The oxygen and carbon isotope composition of Carboniferous fossil components: sea‐water effects

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Abstract
The aragonitic molluscs and lime‐mud of the Pennsylvanian Buckhorn asphalt (Deese Group) of southern Oklahoma precipitated calcium carbonate in oxygen and carbon isotopic equilibrium with ambient sea‐water. In addition, δ18O values indicate that the pelecypods precipitated their shells during the warmer months of the year. The coiled nautiloids probably precipitated their shells in the warm surface water and throughout the year. For the orthocone nautiloids, the δ18O values suggest that they precipitated their shells in deeper/cooler water.The low‐Mg calcite brachiopods of the Mississippian Lake Valley Formation of New Mexico precipitated shells in oxygen and carbon isotopic equilibrium with ambient sea‐water.The δ18O and δ13C values of the Buckhorn and Lake Valley faunas, in conjunction with other published results, suggest that Carboniferous sea‐water was, on a average, depleted in δ18O by 1·5 ± 2‰, PDB, relative to Recent sea‐water. However, the δ13C value of +2.6 ± 2‰, PDB, for average Carboniferous sea‐water is similar to that of Recent ocean water.Early diagenetic alteration of metastable carbonates probably occurs in a meteoric‐sea‐water mixing zone. In this zone the oxygen and carbon isotopic compositions of these components are increased by about 2‐4‰, PDB over their marine composition.