Michael P. Doukas

Soil efflux and total emission rates of magmatic CO2 at the Horseshoe Lake tree kill, Mammoth Mountain, California, 1995–1999

Abstract We report the results of eight soil CO 2 efflux surveys by the closed circulation chamber method at the Horseshoe Lake tree kill (HLTK) — the largest tree kill on Mammoth Mountain. The surveys were undertaken from 1995 to 1999 to constrain total HLTK CO 2 emissions and to evaluate occasional efflux surveys as a surveillance tool for the tree kills. HLTK effluxes range from 1 to >10,000 g m −2 day −1 (grams CO 2 per square meter per day); they are not normally distributed. Station efflux rates can vary by 7–35% during the course of the 8- to 16-h surveys. Disturbance of the upper 2 cm of ground surface causes effluxes to almost double. Semivariograms of efflux spatial covariance fit exponential or spherical models; they lack nugget effects. Efflux contour maps and total CO 2 emission rates based on exponential, spherical, and linear kriging models of survey data are nearly identical; similar results are also obtained with triangulation models, suggesting that the kriging models are not seriously distorted by the lack of normal efflux distributions. In addition, model estimates of total CO 2 emission rates are relatively insensitive to the measurement precision of the efflux rates and to the efflux value used to separate magmatic from forest soil sources of CO 2 . Surveys since 1997 indicate that, contrary to earlier speculations, a termination of elevated CO 2 emissions at the HLTK is unlikely anytime soon. The HLTK CO 2 efflux anomaly fluctuated greatly in size and intensity throughout the 1995–1999 surveys but maintained a N–S elongation, presumably reflecting fault control of CO 2 transport from depth. Total CO 2 emission rates also fluctuated greatly, ranging from 46 to 136 t day −1 (metric tons CO 2 per day) and averaging 93 t day −1 . The large inter-survey variations are caused primarily by external (meteorological) processes operating on time scales of hours to days. The externally caused variations can mask significant changes occurring at depth; a striking example is the masking of a degassing event generated at depth and detected by a soil gas sensor network in September 1997 while an efflux survey was in progress. Thus, occasional efflux surveys are not an altogether effective surveillance tool for the HLTK, and making them effective by greatly increasing their frequency may not be practical.

Three‐year decline of magmatic CO2 emissions from soils of a Mammoth Mountain Tree Kill: Horseshoe Lake, CA, 1995–1997

We used the closed chamber method to measure soil CO2 efflux over a three-year period at the Horseshoe Lake tree kill (HLTK)—the largest tree kill on Mammoth Mountain in central eastern California. Efflux contour maps show a significant decline in the areas and rates of CO2 emission from 1995 to 1997. The emission rate fell from 350 t d−1 (metric tons per day) in 1995 to 130 t d−1 in 1997. The trend suggests a return to background soil CO2 efflux levels by early to mid 1999 and may reflect exhaustion of CO2 in a deep reservoir of accumulated gas and/or mechanical closure or sealing of fault conduits transmitting gas to the surface. However, emissions rose to 220 t d−1 on 23 September 1997 at the onset of a degassing event that lasted until 5 December 1997. Recent reservoir recharge and/or extension-enhanced gas flow may have caused the degassing event.

Emission rates of sulfur dioxide and carbon dioxide from Redoubt Volcano, Alaska during the 1989–1990 eruptions

