Soil Gas CO2 Concentrations and CO2 Fluxes in the Auckland Volcanic Field

Abstract

Numerous studies have shown that anomalously high soil gas CO2 fluxes and concentrations may be precursors to volcanic activity, and that faults or fractures may act as conduits for gases and magma to the surface. The Auckland Volcanic Field (AVF) is a dormant monogenetic basaltic field located in Auckland, New Zealand, on which there have been no previous studies of soil gas CO 2 fluxes or concentrations; atmospheric CO 2 is likewise not monitored. As part of the DEtermining VOlcanic Risk in Auckland project, soil gas CO2 fluxes and concentrations were measured in seven transects spanning various settings (e.g. urban, rural, areas of known or suspected faults, areas of no suspected faults) in the AVF over one week in November 2010. There were two key goals for this pilot study: establish a baseline soil gas CO2 flux and concentration for the AVF; and to attempt to detect subsurface structures such as faults that may act as a control on future vent locations. During this study, 72 measurements of CO 2 concentrations ranged from 393 parts per million (ppm) to 10,140 ppm; 443 fluxes varied from 0 to 108.7 g m-2 d -1 . Using a graphical statistical approach, two populations of CO 2 fluxes were identified. Both may represent the biological CO2 production background flux in the AVF, with the main control attributed to soil permeability. Differences in urban versus rural fluxes may be attributed to greater infiltration of air with higher CO 2 concentrations in urban areas. No faults could be discerned using soil gas CO2 fluxes or concentrations; however, a heteroscedastic 2 tail test indicates that there is a significant difference between fault and no-fault transects. These results hint at interesting variations in soil CO2 across Auckland; future work includes expanding the study area and identifying the sources of CO 2 via 13 C isotope analysis.