Role of mantle carbonation in trench outer-rise region in the global carbon cycle

A nearly balanced carbon budget between subduction input and degassing output has likely controlled the long-term surface environment and habitability of Earth throughout geological history. However, the ongoing extensive hydration and carbonation of the mantle in trench outer-rise regions may affect the global carbon budget. In this study, we show that the carbon content of the lithospheric mantle can be inferred from geophysical data and thermodynamic modeling. Based on the seismic velocity anomaly in trench outer-rise regions, we estimated that the total carbon flux due to mantle carbonation is 7-31 Mt C/year, with possible fluid-to-rock mass ratios of 250-1000. These values are similar to the carbon uptake by altered oceanic crust, indicating that mantle carbonation has a significant effect on the subduction carbon budget. Although there are large uncertainties on the estimates of the subduction and degassing carbon fluxes, secular cooling of the mantle leads to the development of outer-rise faults associated with bending of the oceanic lithosphere and increased mantle carbonation, which may disrupt the self-regulating system of the global carbon cycle on geological timescales.