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

Abstract

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, espec ially in cont inental biotopes, poor knowledge of the lower and middle Scythian faunas from m,UlY environments, and by interpolation of the unknown (lower and middle) Scythian diversity from the known Upper Pennian and Middle Triassic diversity data in many major fossil gro ups. Most of these problems can be reso lved by using the firs t appearance datum (FAD) of Nil1deodlls pan'Ii,I' as either an isochronous PTB, or as an isochronous marker [eve I very dose 10 lhe base of lhe Triassi c; careful studies of fossil-rich, comp[ele conlinental boundary sections (e.g., Dalongkou in Sinkiang), and uti li sation of uninterpoJated 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 arc: (I) Among the marine biota, only the plankton and the wann-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 warmwater benthos, some of the plankton (radiolarians) and the terrestrial plant product ivity was strongly delayed for several million years, and occurred on ly in the upper Ol enekian (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 fossi l groups 90-100% at generic level. The reconstruction of the scenario for the PTB biotic crisis requires not only the conside'rlItion of the uninterpolaled extinction and recovery patterns of all foss il groups across all envi ronments, but must also account for the main feat ures of geological evolution from the Middle Permian to the Lower Triassic. The most important causal factors in the PTB biotic crisis are the ex tinction event at the Guadalupian-Lopingian boundary that restricted the diverse Upper Permian wann-water benthos 10 the Tet hyan shelves, the long-[asling, widespread Siberian Trap volcanism (Dzhulfian - lower Scythian) which was Ihe grealest volcanic event during the Phanerozoic, and (he very strong explosive felsic 10 intermediate volcanism around the PTB, close to the margin between eastern Tethys and Panlhalassa. These volcanic activities resulted in those climatic changes that were dircclly and indireclly (as cause of the oceanic sll peranoxia) responsi ble for the PTB biotic crisis, such as periodic cooling of Ihe climate by volcanic dust and sulphate aerosols (mai nly caused by the Siberian Trap volcanism), acid rain, a 3-6 month vo[canic winter at low lat iIlldes and Ihe strongly reduced inpul of sunl ight during the uppennost Dorashamian (both caused by lhe very strong explosive volcanism at the Tethys/Panthalassa margin), followed by global warming in the lower Scythian, and uppennost Dorashamian to lower Scylhian superanoxia.