Geoscientist's analysis of carbon and oxygen isotopes documents profound human-driven changes arising from agriculture and rail causeway.

Gabriel Bowen

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.

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.”