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 average diagenetic shift in ^dgr¹⁸O of the Read Bay Formation components, which stabilized in a partly closed

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Figure

diagenetic system, is about -1 ppt for the originally low -Mg calcite components and about -2 ppt for the originally high -Mg calcite and aragonite components. Conversely, for the Burlington Limestone components, which stabilized in an open diagenetic system, the average diagenetic shift in ^dgr¹⁸O is about -3 ppt for the originally low -Mg calcite components and about -4 ppt for the originally high -Mg calcite components.

The ^dgr¹³C 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.

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