In the context of the archetypal hyperthermal (the PETM), the end-Permian event presents an extreme comparison. Unlike the relatively benign effects of the PETM on the biota (apologies to the benthic foraminifera), the end-Permian event was the largest mass extinction of animals in Earth history. Yet the two events left similar records in the carbon isotope composition of limestones, reflecting potentially quite similar carbon cycle perturbations, both likely triggered by volcanism. Why was one a mass killer and the other not?
Despite the similar isotope record, there are differences: the end-Permian event occurred at the culmination of Paleozoic-long environmental trends reflected in the sedimentological, isotopic, and geochemical records, reflecting maxima or minima in sea level, continentality, weathering rates, hydrothermal activity, and tectonic uplift. Moreover, the seafloor at the time may have been largely free of the blanket of acid-absorbing calcium carbonate that has existed to varying extents and thicknesses since the Jurassic. Thus, the pre-existing conditions for these two disturbances differ substantially.
While the carbon isotope records are strikingly similar in magnitude and rate of change, they may have resulted from quite different rates of carbon injection: negative carbon isotope anomalies are the consequence essentially of the product of the rate of carbon addition and its isotopic composition: the same perturbation can result from slower addition of a source depleted in the heavy isotope (methane or volatilized organic matter addition triggered by a small volcanic addition) or a faster addition of carbon from a heavier source (e.g., the volcanism itself). Thus the more acute response in the end-Permian may indicate that much of the carbon addition was magma-sourced. Needed is a second proxy, e.g., the B isotope proxy of pH as being applied to the PETM, to constrain the source and rate of C addition. However, more voluminous release of CO2 and consequent warming could have overwhelmed the ocean uptake, seafloor carbonate dissolution, and silicate weathering feedbacks that more effectively damped the PETM perturbation.
The styles of volcanism potentially differed substantially, with greater subaerial eruption and injection of toxic materials into the atmosphere during the end-Permian Siberian Traps event.
Clearly the duration of C cycle disruption, the scale of biotic impact and delay of recover, and the persistence and extent of anoxia distinguish the end-Permian from the PETM. Whether the difference was in the rate and duration of C addition or in the pre-existing conditions remains to be determined.