The clumped and oxygen isotopes of aragonite from serpentinized peridotites 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 from 2017 to 2020 using a Kiel IV carbonate device interfaced with a ThermoFisher Scientific MAT253 isotope ratio mass spectrometer.