A GONDWANAN PERSPECTIVE ON THE PERMIAN ICEHOUSE-GREENHOUSE TRANSITION: EMPIRICAL INSIGHTS INTO THE ROLE OF ATMOSPHERIC CARBON DIOXIDE

The Permian saw one of the most extreme climate transitions of the Phanerozoic, from deep icehouse conditions in the Early Permian, which marked the acme of the late Palaeozoic ice age (LPIA), to the onset of a protracted greenhouse state by Late Permian time. This transition occurred in a stepped manner, with four multimillion-year long glacial intervals (P1-P4), each progressively less-severe, interspersed with periods of non-glacial conditions. The Permian System of eastern Australia provides a unique record of this transition along a glacially influenced marine shelf. We integrate sedimentological data with geochemical proxies for the oxygen isotope composition of local seawater, paleotemperature, and concentrations of atmospheric carbon dioxide (pCO2) to assess the nature of the climate transition and its controlling factors. Results indicate that pCO₂ varied in concert with paleoclimatic changes inferred from the local stratigraphic record. Although each glacial interval began during a time of low pCO₂, carbon dioxide levels began to recover shortly after the onset of glacial conditions. Times of increasing pCO₂ correspond well to shifts from a predominance of proximal to distal glaciogenic facies. Inferred highs in atmospheric pCO₂ coincide with transitions from glacial to nonglacial deposition as well as with evidence for higher temperatures and increased meltwater discharge. Atmospheric pCO₂ gradually declined through each intervening nonglacial interval. As pCO₂ decreased, so did nearshore oceanic temperatures and rates of meltwater discharge. Whereas the stratigraphic record implies that the onset of each glacial interval was abrupt, carbon isotope records show no such abrupt shifts. Rather, the data are more consistent with glacial epochs commencing after concentrations of atmospheric pCO₂ fell below a particular threshold. Rising pCO₂ soon after the onset of glacial intervals suggests that climate feedbacks may have played a role in controlling the duration of glacial periods. Although these relationships do not reveal the causal mechanism for fluctuating pCO₂ during the Permian, they are consistent with the notion of pCO₂ threshold behavior evident in climate-ice sheet models for the LPIA.