The isotopic data of the fossil and matrix/cement components of the Mississippian Burlington Limestone (Iowa and Missouri) and Silurian Read Bay Formation (Arctic Canada) can be reconciled with textural and trace-metal observations of the components only if the 18O content of ancient oceans differed from that of the present. Examination of the 18O content of the least altered low -Mg calcite components of the two formations suggests that the 18O content of Mississippian seawater was, on average, about 1 ppt lighter and that of Silurian seawater was, on average, about 5 ppt lighter than the 18O content of present-day seawater.
The δ13C values of the components are bimodally distributed. This distribution is independent of geologic age, inferred original mineralogy of the component, degree of diagenetic alteration, and the type of diagenetic system, with components from both formations contributing to the light (+0.5 ± 0.4 ppt) and heavy (+3.1 ± 0.7 ppt) 13C groups. Both groups also show a decrease in 13C of about 1 ppt with stabilization of the carbonates in the diagenetic system.