DEFINITION OF THE HYDROGEOLOGIC HISTORY OF THE TRANSGRESSIVE PHASE OF LAKE BONNEVILLE, UTAH BY SR ISOTOPES
Modern values of the sources provide first-order constraints because the modern and late Pleistocene drainage systems of the Bonneville basin are similar. The low Sr ratios of the pre-merger lake in the Sevier basin are consistent with low Sr ratios of the modern Sevier River (0.7070), and the high Sr ratios of the pre-merger lake in the Great Salt Lake basin are consistent with the high Sr ratios of the modern lower Bear River (0.7145), which are largely influenced by additions of highly radiogenic spring water. The Sr ratios in the two subbasins became similar soon after merger, indicating thorough mixing of the lake during intermediate stage. However, disparate values in the two subbasins at full-lake stage indicate poor mixing during this interval. The significant decrease in the Great Salt Lake subbasin during full-lake stage suggests that influx of snowmelt and rainfall from the upper reaches of the Bear River (modern value 0.7089), possibly concomitant with shut-off of springs in the lower reaches from increasing water pressure by the expanding lake, dominated the northern part of the lake. This water did not circulate into the southern arm in the Sevier basin, possible owing to a density contrast.