HYDROTHERMAL VENTS AND SUBLACUSTRINE SILICEOUS SPIRES IN YELLOWSTONE LAKE

The geochemistry of Yellowstone Lake is strongly influenced by sublacustrine hydrothermal vent activity. Sublacustrine hydrothermal vent fluids are strongly enriched in As, B, Cl, CO2, Ge, Hg, H2S, K, Li, Mo, Na, Rb, Sb, Si, and W. Cl concentrations indicate that Yellowstone Lake is about 1% geothermal source fluid and 99% inflowing stream water. The geothermal flux to the lake is about 10% of the total geothermal water flux in Yellowstone National Park. With recent swath-sonar mapping that shows numerous new hydrothermal features, Yellowstone Lake should now be considered one of the most significant geothermal basins in the Park. Hg enrichments in hydrothermal vent fluids and associated fauna contribute to elevated Hg in lake and cutthroat trout, which has important implications for grizzly bear, otter, eagle, and osprey populations that feed on the cutthroat trout that spawn in the rivers. Of particular interest, Hg is enriched in hair of grizzly bears that live near Yellowstone Lake, but not in those living in more remote areas, or in other grizzly bear food sources. Silica-rich hydrothermal deposits occur on the lake bottom near active and formerly active hydrothermal vents. Centimeter- to decimeter-sized pipe- and flange-like deposits contain cemented and recrystallized diatoms and represent pathways for hydrothermal fluid migration. Many lake-bottom hydrothermal vents show physical evidence of collapse and sediments collected from such vents show chemical evidence of leaching of 60-70 wt.% SiO2. Another major type of hydrothermal deposit comprises hard, porous siliceous spires that were discovered near Bridge Bay in 1997. At least 8-10 spires up to 7 m tall consist of diatom-rich areas and fibrous masses and globules of amorphous silica that could be of microbial origin. Bridge Bay spires formed in place by growth of chimney-like features from lake-bottom hydrothermal vents. Chemical analyses indicate siliceous vent deposits are strongly enriched in As, Ba, Cs, Hg, Pb, and Sr, and are often enriched in Fe, Mo, Mn, Nb, Rb, Ta, Th, Tl, and W relative to normal Yellowstone Lake sediments. Oxygen isotope analyses of silica deposits indicate formation at temperature between 80° and 160°C. Chemical reaction modeling indicates that vent fluids precipitate amorphous silica by mixing with cold dilute lake waters.