Diffuse soil CO2 emissions at rift volcanoes: Structural controls and total budget of the Olkaria Volcanic Complex (Kenya) case study

Continental rift systems are first-order emitters of deep-sourced CO2 but their impact on the global budget is largely unknown. For instance, the estimate of the total amount of deep-sourced CO2 released by the East African Rift (EAR) is largely undocumented due to the small number of direct measurements available and the impracticability of remote sensing methods induced by atmospheric noise. This study explores the degassing budget and the structural control of soil CO2 emissions at an active rift volcano aiming to implement the database of available direct measurements of EAR's volcanoes. We investigated soil CO2 degassing at the Olkaria Volcanic Complex, a large, multicentred, geothermally exploited volcanic complex in the southern portion of the Kenyan Rift. A total of 1158 soil CO2 flux measurements were collected on fumarolic fields or crosscutting unaltered major faults. The contribution of multiple sources of CO2 (biogenic background vs magmatic/hydrothermal) was tested using both graphical statistical analysis and the carbon isotopic signature ( 13C-CO2). Soil CO2 fluxes reached up to ~5500 gm-2d-1. The emission of deep-sourced CO2 coupled with the structurally controlled hydrothermal fluid circulation, sums up to ~280 tCO2 released per day by the entire complex. Finally, we provide an estimate of the total budget for soil CO2 of the rift, summing up the contributions of all volcanoes in the EAR.