COMPARING THE SHORELINE AND OFFSHORE SEDIMENTARY RECORDS OF LAKE BONNEVILLE

Acknowledging uncertainties, the record of lake-level changes in Pleistocene Lake Bonneville is well understood, and is largely derived from studies of shoreline geomorphology and stratigraphy -- shorelines can be dated and the lake size can be accurately determined (the Bonneville basin is hydrographically closed and responds sensitively to climate changes). Lake Bonneville, however, existed for only a relatively short time interval (roughly 28 to 10 14C ka), and shorelines of older lakes in the basin are not preserved. Therefore, in order to learn about lake dynamics in pre-Bonneville times shoreline chronologies cannot be used, but cores of pre-Bonneville lake sediments contain direct clues to changes in the chemical, physical, and biological characteristics of the lake. The sedimentology, mineralogy, and geochemistry of Lake Bonneville sediments permit correlations between the basin interior and the independently derived record of lake-level changes, which may then be used to interpret changes pre-Bonneville lake size from core stratigraphy.

Some useful proxies in Lake Bonneville sediments are total inorganic carbon (TIC), carbon and oxygen isotopes, and X-ray diffraction of carbonate minerals. Most work has been done with TIC because TIC results are cheap and easy to produce, and in many cases are correlated with results from more expensive analyses. TIC is dependent on factors such as organic productivity, solute mixing (from different water sources), water temperature, total dissolved solids, and shifts in the relative proportion of clastic vs. carbonate-precipitate in sediments. Despite interpretive difficulties, TIC shifts can be correlated between cores from different locations and with lake-level changes. In many cases the stable isotopes and carbonate minerals in fine-grained carbonates are directly correlated with TIC, but in other cases they provide new information. In general, higher values of TIC and delta 18O, and increases in aragonite, suggest falling lake level in the Great Salt Lake subbasin, but rising lake level in the Sevier subbasin.