Drought Impact on Fumarole Gas Composition at Lassen Volcanic National Park, CA.

Abstract:

Surface expressions of the hydrothermal system at Lassen Volcanic National Park include numerous solfataras to the south and southeast of Lassen Peak. Extensive sampling of gas for chemical and isotope analyses took place from the mid-1970s through 2000. Based on those data the model for the Lassen hydrothermal system suggests the main gas upflow occurs at Sulphur Works, which is part of a circulating hydrothermal flow cell (HC1) that includes Bumpass Hell and Little Hot Springs Valley (Janik & McLaren, 2010). A second, slightly cooler cell (HC2) provides gas and steam to Boiling Springs Lake, Devils Kitchen and ultimately to features at Terminal Geyser.

As part of the USGS volcano monitoring effort, sampling of Lassen gases resumed in 2014. Gas compositions of boiling point and superheated fumaroles at four of the Lassen solfataras contained lower concentrations of meteoric-derived components than most early samples. In 2014 the combined N₂ and Ar concentrations from the individual areas decreased on average by 50 to 65% relative to CO₂. N₂/Ar ratios also shifted away from the ratio in air-saturated water toward values as high as 171, consistent with a decline in meteoric water content. Concentrations of hydrothermal gases CO₂, H₂S, and CH₄, and d¹³C-CO₂ values are similar to the early data. Helium concentrations are variable and show no trends. Helium isotope ratios (Rc/R_A) since 2007 from HC1 are between 6.9 and 7.5 and for HC2 are between 6.0 and 6.5. A 2014 Terminal Geyser sample had a value of 6.0. A longer record of helium isotope data is required to examine temporal trends.

Water in the Lassen hydrothermal system is ultimately derived from rain and snowfall at higher elevations in the park. The area has experienced drought conditions since at least 2012. The 2014 dataset suggests that the drought may influence gas compositions by reducing the input of shallow meteoric water. Lassen gases from the California drought of 1976-1977 exhibit similar behavior.