Past Research Highlights
September 2004
The Chemistry of Fossil Lake Bonneville
William Hart, University of Arizona, Department of Geosciences
Jay Quade, University of Arizona, Department of Geosciences
This week witnessed the publication of a long-term study by Bill Hart and Jay Quade of the chemistry of fossil Lake Bonneville in central Utah. Bill completed this work in fulfillment of his master's thesis requirements in Geosciences at the University of Arizona. Bill's work focused on lakes in the eastern Great Basin of Utah, which have fluctuated dramatically in response to changes in rainfall, temperature, and drainage diversion during the Quaternary. The largest of these, Lake Bonneville, the predecessor of Great Salt Lake, was over 50,000 km2 in area at its high stand and 332 m in depth, covering most of western Utah along with smaller areas of eastern Nevada and southern Idaho. Today, the Great Salt Lake (GSL) occupies the largest northern basin, and Sevier Lake occupies the largest southern basin.
We analyzed tufas (see below) and shells from shorelines (also see below) of known ages in order to develop a relation between 87Sr/86Sr (87-strontium/86-strontium) ratio of carbonates and lake level, which then can be used as a basis for constraining lake level from similar analyses on carbonates in cores. Our analyses show that the 87Sr/86Sr ratio of lacustrine carbonates changes substantially at low- to mid-lake levels but is invariant at mid- to high-lake levels.
Giant tufa heads precipitated along the shoreline of former Lake
Bonneville, near Wendover Utah.
The Provo shoreline appears as a horizontal bench across the lower third
of the photograph. This bench formed by wave action and build-up of
tufa heads in Lake Bonneville about 16,000 years ago. Near
Wendover, Utah.
Sr-isotope mixing models of Great Salt Lake and the Bonneville paleolake system were constructed to explain these variations in 87Sr/86Sr ratios with change in lake level. The model verifies that the integration of the southern Sevier and Beaver rivers with the Bear and others rivers in the north is responsible for the lower 87Sr/86Sr ratios in Lake Bonneville compared to the modern Great Salt Lake. We also modeled the 87Sr/86Sr ratio of Lake Bonneville with the upper Bear River diverted into the Snake River basin and obtained an 87Sr/86Sr ratio of 0.71414. Coincidentally, this ratio is close to the observed ratio for Great Salt Lake of 0.71469. This means that 87Sr/86Sr ratios of >0.714 for carbonate can be produced by climatically induced low-lake conditions or by diversion of the upper Bear River out of the Bonneville basin. This model result also demonstrates that the upper Bear River had to be flowing into the Bonneville basin during highstands of other late Quaternary lake cycles: carbonates from the Little Valley (130-160 ka) and Cutler Dam (59 ± 5 ka) lake cycles returned 87Sr/86Sr ratios of 0.71166 and 0.71207, respectively, and are too low to be produced by a lake without the upper Bear River input.
Hart, W. S., Quade, J., Madsen, D., Kauffman, D., The 87Sr/86Sr ratios of lacustrine carbonates and lake-level history of the Bonneville paleolake basin. Geological Society of America Bulletin 116, no. 9/10, p. 1107-1119.
For more information contact Jay Quade at jquade@geo.arizona.edu
