TEMPORAL VARIATIONS REVEAL HOT SPRING FLUID SOURCES

Hydrothermal springs are geochemically discrete habitats for microbial life, models for global geochemical cycles, and preserve some of Earth’s earliest evidence of life. The history of how a hydrothermal spring attains its geochemical fingerprint is complicated but can be elucidated by combining tracers including stable oxygen and hydrogen isotopes (δ18O, δ2H), and concentrations of major and trace solutes. Nordstrom et al. (2009, J. Appl. Geochem. 24, 191) use sulfate and chloride to infer mixing of fluid endmembers: deep hydrothermal (DH), shallow meteoric (SM), and magmatic gas as the drivers of Yellowstone National Park (YNP) hydrothermal fluid compositions. DH fluid, enriched in major ions and 18-O, is estimated to have been recharged by ancient snowmelt from surrounding mountain ranges, while SM fluid preserves recent dilute meteoric signatures. A decade of geochemical data from the YNP Rabbit Creek thermal area allows us to identify thermal springs exhibiting signatures predominantly of DH (spring RW1), SM (spring RS1), and SM with magmatic gas input (spring RS2). These various fluids are likely to flow on different timescales so we designed our study to investigate the seasonal behavior of the thermal springs. The 3 springs were sampled monthly for 18 months starting July 2019, with increased frequency during peak local river flow. All started above 86˚C; RW1 had consistent temperature (within 5%) and steady δ18O and δ2H exhibiting 0.2‰ and 2‰ variation, respectively. RS1 temperature was erratic; δ18O and δ2H were slightly more variable. RS2 showed hysteretic fluctuations of temperature, δ18O, and δ2H, shifting 30%, 3‰, and 10‰ respectively. Temperature and isotopes at RS2 are correlated with conductivity, showing the lightest isotope signature when the spring is the most dilute, approximately 3 months after peak river flow. pH at RS2 is inversely correlated, indicating the source of the dilution and light isotopes is driving the fluid more neutral, which could be interpreted as an influx of dilute, circumneutral groundwater. These observations provide evidence that some hydrothermal fluids are sensitive to yearly seasonal variation whereas others are not, leading to new questions about how long term climate change will affect hydrothermal features and the microbiological communities inhabiting them.