APPLYING INTEGRATED GEOCHEMICAL DATA TO TRACE THE EFFECTS OF SEASONAL SNOWMELT RECHARGE IN THREE HYDROTHERMAL FLUID TYPES

Surficial fluid compositions in continental hydrothermal systems, driven by subsurface water-rock-gas interactions, provide diverse microbial habitats. Hot spring compositions at Yellowstone National Park (YNP) are defined by initial rain and snow compositions, complex deep hydrothermal reactions with rhyolite and other rocks, and magmatic gas injection. Following Nordstrom et al. (2009, J. Appl. Geochem. 24, 191), we use chloride and sulfate concentrations to infer two fluidic endmembers we call deep hydrothermal (DH) and shallow meteoric (SM), that interact with magmatic gas to produce the varied surficial spring compositions observed in YNP. Combining this approach, total boron versus chloride concentrations, and stable oxygen and hydrogen isotope ratios of hot spring water, we identify signatures of DH fluids, a SM endmember, local snowmelt, and SM with magmatic gas input (SMG) in a single YNP thermal area called Rabbit Creek. We integrate a decade of geochemically diverse Rabbit Creek data with a recent temporal study of seasonal snowmelt-driven meteoric recharge effects on three different fluid signatures (DH, SM, and SMG) from summer 2019 through summer 2020. Sample intervals, based on hydrographic data for local rivers, are approximately 4 weeks apart during baseflow conditions and 2 weeks apart during spring melt and recession limb to maximize non-baseflow resolution. Our hypothesis of DH consistency and meteoric perturbation was field tested using temperature, pH, and conductivity. All springs were 88 ± 2°C in July 2019. The DH signature spring stayed above 86°C throughout the sampling period while the SM and SMG springs cooled to 75 and 63°C, respectively, during the winter. The DH spring mostly stayed at pH 8.2 ± 0.2 while the SM spring varied between pH 6.5 and 7.7, and the SMG spring shifted between a pH of 4.8 and 4.2. The pH and temperature fluctuations, driven by meteoric fluids and magmatic gas input, allow springs to approach the boundary of photosynthetic microbial tolerance, possibly capturing a seasonal microbial primary production rhythm. Temporally examining three hydrothermal fluid types reveals neighboring springs can have differing proportions of deep and surface derived fluids with varying magmatic gas input and can dramatically fluctuate seasonally depending on fluid signature.