The carbon geochemistry of serpentinized peridotites, gabbroic rocks, and sediments recovered during IODP Expedition 357 (October to December 2015) on the Atlantis Massif, Mid-Atlantic Ridge was examined to characterize carbon sources and the fate of dissolved organic (DOC) and inorganic carbon (DIC) in seawater during long-lived hydrothermal circulation and serpentinization. The measurements were performed at the ETH Zurich, Geological Institute from 2017 to 2020 using a GasBench II connected to a Delta V Mass Spectrometer.