PALEOECOLOGIC PERSISTENCE IN THE TROPICS ACROSS THE ONSET OF THE LATE PALEOZOIC ICE AGE: A FIELD-BASED STUDY FROM THE UPPER MISSISSIPPIAN CHESTERIAN ILLINOIS BASIN, USA

The stratigraphic succession of Upper Mississippian (Chesterian) rocks in the Illinois Basin contains a highly resolved record of tropical environmental change and glacio-eustasy across the onset of the Late Paleozoic Ice Age (LPIA). Here, we examine how the diversity and community structure of tropical, shallow-marine, invertebrate assemblages were affected by: 1) the abrupt shift from carbonate dominated sedimentation before the onset of the LPIA to mixed carbonate-siliciclastic dominated sedimentation after, and 2) the initiation of high amplitude glacio-eustasy. To these ends, we sampled fossiliferous facies from nine consecutive fourth-order depositional sequences spanning the onset of the LPIA, yielding a total data set of over 200 collections, 52 taxa and 14,089 fossil individuals.

Results from analytic rarefaction indicate that taxonomic diversity increased by 17 % following the transition to mixed carbonate-siliciclastic sedimentation at the onset of the LPIA. These results are surprising given that recent literature calls for declining diversity both globally and in the tropics during this time interval. The shift to mixed carbonate-siliciclastic dominated sedimentation is associated with little genus-level turnover: greater than 89 % of genera persist into the first depositional sequence across the transition. At a broader scale, over 53 % of all genera, most of them common, are shared between carbonate dominated and mixed carbonate-siliciclastic dominated intervals. Therefore, we attribute the increase in diversity to an increase in the number of rare taxa living in a wider range of habitats on the mixed carbonate-siliciclastic ramp. Following the onset of the LPIA, high amplitude glacio-eustatic changes do not appear to have exerted strong controls on sequence diversity or community structure: in most cases total sequence diversity does not vary significantly through time and cluster analysis and DCA show a high degree of overlap among samples separated by more than 400 kyr and multiple eustatic cycles. These results suggest that, at least in the Illinois Basin, tropical assemblages persisted with relatively little change despite the major environmental changes associated with the onset of the LPIA.