MONTANEZ, ISABEL, University of California-Davis, Davis, CA

ABSTRACT: Evolution of Permian Atmospheric pCO2 and Western Equatorial Pangean Climate: As Recorded by Paleosol Morphologic and Geochemical Proxies

The evolution of atmospheric pCO₂ throughout Earth’s history has been of increasing interest given the role pCO₂ plays in biogeochemical cycling, continental weathering, climate change, and evolutionary events. Several previous geochemical models estimated the evolution of atmospheric pCO₂ levels for the Phanerozoic. The results of these models suggest notably low CO₂ levels, comparable to present-day values (345 ppm), existed during Permian-Carboniferous glaciation. This pCO₂ minimum was followed by a rapid increase in atmospheric pCO₂ initiated sometime during the latter half of the Permian. Peak pCO₂ levels (3 to 9-fold greater than early Permian minimum values) were reached during the Middle to Late Triassic. This modeled increase in pCO₂ levels during the Permian implies that the greenhouse effect was a major, if not dominant, control on climate during the latter half of the Permian. However, this hypothesized evolution of Permian-Triassic atmospheric pCO₂ levels remained relatively untested in the geologic record until recently.

This lecture will present a newly defined record of atmospheric carbon dioxide levels for the Permian estimated from the d¹³C values of pedogenic carbonates, goethites, soil organic matter, charcoals, and reptile teeth from four depositional basins in North America and Europe. This Permian pCO₂ curve suggests that the transition from Late Paleozoic minimum pCO₂ values to greenhouse conditions occurred very early in the Early Permian, significantly earlier than previously suggested. Moreover, the evolution of pCO₂ throughout the Permian was characterized by several rapid and large magnitude variations, including a second pCO₂ minima in the Early Permian. Estimated peak pCO₂ levels (~2000 to 2500 ppmv) in the Early Permian coincide temporally with numerically modeled estimates of the timing of the end of Late Paleozoic glaciation. This suggests that rapidly increasing CO₂ levels in the Early Permian may have been a major factor in terminating Late Paleozoic glaciation. The subsequent fall in pCO₂ levels may record increased continental weathering associated with deglaciation as well as uplift of Himalayan-scale, Alleghanian, Ouachita, and Hercynian mountains in the Early Permian. Paleoatmospheric pCO₂ levels increase to a second peak in the Middle Permian and then decline progressively throughout the Late Permian. What processes brought on the rapid return to peak pCO₂ levels in the Middle to earliest Late Permian remains to be investigated. Significantly, the Permian-Triassic boundary occurs on the falling limb of the pCO₂ curve, which reaches minimum levels in the Early Triassic. Paleosol morphologies and the stable isotope compositions of pedogenic clays (D and d¹⁸O values) document significant changes in paleo-precipitation and atmospheric circulation coincident with these fluctuations in latest Pennsylvanian through Permian atmospheric pCO₂ .

AAPG Search and Discovery Article #90910©2000-2001 AAPG Distinguished Lectures