- Hilton, David Dr.
The aim of the award was to determine the carbon footprint of one passively degassing volcano as a means to evaluate how mantle-derived carbon is partitioned between direct degassing through high-temperature vents and diffuse gas loss via groundwater systems. Lassen Volcano was selected because it is located on an active subduction zone and has both high-temperature vents as well as an extensive groundwater system accessible by wells. A combination of public supply wells and private (household or domestic) wells throughout the Lassen catchment were sampled for CO2, He-isotopes and gas chemistry. Groundwaters were collected from a total of 42 wells. The region of Lassen Peak also has numerous high-temperature degassing vents, and these localities were sampled for He, CO2, and gas chemistry. Using airborne (helicopter) techniques, we measured a cross section of the CO2 and H2S-rich gas plume emitted from the Bumpass Hell hydrothermal area.
Estimates of the magmatic CO2 flux for Lassen are ~3.5 x 107 kg/yr which has been increased to ~4.3 x 107 kg/yr if the Hat Creek-Rising River-Crystal Lake region is added. At face value, therefore, our helicopter-based measurements imply that Bumpass Hell represents ~3.4% of the total magmatic CO2 flux at Lassen. More significant, however, is the observation that if our estimate of the groundwater-borne magmatic CO2 load is correct, then ~26% of the total magmatic CO2 flux ( = 5.8 x 107 kg/yr) enters the groundwater system(s) and is carried away from the summit degassing regions. In this way, ~74% of the true magmatic CO2 flux at Lassen is obtained by using various techniques (e.g., remote sensing) that measure fluxes targeting direct-venting sites – located at or close to the volcano summit. The question now posed is the general applicability of these observations to other volcanoes worldwide used in deriving CO2 flux estimates from the solid Earth.