How does diffuse CO2 flux vary across the East African Rift System?

Magma-rich continental rifts represent a key tectonic setting for natural CO₂ emissions into Earth's atmosphere, and may play a dominant role in modulating Earth's climate on geological timescales. However, the total volume of mantle CO₂ emitted at rift settings is poorly constrained, as is the mechanisms that control variations in CO₂ flux over the lifetime of rifting. In order to test how mantle CO₂ fluxes vary as a function of initial lithospheric conditions, magmatic-volcanic outputs, and the stage of rift evolution, we examined diffuse CO₂ emissions at various settings along the magma-rich Eastern Branch of the East Africa Rift System (EARS) using the soil CO₂ accumulation chamber method. Our results suggest that the volume of diffusely emitted mantle CO₂ at rift settings is dependent on the nature of the carbon source and total magma production. In settings with higher rates of magmatism/volcanism (i.e., Afar Depression, Ethiopia, and Magadi basin, Kenya), greater CO₂ outputs are observed in rift basins that have undergone less stretching, and are hence underlain by a greater volume of subcontinental lithospheric mantle. For early-stage basins (e.g., Natron, Manyara, and Balangida basins of Tanzania), mantle CO₂ fluxes reduce as the EARS crosses from the Proterozoic Mobile Belt to the Tanzanian Craton. This implies that mantle CO₂ flux in the EARS is also dependent on the nature of the initial rifted lithosphere (e.g., craton vs mobile belt). We conclude that annual estimates of mantle CO₂ emissions from continental rift systems must not only consider magma production rates, but also the state of the initial cratonic lithosphere and the volume of subcontinental lithospheric mantle underlying the rift system. Many of these conditions will vary over the course of rift evolution.