End-Permian extinction and volcanism-induced environmental...

alaeoecology 252 (2007) 218–238www.elsevier.com/locate/palaeo

Palaeogeography, Palaeoclimatology, P

End-Permian extinction and volcanism-induced environmental stress:The Permian–Triassic boundary interval of lower-slope facies

at Chaotian, South China

Yukio Isozaki a,⁎, Noriei Shimizu a, Jianxin Yao b, Zhansheng Ji b, Tetsuo Matsuda c,✠

a Department of Earth Science and Astronomy, The University of Tokyo, Meguro, Tokyo 153-8902, Japanb Geology Institute, Chinese Academy of Geological Science, Baiwangzhong, Beijing 100037, China

c Kyoei Consulting Corporation, Toyama, Japan

Accepted 30 November 2006

Abstract

In order to reveal environmental changes across the Permo-Triassic boundary (PTB), the detailed lithostratigraphy of the PTB interval isanalyzed at Chaotian in northern Sichuan, China. The studied section is composed of the Changhsingian (Upper Permian) DalongFormation and the Induan (Lower Triassic) Feixianguan Formation of a lower-slope facies deposited on the northwestern margin of theYangtze carbonate platform. The 12-m-thick interval across the PTB consists mainly of bedded carbonates and mudstone, and islithologically divided into 7 units, i.e., Units A to G, in ascending order. The main extinction horizon of Permian taxa is recognized at theUnit D/E boundary where various fossil metazoans and protists, such as ammonoids, brachiopods, bivalves, conodonts, and radiolarians,rapidly disappeared or became scarce. The complete disappearance of radiolarians at the Unit D/E boundary emphasizes that the PTBextinction affected not only various Late Permian benthic and free-swimming metazoans but also planktonic protozoans. The lowestInduan index conodontHindeodus parvus first occurs at the base of Unit F, marking the biostratigraphically defined PTB horizon. Unit Ecomposed of unique bedded marl between the main extinction horizon and the first occurrence of Triassic taxon represents a period ofstrong environmental stresses that suppressed productivity both of silica- and carbonate-secreting organisms. By changing their size,radiolarians reactedmost sensitively to the environmental change that already started in the lateChanghsingian, appreciably before the finalextinction event. The frequent intercalation of rhyo-dacitic tuff beds, particularly in Unit D and the lower part of Unit E across the mainextinction horizon, suggests that intermittent felsic volcanism and relevant environmental change may have been responsible for the massextinction of the Permian taxa and for the prolonged post-extinction lag time before the initial recovery. The frequent ash falls during thelate Changhsingian indicate that the volcanism-induced environmental change had already started earlier than the main extinction. All thebiological production (carbonate, silica, and organicmatter) collapsed at theUnit D/E boundarywhen the environmental stressesmay havepassed a critical threshold for maintaining ecological stability. The PTB interval between the extinction and the first appearance of Triassictaxon at Chaotian is ca. 1.4 m thick, apparently almost eight times thicker than that at the Global Stratotype Section and Point of PTB inMeishan (19 cm). The Chaotian section, as well as the neighboring Shangsi section in northern Sichuan, may provide a better chance forhigh-resolution chemostratigraphic analyses that may allow detection and correlation of subtle environmental changes across the PTB.© 2007 Elsevier B.V. All rights reserved.

Keywords: Permian–Triassic boundary; Mass extinction; Radiolaria; Environmental stress; Felsic volcanism; South China

⁎ Corresponding author.E-mail address: [email protected] (Y. Isozaki).

✠ Deceased April 23, 2002.

0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.palaeo.2006.11.051

mailto:[email protected]

http://dx.doi.org/10.1016/j.palaeo.2006.11.051

219Y. Isozaki et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 218–238

1. Introduction

The Permo-Triassic boundary (PTB; ca. 252–253 Ma;Mundil et al., 2004) marks the most severe massextinction in Phanerozoic life history, with a large varietyof marine and terrestrial biota being lost (up to 90% at thespecies level; Sepkoski, 1984, 1996). Despite long-termdiscussion of various ideas and hypotheses, its cause stillremains unknown (e.g., Erwin et al., 2002; Bottjer, 2004),although an extraterrestrial cause for the PTB eventappears unlikely (e.g., Isozaki, 2001a; Koeberl et al.,2002; Farley et al., 2005). Regardless of the ultimatecause, there is agreement among researchers that large-scale environmental change, probably of a global scale,occurred at the PTB and terminated various ecologicalhabitats as well as Permian biota.

The main source of information on the PTB extinctionevent has been the thick, fossiliferous sedimentarysequences deposited in the Tethyan domain. One ofthese sections at Meishan in South China was latelyselected as the Global Stratotype Section and Point(GSSP) of the PTB (Yin et al., 2001). In the rest of theworld, however, the marine Upper Permian strata areoften incomplete owing to the presence of a remarkableunconformity, and terrestrial non-marine sequences lackdetailed fossil control for high-resolution correlation ofthe PTB horizon. The exceptional and excellent preser-vation of the continuous PTB interval in South China mayhave been probably related to its unique extension-dominant tectonic setting during the Paleozoic–Mesozoictransition. More than 50 PTB sections have beendescribed to date (e.g., Yin et al., 1992), however, com-plete PTB intervals adequately underpinned by variousindex fossils are quite rare even in South China, owing tothe drastic lateral facies change in local basins sensitive tosubtle sea-level fluctuations.

