DYNAMIC SHIFTS IN GLACIAL-INTERGLACIAL ATMOSPHERIC CO2 AND TROPICAL VEGETATION DURING EARTH’S LAST ICEHOUSE

The Carboniferous—early Permian (325 to 260 Myr) has long been considered a sustained period of low CO₂ coupled with rising pO₂ to a Phanerozoic maximum (26 to 30%) in the early Permian. Such low CO₂ estimates suggest very low radiative forcing given 3% lower solar luminosity at that time, a condition that may have been magnified by pO₂-induced higher atmospheric density. The implication of this reduced radiative forcing on the late Paleozoic climate system is strongly debated, in large part due to poorly constrained atmospheric CO₂ estimates. We present an integrated pedogenic carbonate and fossil cuticle reconstruction of atmospheric CO₂ through 16 million years of the latter half of the Pennsylvanian and earliest Permian developed using a cyclothem series in the Illinois Basin as well as a subset of samples from the Appalachian and Donets basins. Overall, reconstructed CO₂ falls below the modeled threshold (560 ppm) for late Paleozoic glacial inception, well within the range of ice sheet stability during the LPIA (up to 840 ppm). The suborbital resolution record reveals CO₂ variations between ~200 and 700 ppm with an apparent long eccentricity pacing. Short-term CO₂ fluctuations are superimposed on a 10⁶-yr CO₂ trend that varies in-step with major sea level changes and glacial advances and retreats inferred from Donets Basin and Midcontinent stratigraphic trends.

Comparison of the CO₂ reconstruction and inferred sea level with published paleobotanical records for tropical Pangaea suggests a mechanistic relationship between CO₂, glaciation, and major shifts in lowland vegetation, highlighting the important role terrestrial carbon storage likely played in creating the hierarchy of atmospheric CO₂ fluctuations. Specifically, substantial changes in floral composition of paleotropical lowlands within eccentricity glacial-interglacial cycles occurred coincident with 200 to 300 ppm fluctuations in CO₂. Additionally, three ecologic thresholds in the Pennsylvanian were coincident with major CO₂ shifts suggesting a strong role for CO₂-forcing of these ecologic events, whether indirectly through its influence on climate or directly through physiological consequences for photosynthesis and plant viability.