Effects of High Carbon Dioxide Soil-Gas Concentrations and Emission Rates From Mammoth Mountain, California, USA

High concentrations (90 vol %) of carbon dioxide (CO2) are present in shallow soils, and CO2 is emitted to the atmosphere at high rates (1,000 g/d/m2), in several locations around Mammoth Mountain. The CO2 emissions have been diffuse and at ambient temperature. CO2 in the soil has killed most of the coniferous forest in five areas totaling 35 ha around the north, west, and south sides of the mountain at altitudes between 2,600 and 3,000 m. Part of the CO2 has dissolved in ground water, causing acidic conditions and severely corroding steel casings in several wells. The high CO2 emission rates are implicated in the deaths of four people in the past eight years. During winter, a large quantity of CO2 is sequestered in the snow pack on parts of the mountain, posing potential dangers for winter recreation. One U.S. Forest Service campground has been closed and safety plans have been implemented by the local ski resort. Mammoth Mountain is a dormant Quaternary volcanic center, but overlies an area that has been affected by periods of magmatic unrest during the past two decades. Hypocenters of long-period earthquakes indicate that basaltic intrusions reach depths as shallow as 20 to 15 km, from which CO2 has exsolved during decompression and (or) crystallization of these intrusions. CO2 moves to the land surface along fracture zones associated with faults and possibly geologic contacts. The magmatic source of CO2 is confirmed by ¦Ä13C = -3 to -5 PDB, a lack of 14C, and 3He/4He = 4 to 5 R/RA. The present-day high CO2 soil-gas concentrations and emission rates were first documented in 1994; however, anecdotal information and low 14C in post-1989 tree rings suggest that an abrupt increase in both concentrations and emission rates probably began in 1990, following a 6-month period of seismic swarm activity beneath the mountain. Emissions in an area on the south flank of the mountain have been the focus of CO2 monitoring and have shown no indications of abatement between 1994 and 2005, during which estimates of the total CO2 efflux ranged from 90 to 150 MT/d. The variations can be partly attributed to the precision of the techniques and to minor differences in measurement protocols between researchers; variations in soil- moisture and atmospheric conditions alone can cause fluctuations in efflux of ± 10% over periods of hours to days.