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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Abstract
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 18O of Mississippian seawater was, on average, about 1.5 lighter, and that of Silurian seawater was about 5.5 lighter than the 18O of present day ocean water.
The average diagenetic shift in 18O for the Read Bay Formation, which stabilized in a partly closed diagenetic phreatic meteoric system, is about - 1 for the original low-Mg calcite and about -2 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 18O is about -3 for the original low-Mg calcite and about -4 f r the original high-Mg calcite components.
The 13C 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 13C 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% and for the heavy -13C group it is +3.25% (PDB).Both 13C groups also show a decrease in 13C of about 1% with increasing diagenetic alteration (stabilization).
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