Mark Patzkowsky and Rudy Slingerland
Understanding the links between environmental change and the Earth's biota continues to be a main challenge for the earth sciences. In the past few years, most research in this area has concentrated on mass extinction intervals because they represent extreme perturbations to the global environment and because mass extinctions have had an unusually large effect on the composition of the Earth's biota by resetting patterns established during background times. More recent analyses of the fossil record indicate, however, that mass extinctions are only the rare end members in a range of perturbations effecting Earth's biota that vary in magnitude and geographic extent from regional to global (Raup, 1991). Moreover, these more numerous smaller perturbations may have played an important, but previously unheralded, role in major biotic transitions that took place over millions of years (Miller, 1998).
Identifying the direct link between environmental perturbation and extinction at any scale requires intimate knowledge of single, unique events, making the discovery of general rules or patterns elusive. Progress toward a general understanding can be made by studying components of the global environment (e.g., tropical marine ecosystems) that existed for long periods of time and that have seen repeated environmental perturbations produced by similar causes. In this context, ancient epeiric seas provide a well-constrained system for study. Although rare today because of the overall lowstand of sea level, epeiric seas were widespread in the past during highstands of sea level, such as the early and middle Paleozoic, and for many times provide the only record we have of the marine realm. These broad, shallow seas were likely characterized by unusual conditions of temperature, salinity, nutrient concentrations, and circulation patterns. Because small fluctuations in sea level could have profound effects on these environmental conditions, faunas living in these seas were prone to extinction (Johnson, 1974). Furthermore, endemic faunas populated ancient epeiric seas, so that environmental perturbations limited to single continents could still have profound effects on global diversity. The record of rapid extinction and immigrations events in epeiric seas is now well-established (Palmer, 1965, 1981; Brett and Baird, 1996; Walliser, 1996; Patzkowsky and Holland, 1997), but their causes remain poorly understood.
Given the importance of epeiric seas for the record of global marine diversity and given the lack of modern analogues for epeiric seas, a systematic approach to the study of these ancient systems is warranted. We propose to study Middle and Late Ordovician epeiric seas of Laurentia as a case study. Specifically, we will use a quantitative circulation model to test several circulation hypotheses and to test scenarios for rapid environmental perturbations that may have caused known faunal extinction and immigration events. Although our study is limited in temporal and geographic scope, we will learn some general principles about epeiric ecosystems if we can determine reasonable oceanographic mechanisms to explain the Middle and Upper Ordovician faunal patterns.
The main goals of the study are 1) to apply a quantitative circulation model to Middle and Late Ordovician epeiric seas of Laurentia to test various hypotheses of circulation and 2) to identify mechanisms of rapid oceanographic change that caused faunal extinction and immigration events.
Quantitative Circulation Modeling: We will use a quantitative ocean circulation model to establish circulation patterns in the Middle and Late Ordovician epeiric seas and to test several previously proposed hypotheses of circulation based on conceptual models. Our approach will begin with a global atmospheric circulation model (AGCM) to define wind fields across Laurentia. Next, we will use the wind fields to drive an ocean circulation model. We will model three time slices (Chatfieldian, Edenian, Richmondian) in order to cover any long-term changes in circulation associated with the effects of the Taconic Orogeny. Finally, we will test specifically the role of tectonism in driving circulation by looking at changes in windfields and it's effects on circulation associated with the rise of the Taconic highlands, and by looking at the effect of foreland basin development on circulation patterns.
Testing Mechanisms of Rapid Oceanographic Change: We will use the modeling results described above as a baseline for exploring mechanisms to produce rapid paleoceanographic shifts that can explain biotic events. For example, we will investigate how a rise in sea level might change circulation patterns and oceanographic conditions within the epeiric sea. Such investigations will have general application to epeiric sea environments throughout time.