SOUTH TO NORTH FLOW IN LAKE BONNEVILLE: EVIDENCE FROM CARBONATE MINERALOGY AND GEOCHEMISTRY

Secular changes in the mineralogy and chemistry of fine-grained carbonate sediment formed in surface waters define the evolution of water masses in Lake Bonneville. Samples were analyzed from four cores of offshore marl from the two major subbasins of Lake Bonneville, the small and shallow southern Sevier basin and the large and deep northern Great Salt Lake basin. Two cores are located in the center of the northern basin, one core in the center of the southern basin, and the fourth on the north side of the threshold that separates the two basins. All four cores provide a sediment record of the transgressive phase of Lake Bonneville between about 24 and 18 cal ka, and the two cores from the northern basin also record lake history between 30 and 24, and between 18 and 13 cal ka. The curves of the aragonite to calcite ratio (A/C), the total inorganic carbon (TIC), and C, O, and Sr isotopic compositions of endogenic carbonate all indicate that at no time during the transgressive phase when lake level was above the threshold did waters from the two basins become well mixed. The table below shows the average compositions of the sediments in the two basins.

core location	A/C	TIC	87Sr/86Sr	δ13C	δ18O
Sevier basin	1.4	6.5	0.71156	+3.7	-4.6
Threshold	0.2	3.4	0.71127	+0.5	-6.4
GSL basin	0.0	2.1	0.71079	-0.1	-7.3

All values from the core on the north side of the threshold are intermediate between those in adjacent basins, indicating that there must have been surface flow from the southern basin to the north. ⁸⁷Sr/⁸⁶Sr ratios measured in benthic ostracodes from the transgressive phase in all cores are similar and have values close to that of Sevier-basin endogenic carbonate. This similarity indicates that bottom water in the northern basin was probably derived from northward flow from the Sevier basin.