Models of Euramerican cyclothem deposition invoke orbitally driven glacioeustasy to explain widespread cyclic marine and nonmarine late Paleozoic sedimentary sequences. Base-level fluctuations of ∼100+ m have been estimated for the deposition of mid-continent North American subpycnoclinal black shales, subaerial exposure relief of algal bioherms in the Sacramento Mountains, and Russian Platform carbonates. Similar to the Pleistocene, these glacioeustatic fluctuations are thought to be driven by variations in orbital parameters. To evaluate this hypothesis, a coupled general circulation model-ice sheet model was used to simulate the effects of both transient orbital changes and variable atmospheric pCO2 concentrations on late Paleozoic continental ice sheets. In our model, large continental ice sheet inception is simulated at and below pCO2 levels of 280 ppm. Model results predict that while changing orbital parameters results in dynamic ice sheet behavior, the maximum orbitally induced sea-level fluctuation is ∼25 m. The model also demonstrates that the complete ablation of ice sheets formed at 280 ppm (∼7.9 × 107 km3, sea-level change ∼135 m) requires an increase in atmospheric pCO2 to levels >2240 ppm. These results present a potential paradox: while our model is able to simulate widespread Gondwanan glaciation, it is unable to reproduce significant orbitally driven glacioeustatic fluctuations without very large magnitude carbon cycle perturbations. We discuss possible solutions to this paradox.
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