PASQUIER-CARDIN, A., ALLARD, P., FERREIRA, T., HATE, C., COUTINHO, R., FONTUGNE, M., JAUDON, M. (1999) - Magma-derived CO₂ emissions recorded in ¹⁴C and ¹³C content of plants growing in Furnas caldera, Azores. Journal of Volcanology and Geothermal Research, 92, 195-208.

​The environmental impact of fumarolic and soil emanations of magma-derived carbon dioxide across Furnas caldera has been investigated by measuring the ¹⁴C and ¹³C content of 40 specimens of different C3 plants (leaves) growing within and outside the degassing areas. The results demonstrate a significant to large ¹⁴C depletion in many of the plants due to assimilation of ¹⁴C-free endogenous CO₂ during photosynthesis and leading to artificial radiocarbon ageing of up to 4400 years. The extent of ¹⁴C ageing broadly correlates with the intensity of gas manifestations at the sampling sites, as inferred from field observations and measurements of excess CO₂ concentrations in the volcanic ground. It also provides a time-integrated measure of the amount of volcanic CO₂ locally admixed to the ambient air; at several sites this accounts for 15 to 40% of total CO₂ (420 to 600 ppm) in enriched air. In some of the plant species (Azalea, Camellia and fern) ¹⁴C depletion is correlated with an enrichment of ¹³C due to assimilation of magma-derived CO with a 4‰ higher ð¹³C than normal atmosphere. The rate of ¹³C enrichment averages ca. 0.18‰ by percent of volcanic carbon fixed in the plant and includes enhanced ¹³C discrimination during photosynthesis as a consequence of increased ambient pCO₂ inferred at -0.0306‰ per added ppm of volcanic (CO₂) . Furnas is one of the few volcanoes where clear C enrichment in plants due to endogenous degassing has been evidenced. Our results can be used to estimate the local intensity of volcanic soil gas fluxes in the emanating areas of Furnas caldera. They also have implications for radiocarbon dating of past eruptive events in the caldera, since plants artificially aged by previous degassing could be trapped in volcanic deposits.