Great Salt Lake spent about 8% of the past 236,000 years as a deep lake
A USC-led reconstruction from a core drilled in 2000 puts the lake's two giant freshwater phases at about 19,000 years between them. The shallow, salty lake held for the rest of the 236,000-year record.
The Scientist · Science desk
What happened
- A USC Dornsife-led study in Paleoceanography and Paleoclimatology reconstructs nearly 240,000 years of Great Salt Lake from buried sediments and finds its giant lakes were brief departures from a hypersaline state.
- The team dated the layers by measuring radioactive decay in minerals and read salinity from molecules left by microorganisms, whose relative abundance shifts between fresh, brackish and extremely salty water.
- Two interruptions show up: the lake became Lake Bonneville from roughly 30,000 to 16,000 years ago, and an earlier deep lake, Little Valley, existed roughly 140,000 to 135,000 years ago.
- Both ended the same way, with a warming and drying climate pulling the water back, salinity rising and salt deposits forming on the lakebed, though the scientists say the older transition's timing is less precise.
- Records from Nevada, California and Arizona show similar wet-to-dry shifts in the same periods, which the authors take as evidence the lake was responding to regional change.
Why it matters
- Who has to decide decision Any target set for the lake is a choice of baseline, and this record makes the shallow, salty lake the default condition and the deep freshwater lake the exception.
- What it limits constraint At roughly five centimeters of sediment per century, the core can compare events lasting thousands of years; a question about drying within a human lifetime needs a different measurement.
- Who's exposed exposure With no outlet and evaporation as the main exit, the lake takes agricultural diversion and a warmer atmosphere as reductions to the same water balance.
- What it sets up precedent Because the wet phases register across the Great Basin, the same baseline problem applies to other terminal lakes in the region, not only to Utah's.
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About 19,000 of those years are wet. Lake Bonneville lasted roughly 14,000 years [17], and Little Valley, the earlier deep lake, about 5,000 [18]. The core spans about 236,000 years [2], so the two deep-lake phases together come to roughly 8 percent of it [20]. "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," said Rachel So, the study's corresponding author and a recent Earth sciences Ph.D. graduate at USC Dornsife [7]. She compared the record to a puddle that holds its size for an hour, swells into a pond, then shrinks [8].
Sarah Feakins, the senior author and a professor of Earth sciences at USC Dornsife, described a slow fill. Cooler temperatures slowed evaporation while storms came more often, and Bonneville filled "one storm at a time," she said [12]. Shoreline evidence puts both ancient lakes near 1,000 feet deep, though Little Valley may have remained somewhat brackish, and by the dated intervals it lasted about a third as long as Bonneville [5][19].
Bonneville's shorelines are still visible across western Utah, but they record high water, not a continuous history of size and salinity [16]. The core supplies that continuity at a cost in resolution. Its 122 meters average about half a millimeter of sediment a year, which puts a century of lake history in roughly five centimeters [2][21]. That is adequate for comparing events thousands of years long, which is the comparison the authors make. "It allows us to compare the two fresh lake events for their duration and freshness, to see how fragile and fleeting these moments are," Feakins said [10]. The published account of the study describes the order of events in each fresh-to-salty transition without giving a rate for it [22].
The two deep lakes sit inside glacial cycles. "The fact that the core spans two and a half glacial cycles allows us to see two fresh-to-salty transitions," Feakins said [9]. Great Salt Lake has no outlet, and water leaves mainly through evaporation, which makes it sensitive to temperature, precipitation and river flow [13]. The modern lake carries two pressures the ancient ones did not: human water use, mostly for agriculture, reduces what reaches it, and human-caused climate change increases evaporation [14]. "Today we're warming the climate at an unprecedented rate," Feakins said [15].
What to watch
- Tighter dating on the older transition, which the scientists say is the less precise of the two, would narrow Little Valley's roughly 5,000-year window.
- Whether the microbial-molecule proxy can be calibrated to quantitative salinity instead of the fresh, brackish and hypersaline categories it now supports.
- Measured inflow to the modern lake against evaporative loss, which this sediment record does not quantify.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence68
- Adoption
- Not scored
- Hype gap+18
- Incentives
- Not scored
- Confidence65
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
A study led by researchers at the USC Dornsife College of Letters, Arts and Sciences, published in Paleoceanography and Paleoclimatology, reconstructed nearly 240,000 years of Great Salt Lake history from sediments buried beneath the lake and found that the enormous lakes were brief departures from its usual hypersaline state.
- [2]
The researchers used a nearly 400-foot (122-meter) sediment core drilled from Great Salt Lake's bed in 2000, whose layers preserve a record stretching back about 236,000 years.
- [3]
The team dated the sediment layers by measuring radioactive decay in minerals, and analyzed molecules left by microorganisms that once lived in the lake; because the relative abundance of those molecules changes with salinity, the team could broadly gauge whether the water was fresh, brackish or extremely salty.
- [4]
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.
- [5]
Shoreline evidence suggests both ancient lakes approached 1,000 feet (305 meters) deep, although Little Valley may have remained somewhat brackish and lasted less than half as long as Bonneville.
- [6]
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 that the timing of the older transition is less precise.
- [7]
"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," said Rachel So, corresponding author and a recent Ph.D. graduate in Earth sciences at USC Dornsife.
- [8]
So 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, saying "If you scaled this up to the size of the present Great Salt Lake, that's probably what it looked like."
- [9]
"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.
- [10]
"It allows us to compare the two fresh lake events for their duration and freshness, to see how fragile and fleeting these moments are," said Sarah Feakins.
- [11]
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.
- [12]
Feakins says cooler temperatures slowed evaporation while more frequent storms brought additional water into the basin, and that Lake Bonneville filled "one storm at a time."
- [13]
Great Salt Lake has no outlet; water leaves mainly through evaporation, making the lake highly sensitive to changes in temperature, precipitation and river flow.
- [14]
Modern Great Salt Lake faces pressures its ancient predecessors did not: human water use, mostly for agriculture, reduces the amount of water reaching the lake, while human-caused climate change increases evaporation.
- [15]
"Today we're warming the climate at an unprecedented rate," Feakins said.
- [16]
Scientists have long known Lake Bonneville once covered much of western Utah during the last ice age and its ancient shorelines remain visible across the landscape, but those shorelines provide snapshots, not a continuous account of changes in the lake's size and salinity over time.
- [17]
The Lake Bonneville phase lasted roughly 14,000 years.
- [18]
The Little Valley phase lasted roughly 5,000 years.
- [19]
By the dated intervals, Little Valley lasted about a third as long as Lake Bonneville, consistent with the researchers' statement that it lasted less than half as long.
- [20]
The two deep-lake phases together account for about 8 percent of the 236,000-year core record, leaving about 92 percent in the hypersaline state.
- [21]
The core's average sedimentation rate is about half a millimeter per year, so a century of lake history occupies roughly five centimeters of core.
- [22]
The published account of the study gives the sequence of each fresh-to-salty transition (warming and drying, receding water, rising salinity, salt deposition) but no duration or rate for the transition itself.
Sources
2 independent publishers whose own reporting we read for this story.
- phys.orgGreat Salt Lake's ancient past reveals how quickly fresh water can disappear
1 article · September 15, 2026
- sciencealert.comAmerica's 'Dead Sea' Was Once a Vast Freshwater Lake 10 Times Its Size
1 article · October 4, 2026
Topics and entities
Topics
- Great Basin water supply
- Paleoclimatology
Entities
- Little Valley lake
- Paleoceanography and Paleoclimatology
- USC Dornsife College of Letters, Arts and Sciences