The 8,000-year history recorded in Great Salt Lake sediments

(Press-News.org) Over the past 8,000 years, Utah’s Great Salt Lake has been sensitive to changes in climate and water inflow. Now, new sediment isotope data indicate that human activity over the past 200 years has pushed the lake into a biogeochemical state not seen for at least 2,000 years.

A University of Utah geoscientist applied isotope analysis to sediments recovered from the lake’s bed to characterize changes to the lake and its surrounding watershed back to the time the lake took its current shape from the vast freshwater Lake Bonneville that once covered much of northern Utah.

“Lakes are great integrators. They're a point of focus for water, for sediments, and also for carbon and nutrients,” said Gabriel Bowen, a professor and chairman of the Department of Geology & Geophysics. “We can go to lakes like this and look at their sediments and they tell us a lot about the surrounding landscape.”

Sedimentary records provide context for ongoing changes in terminal saline lakes, which support fragile, yet vital ecosystems, and may help define targets for their management, according to Bowen’s new study, published last month in Geophysical Research Letters.

This research helps fill critical gaps in the lake’s geological and hydrological records, coming at a time when the drought-depleted level of the terminal body has been hovering near its historic low.

“We have all these great observations, so much monitoring, so much information and interest in what's happening today. We also have a legacy of people looking at the huge changes in the lake that happened over tens of thousands and hundreds of thousands of years,” Bowen said. “What we've been missing is the scale in the middle.”

That is the time spanning the first arrival of white settlers in Utah but after Lake Bonneville receded to become Great Salt Lake.

By analyzing oxygen and carbon isotopes preserved in lake sediments, the study reconstructs the lake’s water and carbon budgets through time. Two distinct, human-driven shifts stand out:

Mid-19th century – Coinciding with Mormon settlement in 1847, irrigation rapidly greened the landscape around the lake, increasing the flow of organic matter into the lake and altering its carbon cycle. Mid-20th century – Construction of the railroad causeway in 1959 disrupted water flow between the lake’s north and south arms, which turned Gilbert Bay from a terminal lake to an open one that partially drained into Gunnison Bay, altering the salinity and water balance to values rarely seen in thousands of years. The new study examines two sets of sediment cores extracted from the bed of Great Salt Lake, each representing different timescales. The top 10 meters of the first core, drilled in the year 2000 south of Fremont Island, contains sediments washed into the lake up to 8,000 years ago.

[caption id="attachment_100686" align="alignright" width="511"] The view of the Great Salt Lake from Gunnison Island, which has long served as a nesting ground for pelicans. Credit: Brian Maffly[/caption]

The other samples, recovered by the U.S. Geological Survey, represent only the upper 30 centimeters of sediments, deposited in the last few hundred years.

“The first gives us a look at what was happening for the 8,000 years before the settlers showed up here,” Bowen said. “The second are these shallower cores that allow us to see how the lake changed after the arrival of the settlers.”

Bowen subjected these lakebed sediments at varying depths to an analysis that determines isotope ratios of carbon and oxygen, shedding light on the landscape surrounding the lake and the water in the lake at varying points in the past.

“The carbon tells us about the biogeochemistry, about how the carbon cycles through the lake, and that's affected by things like weathering of rocks that bring carbon to the lake and the vegetation in the watershed, which also contributes carbon that dissolves into the water and flows to the lake,” he said.

Bowen’s analysis documented a sharp change in carbon, indicating profound changes that coincided with the arrival of Mormon pioneers in the Salt Lake Valley, where they introduced irrigated agriculture to support a rapidly growing community.

“We see a big shift in the carbon isotopes, and it shifts from values that are more indicative of rock weathering, carbon coming into the lake from dissolving limestone, toward more organic sources, more vegetation sources,” Bowen said.

The new carbon balance after settlement was unprecedented during the 8,000 years of record following the demise of Lake Bonneville.

Next, Bowen’s oxygen isotope analysis reconstructed the lake’s water balance over time.

“Essentially, it tells us about the balance of evaporation and water inflow into the lake. As the lake is expanding, the oxygen isotope ratio goes down. As the lake shrinks, it goes up, basically telling us about the rate of change of the lake volume. We see little fluctuations, but nothing major until we get to 1959.”

That’s the year Union Pacific built a 20-mile causeway to replace a historic rail trestle, dividing the lake’s North Arm, which has no tributaries, from its South Arm, also known as Gilbert Bay, which receives inflow from three rivers. Water flows through a gap in the causeway into North Arm, now rendering the South Arm an open system.

“We changed the hydrology of the lake fundamentally and gave it an outflow. We see that really clearly in the oxygen isotopes, which start behaving in a different way,” he said. Counterintuitively, the impact of this change was to make Gilbert Bay waters fresher than they would have been otherwise, buying time to deal with falling lake levels and increasing salinity due to other causes.

“If we look at the longer time scale, 8,000 years, the lake has mostly been pinned at a high evaporation state. It's been essentially in a shrinking, consolidating state throughout that time. And that only reversed when we put in the causeway.”

The paper, “Multi-millennial context for post-colonial hydroecological change in Great Salt Lake,” was posted online July 22 in the journal Geophysical Research Letters. Gabriel Bowen is the sole author and is supported by grants from the National Science Foundation.

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