COMMENTS AND REPLY 101 Copyright  2004, SEPM (Society for Sedimentary Geology) 0883-1351/04/0019-0101/$3.00 Comment—Contrasting Deep- water Records from the Upper Permian and Lower Triassic of South Tibet and British Columbia: Evidence for a Diachronous Mass Extinction (Wignall and Newton, 2003) GREGORY J. RETALLACK Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, E-mail: gregr@darkwing.uoregon.edu PALAIOS, 2004, V. 19, p. 101–102 ‘‘Happy families are all alike, but every unhappy family is unhappy in its own way’’ Tolstoy (1878). As in this fa- mous first line of Anna Karenina, each Permian–Triassic boundary section is unhappy in its own way. Wignall and Newton (2003) are to be commended for showing that this great mass extinction had different effects at different lo- calities, such as deep oceanic warming in Tibet, but they have not demonstrated their chief claims that (1) mass ex- tinction was globally diachronous by a half million years, or (2) due to dysoxia from oceanic stagnation. Further- more, their foraminiferal disaster taxa are better ex- plained by a methane-outburst hypothesis (Krull and Re- tallack, 1999; Berner, 2002), which they fail to mention. There is a difference between diachronous mass extinc- tion, where most species become extinct at different times in different places, and selective survival, where some spe- cies survive the mass extinction. Wignall and Newton’s (2003) claim for diachroneity comes from range truncation of foraminifera assigned to 7 genera in the basal 35 cm of Triassic beds in the Selong section of Tibet. These are a statistically and taxonomically inadequate basis for de- layed mass extinction, especially considering that none of these Triassic foraminifera were fusulines, which remain the primary foraminiferal casualties of the Permian–Tri- assic life crisis (Stanley and Yang, 1996). The best under- stood Late Permian taxa near Selong are 43 species of bra- chiopods, which dwindled to 8 dwarfed brachiopod species in the ‘‘Waagenites’’ bed, above which no brachiopods were found despite deliberate search (Shen et al., 2000, 2001). All the brachiopods were gone before the first appearance of the conodont Hindeodus parvus just above the ‘‘Waagen- ites’’ bed, which includes a marked carbon isotopic excur- sion (Jin et al., 1996). Brachiopods are evidence of mass extinction coincident with the isotopic excursion and be- fore the boundary-marking H. parvus, as in numerous other Permian–Triassic boundary sections around the world (Erwin, 1993; Jin et al., 1996). Wignall and New- ton’s (2003) Selong foraminifera were thus survivors of the brachiopod mass-extinction 35 cm below. The selective survival of foraminifera is unsurprising within the context of a Permian–Triassic methane-out- burst hypothesis (Krull and Retallack, 1999; Krull et al., 2000). Modern foraminifera thrive around sewage and natural gas outlets, creating large and well-calcified tests (Yanko et al., 1994). Disaster blooms of foraminifera also were associated with the methane-outburst crisis of the Eocene–Paleocene boundary (Thomas, 2003). Many fora- minifera also thrive under dysoxic conditions (10–40 M.kg O2: Patterson et al., 2000; Platon and Sen Gupta, 2001). Modelling of Permian–Triassic methane outburst by Berner (2002) indicates that atmospheric reduction ini- tiated by methane oxidation, added to carbon dioxide from volcanism and ongoing extinctions, could have lowered at- mospheric oxygen levels from a Late Permian high of 35% to an earliest Triassic low of 12% within 20,000 years of the boundary. The difficulties of diminished atmospheric and oceanic oxygen were compounded by hypercapnia from methane oxidation to carbon dioxide. Among marine invertebrates, poorly ventilated corals and brachiopods were preferentially lost compared with more muscular molluscs (Knoll et al., 1996). Among plants, marginally aerated swamp vegetation succumbed, rather than plants of well-drained soils (Retallack et al., 1996). Among verte- brates, the survivors had more effective lungs and nasal passages (Retallack et al., 2003). Supposed diachronous extinctions are linked with inter- preted diachronous oceanic dysoxia by Wignall and New- ton (2003), whereas isotopic evidence for methane out- burst is globally synchronous (Krull et al., 2000). The methane-outburst hypothesis presents a mechanism for dysoxia (Berner, 2002) that is very different from the oce- anic stagnation model invoked by Wignall and Newton (2003). Both stagnation and methanogenic hypoxia can re- sult in well-bedded shales with little evidence of life. The stagnation model is characterized by carbonaceous black shales fed by surface-water biological productivity (Wig- nall, 1994). Methane-induced hypoxia on the other hand would not necessarily produce shales with high organic carbon content. Although Isozaki (1997) and Wignall and Newton (2003) claim that early Triassic shales are carbo- naceous, I have been unable to find any supporting ana- lytical data. Hundreds of published organic carbon analy- ses of marine earliest Triassic shales do not exceed 3 wt %, whereas Late Permian and Middle Triassic shales in the same sequences are much more carbonaceous (Suzuki et al., 1993; Kajiwara et al., 1994, Wang et al., 1994; Wolbach et al., 1994; Wignall et al., 1998; Morante, 1996; Krull et al., 2000). Well-studied oceanic anoxia events of the Cre- taceous produce thick, carbonaceous shales (Wignall, 1994; Kunht et al., 2002) with carbon contents (10 wt % TOC) well in excess of thin carbonaceous shales at the methane outburst crisis of the Paleocene–Eocene bound- ary (Gavrilov et al., 2003; Beniamovski et al., 2003). Ear- liest Triassic oceanic dysoxia was, contrary to Wignall and Newton (2003), neither diachronous nor carbonaceous, but more likely synchronous and oligotrophic (Morante, 1996; Krull et al., 2000). 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