Stable-Isotope and trace-element geochemistry of Molluscan fossils from the Cretaceous Bearpaw Marine Cyclotherm of the Western Interior Basin of North american
The Late Cretaceous Bearpaw Formation and its equivalents (products of deposition during the last major transgressive-regressive marine cycle to affect the Western Interior Basin) contain a molluscan fauna dominated by ammonites and bivalves. Other than molluscs, representatives of the invertebrate megafauna! groups that were prevalent in the open seas and oceans are relatively rare or absent. Usually, this is attributed to brackish-water conditions or oxygen-deficient environments in the Bearpaw epicontinental sea. Investigation of stable-isotope and major- and trace-element compositions of wellpreserved carbonate shells of the ammonites and bivalves from the Bearpaw cyclothem provides valuable information about environmental conditions within the Bearpaw sea. A clear relationship exists between the Î´18O values of Bearpaw zonal baculitids and their biostratigraphic sequence. The baculitids from the zones denoting peak transgression have the lowest average Î´18O values (-2.3 to- 0.7%o), whereas those from the underlying and overlying zones have higher Î´18O values (-0.8 to 0.22%o). This pattern of the Î´18O values can only be explained by fluctuations in temperature, rather than variations in freshwater influx, which may have been reduced due to lower precipitation and run-off under drier, warmer, climatic conditions. Inoceramids usually have the highest Î´13C values (0.6 to 5.0 %o) and lowest Î´18O values (-4.0 to -2.8 %o), whereas coeval baculitids have the lowest Î´13C values (-4.8 to -0.3%o) and the highest Î´18O values (-2.3 to 0.0 %o). The large differences in stable-isotope values imply that these animals must have inhabited isotopically distinct reservoirs within the Bearpaw sea. Thus, the baculitids probably hovered and swam mostly in the upper part of the water column in contrast to the inoceramids which are known to have been benthonic. Didymoceras, with isotopic compositions similar to those of benthonic inoceramids, also probably lived very close to the bottom. The heterogeneity of the stable-isotope compositions of these coeval benthonic and nektonic molluscs points to isotopically distinct, stratified, reservoirs within the seaway, much like those inferred for the older Claggett and Greenhorn seas. Causes of this stratification may have involved modification of bottom water through isotopic exchange reactions between the water and the sediments near or at the bottom of the sea. Relatively constant element/Ca ratios of nektonic baculitids indicate a stable chemical composition for the upper water column of the seaway. This suggests a low fresh-water influx, as indicated also by the stable-isotope compositions. Benthonic fossils have more variable compositions due partly to modification by pore waters derived from the sediments. Both nektonic and benthonic molluscs exhibit relatively high Ce/Ce* values ( -0. 16 to 0. 02 ), similar to the seawater in anoxic and dysoxic environments of modern oceans and seas. Therefore, the seaway was probably dysoxic in both its upper and bottom waters through most of the Bearpaw cycle. Low oxygen-level conditions are reflected by characteristics of the Bearpaw sedimentary rocks and fauna in general. With the seeming absence of brackish waters, dysoxia may account for the rarity of other normal, open ocean, invertebrate organisms.