RECONSTRUCTION OF THE PALEOENVIRONMENTS OF LATE-QUATERNARY LAKE BONNEVILLE, UTAH BETWEEN ~60 AND 11 KA

Paleolimnology provides useful information about the response of lake systems to changing climate patterns. We used a sediment core from the Great Salt Lake, Utah, U.S.A., to reconstruct the ecological changes in the Lake Bonneville basin during the last glacial period (~60,000 to 11,000 yr B.P). This core section contains the record of two prehistoric lakes – the Cutler Dam Alloformation and Lake Bonneville. The chronology of these lake stands is well documented by the elevations of dated shore deposits, but little is known about the period between the lake high-stands. Unlike shoreline deposits, sediment cores contain continuous records of the lake’s environment, including times when the lake was too low to leave distinguishable shoreline deposits.

We conducted a detailed study of a portion of a 121 m sediment core that was taken from Great Salt Lake in 2000 as part of the Global Lakes Drilling project. Samples were extracted at 10 cm increments between 21.1 and 14.4 meters below lake floor, giving an approximate resolution of 600 years. We used percent biogenic silica determination, carbon and nitrogen isotope composition of organic matter, and absence, presence or abundance of microfossil indicators including brine shrimp eggs, brine fly exoskeletons and larvae fragments, preserved algae and ostracodes. Biogenic silica and carbon and nitrogen isotopes are an indication of lake productivity. Quantifying or qualifying changes in microfossil assemblages signifies changes in lake level. Ostracode fossils are valuable paleolimnological tools that denote lake depth and alkalinity. To determine sediment age, we modified an age model from a previous low-resolution study of the core.

We found a possible extension of the Cutler Dam Alloformation, changes in lake ecology affected by input from the Bear River, a clear shift in microfossil and chemical indicators attributed to the onset of Lake Bonneville’s transgressive phase, and a clear indication of the lakes regressive phase. This study showed how the ecology of the lake changed gradually between the established lake stands. In addition, our results contribute to the timing of important responses of lake level to climate change.