Editor's Note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Chunyang Tan, Andrew Fowler, and William Seyfried, all researchers at the University of Minnesota.

The expanse of Yellowstone Lake masks extraordinary geothermal features as diverse as their on-land counterparts—discoveries that have recently been made following remote operated vehicle (ROV) missions to the lake floor. Thanks to the technology afforded by temperature and chemical sensors developed at the University of Minnesota and deployed on the lake floor for an entire year, we’ve made some surprising discoveries about the dynamic and extreme environment on parts of the lake floor (recently published in Journal of Volcanology and Geothermal Research).

Sensor (left) and battery/electronics package (right) at vent location A in the Deep Hole area of Yellowstone Lake. Photo was taken immediately before recovery from the lake floor. Notice the battery/electronics package slightly submerged in sediment/Chunyang Tan, University of Minnesota. Public domain.

The sensors were developed and built in William Seyfried’s lab at the University of Minnesota to record temperature, acidity, and chemical reduction-oxidation (redox) continuously. They were also designed to survive some of the most extreme conditions on earth: the scorching temperatures and enormous pressures on the ocean floor at ‘black smoker’ hydrothermal vents, where equipment must also accommodate the effects of highly corrosive vent fluids. What’s more, the sensors are designed to survive these harsh conditions for months on end.

Two sensors were left in the deepest part of the lake in an area called the “Deep Hole” from August 2017 to August 2018. The sensors were placed on titanium sheaths, which were situated directly on top of vents to prevent the pliable vent sediment from collapsing on the sensors. Fluids discharging from the lake floor vents were collected by the ROV just before deploying the sensors and were found to be unusually hot (up to 150 ⁰C/302 ⁰F). The vent fluids formed where hot steam pushes through clay on the lake floor and then mixes with cold lake water. The location is much like the steam-fed fumaroles of the Mud Volcano area of Yellowstone National Park, but higher temperatures are reached at the lake floor because the weight of overlying lake water acts like a pressure cooker that raises the boiling point at the steam vents.

At each vent, two temperature probes were deployed—one was part of the chemical sensor package in the shallow part of the vent, and another independent temperature logger was placed slightly deeper in the vent.  Surprisingly, the year-long temperature records at the two vents—named “A” and “B”— as measured by the deeper independent thermal loggers were quite different, with one vent showing fairly consistent temperatures, while the other dropped dramatically for months before increasing again.

Year-long (August 2017 to August 2018) temperature records from two hydrothermal vents (“A” and “B”) in the Deep Hole area of Yellowstone Lake/Public domain.

At location A, the temperature measured by the independent sensor, deeper In the vent, was over 50 ⁰C (90 ⁰F) greater compared to higher up at the sensor that is part of the chemical logger.  This is an impressive gradient and implies high temperatures are present very close to the lake bottom. Additionally, the chemical sensor measured acidic (pH 5) and H2S-rich fluid, confirming the origin of the vent as gas-rich steam. More surprising, the PVC-encased battery and electronics placed in cold lake water over a meter (3 feet) from the vent and buried only 3-5 cm (~ 1 inch) in sediment was thermally deformed – the PVC was partially melted! This remarkable outcome revealed an extreme environment where high temperatures in the lake sediment are more widely distributed than previously suspected.

At location B, the chemical sensor had been dislodged from the vent and submerged in about 20 cm of sediment. Despite the carnage, successful chemical measurements were obtained for several months. The independently positioned temperature sensor confirmed the dramatic change in vent activity (heating) and lake water mixing (cooling) that occurred throughout the year. We believe that the great changes at location B—both the temperature changes and the dislocation of the sensor—indicate that slumping of sediment occurred while the sensor was deployed.

Sensor (top), deformed battery/electronics package (bottom), and titanium sheath (left) following recovery from vent location A in the Deep Hole area on the floor of Yellowstone Lake/Chunyang Tan, University of Minnesota. Public domain.

Several factors may have caused the slump. Small hydrothermal explosions are common in Yellowstone’s geyser basins and are also hypothesized to occur on the lake floor. Indeed, such events may be responsible for the formation of the vents in the Deep Hole area on the lake floor where the sensors were deployed. Alternatively, the slump may have been triggered by seismic activity. Given the increase in seismic activity in the Yellowstone region between February and May 2018, and the precarious location of sensor B on a ~15-degree slope, a seismic cause is entirely plausible for triggering a slump. However, direct cause and effect based on the sensor temperature record and known earthquake locations and times does not conclusively prove this.

Whereas the cause of the slumping is speculative, identification of extreme temperatures mere centimeters beneath the lake floor and the tortured journey of the two robust sensor packages are a testament to the dynamic and extreme environment beneath Yellowstone Lake. We are excited to continue exploring this dynamic landscape—a setting that is every bit as wonderous as the geyser basins that are enjoyed by millions of visitors every year!