Fabric-retentive, non-luminescent Helderbergian brachiopods from open marine deposits, analogous to brachiopods used in other studies to determine paleo-oceanic stable isotopic composition, exhibit depleted delta 13 C values immediately below the erosional contact and more enriched values of delta 13 C at greater stratigraphic distances below this surface. In addition, delta 13 C values of these brachiopods display a marked shift across the erosional surface, and values above it are essentially the same as those 30 m or more beneath the surface. Complementing these trends, whole rock analyses, metastable, high-magnesian calcite constituents (echinoderms), as well as early equant calcite cement, also exhibit an increased depletion trend in delta 13 C values as the Helderberg-Oriskany contact is approached from below. Fabric-retentive luminescent, as well as obviously recrystallized brachiopods, display stable isotopic values similar to those of fabric-retentive, non-luminescent brachiopods. Thus, "best-preserved" fabric-retentive, non-luminescent brachiopods exhibit values similar to obviously recrystallized brachiopods, diagenetically altered metastable constituents (echinoderms), stable early meteoric cements, and whole rock samples. Helderberg strata of New York State were affected by early meteoric diagenesis during a Siegenian exposure event prior to deposition of the overlying marine Oriskany Sandstone. Meteoric diagenesis during this exposure gave rise to a characteristic carbon isotopic depletion trend approaching the Helderberg/Oriskany contact. All of the skeletal constituents underwent isotopic exchange during early diagenetic exposure to meteoric waters. Consequently, the practice of using fabric-retentive, non-luminescent brachiopods as indicators of their original chemistry and, by extension, paleo-oceanic chemistry is questioned.