The research, led by scientists at the USC Dornsife College of Letters, Arts and Sciences and published in the journal Paleoceanography and Paleoclimatology, reconstructs nearly 240,000 years of lake history from a sediment core drilled from the lakebed. The team found that the enormous lakes of the past were brief departures from the basin’s usual hypersaline state, an extremely salty condition that has dominated the record for the vast majority of the time. Both giant lakes appear to have followed the same pattern: as the regional climate warmed and dried, the water receded, salinity climbed and salt deposits formed on the lakebed. The scientists note that the timing of the older of the two transitions is less precise than the younger one, but the overall trajectory of shrinkage and salinification is consistent across both events.

Corresponding author Rachel So, a recent PhD graduate in Earth sciences at USC Dornsife, compares the long-term record to watching a puddle that remains nearly the same size for an hour, briefly swells into a pond and then shrinks again. Scaled up to the size of the present Great Salt Lake, she explains, that is essentially what the basin looked like across nearly a quarter of a million years. For most of the past 240,000 years the lake stayed roughly the same size, but on two separate occasions it briefly grew to ten times its modern extent. The comparison captures the central finding of the study: the freshwater phases that produced the region’s most spectacular lakes were geologically fleeting interruptions in an otherwise salty story.

Scientists have long known that Lake Bonneville once covered much of western Utah during the last ice age, and its ancient shorelines remain etched into the landscape today, visible as terraces on hillsides far above the modern waterline. But those shorelines provide snapshots rather than a continuous account of how the lake’s size and salinity changed through time. To build a continuous record, the researchers turned to a nearly 400-foot sediment core drilled from the bed of Great Salt Lake in 2000. Its layered sediments preserve an archive stretching back about 236,000 years, spanning multiple glacial cycles and capturing both of the great freshwater episodes in a single, uninterrupted sequence.

The team dated the sediment layers by measuring radioactive decay in minerals, a technique that functions as a kind of geological clock and anchors the record in time. They also analyzed molecules left behind by microorganisms that once lived in the lake. Because the relative abundance of those molecules changes with salinity, their signatures allowed the researchers to broadly gauge whether the water at any given moment was fresh, brackish or extremely salty. Together, the dating and the biomarker analysis turned the core into a chronicle of the lake’s alternating states, revealing two major interruptions in its long hypersaline history.

The first and most recent interruption came between roughly 30,000 and 16,000 years ago, when the lake expanded into Lake Bonneville. The second, an earlier deep lake known from the Little Valley interval, existed roughly 140,000 to 135,000 years ago. Shoreline evidence suggests both lakes approached 1,000 feet in depth, but they were not identical twins. The Little Valley lake may have remained somewhat brackish rather than truly fresh, and it lasted less than half as long as Lake Bonneville. The most recent freshwater phase was therefore both longer and fresher than its older counterpart, a difference that offers clues about how variable the region’s wet episodes can be.

Senior author Sarah Feakins, professor of Earth sciences at USC Dornsife, emphasizes that the core’s span of two and a half glacial cycles is what makes the comparison possible. Because the record captures two complete fresh-to-salty transitions, the researchers can compare the two freshwater events directly, evaluating their durations and their degrees of freshness, and seeing how fragile and fleeting those moments were in the basin’s history. The record also aligns with evidence from elsewhere in the American West: 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 climate changes felt across the entire region rather than to purely local conditions.

The mechanism behind the lake’s dramatic swings comes down to a balance between water arriving and water leaving. Feakins explains that during the cooler periods, lower temperatures slowed evaporation while more frequent storms delivered additional water into the basin, allowing Lake Bonneville to fill, as she puts it, one storm at a time. As temperatures rose, the balance reversed: evaporation increased, storms became less frequent, and the lakes dwindled. Their remaining water grew progressively saltier until minerals began to form crystals and settle onto the lakebed, leaving salt deposits that the core still preserves today. Great Salt Lake’s lack of an outlet makes it especially sensitive to this balance, since water leaves the basin mainly through evaporation, so any shift in temperature, precipitation or river flow is registered directly in the lake’s level and salinity.

Modern Great Salt Lake faces pressures its ancient predecessors never encountered. Human water use, mostly for agriculture, reduces the amount of water reaching the lake, while human-caused climate change increases evaporation across the region. Feakins notes that today the climate is warming at an unprecedented rate, and that warming makes the atmosphere thirstier, accelerating evaporative drying from soils and lakes throughout the West. Both researchers caution against misreading the ancient record as reassurance. So warns that people should not use the argument that climate changed naturally in the past, and so the lake shrank, to justify treating shrinking lakes today as normal or purely natural phenomena. The geological record shows what a warming and drying climate can do to a closed basin; it does not excuse the additional human pressures now compounding that vulnerability.

The study is not without limitations, and the researchers are candid about them. Dating precision decreases in the core’s older layers, where margins of error span several thousand years, and dating some of the ancient salt deposits proved especially difficult. Attempts to reconstruct past temperatures were also hampered, because the lake’s high salinity made those results unreliable. Despite these challenges, the sediment core delivers a nearly continuous account of a lake responding to major shifts in the West’s water balance, and it demonstrates with unusual clarity how a vast freshwater lake covering thousands of square miles can give way to salt when the climate warms and dries. As the modern West grapples with a shrinking Great Salt Lake, the basin’s own sediments provide a 240,000-year perspective on just how quickly fresh water can disappear.

Subject of Research: Reconstruction of 240,000 years of salinity and lake-level change at Great Salt Lake, Utah, from sediment core evidence

Article Title: Great Salt Lake’s ancient past reveals how quickly fresh water can disappear

Article References: Great Salt Lake’s ancient past reveals how quickly fresh water can disappear. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Great Salt Lake, Lake Bonneville, paleoclimatology, sediment core, salinity, climate change, USC Dornsife, hypersaline lakes, American West, evaporation, glacial cycles, water resources