Origin of the CO2 fluxes of the Icelandic Hotspot

Carbon dioxide plays a major role in climate change and warming of the Earth's atmosphere. In order to assess past and predict future climate perspectives, it is vital to assess all sources of CO₂ contributing to the global fluxes. The deep Earth's CO₂ fluxes have been evaluated based on degassing of volcanic complexes and geothermal systems. However, passive degassing of CO₂ through tectonically active areas has not been included into such models, such degassing potentially contributing significantly to the Earth's CO₂ fluxes. The sources of CO₂ degassing of active and off-rift systems in Iceland were studied using carbon isotope systematics of gases and groundwaters. The range of CO₂ concentrations, δ¹³C-CO₂ and ¹⁴C-CO₂ was large, ~5-75·10⁴ ppm, -27.4 to +2.0 ‰ and 0.6 to 118 pMC, respectively. Sources of CO₂ were evaluated by comparing the measured chemical and isotope composition with those simulated using isotope geochemical models. Three major sources of CO₂ were identified: (1) dissolution of partially degassed basaltic rocks formed at the surface or shallow depths, (2) atmospheric CO₂ through air-water exchange at surface, and (3) input of gas at depth with similar carbon and isotope composition as the pre-erupted melt of the upper mantle and lower crust beneath Iceland. The CO₂ flux of the Icelandic crust was estimated to be ~5- 10·10¹⁰ mol/yr. Similar fluxes of 7-23·10¹⁰ mol/yr have been obtained using CO₂/ ³He values and ³He flux estimates. Passive degassing through tectonically active zones off-axis account for as high as ~50% of the total flux with significant proportion of the CO₂ originating from the mantle. The CO₂ partial pressures within active volcanic and geothermal systems correspond to ~0.5-10 bar with a flux of ~500- 3000·10⁵ mol CO₂/km²/yr, Off-axis, the CO₂ partial pressures of mantle origin were estimated to be ~10^-6 bar, with an average flux of <5·10⁵ mol CO₂/km²/yr, but as high as 125-1600·10⁵ mol CO₂/km²/yr for the most tectonically active ones. The results indicate that diffusive CO₂ fluxes of deep origin in areas in tectonically active areas outside active volcanic regions may be a significant contribute to Earth's CO₂ degassing.