STABLE ISOTOPIC VALUES OF INOCERAMIDS FROM LATE CRETACEOUS BLACK SHALES ON DEMERARA RISE

Inoceramid bivalves are common in the Late Cretaceous laminated black shales on Demerara Rise (paleotropical Atlantic) recovered during Ocean Drilling Program Leg 207. Stable isotopic analyses of calcitic prisms of these inoceramids have average oxygen isotopic values of -2.9‰ (V-PDB) and average carbon isotopic values of 1.2‰ (V-PDB). The lowest inoceramid oxygen value measured to date is -3.3‰. Planktic foraminifera from the same samples exhibit oxygen isotopic values that are ~1.5‰ lower than in the inoceramids but carbon values that are similar to values in inoceramids. No benthic foraminifera were found in these samples.

The occurrence of inoceramids in laminated rocks on Demerara Rise is consistent with many previous studies showing that inoceramids often thrived in settings where bottom water conditions were generally inhospitable. The Demerara Rise inoceramids co-occur with exquisitely well preserved planktic foraminifera, common fish debris, and aragonitic ammonites; however, other benthic taxa (including burrowers) were apparently absent. Also similar to previous studies of inoceramids, the very low oxygen and relatively high carbon isotopic values found on Demerara Rise are difficult to explain.

What distinguishes this study from previous isotopic studies of inoceramids is the quality of preservation of the samples examined. The prisms analyzed were clear, and co-occurring planktic foraminifera exhibit glassy preservation (radial hyaline microstructure preserved, no shell infilling). In studies from several localities including Demerara Rise, analyses of material from samples with glassy preservation have been shown to yield results that are most likely to accurately record depositional values. At face value, results for the inoceramids suggest bottom water temperatures of ~25°C during deposition of the black shales. These values are very warm but are still 6-8°C cooler than estimates for contemporary surface waters.

This paper is co-authored with the Leg 207 Scientific Party, College Station, Texas.