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