Thresholds for Paleozoic ice sheet initiation

Continental drift and atmospheric greenhouse gas concentrations have each, in turn, been proposed to explain the evolution of Paleozoic climate from early era ice-free conditions to late era continental-scale glaciation, despite continually increasing solar luminosity. To assess the relative roles of continental confi guration and atmospheric pCO₂ on the formation of continental-scale ice sheets, we use a coupled ice sheet-climate model to simulate ice sheet initiation at eight different Paleozoic time slices using uniform topography. For each time slice, we simulate the climate at three atmospheric pCO₂ levels (560, 840, and 1120 ppm) and both constant (97.5% of modern) and time-appropriate solar luminosity values. Under constant luminosity, our results indicate that continental confi gurations favor ice sheet initiation in the mid-Paleozoic (400-340 Ma). After accounting for solar brightening, ice sheet initiation is favored in the early Paleozoic (480-370 Ma) simulations. Neither of these results is consistent with geological evidence of continental-scale glaciation. Changes in atmospheric pCO₂ can reconcile these differences. Suffi ciently high (≥1120 ppm) or low (≤560 ppm) pCO₂ overcomes paleogeographic and luminosity predispositions to ice-free or ice age conditions. Based on our simulations and geological evidence of glaciation and atmospheric composition, we conclude that atmospheric pCO₂ was the primary control on Paleozoic continental-scale glaciation, while paleogeographic confi gurations and solar irradiance were of secondary importance.