Problems for Evaluation of the Scenario of the Permian-Triassic Boundary Biotic Crisis and of Its Causes

摘要

Determination of the causes of the Permian-Triassic boundary (PTB) biotic crisis is hindered primarily by the diachronous nature of the used PTB, poor stratigraphic control of compared Upper Permian and Lower Triassic faunas, especially in continental biotopes, poor knowledge of the lower and middle Scythian faunas from many environments, and by interpolation of the unknown (lower and middle) Scythian diversity from the known Upper Permian and Middle Triassic diversity data in many major fossil groups. Most of these problems can be resolved by using the first appearance datum (FAD) of Hindeodus parvus as either an isochronous PTB, or as an isochronous marker level very close to the base of the Triassic; careful studies of fossil-rich, complete continental boundary sections (e.g., Dalongkou in Sinkiang), and utilisation of uninterpolated diversity data, which are based on known Scythian data, for the reconstruction of the extinction and recovery patterns in all fossil groups. The most important features of the PTB biotic crisis are: (1) Among the marine biota, only the plankton and the warm-water benthos, nektobenthos and nekton are strongly affected by the PTB biotic crisis. (2) The recovery of the warm-water nekton and nektobenthos was very fast (after one conodont zone). The recovery of the warm-water benthos, some of the plankton (radiolarians) and the terrestrial plant productivity was strongly delayed for several million years, and occurred only in the upper Olenekian (upper Scythian) and in the Middle Triassic. (3) The number of the Lazarus taxa that re-appeared in the upper Olenekian and above all in the Middle Triassic, is very high (about 50%) and in some fossil groups 90-100% at generic level. The reconstruction of the scenario for the PTB biotic crisis requires not only the consideration of the uninterpolated extinction and recovery patterns of all fossil groups across all environments, but must also account for the main features of geological evolution from the Middle Permian to the Lower Triassic. The most important causal factors in the PTB biotic crisis are the extinction event at the Guadalupian-Lopingian boundary that restricted the diverse Upper Permian warm-water benthos to the Tethyan shelves, the long-lasting, widespread Siberian Trap volcanism (Dzhulfian-lower Scythian) which was the greatest volcanic event during the Phanerozoic, and the very strong explosive felsic to intermediate volcanism around the PTB, close to the margin between eastern Tethys and Panthalassa. These volcanic activities resulted in those climatic changes that were directly and indirectly (as cause of the oceanic superanoxia) responsible for the PTB biotic crisis, such as periodic cooling of the climate by volcanic dust and sulphate aerosols (mainly caused by the Siberian Trap volcanism), acid rain, a 3-6 month “volcanic winter” at low latitudes and the strongly reduced input of sunlight during the uppermost Dorashamian (both caused by the very strong explosive volcanism at the Tethys/Panthalassa margin), followed by global warming in the lower Scythian, and uppermost Dorashamian to lower Scythian superanoxia.