estimating the quantity of co 2 diffusively emitted from the Earth's surface has important implications for volcanic surveillance and global atmospheric co 2 budgets. However, the identification and quantification of non-hydrothermal contributions to CO 2 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 co 2 fluxes, together with their carbon isotope compositions, and at a depth of 80 cm, the co 2 concentration, soil temperature, and the activities of radon and thoron. the contributions of deep co 2 and biogenic co 2 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 co 2 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 CO 2 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 co 2 emissions in volcanic areas, because they can partially-or completely overwhelm the volcanic-hydrothermal signal. The emission of volcanic-hydrothermal fluids from fumaroles and soil diffuse degassing structures (DDS) are prevalent forms of thermal energy release in quiescent volcanoes 1,2 , and their monitoring is of primary importance in understanding volcanic activity 3-6. The amount of CO 2 emitted by volcanic DDS is thought to be, globally, a relevant contributor (likely the most important) to the CO 2 budget from volcanic activity to the atmosphere 7,8. However, the uncertainties in determining the amount of the volcanic diffuse CO 2 emission are significant, as biological activity can also produce abundant CO 2. In fact, over the last 20 years, the definition and characterization of the diffuse degassing processes has been based, with a few exceptions, only on CO 2 flux measurements without differentiating between their possibly disparate deep-(i.e., volcanic-hydrothermal) or shallow (i.e., biogenic) sources. Coupling CO 2 flux measurements with other parameters collected from the surface or within the soil in volcanic areas (e.g. 2,9,10), can be crucial to better decipher the actual fraction of gas emitted from magmatichydrothermal systems. This approach is of fundamental importance in cases where the statistical partitioning 11 and subsequent removal from the total CO 2 output of non-hydrothermal CO 2 flux (of biogenic origin) is not easily applicable or ambiguous. Ultimately, a multi-parametric strategy circumvents over-interpretation in the extent and the amount of deep degassing estimated for active volcanoes worldwide. In addition to monitor...
