CONTROLS ON ICE SHEET INITIATION DURING THE PALEOZOIC
Paleogeography and atmospheric pCO2 concentrations have each been proposed as explanations for why the majority of the Paleozoic era remained ice-free, while the Late Paleozoic experienced continental-scale ice sheets despite higher solar luminosity. To assess the relative roles of continental configuration and atmospheric pCO2 in the formation of continental-scale ice sheets, we use the GENESIS AGCM version 3.0 coupled to the Penn State ice sheet model to simulate eight different time-slices spanning the early Ordovician (480 Ma) through late Permian (250 Ma). For each time-slice we simulate the climate at three different atmospheric pCO2 concentrations, 560 ppm, 840 ppm, and 1120 ppm, with constant solar luminosity of 97.5% of modern. Additional experiments were also performed to assess long-term variations in solar luminosity on glacial inception.
Our results suggest that paleogeography plays an important role in dictating global temperature, as greater high latitude land mass increases planetary albedo through increased snow cover causing cooler temperatures. However, paleogeography alone cannot explain the geologic record as the most favorable continental configurations for ice sheet initiation represent times with little evidence of widespread glaciation and vice versa. Sufficiently high/low atmospheric pCO2 concentrations can prevent/initiate the formation of ice sheets. Our simulations suggest a threshold for glaciation at atmospheric pCO2 concentrations as high as 840 ppm in the Devonian and Early Carboniferous when Gondwana migrated over the South Pole, to as low as 560 ppm during the Late Paleozoic. Accounting for reduced solar luminosity raises the minimum CO2 value for glaciation in the early Paleozoic to 1120 ppm.
We conclude that the primary culprit behind Paleozoic ice ages is reduced pCO2. In all cases CO2 values at or below 840 ppm promote glacial initiation on Gondwana. In the Late Devonian and Early Carboniferous, paleogeography may have significantly raised the minimum CO2 threshold required for glaciation. In the Late Ordovician and Early Silurian, reduced solar luminosity may have been an important factor as well. Lastly, the Late Paleozoic Ice Age is best explained by a reduction in pCO2, as this event occurred when the paleogeography was not conducive to glaciation.