In order to reveal the nature of PTB global envi-ronmental change, we organized a Japan–China jointresearch team in 1997 and conducted detailed stratigra-phical research focused on the PTB interval in several areasof South China. We targeted sections of deep slope/basinfacies rather than those of shallow platform carbonates toensure complete succession. The marine Upper Permianrocks in the northern parts of Sichuan province arecharacterized by fine-grained bedded sequences of arelatively deep slope to basinal facies, likely to recordcontinuous sequence across the PTB without unconformi-ty and hiatus. The studied section in Chaotian in northernSichuan (Fig. 1A) exposes an almost complete successionof Middle Permian to Lowest Triassic rocks. Isozaki et al.(2004) reported the general stratigraphic framework of theChaotian section that contains a well-exposed PTB

interval. Despite its rich paleontological data (Zhao et al.,1978; Yang et al., 1987) and stratigraphic continuity acrossthe PTB, not much attention has been paid to the Chaotiansection relating to the mass extinction.

We analyzed litho- and biostratigraphy of the PTBinterval in the Chaotian section in detail in order todocument the putative PTB environmental change re-sponsible for the greatest mass extinction of thePhanerozoic. This article describes high-resolution litho-and biostratigraphy of the PTB interval of the Chaotiansection including the main extinction horizon of thePermian taxa and that of the first appearance of the Triassicone. We focused particularly on volcanic tuffs frequentlyintercalated in the PTB interval, and on stratigraphicchange in radiolarian density and test size. As radiolarianswere the only zooplanktons that monitored the silicabudget and bioproductivity of surface waters, theirbehavior immediately before the crisis at the PTB issignificant in understanding the environmental changerelevant to a possible cause of the PTB mass extinction.

Isozaki, Matsuda, and Yao mapped and logged theChaotian section, Ji and Yao studied conodont biostra-tigraphy, and Shimizu and Isozaki analyzed microscopictextures and geochemical aspects of the PTB intervalincluding stratigraphic changes in radiolarian abundanceand test size. Refer to Isozaki et al. (2004) for overviewof the Chaotian section, and to Ji et al. (2007-this issue)for details of the PTB conodont biostratigraphy.

2. Geologic setting

In the Late Permian and Early Triassic, the South Chinacraton was isolated from other continental blocks in a low-latitude area around the equator, forming the easternmargin of the Paleo-Tethys (Scotese and Langford, 1995).Throughout most of the Permian period, shallow marinefossiliferous carbonates accumulated thickly over craton toform the Yangtze carbonate platform. The eastern half ofSichuan province corresponds to the northwestern marginof the Permian Yangtze platform, immediately to the southof the Qinling–Dabie collisional suture that formed in theTriassic. The shallowmarine carbonate platformdevelopedextensively in the Middle Permian and early Late Permiantime in the western part of South China craton (Zhu et al.,1999; Fig. 1B) that directly faced to the eastern Paleo-Tethys. In the later part of the Late Permian, the platformshrunk toward south by being replaced by a deeper slopeand basin (Fig. 1C). In northern Sichuan (Fig. 1A), theUpper Permian (Lopingian) and Lower Triassic (Induan)consist of less calcareous and more argillaceous/siliceousfine-grained rocks of a deep-water slope facies. They showa remarkable contrast with the underlying Maokou


Fig. 1. Index map of the Chaotian section in northern Sichuan, South China (A), and the Late Permian (Late Wuchiapingian and Late Changhsingian)paleogeographic maps of the northwestern South China craton (B, C: modified from Zhu et al., 1999). Note the remarkable facies change in northernSichuan during the Late Permian, from shallow marine carbonate platform (B: early Late Permian) to deep-water basinal siliceous limestone (C:Permo-Triassic boundary interval).

220 Y. Isozaki et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 218–238

Formation of the Guadalupian (Middle Permian) age, aswell as with the coeval platform carbonates, that containabundant shallow marine bioclasts.

The Chaotian section is located nearly 20 km to thenorth of Guangyuan city, northern Sichuan (Fig. 1A).Extensive exposures ofMiddle Permian to Lower Triassicrocks are observed along the southbound Jialingjiangriver that forms a narrow gorge called Mingyuexia(Fig. 2A). In the center of the ca. 1-km-long gorge, theaxial part of an E–W trending anticline is observed. Thestudied section is on the eastern side of the river and on the

southern limb of the anticline. The occurrence of variousammonoids and conodonts (Zhao et al., 1978; Yang et al.,1987) were reported from a section on the northern limbof the anticline on thewestern side of the river that is alonga railway track. As the latter is partly covered now, detailscannot be checked, nonetheless, the stratigraphy is by andlarge the same on both sides of the anticline.

