Great Salt Lake’s ancient past reveals how quickly fresh water can disappear
Research led by USC Dornsife Earth scientists traces 240,000 years of change, showing that vast freshwater lakes were fleeting as warming and drying reshaped the basin.
University of Southern California
Key findings:
USC Dornsife researchers studying the history of Utah’s Great Salt Lake identified two brief, deep-lake phases when the water was much fresher than now.
Their analysis of core samples spanning the past 240,000 years shows that the most recent freshwater phase, which formed Lake Bonneville, lasted longer and was fresher than the earlier Little Valley lake phase.
The researchers found that both lakes shrank and became salty as the regional climate warmed and dried, offering possible insights into Great Salt Lake’s future in the face of climate change.
From a mountainside overlooking Utah’s Great Salt Lake, a time traveler turning back the clock 20,000 years would watch a dramatic transformation of the landscape.
The shallow, salty lake below would swell into an immense body of fresh water, at one point reaching nearly 1,000 feet deep and covering more than 10 times the lake’s modern area. Mountain ranges would become islands, and the new shoreline would move far beyond its current boundary.
Travel back 120,000 more years and another giant lake would appear.
A new study led by researchers at the USC Dornsife College of Letters, Arts and Sciences and published in Paleoceanography and Paleoclimatology, traces those transformations through sediments buried beneath Great Salt Lake. The scientists reconstructed nearly 240,000 years of lake history and found that the enormous lakes were brief departures from its usual hypersaline (extremely salty) state.
Both giant lakes appear to have followed the same pattern: As the climate warmed and dried, the water receded, salinity rose and salt deposits formed on the lakebed, although the scientists note the timing of the older transition is less precise.
Corresponding author Rachel So, a recent PhD graduate from Earth sciences at USC Dornsife, compares the record to watching a puddle that remains nearly the same size for an hour, briefly swells into a pond and then shrinks again. “If you scaled this up to the size of the present Great Salt Lake, that’s probably what it looked like,” she says. “For most of the past 240,000 years, the lake stayed about the same size but briefly grew 10 times larger on two separate occasions.”
Ancient sediments reveal a lake transformed
Scientists have long known that Lake Bonneville once covered much of western Utah during the last ice age. Its ancient shorelines remain visible across the landscape. But those shorelines provide snapshots, not a continuous account of the lake’s size and salinity change over time.
For that, the researchers turned to a nearly 400-foot sediment core drilled from Great Salt Lake’s bed in 2000. Its layers preserve a record stretching back about 236,000 years.
The team dated the sediment layers by measuring radioactive decay in minerals, which provides a kind of geological clock. The researchers also analyzed molecules left by microorganisms that once lived in the lake. Because the relative abundance of those molecules changes with salinity, they could broadly gauge whether the water was fresh, brackish or extremely salty.
The record shows two major interruptions in the lake’s long hypersaline history. From roughly 30,000 to 16,000 years ago, it expanded to become Lake Bonneville. And Little Valley, an earlier deep lake, existed roughly 140,000 to 135,000 years ago. Shoreline evidence suggests both approached 1,000 feet deep, although Little Valley may have remained somewhat brackish and lasted less than half as long as Bonneville.
“The fact that the core spans two and a half glacial cycles allows us to see two fresh-to-salty transitions,” said senior author Sarah Feakins, professor of Earth sciences at USC Dornsife. “It allows us to compare the two fresh lake events for their duration and freshness, to see how fragile and fleeting these moments are.”
Various records from Nevada, California and Arizona show similar wet-to-dry shifts during the same periods, suggesting that Great Salt Lake was responding to changes felt across the region.
Lessons for a warming West
Feakins says that cooler temperatures slowed evaporation while more frequent storms brought additional water into the basin. Lake Bonneville filled “one storm at a time.”
As temperatures rose, she explained, evaporation increased and storms became less frequent. The lakes dwindled, and their water grew saltier until minerals formed crystals and settled on the lakebed.
Great Salt Lake has no outlet; water leaves mainly through evaporation, making the lake highly sensitive to changes in temperature, precipitation and river flow.
Modern Great Salt Lake also faces pressures its ancient predecessors did not. Human water use, mostly for agriculture, reduces the amount reaching the lake, while human-caused climate change increases evaporation.
“Today we’re warming the climate at an unprecedented rate,” Feakins said. “That warming makes the atmosphere thirstier, increasing the rate of evaporative drying from the soil and lakes across the region.”
She cautioned against treating the ancient changes as evidence that the lake’s decline today is natural.
“People shouldn’t use the defense that ‘climate change happened naturally in the past and so the lake shrank’ to justify shrinking lakes today as normal or a natural phenomenon,” So added.
Knowing when things happened in the past requires radioisotope based dating methods. Dates become less precise in the core’s older layers, with margins of error spanning several thousand years. Dating some ancient salt deposits proved especially difficult. The researchers also tried to reconstruct past temperatures, but the lake’s high salinity made the results unreliable.
Despite those limitations, the sediment core provides a nearly continuous record of a lake responding to major shifts in the West’s water balance and shows how a vast freshwater lake can give way to salt when the climate warms and dries.
About the study
In addition to So and Feakins, study authors include Elliot Jagniecki of the Utah Geological Survey; Tim Lowenstein of Binghamton University; Adam Jost and David McGee of MIT; Christopher Kinsley of the Berkeley Geochronology Center; Kristian Olson of Alfred University; and Jessica Tierney of the University of Arizona.
The research was supported by National Science Foundation grants 2152630, 2218544 and 2218547, USC Women in Science and Engineering, the USC Wrigley Institute Graduate Fellowship, and the Packard Fellowship for Science and Engineering.
Journal
Paleoceanography and Paleoclimatology
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Fresh to Hypersaline Transitions Across Two Glacial Cycles at Great Salt Lake, Utah
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