George E. Hilley David R. Shelly John C. King John P. McGeehin Margaret T. Mangan William C. Evans Jennifer L. Lewicki 2014 Unrest at Mammoth Mountain over the past several decades, manifest by seismicity, ground deformation, diffuse CO₂ emissions, and elevated ³He/⁴He ratios in fumarolic gases has been driven by the release of CO₂-rich fluids from basaltic intrusions in the middle to lower crust. Recent unrest included the occurrence of three lower-crustal (32–19 km depth) seismic swarms beneath Mammoth Mountain in 2006, 2008 and 2009 that were consistently followed by peaks in the occurrence rate of shallow (≤10 km depth) earthquakes. We measured ¹⁴C in the growth rings (1998–2012) of a tree growing in the largest (∼0.3 km²) area of diffuse CO₂ emissions on Mammoth Mountain (the Horseshoe Lake tree kill; HLTK) and applied atmospheric CO₂ concentration source area modeling to confirm that the tree was a reliable integrator of magmatic CO₂ emissions over most of this area. The tree-ring ¹⁴C record implied that magmatic CO₂ emissions from the HLTK were relatively stable from 1998 to 2009, nearly doubled from 2009 to 2011, and then declined by the 2012 growing season. The initial increase in CO₂ emissions was detected during the growing season that immediately followed the largest (February 2010) peak in the occurrence rate of shallow earthquakes. Migration of CO₂-rich magmatic fluids may have driven observed patterns of elevated deep, then shallow seismicity, while the relationship between pore fluid pressures within a shallow (upper 3 km of crust) fluid reservoir and permeability structure of the reservoir cap rock may have controlled the temporal pattern of surface CO₂ emissions. application/pdf 10.1016/j.epsl.2013.12.035 en Elsevier Crustal migration of CO2-rich magmatic fluids recorded by tree-ring radiocarbon and seismicity at Mammoth Mountain, CA, USA article