Abstract:
Owing to the fact that low-Mg calcite fossil shells are so important in
paleoceanographic research, 249 brachiopod, cement and matrix specimens
from two neighboring localities (Jemez Springs and Battleship Rock), of the
Upper Pennsylvanian Madera Formation were analyzed. Of which, about 86%
of the Madera brachiopods are preserved in their pristine mineralogy,
microstructure and geochemistry. Cement and matrix samples, in contrast, have
been subjected to complete but variable post-deposition~1 alteration. It is
confirmed that the stable isotope data of brachiopods are much better than that
of matrix material in defining depositional parameters. Because there is no
uniform or constant relationship between the two data bases (e.g., from 0.1 to
3.0%0 for 0180 and from 0.2 to 6.7%0 for 013C in this study), it is not possible to
make corrections for the matrix data. Regarding the two stratigraphic sections,
elemental and petrographic analyses suggest that Jemez Springs is closer to
Penasco Uplift than Battleship Rock. Seawater at Jemez Springs is more
aerobic, and the water chemistry is more influenced by continental sources than
that at Battleship Rock. In addition, there is a relatively stronger dolomitization
in the mid-section of the Battleship Rock.
Results further suggest that no significant biogenic fractionation or vital
effects occurred during their shell secretion, suggesting that the Madera
brachiopods incorporated oxygen and carbon isotopes in equilibrium with the
ambient seawater. This conclusion is not only drawn from the temporal and
spatial analyses, but also supported by brachiopod inter-generic comparison
(Composita and Neospirifer) and statistical analysis ( t-test).
