Late Ordovician (Hirnantian) glaciation, indicated by periglacial paleosols, tillites, and glacial pavements in Saharan Africa, has been attributed to advances in weathering and carbon sequestration due to evolution of early land plants, in the same way that Devonian-Permian glaciation has been attributed to evolution of forests and Quaternary glaciations to evolution of grasslands. Two problems for carbon cycle explanations of this glaciation are an Hirnantian CO2 greenhouse estimated at 4035 ppmv from mass balance models and possibly related Hirnantian mass extinctions. This estimate of high CO2 is a 10-m.yr. average, here evaluated against high-resolution sequences of paleosols that give estimates not only of CO2 but also of paleoclimate and vegetation through the Late Ordovician. Katian paleosols of the eastern United States show increases in depth and in degree of carbon consumption and storage comparable with those preceding glaciation of the Devonian-Permian and Quaternary. Early thalloid land plants with rhizoids may have drawn down atmospheric CO2 to only 166 ± 83 ppmv during the Hirnantian, as estimated here from a pedogenic CO2 paleobarometer. Ordovician paleosols also reveal a short-term greenhouse spike to as much as 4670 ppmv immediately before Hirnantian glaciation. This greenhouse spike may be due to thermogenic methane release from a large igneous province and is comparable with greenhouse spikes during other mass extinctions, such as those of the Late Permian and Late Jurassic.

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