STRATIGRAPHIC CONTROLS ON REGIONAL DIVERSITY IN THE LATE ORDOVICIAN: CORRECTING FOR THE EFFECTS OF FACIES AVAILABILITY AND SAMPLING INTENSITY

Dissecting global diversity into its regional components is important for understanding the processes that drive global diversity. However, regional diversity contains a set of biases that are usually not considered in synoptic global studies and that must be accounted for before making interregional comparisons. For example, depositional sequence architecture and outcrop availability both determine the range of environments available for sampling at localities within a basin. Comparison of diversity among regions must account for such differences. As an illustration, we examined diversity between two regions on the Upper Ordovician epeiric platform of the eastern United States. Recent synoptic studies indicate that the Upper Mohawkian strata on the Cincinnati Arch (Lexington Limestone) contain nearly twice the diversity of genera as observed in the correlative strata on the Nashville Dome. We analyzed diversity in these two regions based on measured sections from two 3rd-depositional sequences. Samples from each region span a similar range of depositional environments from shallow subtidal to offshore. After normalizing for environmental coverage the Cincinnati Arch contains only about 30 percent more genera than the Nashville Dome. In addition, we used rarefaction on specimen counts to standardize sample size between regions. After rarefaction, the Cincinnati Arch contains only about 15 to 20 percent more genera than the Nashville Dome. Alpha diversity is identical in each region so the small increase in diversity on the Cincinnati Arch is accounted for by an increase in beta diversity. Thus, our preliminary analyses indicate that the diversity differences between the Cincinnati Arch and the Nashville Dome suggested by the synoptic studies is likely a product of sampling more specimens and/or more environments rather than due to major differences in alpha or beta diversity between regions.