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Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Petrology
Vol. 51 (1981)No. 3. (September), Pages 987-997

Chemical Diagenesis of a Multicomponent Carbonate System -2: Stable Isotopes

Uwe Brand (2), Jan Veizer

ABSTRACT

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 18O content of ancient oceans differed from that of the present oceans. This postulated secular variation in the 18O content of seawater requires a separate evaluation of the Mississippian and Silurian carbonate components. Examination of the 18O content of the least-altered low-Mg calcite fossil components of the two formations suggests that the ^dgr18O of Mississippian seawater was, on average, about 1.5^pmil lighter, and that of Silurian seawater was about 5.5^pmil lighter than the ^dgr18O of present day ocean water.

The average diagenetic shift in ^dgr18O for the Read Bay Formation, which stabilized in a partly closed diagenetic phreatic meteoric system, is about - 1^pmil for the original low-Mg calcite and about -2^pmil 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 ^dgr18O is about -3^pmil for the original low-Mg calcite and about -4^pmil f r the original high-Mg calcite components.

The ^dgr13C values of the originally low-Mg calcite (or high-Mg calcite with less than 7 mole % MgCO3) 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 ^dgr13C 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 13C groups. The mode of the light -13C group is +0.25%^pmil and for the heavy -13C group it is +3.25%^pmil (PDB).Both 13C groups also show a decrease in 13C of about 1%^pmil with increasing diagenetic alteration (stabilization).


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