SEEING BELOW THE SURFACE: USING GEOMORPHOLOGY, GEOLOGY, AND GEOCHEMISTRY TO UNDERSTAND THE PAST, PRESENT, AND FUTURE OF HOT SPRING SYSTEMS IN YELLOWSTONE

Hot spring systems involve the interaction of deep hydrothermal fluids, volcanic gases, and surface meteoric water. Their subsurface plumbing is controlled by the geology, and both the subsurface structure and the geomorphology of the landscape influences the degree of mixing with surface waters. This study focuses on the Greater Obsidian Pool Area in Yellowstone National Park, where a geochemically diverse group of hot springs have been developing in a meadow between glacial deposits on the edge of a lake. 20 years of geochemical data are combined with geologic and geomorphic field observations to create a spatial and temporal view of the hot springs in the region. Chloride concentration is used to infer the depth of fluid sources and mixing with surface water, while sulfate concentration is used to infer volcanic gas input. A geochemical map of the region suggests a pattern of concealed fractures in the subsurface as well as the geomorphologic influence of incoming groundwater diluting hot springs away from the hypothesized fractures. Field mapping reveals glacial deposits flanking the valley, alluvial deposits composing the meadow, and a previously unmapped hydrothermal explosion crater sitting among acidic hot springs. Geologic analysis combined with historical aerial imagery allows us to interpret the long-term and geologically recent history of this dynamic hot spring region: fractures allowed hydrothermal fluids and gas to rise to the surface, altering the geology, forming hot springs, and creating a hydrothermal explosion that ripped through the middle of a valley that had once been glaciated. The subsurface fractures that may have acted as a conduit for the focused rise of deep hydrothermal fluids allowed for the development of a series of hot springs that receive varying inputs of meteoric fluid corresponding to their position relative to the conduit. New hot springs continue to form in the region to this day, and understanding their geochemical context is crucial to understanding their future. The future of the hot springs is tied to the sourcing of the fluids that compose them: the meteorically influenced hot springs are vulnerable to climatic variations, while the deeply sourced hot springs are likely to remain more consistent with time.