EFFECT OF PRESERVATION ON THE GEOCHEMISTRY OF LATE CRETACEOUS MOLLUSKS FROM THE WESTERN INTERIOR SEAWAY OF NORTH AMERICA. PART II: MINOR ELEMENTS, O, AND C ISOTOPES

Isotopic ratios of oxygen and carbon from mollusk shells offer data for the reconstruction of the Western Interior Seaway's (WIS) paleoceanographic and paleoclimatic dynamics. However, some WIS fossil material is diagenetically altered, placing into question the reliability of interpretations based on O and C isotopic values. This study investigates two minor-element proxies, Mg/Ca and Sr/Ca ratios, to ascertain diagenetic alteration in samples of ammonite and inoceramid shell from the Baculites compressus and Baculites cuneatus biozones from the Upper Campanian (Late Cretaceous) Pierre Shale. Anomalously low δ¹⁸O values occur within shell samples where the Sr/Ca ratios are less than 2 mMol/Mol or Mg/Ca ratios are greater than 6 mMol/Mol. These minor-element concentrations are more variable in specimens with yellow or orange shell color, and in those with a dull luster. Many of the specimens with an SEM Preservation Index of “fair” to “poor” (2 to 1 on a scale of 5, with 5 equivalent to “excellent” implying ammonites with shell material identical to that of Recent Nautilus) have such minor element concentrations. Presence of cement inside the shell and sampling location (septum versus phragmacone) does not significantly influence Mg/Ca and Sr/Ca ratios. Furthermore, minor elements prove useful in separating samples taken from shells, cements, and concretions. The matrix of concretions has lower Sr/Ca ratios and a broader range of Mg/Ca ratios than shells inside the concretions. Cements (i.e., sparry calcites) are characterized by low δ¹³C and δ¹⁸O values, also have low Sr/Ca and Mg/Ca ratios, distinct from the higher ratios of the shell samples. Well-preserved shell material shows distinct ranges of isotopic variation dependent on taxonomic affinity. These results suggest that care must be taken in selecting material for isotopic analysis, and geochemical as well as SEM examination can reveal suspect shell material.