Timing of Permian-Triassic Anoxia

Yukio Isozaki outlines thresholds in deepening oceanic anoxia and subsequent oxygenation across the Permian-Triassic boundary (Reports, [11 Apr., p. 235][1]). He implies that mass extinction at this time was a product of oceanic stagnation many millions of years in the making. There is, however, continuing uncertainty about the position of the Permian-Triassic boundary in the Japanese section studied. That section lacks critical ammonites and conodonts, or even geochemical markers, such as the change in carbon-13 isotopic lightening, that characterize the boundary elsewhere ([1][2], [2][3]). The Permian-Triassic boundary could have equally preceded or coincided with the onset of “climax superanoxia,” rather than postdated it, as proposed in Isozaki's stratigraphic section. If “climax superanoxia” postdated the Permian-Triassic boundary, as in the best documented sections elsewhere ([1][2]), then there are alternative hypotheses to long-term descent into “superanoxia” and recovery. Instead, Late Permian anoxia could have been related to an extinction event during the Late Permian (end-Guadalupian extinction) ([3][4]), with “climax superanoxia” and delayed recovery forced by the greatest of all mass extinctions at the end of the Permian (end-Changxingian extinction) ([1][2]). Oceanic anoxia could then have been a consequence of death, decay, and boom-or-bust population cycles (4) forced by other agencies (5), such as voluminous volcanic eruptions (6), impact of an unusually large bolide (7), or both (8). 1. [↵][5]W. T. Holser and H. P. Schonlaub, Eds., Abd. Geol. Bund. Autr. 45 , 213 (1997); H. F. Yin, Ed., The Paleozoic-Mesozoic Boundary (China Univ. Geosciences Press, Wuhan, China, 1996). 2. [↵][6]1. K. Wang, 2. H. H. J. Geldsetzer, 3. H. R. Krouse , Geology 22, 580 (1994). [OpenUrl][7][Abstract/FREE Full Text][8] 3. [↵][9]1. S. M. Stanley, 2. X. Yang , Science 266, 1340 (1996). [OpenUrl][10][CrossRef][11][Web of Science][12] 4. R. J. Helby, R. Morgan, A. D. Partridge, Mem. Assoc. Aust. Paleontol. 4 , 1 (1987). 5. 1. M. R. Rampino , Science 274, 1551 (1996). [OpenUrl][13] 6. 1. P. R. Renne, 2. Z. Zichao, 3. M. A. Richards, 4. M. T. Black, 5. A. R. Basu , ibid. 269, 1413 (1995). [OpenUrl][14] 7. J. D. Gorter, Petrol. Explor. Soc. Aust. News 4 , 32 (1996); G. J. Retallack, A. Seyedolali, W. Holser, D. Krinsley, E. S. Krull, Abstr. Annu. Meet. Geol. Soc. Am. 28 , 368 (1996). 8. M. B. Boslough, E. P. Chael, T. G. Trucano, D. A. Crawford, D. L. Campbell, Spec. Publ. Geol. Soc. Am. 307 , 541 (1996). # {#article-title-2} Response: Concerning the age assignment of the Permian-Triassic boundary section in Japan, microfossil age constraints have recently been enhanced: (i) conodonts ( Neogondolella chanxingensis and N. subcarinata ) of the Changxingian stage were found in lower gray cherts below the claystones; (ii) three radiolarian zones were newly defined for the interval of the late Wuchapingian and Changxingian in the uppermost part of the lower gray chert; (iii) Paleozoic-type radiolaria Follicucullus species were found (although few) from the black “boundary” claystone without any Triassic signature (1). Although there is still uncertainty, age assignment of the section shown in figure 3 of my report still appears reasonable. Secular change in carbon-13 values of the section looks unavailable at present because (i) the primary depth of chert sedimentation below carbonate compensation depth (usually more than 3000 meters) ousted carbonates from the section (2), and (ii) preliminary measurements of organic carbon show highly variable (thus, controversial) carbon-13 values through the section. With regard to the comment by Retallack and Holser about the subtle timing between the onset of “superanoxia” and the real boundary, I will refrain from being too specific about whether oceanic anoxia or an apparently stratified ocean was the real culprit of the global mass extinction until more precise geochronologic data are available for the section. The apparent oceanic stratification may have been a consequence, rather than a cause, of all the unique changes across the boundary. The deposition of organic claystone around the boundary, however, was probably a result of transient blooming of anaerobic biota during the “superanoxia,” not of the one-time mass dying at the end-Changxingian, because the Lower Triassic siliceous claystone also intercalates many organic claystone layers up to the Spathian horizons (1, 3). Likewise, the end-Guadalupian extinction alone may not have been responsible for the whole Late Permian anoxia of nearly 8 million years duration. 1. A. Yao and K. Kuwahara, Newsl. Osaka Micropaleont. 10 (spec. vol.), 87 (1997); K. Sugiyama, Proc. Palaeont. Soc. Jap. (new series) 167 , 1180 (1992). 2. T. Matsuda and Y. Isozaki, Tectonics 10 , 475 (1991). 3. Y. Itozaki, Can. Soc. Petrol. Geol. Mem. 17 , 805 (1994). 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