The solid Earth breathes as volcanoes “exhale” gases like carbon dioxide (CO2)—which are essential in regulating global climate—while carbon ultimately from CO2 returns into the deep Earth when oceanic tectonic plates are forced to descend into the mantle at subduction zones. However, the amount of carbon in the sediments and ocean crust that subducts is poorly constrained, as is the fraction of that breaks down in the mantle and contributes to volcanic CO2.