Soil efflux and total emission rates of magmatic CO2 at the Horseshoe Laketree kill, Mammoth Mountain, California, 1995-1999

We report the results of eight soil CO2 efflux surveys by the closed circul ation chamber method at the Horseshoe Lake tree kill (HLTK) - the largest t ree kill on Mammoth Mountain. The surveys were undertaken from 1995 to 1999 to constrain total HLTK CO2 emissions and to evaluate occasional efflux su rveys as a surveillance tool for the tree kills. HLTK effluxes range from 1 to > 10.000 g m(-2) day(-1) (grams CO, per square meter per day): they are not normally distributed. Station efflux rates can vary by 7-35% during th e course of the 8- to 16-h surveys. Disturbance of the upper 2 cm of ground surface causes effluxes to almost double. Semivariograms of efflux spatial covariance fit exponential or spherical models; they lack nugget effects. Efflux contour maps and total CO2 emission rates based on exponential, sphe rical, and linear kriging models of survey data are nearly identical; simil ar results are also obtained with triangulation models, suggesting that the kriging models are not seriously distorted by the lack of normal efflux di stributions. In addition, model estimates of total CO2 emission rates are r elatively insensitive to the measurement precision of the efflux rates and to the efflux value used to separate magmatic from forest soil sources of C O2. Surveys since 1997 indicate that, contrary to earlier speculations, a t ermination of elevated CO2 emissions at the HLTK is unlikely anytime soon. The HLTK CO2 efflux anomaly fluctuated greatly in size and intensity throug hout the 1995-1999 surveys but maintained a N-S elongation, presumably refl ecting fault control of CO2 transport from depth. Total CO2 emission rates also fluctuated greatly, ranging from 46 to 136 t day(-1) (metric tons CO2 per day) and averaging 93 t day(-1). The large inter-survey variations are caused primarily by external (meteorological) processes operating on time s cales of hours to days. The externally caused variations can mask significa nt changes occurring at depth; a striking example is the masking of a degas sing event generated at depth and detected by a soil gas sensor network in September 1997 while an efflux survey was in progress. Thus, occasional eff lux surveys are not an altogether effective surveillance tool for the HLTK, and making them effective by greatly increasing their frequency may not be practical. Published by Elsevier Science B.V.