Seasonal Environmental Controls on Soil CO2 Dynamics at a High CO2 Flux Sites (Piton de la Fournaise and Mayotte Volcanoes)

Environmental parameters drive seasonal soil CO2 efflux toward the atmosphere. However, their influence is not fully understood in contexts of high CO2 fluxes where CO2 accumulates in the subsurface. A prime example are volcanoes subject to continuous CO2 diffuse degassing rising from deep magmatic reservoirs, through the subsurface and up to the atmosphere. For many of these volcanoes where soil CO2 is monitored, a seasonal influence of the atmosphere and water table is observed but not characterized. Here, we compare variations of air temperature, atmospheric pressure, rainfall and water table level with near‐surface soil CO2 concentration by performing a time‐lagged detrended cross‐correlation analysis on years‐long time series from the volcanoes of Piton de la Fournaise and Mayotte. At Piton de la Fournaise, soil CO2 variations correlate best with air temperature variations (0.81) and water table variations (0.74). In Mayotte, soil CO2 variations correlate best with atmospheric pressure variations (−0.95). We propose that at Piton de la Fournaise, the thick vadose zone and high permeability favor CO2 transfer by thermal convection. Additionally, energy transfer is decoupled from mass transfer. Slow heat transfer from the atmosphere down to the accumulated CO2 layers in the subsurface results in a delayed influence of air temperature and of the water table level on the thermal gradient between the subsurface and the atmosphere, and consequently on the efficiency of the CO2 transfer. In Mayotte, we propose that the thin vadose zone and the presence of a network of large fractures favor CO2 transfer by barometric pumping. Plain Language Summary The soil is a major reservoir of gaseous CO2 at the scale of the planet. This soil CO2 is mobile and can escape to the atmosphere and contribute to global warming. It thus important to understand what controls its mobility. In contexts of high soil CO2 contents, environmental parameters can drive soil CO2 transfers toward the atmosphere, but their influence is still poorly understood at timescales beyond a few days. To address this issue, we studied how air temperature, atmospheric pressure, rainfall, and water table level affect soil CO2 efflux in volcanic environments where the soil CO2 content is naturally high due to magmatic degassing. Our study sites are the volcanoes of Piton de la Fournaise and Mayotte. We performed a time‐lagged detrended cross‐correlation analysis which compares the similarity between dif