannies. Quade brought them home, stashed them in the trunk, and forgot about them. But by the time he rediscovered them in 2007, they had become valuable clues for deciphering Bonneville's history and the story of how water in the West responded to temperature changes in the past.

Those rock layers were laid down as Lake Bonneville rose and flooded the cave. The calcium, magnesium, iron, and carbonate in the water gradually precipitated out of it and coalesced into crystals -- much as mineral deposits form bathtub rings. The layers represented thousands of years of history compressed into six inches. New methods of analysis would allow that history to be read at an accuracy of 30 to 50 years -- 10 times better than would have been possible with standard radiocarbon dating. If Quade could find those same mineral bathtub rings in other caves around Utah, then he could build the most detailed account yet of how Bonneville contracted and expanded as temperatures rose and fell.

Broecker helped Quade snag funding for the project from the National Science Foundation. Quade teamed up with David Madsen, the archaeologist from the University of Texas at Austin who first brought him to the cave where he found the minerals in 1994. He also connected with Broecker's former Ph.D. student, McGee.

The group visited the Silver Island Range this spring in search of more bathtub rings. They spent nights in Wendover, a town on the Nevada-Utah border with a split personality, where a 50 foot-tall cowboy in flashing red lights welcomes you to the gambling-and-massage side of town. Each morning, they drove out to the mountains past the Bonneville Salt Flats, where rocket-propelled automobiles have blasted out numerous land-speed records.

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