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 chemical, physical and biological diversity of hot spring activity beneath Yellowstone Lake become appreciated. These discoveries have occurred in connection with our NSF funded multidisciplinary project called the Hydrothermal Dynamics of Yellowstone Lake. The highly integrated project explored previously unknown geothermal phenomena on the Lake floor. 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 from within the area of active venting. To do this, specially designed samplers and chemical sensors were deployed. Many instruments were made from titanium to sustain high temperatures and pressures, and are largely similar to instruments used successfully to acquire "black smoker" fluids on the seafloor at deep sea vents.

The hottest vents measured were found in the Deep Hole just east of Stevenson Island, the deepest area of Yellowstone Lake at 125 m (410 feet) (Figure 1). The hottest vent measured was 174?C (345 F). 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 higher temperatures to be reached. Indeed, these are the hottest hydrothermal vents measured in a lake anywhere in the world to date. The hot water and gas bubbles discharge from small ~10 cm (3-inch) openings in the lake floor, generally aligned along a series on conical depressions, which in turn appear to be associated with linear fractures on the lake floor (Figure 1 and 2).  Curiously, the water is even more dilute than the surrounding lake water and contains abundant gasses, such as hydrogen sulfide and carbon dioxide, which tells us that the vents are powered by steam and support a rich microbial ecosystem (. Much like steam heated areas at the surface in YNP, the lake floor around the Deep Hole vents is almost entirely made of a type of clay called kaolinite owing to the highly acidic nature of the venting fluids (Figure 2).

In addition to studying the geochemical and geophysical controls manifest by lake-floor vent fluid diversity, project goals entailed assessing the role to which seasonal lake level changes affect the temperature and chemistry of the steam-heated Deep Hole vents? Similarly, we wished to know how 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 parameters of the hot spring waters and deployed them for one year. The sensors and data loggers were recovered from the lake floor after more than one year with unexpected results. The high-durability PVC  data logger for the sensor that was partly submerged (~ 5 cm) in lake sediment 1 meter (~3 feet) from an active vent showed evidence of melting, indicating that the high temperatures in the Deep Hole are more widespread than thought (Figure 3). This underscores the challenges of long term monitoring of hot and corrosive vent fluids. The data logger record, however, was still complete, and has provided important chemical and physical data that is currently under study. Indeed, the one year deployment of the sensor system will provide much useful information

It is now apparent that the breathtaking range of geothermal features that draw millions of visitors to Yellowstone National Park each year also extend 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 funding from the National Science Foundation and support from the USGS, the National Park Service, the Global Foundation for Ocean Exploration, and Yellowstone Forever.

Last Modified: 09/02/2021
Modified by: William E Seyfried