<p><strong>Abstract.</strong> Earth's penultimate icehouse, the Late Paleozoic Ice Age (LPIA), was a time of dynamic glaciation and repeated ecosystem perturbation, under conditions of substantial variability in atmospheric <i>p</i>CO<sub>2</sub> and O<sub>2</sub>. Improved constraints on the evolution of atmospheric <i>p</i>CO<sub>2</sub> and O<sub>2</sub> : CO<sub>2</sub>

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... uring the LPIA and its subsequent demise to permanent greenhouse conditions is crucial for better understanding the nature of linkages between atmospheric composition, climate, and ecosystem perturbation during this time. We present a new and age-recalibrated <i>p</i>CO<sub>2</sub> reconstruction for a 40-Myr interval (~313 to 273 Ma) of the late Paleozoic that (1) confirms a previously hypothesized strong CO<sub>2</sub>-glaciation linkage, (2) documents synchroneity between major <i>p</i>CO<sub>2</sub> and O<sub>2</sub> : CO<sub>2</sub> changes and compositional turnovers in terrestrial and marine ecosystems, (3) lends support for a modeled progressive decrease in the CO<sub>2</sub> threshold for initiation of continental ice sheets during the LPIA, and (4) indicates a likely role of CO<sub>2</sub> and O<sub>2</sub> : CO<sub>2</sub> thresholds in floral ecologic turnovers. Modeling of the relative role of CO<sub>2</sub> sinks and sources, active during the LPIA and its demise, on steady-state <i>p</i>CO<sub>2</sub> using an intermediate complexity climate-C cycle model (GEOCLIM) and comparison to the new multi-proxy CO<sub>2</sub> record provides new insight into the relative influences of the uplift of the Central Pangaean Mountains, intensifying aridification, and increasing mafic rock to-granite rock ratio of outcropping rocks on the global efficiency of CO<sub>2</sub> consumption and secular change in steady-state <i>p</i>CO<sub>2</sub> through the late Paleozoic.</p>