We have characterized the stable isotopic composition of marine cements from four Middle Ordovician units from Laurentia and Baltica. A common working assumption of isotope stratigraphy is that marine d¹³C values are much more resistant to diagenetic resetting and therefore should retain isotopic compositions indicative of ambient marine DIC d¹³C. However, recent studies of Cenozoic shoal carbonates have demonstrated the ability of microbial activity to significantly affect marine carbon values. Consequently, it is important to develop criteria that can be applied to ancient carbonates to delineate early diagenetic microbial activity that can cause deviations in the marine carbon record. Low-Mg calcite marine cement from the Chickamauga Formation (Alabama) provide an example of this pitfall; this cement was associated with Mn reduction. Chickamauga cement has marine d¹⁸O values and yet is brightly luminescent with Mn values approaching 1 wt.%. This cement has more negative d¹³C values than most of the marine cements from the nearly coeval Holston (Tennessee), Kullsberg (Sweden), and Effna (Virginia) formations. Another example comes from marine cement of the Effna Formation that is interpreted to have formed during sulfate reduction. This cement has more negative d¹³C values compared with other marine cements from this unit. In conclusion, it is recommended that when possible marine cements should be examined for early diagenetic microbial effects and that if present metazoan skeletal components (e.g. brachiopods) that were likely to have formed under aerobic conditions should be utilized for d¹³C chemostratigraphy.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah