Science1 publisher2 min readPublished
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.
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Why it matters
- 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.
- 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.
- 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.
- 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.
About 19,000 of those years are wet. Lake Bonneville lasted roughly 14,000 years [1], and Little Valley, the earlier deep lake, about 5,000 [2]. The core spans about 236,000 years [2], so the two deep-lake phases together come to roughly 8 percent of it [3]. "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 [9]. She compared the record to a puddle that holds its size for an hour, swells into a pond, then shrinks [10].
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 [14]. 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 [6][4].
Bonneville's shorelines are still visible across western Utah, but they record high water, not a continuous history of size and salinity [18]. 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][5]. 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 [12]. The published account of the study describes the order of events in each fresh-to-salty transition without giving a rate for it [6].
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 [11]. Great Salt Lake has no outlet, and water leaves mainly through evaporation, which makes it sensitive to temperature, precipitation and river flow [15]. 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 [16]. "Today we're warming the climate at an unprecedented rate," Feakins said [17].
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.