Airborne measurements of sulfur dioxide emission rates in the gas plume emitted from fumaroles in the summit crater of Redoubt Volcano were started on March 20, 1990 using the COSPEC method. During the latter half of the period of intermittent dome growth and destruction, between March 20 and mid-June 1990, sulfur dioxide emission rates ranged from approximately 1250 to 5850 t/d, rates notably higher than for other convergent-plate boundary volcanoes during periods of active dome growth. Emission rates following the end of dome growth from late June 1990 through May 1991 decreased steadily to less than 75 t/d. The largest mass of sulfur dioxide was released during the period of explosive vent clearing when explosive degassing on December 14–15 injected at least 175,000 ± 50,000 tonnes of SO2 into the atmosphere. Following the explosive eruptions of December 1989, Redoubt Volcano entered a period of intermittent dome growth from late December 1989 to mid-June 1990 during which Redoubt emitted a total mass of SO2 ranging from 572,000 ± 90,000 tonnes to 680,000 ± 90,000 tonnes. From mid-June 1990 through May 1991, the volcano was in a state of posteruption degassing into the troposphere, producing approximately 183,000 ± 50,000 tonnes of SO2. We estimate that Redoubt Volcano released a minimum mass of sulfur dioxide of approximately 930,000 tonnes. While COSPEC data were not obtained frequently enough to enable their use in eruption prediction, SO2 emission rates clearly indicated a consistent decline in emission rates between March through October 1990 and a continued low level of emission rates through the first half of 1991. Values from consecutive daily measurements of sulfur dioxide emission rates spanning the March 23, 1990 eruption decreased in the three days prior to eruption. That decrease was coincident with a several-fold increase in the frequency of shallow seismic events, suggesting partial sealing of the magma conduit to gas loss that resulted in pressurization of the shallow magma system and an increase in earthquake activity. Unlike the short-term SO2 decrease in March 1990, the long-term decrease of sulfur dioxide emission rates from March 1990 through May 1991 was coincident with low rates of seismic energy release and was interpreted to reflect gradual depressurization of the shallow magma reservoir. The long-term declines in seismic energy release and in SO2 emission rates led AVO scientists to conclude on April 19, 1991 that the potential for further eruptive activity from Redoubt Volcano had diminished, and on this basis, the level of concern color code for the volcano was changed from code yellow (Volcano is restless; earthquake activity is elevated; activity may include extrusion of lava) to code green (Volcano is in its normal ‘dormant’ state).

The reawakening of Alaska's Augustine volcano

Augustine volcano, in south central Alaska, ended a 20-year period of repose on 11 January 2006 with 13 explosive eruptions in 20 days. Explosive activity shifted to a quieter effusion of lava in early February, forming a new summit lava dome and two short, blocky lava flows by late March (Figure 1). The eruption was heralded by eight months of increasing seismicity, deformation, gas emission, and small phreatic eruptions, the latter consisting of explosions of steam and debris caused by heating and expansion of groundwater due to an underlying heat source.

Evidence for dike emplacement beneath Iliamna Volcano, Alaska in 1996

Two earthquake swarms, comprising 88 and 2833 locatable events, occurred beneath Iliamna Volcano, Alaska, in May and August of 1996. Swarm earthquakes ranged in magnitude from −0.9 to 3.3. Increases in SO2 and CO2 emissions detected during the fall of 1996 were coincident with the second swarm. No other physical changes were observed in or around the volcano during this time period. No eruption occurred, and seismicity and measured gas emissions have remained at background levels since mid-1997. Earthquake hypocenters recorded during the swarms form a cluster in a previously aseismic volume of crust located to the south of Iliamna’s summit at a depth of −1 to 4 km below sea level. This cluster is elongated to the NNW–SSE, parallel to the trend of the summit and southern vents at Iliamna and to the regional axis of maximum compressive stress determined through inversion of fault-plane solutions for regional earthquakes. Fault-plane solutions calculated for 24 swarm earthquakes located at the top of the new cluster suggest a heterogeneous stress field acting during the second swarm, characterized by normal faulting and strike-slip faulting with p-axes parallel to the axis of regional maximum compressive stress. The increase in earthquake rates, the appearance of a new seismic volume, and the elevated gas emissions at Iliamna Volcano indicate that new magma intruded beneath the volcano in 1996. The elongation of the 1996–1997 earthquake cluster parallel to the direction of regional maximum compressive stress and the accelerated occurrence of both normal and strike-slip faulting in a small volume of crust at the top of the new seismic volume may be explained by the emplacement and inflation of a subvertical planar dike beneath the summit of Iliamna and its southern satellite vents.

Geochemical evidence for a magmatic CO2 degassing event at Mammoth Mountain, California, September–December 1997

Recent time series soil CO2 concentration data from monitoring stations in the vicinity of Mammoth Mountain, California, reveal strong evidence for a magmatic degassing event during the fall of 1997 lasting more than 2 months. Two sensors at Horseshoe Lake first recorded the episode on September 23, 1997, followed 10 days later by a sensor on the north flank of Mammoth Mountain. Direct degassing from shallow intruding magma seems an implausible cause of the degassing event, since the gas released at Horseshoe Lake continued to be cold and barren of other magmatic gases, except for He. We suggest that an increase in compressional strain on the area south of Mammoth Mountain driven by movement of major fault blocks in Long Valley caldera may have triggered an episode of increased degassing by squeezing additional accumulated CO2 from a shallow gas reservoir to the surface along faults and other structures where it could be detected by the CO2 monitoring network. Recharge of the gas reservoir by CO2 emanating from the deep intrusions that probably triggered deep long-period earthquakes may also have contributed to the degassing event. The nature of CO2 discharge at the soil-air interface is influenced by the porous character of High Sierra soils and by meteorological processes. Solar insolation is the primary source of energy for the Earth atmosphere and plays a significant role in most diurnal processes at the Earth surface. Data from this study suggest that external forcing due largely to local orographic winds influences the fine structure of the recorded CO2 signals.