The Chaotian section, over 200m in total thickness, iscomposed of Middle Permian Maokou Formation,Upper Permian Wujiaping and Dalong formations, andlowermost Triassic Feixianguan Formation, in ascending

Fig. 2. The Permo-Triassic boundary (PTB) section along the Jialingjiang River at Chaotian, and the rocks of the PTB interval. (A) Distant view of the entire Chaotian section from the western side ofthe river. Note a car (a small white dot in a circle) for scale. (B) Close-up view of the PTB interval, showing the horizons of the main extinction of Permian taxa (yellow line) and the first occurrence ofTriassic taxon (red line). (C) Close-up view of the main extinction horizon between Unit D (Dalong Formation) and Unit E (Feixianguan Formation). (D) Close-up view of the horizon of the firstoccurrence of Hindeodus parvus (the index conodont of the lowest Triassic) at the base of Unit F. A pen in red oval is for scale. 221

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order (Figs. 2A and 3). The PTB is located in thelowermost part of the Feixianguan Formation (Fig. 2B).As on the southern limb of the E–W trending anticline,

all the Permian and Triassic strata are dipping gently tothe south at 10°–20°. The Permian and Lower Triassicrocks are composed mostly of well-bedded, fine-grained


limestone, siliceous limestone, and mudstone, withminor amount of marl and fine-grained sandstone,suggesting a distal sedimentary setting. The occurrenceof gravity-induced slump beds in lower DalongFormation and limestone breccia beds (debris flowdeposits) in Feixianguan Formation further indicatesunstable depositional settings, such as a lower slope tobase-of-slope even on a continental shelf that was muchdeeper than shallow-water carbonate platforms. Theslope was probably facing to NNE with a land area andcarbonate platform to the south/southwest in west-central Sichuan (Fig. 1C). As there was anothercarbonate platform in southern Shaanxi during the LatePermian (Lu, 1956; Rui et al., 1984), the northernSichuan formed a deep trough-like basin extending in theNW–SE direction. The scarceness in shallow marinebioclasts and dominance in deep-water-type fossilassemblage (ammonoids, conodonts, and radiolarians)are concordant with such sedimentary characteristics ofthe Upper Permian and Lower Triassic rocks at Chaotian.The Shangsi section about 60 km to the WSW (Fig. 1A)exposes a stratigraphic package of a similar age rangeand facies (e.g., Li et al., 1989; Wignall et al., 1995; Laiet al., 1996), although the originally described sectionwas later concealed by a local farmer's house.

The Maokou Formation (over 75 m thick) is com-posed mostly of thickly bedded, black to dark graybioclastic limestone (mostly wackestone) with abundantfossils including brachiopods, fusulines, rugose corals,calcareous algae, and conodonts. It is correlated with theWordian to Capitanian (upper Guadalupian) in Texas.

The Wujiaping Formation (68 m thick) is composedmainly of dark-gray limestone, mostly wackestone andpackstone, with chert nodules/lenses. The limestoneyields various fossils including fusulines, smallerforaminifers, calcareous algae, brachiopods, gastropods,crinoids, rugose corals, and conodonts. This formationis correlated with the Wuchiapingian (Early Lopingian).

The Dalong Formation (26 m thick) consists mostly ofblack bituminous mudstone (22 m thick) with micritic,gray limestone at the top (4 m thick). The black mudstoneis bedded, and it intercalates thin (10–20 cm) layers/lenses of marl. Slump beds occur in the lower half of themudstone. The uppermost (2 m thick) part of theformation comprises gray, micritic limestone rhythmical-ly bedded in 5–10 cm with nodular surfaces, andintercalates several beds of felsic tuff (Fig. 4). Both the

Fig. 3. Stratigraphic column of the PTB interval at Chaotian with distribution oCaO, TOC) by bulk geochemical analysis. Half-tone patterns in the right column(the same legend for Figs. 4, 6 and 7). Bars in the lower diagrams indicate rangeand abundance at the top of Unit D (top of the Dalong Formation) immediately bof the Triassic conodont, biodiversity remained still low in the overlying units.

mudstone and limestone of the upper parts are rich inammonoids, conodonts, and radiolarians (Fig. 3). Themain part of the formation is correlated with theWuchiapingian while the uppermost part with theChanghsingian (Late Lopingian) respectively. Previousworks (Zhao et al., 1978; Yang et al., 1987) described theabove-mentioned main mudstone as belonging to theWujiaping Formation based on the ammonoid age,however, this part is assigned here in the DalongFormation according to its mudstone-dominant lithology.The Wujiaping Formation at its type locality in southernShaanxi province is composed mostly of limestone.

The Feixianguan Formation (over 30 m thick)consists mainly of thinly bedded, light-gray, micriticlimestone. The lowermost 1.4 m is made up of adistinctive bed of gray to olive-gray, faintly laminatedmarl that is unique to this horizon in the Chaotian section(Figs. 3 and 4). This marl is almost barren of fossils,except for the basal part that rarely yields smallammonoids, brachiopods, and bivalves. The marl lackscalcareous/phosphatic bioclasts or radiolarian testsobservable even in thin section. Thin layers of acidictuff, less than 3 cm thick, occur at several horizons. Theoverlying limestone is thinly bedded, and partlyinterbedded with thin marl. Nearly 15–20 m above thebase of the formation, several matrix-supported lime-stone breccia beds of debris flow origin occur.

3. Lithology of the PTB interval

In order to check lithologic change across the PTB indetail, we analyzed fine-scale stratigraphy of a 12-m-thick interval across the PTB that is exposed along asmall stream on the eastern side of the Jialingjiang River(Figs. 2A and B). During the field research, we collectedrock samples from all beds (124 beds) of this interval foranalyses, prepared polished slabs for each sample, andmade more than 200 thin sections in total. This intervalis here lithologically subdivided into 7 units, Units A toG, in ascending order (Fig. 3). Units A–D correspond tothe uppermost Dalong Formation, and Units E–G to thelowermost Feixianguan Formation, respectively. Herewe describe these units in ascending order. On the basisof field, microscopic, and geochemical analyses, theoccurrence and distribution of bioclasts and contents ofrepresentative components (SiO2, CaO, and organiccarbon) are summarized in Fig. 3.

