ABSTRACT

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 δ18O of the Read Bay Formation components, which stabilized in a partly closed 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 δ18O is about —3 ppt for the originally low -Mg calcite components and about —4 ppt for the originally high —Mg calcite components.

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.

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