DEFINITION OF THE HYDROGEOLOGIC HISTORY OF THE TRANSGRESSIVE PHASE OF LAKE BONNEVILLE, UTAH BY SR ISOTOPES

Secular variation in Sr isotopes of calcium carbonate minerals in offshore marl from two cores in different subbasins of Lake Bonneville define changes in water sources and water mixing during transgression of the lake. Water in the smaller, shallower Sevier subbasin had a value of 0.7101 prior to merging at the threshold with water of the larger and deeper Great Salt Lake subbasin. Following the merger, values in the Sevier basin abruptly increased and varied between 0.7110 and 0.7106 for a short time before becoming remarkably constant at 0.7115 throughout most of the large-lake phase. The sample overlying the Bonneville flood layer decreased to 0.7103. Prior to merger, water in Great Salt Lake subbasin had a value of 0.7127. Following merger during the intermediate-volume lake stage, values varied between 0.7112 and 0.7119, with an average of 0.7116. During maximum, full-lake stage, values abruptly decreased and were relatively constant at 0.7107. Immediately following the Bonneville flood, values steadily increased from 0.7115 to 0.7119.

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.