f various fossil groups and contents of representative components (SiO2,display colors of rocks; dark: black, medium: dark gray, white: light grayof measurements with the average. Note the sharp decline in biodiversityeneath the unique boundary marl (Unit E). Even after the first appearance


3.1. Unit A

This N0.9-m-thick unit comprises 14 beds (A1–A14),4–10 cm thick for each bed. Its base is not exposed. It ismainly composed of black, slightly calcareous mudstonewith radiolarians and shell fragments of bivalves andgastropods. Large radiolarians (60–90 μm in diameter)occur abundantly. The silica (SiO2) and carbonate (CaO)contents of 3 samples (A1, A5, and A9) are 73.7–83.9and 11.6–22.3 wt.% (on average 77.8, 16.7 wt.%),respectively (silica and carbonate contents of bulkpowdered sample were analyzed by XRF). The totalorganic carbon (TOC) of two samples (A3 and A13) is1.05 and 3.35 wt.%. Five beds of ca. 5-cm-thick, blackbituminous shale are intercalated. Except for calcareousbioclasts, coarse grains of sand size are absent.

3.2. Unit B

This 2.8-m-thick unit consists of 35 beds (B1–B35), 5–14 cm thick for individual beds. It is mainly composed ofblack calcareous mudstone that yields abundant ammo-noids. The silica and carbonate contents of four sam-ples (B8, B15, B28, and B34) are 53.9–60.1 and 33.4–39.5 wt.% (on average 56.6, 36.7 wt.%), respectively. TheTOC of two samples (B6 and B28) is 1.06 and 3.57 wt.%.These rocks are laminated and lack bioturbation. Severalbeds of black shale and felsic tuff are intercalated. Bioclastsrange in size from 1 to 10mm, and include shell fragmentsof brachiopods, bivalves, gastropods, ammonoids, andostracodes. In addition, radiolarians (40–60 μm indiameter) occur abundantly. These bioclasts often formdistinct laminae. In addition, beds of black mudstone andgray tuff are occasionally intercalated. Except forcalcareous bioclasts, coarse grains of sand size are absent.

3.3. Unit C

This 1.4-m-thick unit comprises 9 beds (C1–C9), fivegray limestone beds (10–20 cm thick) and four blackshale beds (5–10 cm thick) (Fig. 4). Limestone isclassified as wackestone enriched with bioclasts (shellfragments of bivalves, brachiopods, and ammonoids)that range in size mostly from 2 to 3 mm in length.Radiolarians (40–50 μm in diameter) occur but are not soabundant as in Units A and B. These rocks are laminatedand often feature Zoophycus-like, 1- to 2-cm-deepburrows. The silica and carbonate contents of twosamples (C1 and C7) are 2.6 and 6.7, and 80.2 and91.7 wt.% (on average 4.7, 86.0 wt.%), respectively. TheTOC of two samples (C6 and C9) is 1.08 and 1.50 wt.%.Coarse-grained terrigenous clastics are almost absent.

3.4. Unit D

This 2.3-m-thick unit consists of 24 beds (D1–D24),3–10 cm thick for each bed (Fig. 4). All of the beds arewavy-bedded gray limestone, except for the 7-cm-thickbed D1 composed of felsic tuff. Limestone is classifiedmostly as limemudstone with various bioclasts supportedby micritic matrices. A minor amount of wackestone ispartly included. Bioclasts, ca. 5 mm long on average,include shell fragments of bivalves, gastropods, brachio-pods, ammonoids, and ostracodes. Radiolarians (50–80μm in diameter) occur abundantly. Burrows (ca. 1 cmdeep) are often observed. Pyrite (0.1–1 mm in diameter)occurs ubiquitously throughout Unit D and is mostlycubic but large framboidal forms up to 5 mm in diameteroccur in D22. The silica and carbonate contents of sixsamples (D5, D11, D15, D19, D21, and D24) are 3.7–11.2 and 82.9–91.2 wt.% (on average 6.8, 90.5 wt.%),respectively. The TOC of six samples (D3, D11, D19,D21, D22, and D24) ranges between 0.02 and 0.14 wt.%.Coarse-grained terrigenous clastics are absent. In additionto D1, much thinner layers of acidic tuff, less than 5 cmthick, occur at 6 horizons.

3.5. Unit E

This 1.4-m-thick unit is composed of 20 beds (E0,E1.5, E1–E18), 5–10 cm thick for each (Fig. 4). It iscomposed mostly of gray marl (Fig. 6) with a minoramount of felsic tuff. Beds are planar in sharp contrast tothe underlying wavy-bedded limestone of Unit D. Marl iswell laminated in sub-millimeter scale. Except small-sized bivalves and ammonoids, this unit is almost barrenof fossils. Even under the microscope, no radiolarianswere recognized. Bioturbation was not observed. Cubicpyrite, less than 0.1 mm in diameter, sporadically occurs.The silica and carbonate contents of two samples (E5 andE9) are 31.7 and 42.3, and 35.6 and 47.2wt.% (on average37.0, 41.4 wt.%), respectively. This unit is enriched withaluminum (Al2O3: 8.5–10.5 wt.%) and total iron (Fe2O3:5.7–6.2 wt.%) with respect to the rest of the PTB interval.The TOC of four samples (E2, E5, E7, and E11) rangesbetween 0.06 and 0.60 wt.%. Coarse-grained terrigenousclastics are absent. Thin layers of felsic tuff, less than 5 cmthick, occur at 4 horizons and the 3.5-cm-thick bed E1(TE2) close to the base of Unit E is the thickest.

