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Earth surface temperatures warmed by ~5ºC during an ancient (~56 Ma) global warming event referred to as the Paleocene-Eocene thermal maximum (PETM). A hallmark of the PETM is a carbon isotope excursion (CIE) signaling the release of massive amounts of 13C-depleted carbon into the ocean-atmosphere system, but substrate-specific differences in the CIE magnitude are a source of uncertainty for estimating the mass of carbon emitted. Here we report that SIMS-based in situ measurements of d13C in minute (7 um) domains of planktic foraminifer shells (ODP Site 865, central Pacific Ocean) yield a CIE that is ~2‰ larger than that delineated by conventional ‘whole-shell’ d13C values for this same PETM record. SIMS-based measurements on diagenetic crystallites yield d13C values (~2.8‰) that fall between those of pre-CIE and CIE planktic foraminifer shells, indicating that the crystallites are an amalgamated blend of pre-CIE and CIE carbonate. This suggests that diagenesis shifts the whole-shell d13C compositions of pre-CIE and CIE foraminifers found in samples straddling the base of the PETM interval toward the intermediate d13C composition of the crystallites thereby dampening the amplitude of the isotopic excursion. The diagenetic process envisioned would be most consequential for carbonate-rich PETM records that have suffered chemical erosion of pre-CIE carbonate. Given that the domains targeted for SIMS analysis may not be pristinely preserved, we consider the 4.6‰ excursion in our SIMS-based d13C record to be a conservative estimate of the full CIE for surface-ocean dissolved inorganic carbon.