Gases in Brimstone Basin Indicate Heated Pasts (2013)February 19, 2013
In Fall of 2012, Deborah Bergfeld (USGS) and colleagues published a paper in the journal Chemical Geology on an unusual acid-altered region along the southeast shore of Yellowstone Lake. The site, Brimstone Basin, looks like a typical Yellowstone thermal area—a region where heat is released from the ground. However, Brimstone Basin lacks any evidence for obvious heat discharge such as steaming ground or hot (or warm) springs.
Yellowstone contains many regions of acid-altered terrain similar to Brimstone Basin. Their ground surface is altered and bleached to various shades of white and gray. At most locations, geothermal gases, like carbon dioxide (CO2) plus or minus hydrogen sulfide (H2S), emerge through the ground in amounts that relate to the amount of heat released through the ground.
Like most thermal areas, Brimstone Basin has little vegetation, and the surface rock is bleached white with zones of yellow or black discoloration. These areas are associated with high concentrations of sulfur-bearing minerals and the discharge of H2S (rotten egg gas). The lack of thermal features seemed to indicate that the area is a prehistoric thermal area, but without any current activity.
A 2008 field campaign to Brimstone Basin found that a surprisingly large discharge of gas coming up through the soil (diffuse flux) is emitted from the ground surface—about 277 metric tons of CO2 and 0.6 metric tons of H2S per day. Diffuse flux is measured with special equipment that detects the concentrations of gas coming up through soil. Bergfeld and her crew based their gas flux estimates on measurements at about 300 individual sites. They found that the cold gas comes out of the ground at similar rates to gas emissions in much hotter "acid-sulfate" areas of Yellowstone.
They also found evidence for heat in the subsurface at Brimstone Basin. Part of that evidence was found in the chemistry of the emerging gas. They collected gas samples along Alluvium Creek, a cold acidic stream that passes through the main areas of gas upflow. Bergfeld and her crew found several sites where gas emerges through the shallow water with sufficient upflow to cause the water to roil above the surface of the stream. Analyses of the gas show that, compared with other acid sulfate areas at Yellowstone, it is enriched in CO2 relative to H2S and has high concentrations of helium.
Results from the gas chemistry provide several lines of evidence that temperatures beneath Brimstone Basin are still warm. The ratio of isotopes of carbon and helium in the gases indicate that some of the gas at Brimstone Basin originates from magma. In addition, the methane (CH4) and ethane (C2H6) concentrations and the carbon isotope ratio in methane indicate that other gases emerge from heated sedimentary rocks. The gas and isotope ratios are consistent with a high temperature source for the formation of the studied gases.
An additional finding was that the local gas discharge is sufficiently high that it strongly affects the isotopic ratio of oxygen in waters of the local streams. Normally, surface waters all over the world fall along what is known as the meteoric water line. The abundant rising CO2 at Brimstone Basin strips heavy oxygen (18O) from the groundwater, resulting in an unusual isotopic chemistry of the water. Bergfeld and colleagues showed that this phenomenon would only occur if the CO2 was initially at or near boiling temperatures (about 90°C), which provides additional evidence for warm conditions in the nether regions beneath Brimstone Basin.