3.6. Unit F

This 1.7-m-thick unit consists of 14 beds (F1–F14),ca. 10-cm thick for each, except for over 20-cm-thick F3and F13 (Fig. 4). All of these beds are composed of

Fig. 4. Detailed stratigraphic column of Units C–F across the Permo-Triassic boundary at Chaotian. Inset is an enlargement of the interval across themain extinction horizon of Permian taxa. Note the frequent intercalation of felsic tuff, particularly in Unit D and the lower part of Unit E.



bedded, faintly laminated gray limestone with minoramount of gray marl and shale. These beds are classifiedas lime mudstone. No bioclasts are observed under themicroscope except for rare conodonts. In addition, blackcarbonaceous grains, less than 0.1 mm in diameter, areoften concentrated to form a 1-cm-thick dark coloredlayer. Lamination is faint and no bioturbation isrecognized. Cubic pyrite ca. 0.2 mm in diameter occursubiquitously. The silica and carbonate contents of foursamples (F1, F4, F9, and F13) are 1.9–13.2 and 77.3–95.1 wt.% (on average 6.1, 88.1 wt.%), respectively.The TOC of eight samples (F1, F3, F5, F7, F9, F11, F12,and F13) ranges between 0.02 and 0.19 wt.%. Coarse-grained terrigenous clastics are absent.

3.7. Unit G

This N1.4-m-thick unit comprises 9 beds (G1–G9),ca. 10-cm thick for each. It consists mainly of bedded,dark gray limestone with some marl beds. Limestone isclassified in lime mudstone. Except rare conodonts, nobioclasts were observed even under the microscope butthe limestone contains plenty of black, carbonaceous,0.1- to 1-mm-long flake-like objects. Lamination isfaint, and no bioturbation developed. The silica andcarbonate contents of three samples (G1, G4, and G8)are 1.8–3.3 and 92.9–95.9 wt.% (on average 2.7,93.8 wt.%), respectively. The TOC of four samples (G2,G4, G6, and G8) ranges between 0.04 and 0.07 wt.%.Coarse-grained terrigenous clastics are absent.

All of the tuff beds are several centimeters thick andstrongly weathered into soft clay (mostly illite detectedby XRD), but their primary felsic (of rhyolite/dacitetype) composition is proved by the abundant occurrenceof igneous euhedral phenocrysts of quartz, plagioclase,apatite, and zircon (Isozaki et al., 2004). Details of thefelsic tuff beds of the PTB interval in Chaotian will bereported elsewhere.

4. Biostratigraphy

We newly obtained various mega- and microfossilsfrom the Chaotian section, and these new data in additionto those reported by the previous workers (Zhao et al.,1978; Yang et al., 1987) allowed us to determine theprecise ages of the formations in Chaotian. Fig. 5 listsrepresentative fossils from the Middle–Upper Permianand the lowermost Triassic rocks in Chaotian thatinclude fusulines, brachiopods, bivalves, rugose corals,ammonoids, and conodonts. According to the lithofa-cies, theMaokou andWujiaping formations (carbonates)are enriched with shallow marine megafossils and their

fragments, such as fusulines, rugose corals, andcalcareous algae, whereas the Dalong Formation (mud-stone) is replete with remains of relatively deep-waterbiota such as radiolarians, conodonts, and ammonoids.Fossil abundance and diversity are very low in theFeixianguan Formation (Fig. 3).

The Maokou Formation yields Capitanian (upperGuadalupian) fusulines (Yabeina, Chusenella), rugosecorals (Waagenophyllum), brachiopods (Neoplicati-fera), and conodonts (Jinogondolella). The WujiapingFormation yields Wuchiapingian (lower Lopingian)fusulines (Codonofusiella, Reichelina), rugose corals(Lianshanophyllum, Waagenophyllum), conodonts(Clarkina), and ammonoids (Araxoceratid).

The Dalong Formation ranges from the upperWuchiapingian to upper Changhsingian (Fig. 5). Thelower part of the Dalong Formation yields rareammonoids of the Araxoceras–Konglingites Zone(upper Wuchiapingian). Units A and B of the upperDalong Formation (Fig. 3) are characterized by abun-dant ammonoids of the Pseudostephanites–Tapasha-nites Zone (lower Changhsingian), whereas Units C, Dby those the Pseudotirolites–Pleuronodoceras Zone(upper Changhsingian). In terms of conodonts, Unit A,Unit B, and the lower part of Unit C belong to theClarkina changxingensis changxingensis–Clarkina sub-carinata Zone. The upper part of Unit C and lower UnitD belong to the Clarkina postwangi–Clarkina sp. BZone, and middle–upper Unit D to the Clarkinataylorae–Clarkina zhejianensis–Clarkina changxingen-sis yini Zone, respectively. See Ji et al. (2007-this issue)for details of conodonts. Radiolarians, mostly sphericalforms, occur abundantly from the Dalong Formation(Units A–D) but they are not diagnostic for dating. Thepattern of radiolarian occurrence in the PTB interval islater discussed separately.

