Date of this Version
The geochemical composition of Yellowstone Lake water is strongly influenced by sublacustrine hydrothermal vent activity. The evidence for this conclusion is twofold. First, mass-balance calculations indicate that the outflow from Yellowstone Lake is enriched in dissolved As, B, Cl, Cs, Ge, Li, Mo, Sb, and W relative to inflowing waters. Calculations involving stable isotopes of hydrogen and oxygen (δD and δ18O, respectively) and mass-balances indicate about 13 percent evapoconcentration in the lake, which is inadequate to account for the enrichment of these elements in the water column. Second, linear relationships between the concentration of Cl and many other elements in the lake and in hydrothermal vent fluids suggest that Yellowstone Lake water is a mixture of inflowing surface water and hydrothermal source fluid. The conservative behavior of many elements is further demonstrated in mixing experiments that utilize subaerial geyser fluids and Yellowstone River water sampled at the lake outlet.
The hydrothermal source fluid feeding the lake is identified by comparing theoretical predictions of the Cl and δD content of boiled, deep, thermal-reservoir fluid with observed compositions of water-column, pore-water, and vent samples from Yellowstone Lake. This comparison indicates that the hydrothermal source fluid has a temperature of 220°C and a Cl content of 570 mg/kg (~16 mM or millimoles per liter) and it evolved by boiling of a deep reservoir fluid with δD equal to –149 per mil and Cl content of 310 mg/kg. The concentrations of other elements in the hydrothermal source fluid are estimated using the observed linear relationships between Cl and other elements in lake and hydrothermal vent fluids. These concentrations indicate strong enrichment of Cl, Si, B, Li, Na, K, Rb, As, Ge, Mo, Sb, and W in sublacustrine hydrothermal vent fluids. In general, the composition of the hydrothermal source fluid is similar to the composition of subaerial geyser water in Yellowstone National Park (the Park).
The Cl concentration in the hydrothermal source fluid indicates that Yellowstone Lake water is about 1 percent hydrothermal source fluid and 99 percent inflowing stream water. The flux of hydrothermal source fluid into the lake is about 8 x 109 kg of water per year, based on mass-balance calculations for Cl. If the concentration of Cl in deep reservoir fluid, rather than in hydrothermal source fluid, is used, then the flow is calculated to be 1.5x1010 kg of water per year. Using the latter estimate, sublacustrine vents in Yellowstone Lake account for ~10 percent of the total flux of deep, thermal reservoir water in the Park, as estimated from Cl in streams (Friedman and Norton, 2000, this volume). Although the volumetric input of water into the lake from hydrothermal vents is small, the impact of the vent fluids on the geochemistry of Yellowstone Lake is large because of the great enrichment of many elements in these fluids. Because about 41 million kg per day of element-enriched deep thermal water flows into the lake, and recent swath sonar studies show the presence of numerous newly recognized hydrothermal features, Yellowstone Lake should be considered one of the most significant hydrothermal basins in the Park.