PALEOHYDROLOGY OF LAKE BONNEVILLE DETERMINED BY MINERALOGY AND C, O, AND SR ISOTOPE COMPOSITIONS OF AUTHIGENIC CARBONATES

Secular variation in endogenic calcium-carbonate species (aragonite-to-calcite ratio) and their C, O, and Sr isotope compositions record changes in water chemistry during the expansion of Lake Bonneville into a large, multi-basin lake system between 23-14.5 ka. At each of three core locations, one in the center of the deep northern basin, one in the center of the shallow southern basin, and one on the north side of the threshold separating the two basins, the shapes of all four curves for surface-water precipitates are remarkably similar. The simultaneous shifts indicate that fundamental changes in basin hydrology exerted the dominant control on lake chemistry. However, the shapes of the curves at each location differ significantly, demonstrating that surface-water masses in the two basins did not mix, even when lake elevation exceeded the threshold. C, O, and Sr isotope data from limited ostracode samples in each core provide partial insight into the formation and circulation of deep water. In all cores, d¹³C values of ostracodes are lighter than those of surface-water precipitates, suggesting organic matter formed in surface water decomposed in the water column. In contrast, d¹⁸O values of ostracodes are heavier than those of surface-water precipitates, indicating that the bottom-water mass was colder, and possibly more saline owing to evaporation, than the surface water. Finally, unlike surface-water precipitates, similarity in C, O, and Sr isotope compositions of ostracodes at all locations suggests well mixed deep water. Significant shifts in mineralogy and isotopic values occur in all three locations at 20 ka, when lake elevation reached the threshold elevation; and at 14.5 ka, immediately following the Bonneville flood. Both core locations in the northern basin show major shifts in C, O, and Sr in surface precipitates and ostracodes at 16.4 ka that are not or barely detectable in the core location in the southern basin. This event is interpreted to indicate formation of a meromictic lake in the deep northern basin. Although direct paleoclimate interpretation (e.g., evaporation index from d¹⁸O) cannot be made in a hydrologically complex lake system such as Bonneville, determination of the development of water masses within the system provides some indirect insight into paleoclimate history.