REFINING THE RECORD OF PLEISTOCENE LAKE LEVEL CHANGE, LAKE BONNEVILLE, UTAH; EVIDENCE OF CLIMATE-DRIVEN OSCILLATIONS FROM THE PROVO SHOREZONE

Shorezone deposits from Lake Bonneville contain evidence of the lake's rapid response to changing climate conditions during the late Pleistocene. The lake began to transgress about 28 ¹⁴C ka, reached its highstand about 15.5 ¹⁴C ka, and then fell dramatically to the Provo level about 14.5 ¹⁴C ka. The initial regression to the Provo level was controlled by physical processes at the lake's outlet, however, geomorphic and stratigraphic evidence indicate that the late stages of the Provo shoreline were influenced by changes in the evaporation to precipitation ratio in the Bonneville basin. Establishing the onset of climate-induced lake level changes during late Provo time is significant in that it may provide a link between continental and marine records of global climate change. Radiocarbon ages determined from gastropods indicate that the final oscillation of the Provo level of Lake Bonneville occurred about 13.6 ¹⁴C ka, approximately coincident with the termination of Heinrich ice-rafted debris event H1 in the North Atlantic.

Topographic profiles of shorelines from select sites in the Bonneville basin provide a detailed record of the magnitude and relative rate of change in lake level during the formation of the Provo shoreline. A series of prograding and aggrading beach ridges with abrupt terminations occur during the early stages of the Provo shoreline suggesting that lake level was being controlled by outlet dynamics during this time. In contrast, shorezone deposits from the late stages of the Provo shoreline terminate in a more gradual manner, indicating the onset of climatically influenced lake oscillations and marking the initiation of the lake's regressive phase to the pre-Holocene lowstand. Tufa may also be a useful indicator of climate influence on lake level during Provo time. At the studied locations, tufa occurs solely on the late stages of the Provo shoreline. This is consistent with probable higher total dissolved solids as the Bonneville basin water balance became increasingly negative during this time.