THE MICROSTRUCTURE OF PYRITE BLACKENING IN FOSSIL SHELLS

Based on its correlation with taphonomic proxies such as fragmentation and abrasion in large brachiopods, shell blackening in Upper Ordovician (Cincinnatian) brachiopods has previously been identified as a sign of long residence on the sea floor, and has been attributed to the accumulation of iron sulfides and organics in microborings, which suggests dysoxic microenvironments within shells.

In the process of examining hundreds of petrographic thin sections from Cincinnatian limestones, we observed that fragments of brachiopods, trilobites, bryozoans, and echinoderms ossicles commonly contain finely-disseminated pyrite. This pyrite is found in skeletal pores characteristic of the taxon, in microborings and along growth-line boundaries.

To further document the mineralogical basis of blackening, and its progress of development in shells, we prepared transverse thin-sections from a collection of dorsal valves of the brachiopod Cincinnetina meeki collected from the Waynesville Formation of southeastern Indiana. We arrayed the shells by visible degree of blackening and examined them petrographically to determine the microscale distribution of pyrite and elucidate the process.

We observed a progressive degree of pyritization of shell material associated with increasing blackening. Even the lightest colored shells have some pyrite but this is limited to fillings of the larger pore spaces of the punctae. More filled punctae, and more completely-filled punctae correspond to darker brachiopods. Further darkening is associated with pyrite occurring between growth lamellae, and culminates in finely disseminated pyrite throughout the shell associated with the darkest shells.

This structurally-mediated progression confirms the taphonomic significance of shell blackening and suggests a linkage between blackening and shell decay associated with the breakdown and pyritization of more stable organic shell components as well as invading microndolithic fungi and/or bacteria. The dysoxic conditions required for pyrite generation may have occurred as organic matter in pores decayed. As the most blackened shells were likely those exposed longest on the seafloor, we suggest that dark coloration was mediated by decay of colonizing endoliths, which could only attack exposed shells.