Over the past 8,000 years, Utah’s Great Salt Lake has been sensitive to changes in climate and water inflow. New sediment isotope data now 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. This transformative shift is highlighted by recent research conducted by a University of Utah geoscientist.
Gabriel Bowen, a professor and chairman of the Department of Geology & Geophysics at the University of Utah, applied isotope analysis to sediments recovered from the lake’s bed. This analysis characterizes changes to the lake and its surrounding watershed dating back to when the lake took its current form from the vast freshwater Lake Bonneville, which 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 Bowen. “We can go to lakes like this and look at their sediments and they tell us a lot about the surrounding landscape.”
Understanding the Lake’s Historical Context
Sedimentary records provide context for ongoing changes in terminal saline lakes, which support fragile yet vital ecosystems. Bowen’s study, published in Geophysical Research Letters, aims to fill critical gaps in the lake’s geological and hydrological records. This research comes at a time when the drought-depleted level of the terminal body has been hovering near its historic low.
Bowen emphasizes the importance of understanding the middle scale of changes—the period spanning the first arrival of white settlers in Utah 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 Major Human-Driven Shifts
Two distinct, human-driven shifts stand out in the study:
Mid-19th Century: The Mormon Settlement
Coinciding with Mormon settlement in 1847, irrigation rapidly transformed the landscape around the lake, increasing the flow of organic matter into the lake and altering its carbon cycle. This period marked a significant change in the lake’s carbon balance, unprecedented during the 8,000 years of record following the demise of Lake Bonneville.
“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 explained.
Mid-20th Century: The Railroad Causeway
The construction of the railroad causeway in 1959 disrupted water flow between the lake’s north and south arms. This change 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.
“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,” Bowen noted.
Examining the Sediment Cores
The 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 2000 south of Fremont Island, contains sediments washed into the lake up to 8,000 years ago. Meanwhile, 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.
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.
Implications for the Future
The findings from Bowen’s study provide a crucial understanding of how human activities have altered the Great Salt Lake’s natural state. The research underscores the need for informed management strategies to address ongoing and future changes in the lake’s ecosystem. As climate change continues to impact water levels and salinity, these insights could be vital for conservation efforts.
The paper, titled “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.
