Studies of the Permian-Triassic Boundary and Mass Extinction in South China

Ian Metcalfe in collaboration with: R.S. Nicoll, L.P. Black & C.B. Foster (AGSO, Canberra) R. Mundil, P. Renne, J. Glen & K. Ludwig (BGC, Berkely, USA) Wang Cheng-yuan (Nanjing, China)


Permian-Triassic boundary beds at Section D, Meishan, South China. The biostratigraphically defined boundary occurrs in the middle of Bed 27 at the first appearance of the conodont Hindeodus parvus. This level is bracketed by two bentonitic clay beds, Beds 25 and 28 plus other bentonitic clays higher and lower in the section.

THE BOUNDARY BETWEEN THE PERMIAN AND TRIASSIC PERIODS shows the most significant change in global biota in the Phanerozoic (and perhaps in the entire history of life on earth) and marks the boundary between the Palaeozoic and Mesozoic Eras. More than 90 per cent of shelly marine genera were wiped out in this event and animals and plants on land suffered similarly. Possible global catastrophic events suggested as causes of the P-T mass extinction include bolide impact, melting of gas hydrates flooding the world's atmosphere with methane, major marine regression, massive volcanism, oceanic anoxia and overturn of stagnant deep ocean waters (e.g. Erwin, 1993, 1994; Wignall and Twichett, 1996; Knoll et al., 1996; Hallam and Wignall, 1997). The transition between the Permian and Triassic has proved to be notoriously difficult to study because there is a stratigraphic gap at this level in many parts of the world. Biostratigraphic correlation of marine and terrestrial sequences has proved inadequate to answer the question of whether the mass extinction events in the sea and on land were synchronous or not, or if the extinctions took place over a very short or longer time period. Magnetostratigraphic correlation of marine and terrestrial P-T boundary sequences is still not refined enough to effect precise correlation of mass extinction events and P-T boundary sequences belong to the Permo-Triassic Illawarra Mixed Megazone (Menning, 1995). The position of the P-T boundary and mass extinction level, with respect to magnetic polarity is however equivocal, with some authors placing this within a normal chron slightly above the reversal boundary (eg. Menning, 1995) and others placing the boundary precisely coincident with a reversed to normal boundary (eg. Steiner et al., 1989). At Meishan, the P-T boundary, and the mass extinction events occur within a normal chron. Further detailed magnetostratigraphic studies, integrated with detailed biostratigraphy are being undertaken by our group to resolve this and to assist in precise correlation of the P-T boundary and mass extinction level(s) in marine and terrestrial sequences. It has also been claimed that a "spike" or shift in carbon isotope compositions of marine carbonates that immediately post-dates the mass extinction in the marine environment (Baud et al., 1989) can also be used in terrestrial sequences to identify the P-T boundary (Morante et al., 1994). Foster et al., (1997) have however issued a warning regarding such correlations, as isotopic composition of organic matter in terrestrial sediments appears to be overprinted by contributions from different parent plant types. In continuous marine sequences which span the P-T boundary, a transitional zone has been identified where the earliest typical Mesozoic fossils co-exist with Palaeozoic relict faunas. The co-existence of Permian relic faunas with typical Triassic biota in a transitional zone has led to much debate as to where to place the P-T boundary in terms of biostratigraphy.


SEM micrographs of the Pa element of the conodont Hindeodus parvus the first appearance of which is used to mark the base of the Triassic.

A global boundary stratotype (GSSP) has now been ratified at Meishan, China (Yin et al., 1996) with the boundary between the Permian and Triassic defined by the first appearance of the conodont species Hindeodus parvus (Kozur & Pjatakova) within the evolutionary cline Hindeodus latidentatus - Hindeodus parvus - Isarcicella turgida - Isarcicella isarcica. The GSSP has been ratification by a formal vote of the Boundary Working Group and by the ICS and IUGS. Extinction of Permian marine faunas in South China has been shown to have occurred in three phases, two earlier phases in the uppermost Permian and one other phase in the earliest Triassic (Yin et al., 1996). The time period over which these extinction phases and biotic recovery occurred has recently been suggested to be extremely short (less than 165,000 years) by Bowring et al. (1998), but our own isotopic geochronological studies indicate that this conclusion is questionable (Mundil et al., 2001). Our ongoing studies on the Permian-Triassic transition in South China invole SHRIMP and IDTIMS U/Pb isotopic dating of bentonite layers (see below), and biostratigraphic, chemostratigraphic and magnetostratigraphic studies of the Permian-Triassic boundary beds at various localities in China.


Dating of bentonitic clay layers such as the one shown here is providing vital new information for constraining the age of the P-T boundary and the tempo of extinction events which have important implications for proposed causative mechanisms for the P-T mass extinction.

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