Annual cycle of magmatic CO 2 in a tree-kill soil at Mammoth Mountain, California: implications for soil acidification

Abstract

Time-series sensor data reveal significant short-term and seasonal variations of magmatic CO 2 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 CO 2 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 CO 2 , rather than direct magma degassing. Hydrologic processes closely modulated seasonal variations in CO 2 concentrations, which rose to 65%-100% in soil gas under winter snowpack and plunged more than 25% in just days as the CO 2 dissolved in spring snowmelt. The high efflux of CO 2 through the tree-kill soils acts as an open-system CO 2 buffer causing infiltration of waters with pH values commonly of < 4.2, acid loading of up to 7 keq H+. ha -1. yr -1 , mobilization of toxic Al 3+ , and long-term decline of soil fertility.