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.
