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DECADE - The Carbon Footprint of a Passively Degassing Volcano
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Carbon is degassed from active volcanoes prior to and during magmatic eruptions, but it has been recently discovered that magmatically inactive volcanoes may degas a significant amount of C passively. We are studying the carbon concentration of groundwater in several locations surrounding Lassen Peak in order to determine the extent to which deep carbon may be degassed into the water table surrounding a passively degassing volcano. 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.


Groundwaters: 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 distribution of sampling points within a 60 km radius of Lassen Peak is given in the figure (left). 3He/4He ratios (R) > the atmosphere values (RA) indicate the presence of mantle-derived He throughout the region. This is consistent with widespread dispersal of magmatic volatiles from the volcanic system.


It is important to note that dissolved CO2 in groundwater is composed of different end members. Our measurements of dissolved inorganic carbon (DIC) can be corrected to "external" carbon by subtracting the carbonate-derived carbon obtained through the water chemistry. We determined the water chemistry for all groundwater samples and used a simple mixing model to resolve external carbon into organic and endogenic (deep or mantle) CO2, assuming fixed isotope and concentration characteristics.


Using this approach, we find that between 2% and 74% (average = 31±19%) of the total DIC is composed of endogenic CO2. Thus, using our measurements of total DIC, we can calculate the median concentration of endogenic CO2 in groundwaters of the catchment ~ 7 x 10-4 moles/L.


Extensive studies have been carried out on groundwater fluxes in the Lassen region with cold water flow rates estimated at ~8000 L/s (excluding Hat Creek region and eastern Lassen). This value yields an endogenic CO2 flux of 7.8 x 106 kg/yr. Adding this value to prior estimates of Hat Creek (7.6 x 106 kg/yr) gives a total groundwater endogenic CO2 flux of ~ 1.5 x 107 kg/yr.


Thirty groundwater samples were analyzed for gas chemistry and almost all samples are dominated by N2. CO2 contents range from 1% to 99.6% with the majority of samples between 5% and 25% CO2. The highest CO2 contents are found at significant distances from the crater (~30 km). CH4 ranges from 0 to 8.6%. The average CH4/CO2 is 0.22 for groundwaters resulting in a CH4 flux of 3.3 x106 kg/yr (using 1.5 x 107 CO2 flux). Therefore, although the high temperature areas dominate total C emissions from Lassen, in terms of CO2, the ground water flux of CH4 dwarfs the high temperature CH4 flux by six orders of magnitude.


High-T Vents:  The region of Lassen Peak has numerous high-temperature degassing vents, and these localities were sampled for He, CO2 and gas chemistry.


The He-CO2 systematics clearly show the distinction between HT volatiles and groundwaters. The HT vents emit volatiles with arc-like He-CO2 characteristics.


Thirteen gas samples collected at the high-temperature sites were analyzed for chemistry. Samples are dominated by CO2 (87-97 mol% dry gas with one at 53%). CH4 ranges from 4 x10-5 to 0.007 mol% dry gas. The average CH4/CO2 is 1.6 x10-7, which results in a CH4 flux of 7 kg/yr (using 4.3 x107 CO2 flux). 


CO2 Flux Measurements: Using airborne (helicopter) techniques, we measured a cross section of the CO2 and H2S-rich gas plume emitted from the Bumpass Hell hydrothermal area. When combined with our measured wind speed (5.8 m/s), and the CO2/H2S ratio of the gases, the results indicate that Bumpass Hell emits about 4 metric tons per day (T/d) or 1.5 x 106 kg/yr of CO2 and 0.2 (T/d) H2S. 


CO2 Mass Balance: 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% only of the true magmatic CO2 flux at Lassen is obtained by using various techniques (e.g., remote sensing) which 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


The volcano mass spec (V-café) was deployed in the Lassen HT area and CO2, N2, Ar abundance data were collected continuously for 72 hours. CO2/N2 ratios measured are an order of magnitude lower than those of collected gas samples indicating significant mixing with air.

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Reporting Year 2012 Click to expand

  • RY2012-1 - submitted on Jan 01, 2012

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    [2012-01-01] Assembled research team.
  • RY2012-2 - submitted on Jul 01, 2012

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    [2012-07-01] Groundwater sampling at Mt. Lassen.
  • RY2012-3 - submitted on Sep 01, 2012

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    [2012-09-01] Groundwater sampling at Mt. Lassen.

Reporting Year 2014 Click to expand

  • RY2014-1 - submitted on Dec 01, 2011

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    [2011-12-01] Presented related work at International Conference on Gas Geochemistry in La Jolla, CA, USA.
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