Abstract: Ordovician Carbon Isotope Stratigraphy: Some Lessons from Detailed Unit Characterization
TOBIN, K.J., Department of Geosciences, Guyot Hall, Princeton University, Princeton, NJ, 08831; K.R. WALKER, Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996
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