TIMING AND PALEOCLIMATE SIGNIFICANCE OF THE LATE REGRESSIVE PHASE OF LAKE BONNEVILLE AND THE GILBERT EPISODE, GREAT SALT LAKE BASIN, UT

The regressive phase of Lake Bonneville has not been as well dated as the transgressive phase. Many of the radiocarbon ages for regressive-phase deposits are for carbonate materials, such as gastropod shell or tufa. The ages theoretically date the lake at or near the collection elevation, but the potential effects of radiocarbon reservoirs in the lake water and/or post-depositional geochemical changes for most samples are poorly known making the ages hard to interpret. On the other hand, radiocarbon ages of terrestrial organics (mostly wood and charcoal) from the transgressive phase are generally more reliable. In these cases, the living plant received its carbon directly from the atmosphere and post-depositional geochemical changes are minor, did not occur, or can be accounted for in sample preparation. Basal wood or charcoal ages do not directly date the lake at a particular elevation, but provide limits to the age of the transgression at that elevation.

The age of the final regression of Lake Bonneville to lake levels similar to those of modern Great Salt Lake (GSL; roughly 1280 m) is determined by shell ages at elevations between about 1400 m and 1450 m, and by limits imposed by ages of wetland or other organics at elevations below ~1350 m. Based on the available ages, the final regression to elevations similar those of modern GSL was at about 13 cal ka. During the Gilbert episode, the lake rose about 15 m higher than the approximate post-Bonneville lake level of 1280 m, and culminated at about 11.5 cal ka.

The final regression of Lake Bonneville is approximately coincident with Bølling-Allerød hemispheric-scale climate warming (~14.7 – 12.7 cal ka) at the end of Marine Oxygen Isotope Stage 2. The Gilbert episode may or may not be related to the Younger Dryas (YD) (~12.9 – 11.6 cal ka) - the Gilbert episode began at about 12 cal ka and culminated at about 11.5 cal ka, at the end of the YD, however, the Gilbert-episode lake was the highest lake rise in the GSL basin in post-Bonneville times.