EXAMINING THE TROPICAL BIOTIC RESPONSE TO CLIMATE CHANGE AT THE ONSET OF THE LATE PALEOZOIC ICE AGE: A CASE STUDY USING CHESTERIAN BIOFACIES FROM THE ILLINOIS BASIN, USA

The Upper Mississippian (Chesterian) marks the onset of a prolonged interval of cool and seasonal climates associated with the Late Paleozoic ice age (LPIA). Previous research has shown that this climate shift coincided with a 28% reduction in global marine generic diversity, a biotic consequence that was felt largely within the paleotropics. To date, an investigation of how these large-scale biotic effects play out at the scale of individual tropical basins is lacking. In that context, the aim of this study is to test whether: 1) regional and/or local diversity track global diversity through this interval and 2) the diversity and dominance structures of tropical marine biofacies change significantly across the onset of the LPIA. The Illinois Basin of southern Indiana and Kentucky provides an ideal setting for this study because it preserves a record of 11 successive depositional sequences that span the inferred start of the LPIA. Species diversity and abundance data were collected from skeletal grainstone/packstone and shale facies from four fourth-order sequences across the climate transition. Cluster and ordination analyses recognize two broadly overlapping biofacies that are differentiated based upon their species diversity and dominant taxa: a high diversity, spirifer and productid brachiopod biofacies and a lower diversity, productid brachiopod biofacies. Preliminary comparisons of within facies diversity reveal that it remains largely unchanged across the onset of the LPIA. Furthermore, each biofacies appears to maintain a similar species composition despite undergoing some dominance restructuring. These preliminary results suggest that although species turnover may have been limited, biofacies structure was not rigidly maintained through time. Moreover, regional species diversity, at least in the Illinois Basin, does not appear to track global diversity across the onset of the LPIA.