THE INFLUENCE OF SHELL MICROSTRUCTURE ON THE PRESERVATION AND ISOTOPIC COMPOSITION OF LATE CRETACEOUS BIVALVE AND GASTROPOD SHELLS

The isotopic composition of shell carbonate is widely accepted as a proxy for paleoenvironmental conditions, such as temperature, salinity and productivity. In order to obtain reliable signals from shell carbonate, the degree of diagenetic alteration must first be assessed. As this and other studies show, specimens that are visually well preserved are not necessarily unaltered and require further investigation using, for example, high-powered microscopy to examine shell microstructure.

As part of a comprehensive paleoecological study of the Late Cretaceous Owl Creek Formation type locality in Mississippi, visually well-preserved bivalves and gastropods were collected from a 9-meter thick sequence of micaceous muds and silts. Shell flakes were manually removed from specimens and their cross-section and surface examined at 15,000 magnification using scanning electron microscopy to determine the type of microstructure and degree of preservation. We created a qualitative scale of preservation, with pristine microstructure, characterized by distinct and well-defined crystals, ranking as 5. With increasingly poor preservation, crystals show fusion and have irregular boundaries, with a value of 1 appearing as almost entirely fused and indistinct. Once these determinations were made, the carbon and oxygen isotopic compositions of the shell carbonate were analyzed.

As expected, the data show that with decreasing quality of preservation, isotopic values of both cross-lamellar and laminar specimens display a directional change, increasingly departing from the values of well-preserved material. In addition, the data suggest that shell microstructure plays a role in the susceptibility to diagenetic alteration. Bivalve samples with laminar microstructure are better preserved than samples of bivalves and gastropods with cross-lamellae. Combined, these results demonstrate the importance of examining shell microstructure to procure reliable proxy estimates.