Abstract
Estimating the quantity of CO2 diffusively emitted from the Earth’s surface has important implications for volcanic surveillance and global atmospheric CO2 budgets. However, the identification and quantification of non-hydrothermal contributions to CO2 release can be ambiguous. Here, we describe a multi-parametric approach employed at the Nisyros caldera, Aegean Arc, Greece, to assess the relative influence of deep and shallow gases released from the soil. In April 2019, we measured diffuse soil surface CO2 fluxes, together with their carbon isotope compositions, and at a depth of 80 cm, the CO2 concentration, soil temperature, and the activities of radon and thoron. The contributions of deep CO2 and biogenic CO2 fluxes were distinguished on the basis of their carbon isotope compositions. A Principal Component Analysis (PCA), performed on the measured parameters, effectively discriminates between a deep- and a shallow degassing component. The total CO2 output estimated from a relatively small testing area was two times higher with respect to that observed in a previous survey (October 2018). The difference is ascribed to variation in the soil biogenic CO2 production, that was high in April 2019 (a wet period) and low or absent in October 2018 (a dry period). Accounting for seasonal biogenic activity is therefore critical in monitoring and quantifying CO2 emissions in volcanic areas, because they can partially- or completely overwhelm the volcanic-hydrothermal signal.
Highlights
Estimating the quantity of CO2 diffusively emitted from the Earth’s surface has important implications for volcanic surveillance and global atmospheric CO2 budgets
Values of soil T and CCO2 range over large intervals, noting the different processes controlling each of these variables
Such processes were investigated applying the graphical statistical approach (GSA11, see “Methods”), that entails analyzing the distribution of the data in log-probability plots, where a normal population delineates a straight line, while n normal overlapping populations define curves characterized by n−1 inflection points
Summary
Estimating the quantity of CO2 diffusively emitted from the Earth’s surface has important implications for volcanic surveillance and global atmospheric CO2 budgets. In active volcanic areas and fault zones, the relatively fast advective transport of fluids along permeable structures may mobilize the radon gas produced in the crust for longer d istances[15,16] According to this scenario, the gas measured from volcanic soil may be interpreted as a mixture between two different contributions[9]: (1) shallow radon gas production (i.e., low 222Rn/220Rn ratio) within an undisturbed soil and (2) deep radon gas production (i.e., high 222Rn/220Rn ratio) with transport towards the surface in the presence of carrier gases (e.g., CO2). From the 1st to the 9th of April 2019, we monitored emissions at 55 locations (Fig. 1c): (1) 32 sampling points located at the DDS 9 (samples A); (2) 16 sampling points located at the east of DDS 9 (samples B); (3) 7 sampling points randomly located at the hydrothermal craters (samples C)
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