HR: 0830h
AN: V21C-0522    [PDF]
TI: Acid Loading of Soils by Magmatic CO$_{2}$ at Mammoth Mountain, California
AU: * McGee, K A
EM: kenmcgee@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Court 100, Vancouver, WA 98683 United States
AU: Gerlach, T M
EM: tgerlach@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Court 100, Vancouver, WA 98683 United States
AU: Doukas, M P
EM: mdoukas@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Court 100, Vancouver, WA 98683 United States
AB: Areas of tree kill appeared in the early 1990's after a shallow intrusion of magma under the south flank of Mammoth Mountain, California. Subsequent field measurements have revealed high concentrations of soil CO$_{2}$ in these areas, the locations of which are controlled by faults and fractures that serve as conduits for magmatic CO$_{2}$ streaming to the surface from depth. Detailed surveys at the largest of these tree-kill areas, Horseshoe Lake, about 14 ha in size, have consistently shown soil CO$_{2}$ concentrations that range up to 90% or greater in the shallow soil layers. Continuous soil CO$_{2}$ monitoring stations established in 1995 at Horseshoe Lake reveal a pattern of both short-term and seasonal variations in magmatic CO$_{2}$. Because the pressure of CO$_{2}$ is externally fixed by CO$_{2}$ streaming to the surface, carbonic acid activity is constrained by open-system buffering of magmatic CO$_{2}$. Eight years of intensive soil CO$_{2}$ monitoring have documented a consistent pattern whereby pH values as low as 4 can be achieved in the soil solution during spring melting of the winter snow pack. Coupled with the seasonal drop in pH, aluminum, which can also be toxic to forest ecosystems, is released from soils in those areas with the highest CO$_{2}$ concentrations. After more than a decade of exposure to elevated levels of CO$_{2}$ and repeated cycles of acid loading, along with nearly complete tree and vegetation mortality and the release of Al$^{3+}$, the soils at Horseshoe Lake and the other areas of tree kill may not recover their ability to sustain any significant level of forest production for several years, even if the CO$_{2}$ degassing should stop immediately. The level of in-situ acid loading by magmatic CO$_{2}$ in the tree kill areas around Mammoth Mountain rivals that of the better known process of rain-out of acid gases from volcanic plumes in the troposphere.
DE: 1030 Geochemical cycles (0330)
DE: 8400 VOLCANOLOGY
DE: 8419 Eruption monitoring (7280)
DE: 8424 Hydrothermal systems (8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting