Mudpot Geochemistry Reveals Subsurface Geologic Activity

Present day mudpot geochemistry provides a glimpse of past subsurface activity and processes in continental hydrothermal systems. Integrated geochemical, mineralogical, and microbiological sampling of rhyolite hosted mudpots, particularly in chemically distinct subregions of an area, has remained scarce. We examined and compared geochemical parameters in 15 mudpots at eight thermal areas in Yellowstone National Park to nearby hot springs of similar pH and temperature to discern identifying mudpot characteristics. Aqueous trace element relative abundances and sediment collected for mineralogical analysis revealed that mudpots with approximate viscosities of 5–100 cP (mPa·s) contained trace element geochemical “fingerprints” linked with underlying rock units. Alterations in subsurface hydrothermal fluid fractionation can be reflected in relationships between δD, pH, and total dissolved chloride in surficial hot springs in a thermal area. The clay-rich mudpots contained uncharacteristically low aqueous boron compared to other thermal features making boron a less reliable proxy for water–rock interaction in mudpots than previously thought. Turbid hot springs and mudpots of similar pH and temperature lacked consistent chemical similarity, however, mudpots exhibited major anion and cation similarity to hot springs from shared thermal source waters, as determined by dissolved chloride and δD. Mudpot habitats hosted microbial communities dominated by a few phyla with immense diversity at the species level and methanogenesis, nitrogen cycling, and heterotrophic capabilities. Mudpots, formed by acidic steam altering volcanic rock, serve as indicators of underlying geology that may also provide geochemical and microbial insight as modern analogs to ancient habitats.