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 FlashEA 1112 Elemental Analyzer (EA) interfaced via a Conflo IV to a Delta V Plus Isotope Ratio Mass Spectrometer for total carbon (TC) and total non-carbonate carbon (NCC) content and isotope composition, a CM 5012 CO2 coulometer interfaced with a CM 5130 Acidification Module for the total inorganic carbon (TIC) content, and a GasBench II connected to a Delta V Mass Spectrometer for the total inorganic carbon isotope composition. The Results are reported as replicates.