LATE PERMIAN WARMING, THE RAPID LATEST PERMIAN TRANSGRESSION, AND THE PERMO-TRIASSIC EXTINCTION

Permian waning of the Alleghenian/Hercynian/Variscan orogenic assembly of Pangea launched a complex web of feedbacks that led to intense warming in the Late Permian and Early Triassic. The shutting down of uplift and exhumation cut down the availability of chemically weatherable fresh silicate rock. This reduced carbon dioxide drawdown from the atmosphere and diminished the release of nutrients to primary producers that then drew down and buried less carbon. The subsequent warming and melting of polar ice curtailed the sinking of nutrient-rich, cold brines. Consequently weakened wind shear reduced upwelling and further retarded productivity and carbon burial.

As Pangea warmed, dry climates extending to mid-latitudes may have resulted from expansion of the Ferrel circulation cell. Increased coastal evaporation generated warm, nutrient-deficient sinking brines and delivered them to a weakly-circulating deep ocean that became increasingly anoxic and finally euxinic. This warm, saline bottom water delivered heat to high latitudes. Weakening of the thermocline allowed cyclonic storms to reach to great depth, upwelling heat that warmed polar climates. Moist high-latitude paleoclimates reflect shrinkage or breakdown of the polar circulation cell in the Late Permian and Early Triassic as atmospheric moisture and cloud cover was increased to help sustain high-latitude warmth.

If sinking warm mid-latitude brine quickly crossed a threshold to supplant weakened sinking of polar cold brines as the dominant driver of thermhaline circulation in the latest Permian, several feedbacks may be of critical importance. In particular, rapid heating of an already warm Late Permian ocean could generate about 20 meters of thermal expansion, enough to explain the heretofore puzzling boundary transgression. The oceanic warming could have released clathrate-stored oceanic methane to quickly intensify atmospheric heating, possibly triggering the rapid extinction.