The Upper Ordovician of the Cincinnati, Ohio, Region: A Natural Laboratory for Studying the Sedimentological and Biological Effects of Calcite Sea Chemistry

The Cincinnatian Group (Upper Ordovician, Caradoc) of limestones and shales in the upper midwest of the United States is an excellent subject for calcite sea studies because it is easily accessible and contains numerous paleontological and sedimentological features which resulted from this characteristic seawater chemistry. The most prominent evidence for early low magnesium calcite (LMC) cementation is the large number of carbonate hardgrounds throughout the group. These hardgrounds are often very common (up to a dozen in just one vertical meter of section) and thoroughly bored and encrusted. Most of these hardgrounds are biosparite grainstones with many molds of aragonitic mollusks which likely supplied the LMC cement after dissolution of their shells. Some hardgrounds are almost pure micrite with few shells of any kind in the matrix, which raises questions about where the cement for these hardgrounds originated and how it circulated in these tight sediments. It is possible there was a significant amount of aragonitic bioclastic mud present as a cement source, although we do not yet have evidence for it. The hardgrounds provided significant hard substrate attachment and boring space for a variety of marine invertebrate taxa, showing a direct contribution of calcite sea chemistry to ecological diversification, especially of sclerobionts. Evidence for early seafloor aragonite dissolution is common in the Cincinnatian, including abundant bored and encrusted internal and external molds, sediment-filled external molds, and attachment-surface secondary encrustation. Mollusks in the Cincinnatian which had aragonite shells show a variety of characteristics which may have been adaptations to corrosive calcite sea conditions, including bimineralic shells (aragonite interiors and calcite exteriors) and thick periostraca.