This study examines the oxygen and carbon isotopic distribution in the brachiopods, crinoids, rugose corals, and matrix/cement from the Mississippian Burlington Limestone (Iowa and Missouri) and the Silurian Read Bay Formation (Arctic Canada). The isotopic data can be reconciled with textural and trace element observations only if the 18 O content of ancient oceans differed from that of the present oceans. This postulated secular variation in the 18 O content of seawater requires a separate evaluation of the Mississippian and Silurian carbonate components. Examination of the 18 O content of the least-altered low-Mg calcite fossil components of the two formations suggests that the delta 18 O of Mississippian seawater was, on average, about 1.5 per thousand lighter, and that of Silurian seawater was about 5.5 per thousand lighter than the delta 18 O of present day ocean water. The average diagenetic shift in delta 18 O for the Read Bay Formation, which stabilized in a partly closed diagenetic phreatic meteoric system, is about - 1 per thousand for the original low-Mg calcite and about -2 per thousand for the original high-Mg calcite and aragonite components. Conversely, for the Burlington Limestone, which stabilized in an open diagenetic phreatic meteoric system, the average diagenetic shift in delta 18 O is about -3 per thousand for the original low-Mg calcite and about -4 per thousand for the original high-Mg calcite components. The delta 13 C values of the originally low-Mg calcite (or high-Mg calcite with less than 7 mole % MgCO 3 ) brachiopods and rugose corals, the originally high-Mg calcite crinoids, and the originally aragonite lime mud of the Burlington Limestone and Read Bay Formation are bimodally distributed. This bimodal delta 13 C distribution is independent of geologic age, inferred original mineralogy of the component, degree of diagenetic alteration, and the type of diagenetic meteoric system, with components from both the Read Bay Formation and Burlington Limestone contributing to both 13 C groups. The mode of the light - 13 C group is +0.25% per thousand and for the heavy - 13 C group it is +3.25% per thousand (PDB).Both 13 C groups also show a decrease in 13 C of about 1% per thousand with increasing diagenetic alteration (stabilization).