CLIMATE CHANGE ACROSS THE PERMIAN/TRIASSIC BOUNDARY

Abstract

The Permian- Triassic interval was a critical period of time for most of Earth's biosphere and witnessed the most severe extinction event of the Phanerozoic. Present evidence suggests that extraterrestrial influences had little, if anything, to do with these dramatic changes in the biota. Instead, evidence is growing that climate change was a major factor. Late Permian and Early Triassic glacial deposits are unknown and there is no geological evidence for even a brief episode of global cooling at this time. Studies of high latitude palaeosols and data from isotope analyses of biogenic and abiogenic marine carbonates suggest that the Permian-Triassic was a time of global warming, although some of the isotope data are derived from samples that have clearly been altered during diagenesis. Many authors have attributed Permian-Triassic temperature rise to a runaway greenhouse driven by elevated atmospheric CO2 levels, causing temperature rise and breakdown of methane hydrates, which in turn led to methane release and oxidation, thus further increasing atmospheric CO2. Apart from a negative d 13 C isotope excursion, which could have been the result of many other factors, no evidence exists for methane release. In some sections, the isotope shift post-dates the extinction level. There is excellent geological evidence for oceanographic changes such as ocean stagnation, widespread development of anoxia, loss of productivity and sea-level rise. These have all been attributed to rising global temp- eratures, a conclusion supported by results from recent climate simulation models. Re-analysis of the Sr isotope data demonstrates that seawater 87Sr/86Sr ratio varied little through the end-Permian event and immediate aftermath, although values were rising prior to the event and again from the late Induan. This may indicate a temporary decrease in riverine influx to the oceans, which may be climate-related. Overall, the increasing correspondence between computer-generated predictions of Permian- Triassic climate and geological evidence bodes well for future studies on the palaeo- climate of this critical interval in Earth history.