To understand the fate of carbon in subducted oceanic sediments and crust, we studied carbonate phases in rocks from the Mariana subduction zone. These show that carbon is liberated from the downgoing plate at depths less than 20 km. Some of the carbon is subsequently trapped in minerals and likely subducts to greater depths, whereas fluids carry the other part back into the ocean. Our findings imply that shallow subduction zone processes may play an important role in the deep carbon cycle.