PERMIAN – TRIASSIC BOUNDARY EVENTS: A POSSIBLE MANTLE CONNECTION

Central Tethyan Permian – Triassic sections display remarkable similarities, typified by lithological and geochemical successions from the Shahreza section of Iran. Here, Upper Permian strata consist of highly fossiliferous, relatively pure limestone. The Transition Zone (TZ) layers are red in color, highly clay rich, dolomitic, poor in calcite, and barren in fossils. Lower Triassic rocks are light-colored, pure limestone having abundant cement – microbial features. Geochemically, δ¹³C and Sr values decline from the Upper Permian toward the TZ and increase in the Lower Triassic. Mg, Mn, and Fe concentrations increase from the Upper Permian toward the TZ and decrease above.

We suggest that the events associated with the Paleozoic to Mesozoic transition – the mass extinction, the eruption of Sieberian Traps volcanism, and major changes in ocean-atmospheric system – were all caused by the heating of the crust by the mantle possibly related to processes which eventually led to the break-up of the Pangea. There need not be any cause and effect relationships between the boundary events themselves.

In this scenario, crustal warming produced a rise in global temperature ending the equable Late Permian condition. Accompanied by igneous intrusions, crustal warming resulted in destabilization of methane gas hydrates, maturation of sequestered organic matter, cracking of liquid hydrocarbons, and heating of subsurface brines. Upward migration of these fluids altered oceanic and atmospheric chemistries. Seawater became toxic and even corrosive to carbonates. Saturation of seawater with CO₂ - CH₄ lowered partial pressure dissolved O₂ which, in combination with high Ca²⁺ and high CO₃^- concentrations, caused the marine mass mortality. Conditions remained inhospitable for carbonate producing organisms during the Lower Triassic promoting carbonate deposition primarily by microbial and abiotic processes.