Fossil brachiopod shells are commonly used as a faithful record of the chemical and isotopic composition of the ancient ocean water in which they grew, owing to their stable mineralogy and their presence in marine strata throughout Phanerozoic time. The utility of these fossils for preserving ocean-water compositions is evaluated through analyses of multiple specimens that grew in contemporaneous sea water at individual stratigraphic horizons in the Mississippian-age Burlington-Keokuk Formation of mid-continent North America. Significant variations in Sr, C, and O isotopic compositions are observed in contemporaneous, non-luminescent shells, indicating that apparently pristine brachiopod shell material has been altered by post-depositional processes of diagenesis. The least-altered compositions at each horizon are used to estimate secular variations of 87Sr/86Sr in Early Mississippian ocean water. The estimated rates of change of ocean-water 87Sr/86Sr are comparable in magnitude to the rapid Sr isotopic changes documented for Cenozoic ocean water, for which high-frequency glacial cycles are inferred as a driving mechanism. The rigorous assessment of diagenetic effects on the isotopic signatures of marine phases is requisite for the advancement of high-resolution reconstructions of paleo-ocean chemistry.