Airborne detection of diffuse carbon dioxide emissions at Mammoth Mountain, California

We report the first airborne detection of CO2 degassing from diffuse volcanic sources. Airborne measurement of diffuse CO2 degassing offers a rapid alternative for monitoring CO2 emission rates at Mammoth Mountain. CO2 concentrations, temperatures, and barometric pressures were measured at ∼2,500 GPS-referenced locations during a one-hour, eleven-orbit survey of air around Mammoth Mountain at ∼3 km from the summit and altitudes of 2,895–3,657 m. A volcanic CO2 anomaly 4–5 km across with CO2 levels ∼1 ppm above background was revealed downwind of tree-kill areas. It contained a 1-km core with concentrations exceeding background by >3 ppm. Emission rates of ∼250 t d−1 are indicated. Orographic winds may play a key role in transporting the diffusely degassed CO2 upslope to elevations where it is lofted into the regional wind system.

Quiescent hydrogen sulfide and carbon dioxide degassing from Mount Baker, Washington

Volcanic H2S emission rate data are scant despite their importance in understanding magma degassing. We present results from direct airborne plume measurements of H2S and CO2 on a 21-orbit survey at eleven different altitudes around Mount Baker volcano in September 2000 utilizing instrumentation mounted in a light aircraft. Measured emission rates of H2S and CO2 were 5.5 td−1 and 187 td−1 respectively. Maximum concentrations of H2S and CO2 encountered within the 4-km-wide plume were 75 ppb and 2 ppm respectively. Utilizing the H2S signal as a marker for the plume allows the corresponding CO2 signal to be more easily and accurately distinguished from ambient CO2 background. This technique is sensitive enough for monitoring weakly degassing volcanoes in a pre-eruptive condition when scrubbing by hydrothermal fluid or aquifers might mask the presence of more acid magmatic gases such as SO2.

Chlorine degassing during the lava dome-building eruption of Mount St. Helens, 2004-2005: Chapter 27 in A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006

Remote measurements of volcanic gases from the Mount St. Helens lava dome were carried out using Open-Path Fourier-Transform Infrared spectroscopy on August 31, 2005. Measurements were performed at a site ~1 km from the lava dome, which was used as a source of IR radiation. On average, during the period of measurement, the volcanic gas contained 99 mol percent H2O, 0.78 percent CO2, 0.095 percent HCl, 0.085 percent SO2, 0.027 percent HF, 4.8×10-4 percent CO, and 2.5×10-4 percent COS close to the active vent. The fluxes of these species, constrained by synchronous measurements of SO2 flux, were 7,200 t/d H2O, 140 t/d CO2, 22 t/d SO2, 14 t/d HCl, 2.0 t/d HF, 54 kg/d CO, and 59 kg/d COS, ±20 percent. Observations of H2O/Cl in the vapor and melt are compared to models of closed- and open-system degassing and to models where a closed system dominates to depths as shallow as ~1 km, and gases are then allowed to escape through a permeable bubble network. Although several features are consistent with this model—for example, (1) H2O/Cl in the gases emitted from stagnant parts of the lava dome, (2) the concentration of Cl in the matrix glass of erupted dacite, and (3) the glass H2O/Cl—the gases emitted from the active part of the lava dome have much higher H2O/Cl than expected. These higher H2O/Cl levels result from a combination of two factors (1) the addition of substantial amounts of ground water or glacier-derived H2O to the gases at shallow depths, such that only ~10 mol percent of the measured H2O is magmatic, and (or) (2) some Cl present as alkali chloride (NaCl and KCl) in the gas phase. The mean molar Cl/S is similar to gases measured at other silicic subduction-zone volcanoes during effusive activity; this may be due to the influence of Cl in the vapor on S solubility in the melt, which produces a solubility maximum for S at vapor Cl/S