The Feixianguan Formation ranges from the upper-most Changhsingian to lower Induan. This formation isalmost barren of fossils except for small ammonoids,brachiopods, and bivalves from the lower Unit E and rareconodonts from limestone in higher horizons (Units F,G) (Figs. 3 and 5). Themudstone in the lowermost part ofthe formation (E1.5) yields the latest Changhsingianammonoid Hypophiceras together with Huananocerasand Pleuronodoceras. This ammonoid assemblageuniquely characterizes the topmost Changhsingianhorizon in major PTB sections in China (Yang et al.,1996). The limestone of Unit F and lower Unit G belongto theHindeodus parvus (conodont) Zone, and the upperUnit G to the Isarcicella Zone, respectively (Figs. 3 and5). The H. parvus Zone (defined by the occurrence of H.parvus without accompanying Isarcicella isarcica) is


Fig. 5. List of the Permian and Triassic fossils from the Chaotian section (compiled from Zhao et al., 1978; Yang et al., 1987; Isozaki et al., 2004, this study) fossil zones, and their ages. 227Y.

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generally regarded as the lowermost zone of the Triassic(Yin et al., 2001). Radiolarians are completely absent inthe Feixianguan Formation. The occurrence of Ophi-ceras sp. was previously reported from the basalFeixianguan Formation on the northern limb of theanticline (Yang et al., 1987), however, its precise horizoncan hardly be checked because it is covered, and thisneeds further re-evaluation.

Above-mentioned fossil data clearly indicate that themain extinction of the Permian biota occurred at the UnitD/Unit E boundary, i.e., the boundary between theDalong Formation and Feixianguan Formation, whereasthe radiation of the Triassic biota started from the base ofUnit F (Figs. 3, 4 and 5).

Fig. 6 illustrates detailed stratigraphic distribution ofthe critical fossils for assigning the PTB interval, i.e.,ammonoids and conodonts, from Unit C to Unit Gacross the PTB. Most of the Changhsingian ammonoidsand conodonts show their final occurrence in the bedD24 (the top of Unit D) except for some holdover taxa.On the other hand, the first bona fide Triassic taxon H.parvus appeared for the first time in the bed F1 (the baseof Unit F). Although uncertainty remains in the middleto upper parts of Unit E, the PTB horizon surely existssomewhere in the lowermost Feixianguan Formation inthe Chaotian section. The horizon of the first occurrence(FO) of H. parvus at Chaotian probably corresponds tothe first appearance datum (FAD) of the taxon that isgenerally used as the key criterion to identify the PTB inmany sections (e.g., Yin et al., 2001). Thus, Unit Ebetween the main extinction horizon and that of the firstoccurrence of H. parvus may represents a transitionalinterval that probably experienced a unique stressfulperiod to various biota of a global context across thePTB (Figs. 3, 5 and 6).

5. Changes in radiolarian population and size

In order to clarify the stratigraphic change ofplankton productivity across the PTB, radiolarianoccurrence was examined in terms of their populationand test (siliceous skeleton) size. By observing morethan 200 thin sections (aligned perpendicular to thebedding) from the study interval across the PTB, wecounted the number of radiolarian individuals per ar-ea and measured their average diameter. For popula-tions, radiolarian numbers were counted in a domain of3–5 mm2 of each sample and standardized per cm2. Forsize, the diameter of each radiolarian test (mostlyspherical) was measured for at least 100 individuals ineach sample. The average diameter of radiolarian testand standard deviation were calculated using the

analytical method for sandstone grains in thin section(Friedman, 1958). Fig. 7 illustrates stratigraphic changesin radiolarian population and test size in the PTB inter-val at Chaotian.

In terms of radiolarian populations, the study intervalcan be divided into three segments, i.e., Units A, B,Units C, D, and Units E to G, separated by two steps ofsharp decline (Fig. 7). In Units A and B, the averagepopulation of radiolarians is 2900–3200 individuals percm2, while in Units C and D it declines to almost half at1300–1500 per cm2. Across the Unit D/E boundary,radiolarians disappeared completely. The disappearanceof radiolarians was abrupt as populations were almostconstant in the underlying Unit D. Thus, the Unit D/Eboundary represents the major extinction level ofPermian radiolarians as well as other fossil groupssuch as ammonoids and conodonts (Figs. 3, 5 and 6)).

As no radiolarians occur in Units E to G, measure-ment was provided solely for Units A–D. In allmeasured samples, radiolarian sizes (diameter ofspherical test) show normal distributions with standarddeviations mostly between 11 and 19 μm. This indicatesthat no mixing has occurred between plural radiolarianfaunas with different test sizes, and that the mean valuerepresents an average size of radiolarians in eachsample. As shown in Fig. 7, there is a clear stratigraphicchange in radiolarian size in the study interval. In UnitA, the average radiolarian size declined by nearly 30%gradually from 80 μm, and in Units B and C the averagevalues stayed almost constant at around 50 μm. In UnitD on the other hand, the average values increasedgradually again up to 80 μm, and stayed constant in thetopmost 1-m-thick interval (beds D21–24) immediatelybefore their final occurrence.

6. Discussion

6.1. Change in sedimentary regime across the PTB

There are two remarkable lithologic changes in theUpper Permian at Chaotian, i.e., (1) the transition fromthe terrigenous mudstone-dominant (Units A, B) to thecarbonate-dominant facies (Units C–G), and (2) theintercalation of unique marl beds (Unit E) within thecarbonate-dominant facies. The first one was probablylinked to local tectonics with respect to slope/basingeometry, whereas the second one may have reflected aglobal environmental change in biosphere thus appearsmore important in the PTB study.

