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

Yellowstone National Park has one of the highest concentrations of continental geothermal activity on Earth, and includes an extraordinary collection of geysers, acid mud pots, steam fumaroles, and silica deposits. Only recently, however, has the diversity of hot spring activity beneath Yellowstone Lake become appreciated. This isn't surprising, considering the impressive size of Yellowstone Lake—at about 130 square miles, it is the largest lake in North America above an altitude of 7000 ft (2134 m)! This work is thanks, in large part, to the use of remotely operated vehicles (ROVs) to investigate the lake bottom.

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Geothermal activity in Yellowstone Lake was reported by the Hayden expedition as far back as 1878, and known about by Native Americans for millennia. Perhaps the most well-known feature is "Fishing Cone"—a silica cone along the shore of West Thumb that releases warm "salty" water with a much different composition than Yellowstone Lake water. However, geothermal activity in deeper portions of the lake has only been studied for the past 20 years, since scientists from the USGS and the University of Wisconsin-Milwaukee conducted the first ROV-based studies of the lake floor.

A large multidisciplinary project called the Hydrothermal Dynamics of Yellowstone Lake (HD-YLAKE) recently completed a 3-year study of hydrothermal vents in the lake. A team from the University of Minnesota has focused on measuring hot spring temperatures and fluid chemistry on the lake floor, while also studying rock and sediment samples. To do this, specially designed samplers and chemical sensors were deployed, much like those used to investigate "black smoker" fluids on the seafloor at deep-sea vents.

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The hottest vents measured during the HD-YLAKE study were found in the Deep Hole area just east of Stevenson Island, the deepest area of Yellowstone Lake at 410 feet (125 m). The hottest vent measured was a whopping 345°F (174°C). This is much hotter than any surface hot spring at Yellowstone, because the weight from the overlying lake water acts like a pressure cooker lid and allows temperatures higher than boiling to be reached. Indeed, these are the hottest hydrothermal vents measured in a lake anywhere in the world! The hot water and gas bubbles discharge from small ~10 cm (3-inch) openings in the lake floor, which are very different from the mounds made of silica like the "Fishing Cone". The water from these vents contains lots of gasses like hydrogen sulfide and carbon dioxide, which tells us that the vents are powered by steam. Much like steam-heated areas at the surface in places like the Mud Volcano at Yellowstone, the lake floor around the Deep Hole vents is almost entirely made of a type of clay called kaolinite. Additional details are provided in a recent publication.

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Do seasonal lake-level changes affect the temperature and chemistry of the steam-heated Deep Hole vents? And do the vent fluids respond to seismic events in and around the Lake? To answer these and related questions, University of Minnesota researchers developed underwater sensors capable of logging the temperature and chemical characteristics of the hot spring waters and deployed them for one year. The sensors and data loggers were recovered in August 2018, with unexpected results. The high-durability PVC data logger for the sensor that was partly submerged (by ~2 in, or 5 cm) in lake sediment well away from an active vent showed evidence of melting, indicating that the high temperatures in the Deep Hole are more widespread than previously thought. This underscores the challenges of long-term monitoring of hot and corrosive vent fluids. Considering the softening temperature for PVC is about 212°F (100°C), it is remarkable such high temperatures are present in sediment bathed in 39°F (4°C) lake water. The data logger record, however, was complete, and has provided important chemical and physical data that is currently under study.

In addition to recovering the dataloggers from the Deep Hole, August 2018 work also involved sampling hydrothermal vents in the West Thumb area at a depth of 175 ft (53 m). There, the hottest water was a more modest 286°F (141°C), which is still much hotter compared to surface hot springs. Rather than being powered by steam like the vents in Deep Hole, the water in the West Thumb vents is more like the water that deposits silica in many of the onshore geothermal areas in Yellowstone.

It is now apparent that the breathtaking range of geothermal features that draw millions of visitors to Yellowstone National Park each year also extends below Yellowstone Lake, forming the most diverse range of lake-bottom geothermal features known anywhere. More exciting discoveries and details are sure to come from the HD-YLAKE project, thanks to support from the National Science Foundation through award EAR-1516361, entitled "Collaborative Research: The Response of Continental Hydrothermal Systems to Tectonic, Magmatic, and Climatic Forcing", the U.S. Geological Survey, the Fisheries group and Yellowstone Center for Resources at Yellowstone National Park, the Global Foundation for Ocean Exploration, and Yellowstone Forever. All work in Yellowstone National Park was completed under Yellowstone research permit YELL-2017-SCI-7018.

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