ALTERNATIVE HYPOTHESIS FOR THE ORIGIN OF THE BRINE IN BRISTOL DRY LAKE, WESTERN USA

The geochemical evolution of the Bristol Dry Lake (BDL) Basin, located in the Mojave Desert, USA, has been the subject of considerable debate since BDL was included as an end member of closed basin geochemical evolution proposed by Hans Eugster and Lawrence Hardie in the 1970’s. The Na-Ca-Cl brine is unusual for continental environments and is difficult to evolve from freshwater because most freshwater contains bicarbonate in excess of calcium. Once calcium carbonate minerals begin to form during evaporation, Ca becomes the limiting ion and the brine evolves towards a Na dominated brine composition with little Ca. One argument for the origin of the BDL brine is that geothermal fluids are needed to create a brine of this composition; however, the BDL basin is devoid of geothermal indicators other than a recent cinder cone that is <100,000 years old. Given that the basin is >4 million year old, it is unlikely that this had an influence on the brine evolution of the basin for much of the basin’s history, if at all. Cinder cones are relatively dry heat sources and would not contribute deeply heated fluids. An alternate hypothesis, presented here, is that Ca-rich water flowed into the basin during Bouse Formation time (around 5 mya) in a relatively open basin and that residual Ca-rich fluids from this incursion contributed to the brine composition. Thus, the chemical evolution of the water in the basin changed from a Ca-rich Bouse Formation water, to a Na-Ca-rich closed basin water. While it is clear that the composition of the brine is not likely from the evaporation of solely meteoric water, geothermal Ca-Cl rich brines have not been documented and heated water is not present in 500 m deep wells in the center of the basin. It is, therefore, possible that the calcium component of the brine comes from either residual brines or diagenetically altered fluids from the dissolution of calcium carbonate, derived from Bouse Formation sediments at depths below the deepest wells in the basin (537 m below land surface). Given the contradictory nature of the evidence for the origin of the brine, further work is needed to elucidate whether geothermal heating or residual fluids from Bouse Formation sediments is the likely source of the brine.