After the long-term deposition of black terrigenousmudstone of the Dalong Formation (including Units A,B) during the late Wuchiapingian and most of

Fig. 6. Stratigraphic distribution of diagnostic conodonts and ammonoids in identifying the PTB horizon in the Chaotian section (modified from Isozaki et al., 2004). Note the frequent occurrence ofammonoid and conodonts in Units C and D in a clear contrast with almost barren Units E and F. Thin beds of felsic tuff (☆) are concentrated in Unit D and the lower half of Unit E. 229

Y.Isozaki

etal.

/Palaeogeography,

Palaeoclim


252(2007)

218–238

Fig. 7. Secular change in radiolarian density and test diameter in the uppermost Permian at the section. Left: secular change in radiolarian population;Right: secular change in radiolarian test size (diameter). Note the discordant behaviors between population and size.


Changhsingian time, the deposition of carbonate startedat the base of Unit C, as marked by a sharp increase ofCaO mirrored in a decline of SiO2 (Fig. 3). From Unit Cto Unit D, the bedding mode changed from planar towavy/nodular one, and TOC drops rapidly in Unit D bymore than one order of magnitude (Fig. 3). Theseobservations suggest that the sedimentary environmentat Chaotian has started to shift to a shallower, more

oxygenated, and more pelagic setting during the lateChanghsingian. The upsection increase in fossil abun-dance (ammonoids, conodonts, and other fossils; Fig. 3)supports this trend. Within Unit D, the lithology andrhythm of bedding are almost constant (Fig. 4), and theammonoid and conodont faunal compositions remaineduniform in the middle–upper part of the unit (Fig. 6).The radiolarian population also stayed constant in Unit


D (Fig. 7). The deposition of bedded limestone andbiological production likely continued almost until thefinal moment of the Changhsingian.

At the top of Unit D, in turn, most of the Permianfossils, including ammonoids, conodonts, radiolarians,and other invertebrates, disappeared very abruptly.Immediately above Unit D, bedded marl (Unit E)appeared for the first time in the Chaotian section,marking a sharp break in continuous deposition oflimestone and fossil abundance/lineage (Fig. 2C). In theChaotian section, the occurrence of bedded marl isrestricted to the PTB interval; i.e., mostly in Unit E andpartly in Units F and G (Fig. 4). The average bulkchemical composition of Unit E shows less calcareousbut more argillaceous nature with respect to theneighboring limestone, and its TOC records the lowestin the Chaotian section (Fig. 3). In addition, highconcentration of Al and Fe is another unique geochem-ical signal of Unit E that may imply the apparentabundance due to a lesser amount of carbonate or extrainput of Al–Fe-enrichedmaterial, such as volcanic ash orvolcaniclastics. These geochemical characteristics high-light the uniqueness of Unit E in the carbonate-dominantfacies across the PTB at Chaotian.

Unit E is free from burrowing and almost barren ofmega-/microfossils, except for the lowermost blackshale with tiny ammonoids and brachiopods. As thebase of Unit E corresponds to the main extinction ho-rizon of various Permian taxa, this unique marl re-presents an interval that recorded a highly stressfulperiod immediately after a drastic environmental changein the latest Changhsingian, in particular, the appearanceand persistence of a harsh condition that has driven themain extinction. It is noteworthy that the rare ammo-noids and brachiopods from the basal Unit E are allsmall-sized (Fig. 6) with respect to those from Unit Dimmediately below. This may imply the post-extinction“Lilliput effect” (Urbanek, 1993; Twitchett, 2006), asdwarfism in marine community generally reflects theappearance of a strong environmental stress. The rapiddecrease in CaO content (Fig. 3) further indicates thatthe bioproductivity of other metazoans forming carbon-ate skeleton was also sharply reduced at the Unit D/Eboundary. In addition, the sharp disappearance ofradiolarians occurred at the top of Unit D (Fig. 7).Thus at the time of Unit D/E boundary, production ofboth major carbonate-stabilizing benthos/nekton andsilica-stabilizing plankton (radiolarians) was all sup-pressed remarkably. Changes in seawater temperature,redox, acidity, etc., and/or shortage in nutrients arepossible drivers, however, it is difficult to specify thecritical cause.

At the Unit E/F boundary, the lithology changed fromthe topmost marl (E18) back to the bedded limestone (F1)from where H. parvus (the index taxon for the lowestTriassic) first appeared. Units F and G comprise beddedmicritic limestone but completely lack radiolarians andcalcareous bioclasts. The re-appearance of limestoneindicates that the marine carbonate production, onceaborted at the Unit D/E boundary, had revived by certainorganism after the suppressed interval. Under themicroscope, no bioclasts were identified in the fine-grained micritic limestone of Units F and G, and, there-fore, the origin of carbonates is not identified at present.Nonetheless, coccoid calci-spherules produced by cya-nobacteria, a common member of post-extinction disastercommunity, may be a candidate for such post-extinctioncarbonates (Wignall et al., 1995). In the form ofanachronistic (Precambrian-like) calcimicrobialite, thedevelopment of a disaster community was documentedin the H. parvus and I. isarcica zones of various shallowmarine PTB sections (Sano and Nakashima, 1997; Baudet al., 1997; Kershaw et al., 1999; Lehrmann et al., 2003;Ezaki et al., 2003). The lower slope facies at Chaotianwastoo deep to grow in situmicrobialite/stromatolite, but thisenvironment could receive lime mud derived from thecoeval shallow platforms. Units F, G presumably rep-resent lateral equivalents of the shallow marine calcimi-crobialite, and the microscopic grainy/flaky organicmaterial identified in Units F, G were likewise derivedfrom cyanobacterial mat flourished in shallow-watercarbonate ramps. Calcareous components of Unit E mayhave also derived from shallow-marine calcimicrobialiteformed by pioneer cyanobacteria that appeared prior toH.parvus. The PTB calcimicrobialite often started to depositbefore the first appearance of H. parvus, e.g., inPanthalassa, South China, and Oman. In addition to thebivalve Claraia, the occurrence of a peculiar brachiopodLingula (Fig. 5) further supports the existence of such adisaster fauna (Rodland and Bottjer, 2001) as pointed outby Isozaki et al. (2004). Although still in a post-extinctionharsh condition, the severe environmental stresses thatappeared at the top of Unit D may have been partly easedby the time of Unit E/F boundary. In a sharp contrast tocalcareous biota, radiolarians could not recover at all inthe early Induan (Fig. 7), suggesting that the environmenthad still remained in a survival phase, i.e., a harshcondition that not yet allowed the revival of many groupsof organisms except the disaster community.

