Variation in the composition of Lake Bonneville marl: a potential key to lake-level fluctuations and paleoclimate

Lake Bonneville marl provides a stratigraphic record of lake history preserved in its carbonate minerals and stable isotopes. We have analyzed the marl in shallow cores taken at three localities in the Bonneville basin. Chronology for the cores is provided by dated volcanic ashes, ostracode biostratigraphy, and a distinctive lithologic unit believed to have been deposited during and immediately after the Bonneville Flood.A core taken at Monument Point at the north shore of Great Salt Lake encompasses virtually the entire Bonneville lake cycle, including the 26.5 ka Thiokol basaltic ash at the base and deposits representing the overflowing stage at the Provo shoreline at the top of the core. Two cores from the Old River Bed area near the threshold between the Sevier basin and the Great Salt Lake basin (the main body of Lake Bonneville) represent deposition from the end of the Stansbury oscillation ( 20 ka) to post-Provo time ( 13 ka), and one core from near Sunstone Knoll in the Sevier basin provides a nearly complete record of the period when Lake Bonneville flooded the Sevier basin (20–13 ka).In all cores, percent calcium carbonate, the aragonite to calcite ratio, and percent sand were measured at approximately 2-cm intervals, and ¹⁸O and ¹³C were determined in one core from the Old River Bed area. The transgressive period from about 20 ka to 15 ka is represented in all cores, but the general trends and the details of the records are different, probably as a result of water chemistry and water balance differences between the main body and the Sevier basin because they were fed by different rivers and had different hypsometries. The Old River Bed marl sections are intermediate in position and composition between the Monument Point and Sunstone Knoll sections. Variations in marl composition at the Old River Bed, which are correlated with lake-level changes, were probably caused by changes in the relative proportions of water from the two basins, which were caused by shifts in water balance in the lake.This is the second paper in a series of papers published in this issue on Climatic and Tectonic Rhythms in Lake Deposits.