Annual cycle of magmatic CO2 in a tree-kill soil at Mammoth Mountain, California: Implications for soil acidification

Time-series sensor data reveal significant short-term and seasonal variations of magmatic CO₂ in soil over a 12 month period in 1995–1996 at the largest tree-kill site on Mammoth Mountain, central-eastern California. Short-term variations leading to ground-level soil CO₂ concentrations hazardous and lethal to humans were triggered by shallow faulting in the absence of increased seismicity or intrusion, consistent with tapping a reservoir of accumulated CO₂, rather than direct magma degassing. Hydrologic processes closely modulated seasonal variations in CO₂ concentrations, which rose to 65%–100% in soil gas under winter snowpack and plunged more than 25% in just days as the CO₂ dissolved in spring snowmelt. The high efflux of CO₂ through the tree-kill soils acts as an open-system CO₂ buffer causing infiltration of waters with pH values commonly of <4.2, acid loading of up to 7 keq_H+ṁha⁻¹ṁyr⁻¹, mobilization of toxic Al³⁺, and long-term decline of soil fertility.