Heat and mass transport in sublacustrine vents in Yellowstone Lake, Wyoming: In-situ chemical and temperature data documenting a dynamic hydrothermal system

Recent advances in the development of in-situ chemical and temperature sensors have shown great promise as a means to investigate time series changes in marine and terrestrial hydrothermal systems. Here we apply this technology to assess chemical and physical controls on sublacustrine vent fluids in Yellowstone Lake, WY. Autonomous sensor systems were deployed in August 2017 using ROV assets to position instruments at two sites in the Deep Hole region of the Lake, off-shore from Stevenson Island. Earlier studies have documented that fissure related depressions (~120 m water depth) in this region host numerous vents issuing high enthalpy, CO₂ saturated and H₂S bearing vapor, at temperatures in excess of 150 °C. The YSZ (ceramic)-based in-situ chemical sensors deployed here were designed to measure pH and redox at 60 s intervals for up to one year. Two titanium-sheathed thermocouples, one coupled directly to the electrochemical sensor and another, independently powered and positioned slightly deeper in vents at deployment sites, provided insight on maximum temperature in the near surface region. In addition to in-situ chemical and temperature data, vent fluid samples were acquired at the outset and during recovery with a novel isobaric system that maintains lake bottom pressure, precluding fluid-sample de-gassing. At the same time, push cores of vents and in the near vent region were also acquired. The mineralized cores provide evidence of long term mass transfer processes, while also facilitating insertion and stabilization of the sensor units on the lake floor. Recovery of the sensors documented similar chemical controls at both sites, suggesting compositionally invariant vapor influx, characterized by moderately low pH (~4–5) and reducing conditions, buffered by CO₂ and H₂S, respectively. Accordingly, the diatomaceous sediment was extensively altered to kaolinite and pyrite. Temperature variability at the two sites was especially significant. One site (Site B), situated on the slope of what might be a hydrothermal domal structure, demonstrated noteworthy cooling and heating episodes that may be associated with hydrothermally or seismically triggered sediment slumping events. Upon instrument recovery in 2018, the vent site was largely sediment covered and the sensor insert showed evidence of dis-location and melting by conductive heating under the hot sediment overburden. The initially active and high temperature vent fluid (~148 °C) had largely ceased and replaced by diffuse flow venting at the margin of the previous up-flow zone. The time series in-situ chemical and physical data obtained in the course of the study document the existence of a dynamic hydrothermal system in time and space, while underscoring the challenges of research of this type in such environments.