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.

Automated summary

Environmental parameters have a significant impact on the seasonal soil CO2 efflux to the atmosphere, especially in cases where CO2 accumulates in the subsurface. Through time-lagged detrended cross-correlation analysis, it was found that variations in air temperature, atmospheric pressure, rainfall, and water table level are related to near-surface soil CO2 concentration in the volcanoes of Piton de la Fournaise and Mayotte. The transfer mechanism of CO2 differs in these two regions: thermal convection in Piton de la Fournaise and barometric pumping in Mayotte.