Investigations Into Methane and Carbon Dioxide Emissions, Sources, and Pathways in Quaternary Volcanic Calderas in the Western United States

Abstract

Diffuse CO2 emissions from Quaternary volcanic calderas in the western United States have been well characterized at Yellowstone National Park (YNP) but not Valles Caldera National Preserve (VCNP), and diffuse CH4 emissions have not been measured at either caldera, despite the potential for elucidating subsurface dynamics and climate impacts. A cavity ring-down spectrometer (CRDS) and automated chamber were used to measure fluxes and carbon isotope compositions of CO2 and CH4 at multiple sites using a survey style in each caldera. Additionally, a time series of more than a dozen measurements was collected prior to and during an eruption at Steamboat Geyser, marking an extremely unique observation.

The survey measurements showed similar flux ranges at the two calderas despite YNP having a more active hydrothermal system. CO2 fluxes in g m-2 day-1 were -9 to 820 at VCNP and -3 to 672 at YNP. CH4 fluxes in mg m-2 day-1 were -8 to 1554 at VCNP and -6 to 465 at YNP. Impermeable silica sinter at the surface kept fluxes from neutral-chloride soils in YNP low. δ13C-CH4 at VCNP was < -60‰ at some sites but isotopically heavier at others, consistent with a deep source. δ13C-CO2 at VCNP were slightly heavier than atmospheric. At YNP, δ13C-CH4 along the eastern portion of the caldera highlighted active degassing of heavy carbon likely related to a deep source, whereas measurements along the western portion presented a lighter signature indicating a mixed shallow source.

The time series data at Steamboat Geyser showed steady fluxes for 2.5 hours, followed by a sharp decrease of 58% for CH4 and 50% for CO2 10-25 minutes before the eruption and then a quick return to previously measured values. Similarly, precursor δ13C-CH4 and δ13C-CO2 were both distinctly lighter (-35.7 ± 2.1‰ and -6.2 ± 0.4‰, respectively) than the non-precursor measurements (-27.5 ± 0.3‰; -3.9 ± 0.1‰, respectively) that reflected a deep source.

The utility of CRDS for quantifying emissions and isotopic compositions simultaneously and in near-real time of diffuse gases and identifying geyser eruption precursors has been demonstrated.