Although hidden from the view of many visitors to Yellowstone National Park, the floor of Yellowstone Lake has many hydrothermal features. In fact, it is the third-largest thermal basin in the park. Scientists study the lake floor using a variety of sophisticated tools, including sonar imaging of the lake bottom, magnetic surveys, heat-flow surveys using thermal probes, and direct sampling of rocks, sediment and biota using remotely operated vehicles (ROVs) and sediment coring devices. These tools provide a better understanding of the geologic, geochemical and geophysical processes operating beneath the floor of Yellowstone Lake.
In August, a group of geoscientists ventured out onto the lake on the R/V Annie, a specially built, 40-ft-long boat designed for lake research, to collect relatively short (less than 1 meter/3 feet) sediment cores from carefully selected target areas. The goal was to improve knowledge of lake-bottom hydrothermal hot springs, hydrothermal alteration of sediments, hydrothermal explosion craters and deposits, and structures (faults, fractures, fissures) that cut the lake floor.
Sediments contained within the short cores represent hundreds of years of lake history, where the oldest sediment is at the bottom of the core and the youngest at the top. The mineralogy and geochemistry of the collected sediments will be studied to understand variations brought about by interactions with hot fluids or hydrothermal explosion ejecta. Pore water contained within the sediments also was extracted to determine whether the fluids have compositions representing normal lake water, or rather are hydrothermal fluids resulting from high-temperature reactions with sediment or rocks deep beneath the lake floor.
The coring targets included the deepest part of the 13,000-year-old Mary Bay and 8,000-year-old Elliott’s hydrothermal explosion craters in the northern part of Yellowstone Lake. The greatest depths in these large craters are actually in smaller craters that were formed after the main crater-forming explosions. Both Mary Bay and Elliott’s Crater continue to have active hydrothermal vents today. The team also targeted:
- A vent area on the North Basin Hydrothermal Dome to sample stiff, hydrothermally altered mud that caps the dome.
- The extensional fissures west of Stevenson Island, which are suspected of once being a hydrothermal area but have low heat flow values today.
- The Bridge Bay spire area that includes an area of inactive silica-rich spires rising about 8 meters (25 feet) above the lake floor from depressions interpreted as former hydrothermal craters.
Sediment cores were collected using a gravity corer connected to the boat by a Kevlar line on a winch. The coring device was lowered to near the lake bottom and then allowed to free-fall about 5–10 meters (15–30 feet) into the lake mud. This study collected thermal data by fitting the coring barrel with outriggers that carried temperature-measuring devices. These thermal measurements provide information on present-day heat flow in the subsurface that can reflect ongoing hydrothermal fluid flow. During the work, the team collected nine cores that varied in length from 14 to 59 centimeters (5.5 to 23.2 inches).
Although the analysis is just beginning, the cores already are providing important insights. Two hot cores with temperatures up to 91°C (196°F) were collected from the Mary Bay explosion crater. In contrast, two cores from Elliott’s Crater were cold, but the cores may have interesting chemical signatures in the pore fluids or sediments. Three additional cores were collected from the area west of Stevenson Island and should provide a good basis for determining if the region was formerly a hydrothermally active area. Two cores from the Bridge Bay spire area will provide evidence for its hydrothermal history. Preliminary results indicate pH values of pore fluids from 6.0–7.5, slightly acid to slightly alkaline (neutral pH is 7.0). Some of the pH values are outside the normal range for lake water, suggesting that the samples may be alkaline-chloride hydrothermal fluids, while others may be influenced by acidic vapor-dominated fluids.
During the winter of 2021–2022, Yellowstone Volcano Observatory scientists will complete a full analysis of the sediment composition from the sampled cores, as well as the fluids those cores contain. Analysis of heat flow data also will provide new understanding of thermal activity and thermal history of these important sites. The lessons learned should provide new insights into the dynamic hydrothermal system hidden from view on the floor of Yellowstone Lake.
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Pat Shanks and Lisa Morgan, scientists emeriti with the U.S. Geological Survey, and Rob Harris, professor in the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University.