The lithologic change across the PTB in the Shangsisection about 60 km to the WSW (Fig. 1A; Li et al.,1989; Wignall et al., 1995; Lai et al., 1996) appearsquite similar to that in Chaotian, thus above-describedstratigraphic change in environmental regime across the


PTB was more or less the same throughout the northernSichuan along the northwestern continental margin ofSouth China that directly faced the Paleo-Tethys.

6.2. Radiolarian response

Among all the fossil organisms from Chaotian,radiolarians reacted most sensitively to the putativeenvironmental change across the PTB. They occurabundantly and ubiquitously throughout the UpperChanghsingian regardless of lithofacies, however, theydisappeared suddenly at the Unit D/E boundary, andbecame completely barren in Units E–G (Fig. 7). Thisindicates that radiolarians, a major plankton group of thePaleozoic, suffered the PTB environmental change verybadly, and that silica-stabilization in the low-latitudePaleo-Tethys has run into a crash. The extinction of thePermian radiolarians at the PTB was recognized inPanthalassa, particularly in the tropical domains and thenorthern hemisphere (Isozaki, 1994, 1997; Kozur, 1998;Yao and Kuwahara, 1999; Ezaki and Yao, 2000; Fenget al., 2000; Xia et al., 2004). This phenomenon isprobably of global scale except for the partly survival inthe Southern Hemisphere (Takemura et al., 2003).

Their behavior immediately before the extinction atthe Unit D/E boundary is of particular interests. First,the radiolarian population changed in two acute steps inthe Upper Changhsingian in Chaotian, i.e., at the Unit B/C boundary and at the Unit D/E boundary (Fig. 7). Atthe Unit B/C boundary, radiolarian population suddenlydecreased to a half, whereas at the Unit D/E boundary allradiolarians became extinct. Concerning the first de-cline, there are two possible explanations; i.e., 1)radiolarian productivity per se declined to a half, and2) radiolarian productivity was constant but accumula-tion rate of other material doubled to apparently diluteradiolarian population. As described above, the Unit B/C boundary accommodates a sharp lithologic changefrom mudstone to limestone probably with a change insedimentation rate as well as the composition. Thepopulation shows quite stable attitude not only withinUnits A, B and within Units C, D, but also in the vicinityof the Unit B/C boundary, suggesting that the radiolarianproduction was almost constant throughout the lateChanghsingian. Thus the second option for a highersedimentation rate mainly by carbonate mud appearslikely.

At the Unit D/E boundary, radiolarians totallydisappeared, and never returned at least in the lowerInduan interval at Chaotian. This suggests that a crucialcondition developed over the continental shelf inSichuan at the end of Changhsingian, although some

Permian ammonoids survived the crisis. Radiolarianswere likely less tolerant to this environmental change inthe latest Changhsingian. As radiolarians were the mainplankton group in the Permian ocean, their extinctionmay have led cascaded extinction of various higherpredator taxa in the marine food web.

On the other hand, the stratigraphic change inradiolarian size within the upper Changhsingian demon-strates an independent pattern from their population(Fig. 7). The average size decreased from Unit A to UnitB, stayed almost constant throughout Units B and C, andincreased again within Unit D. The radiolarian sizechanged considerably within the same lithology (e.g., inUnit A and Unit D), whereas they stayed constantregardless of the drastic lithologic change (across the UnitB/C boundary). Thus the radiolarian size changedindependently on the facies change, therefore, their sizewas controlled by other factor(s). It is noteworthy thattheir size increased from50 to 80μm, i.e., almost doubled,immediately before their total disappearance. The similarpattern was also confirmed in the topmost 1.5-m-thickinterval at Shangsi (unpublished data), therefore, this sizeincrease prior to their abrupt disappearance is a commonphenomenon in the northwestern continental shelf inSouth China and was probably related with theenvironmental change that drove the main extinctionevent at the Unit D/E boundary.

For modern radiolarians, it is generally regarded thattheir average size changes according to various factorsof seawater, e.g., temperature, salinity, dissolvedoxygen, dissolved silica, and other nutrients (e.g.,Anderson, 1983). It has been proposed that largerradiolarians tend to dominate in cool waters of high-latitude or deep seas (Anderson, 1983; Granlund, 1986);however, the critical factors and physiological mechan-isms controlling radiolarian size have not yet been fullyclarified (Kozo Takahashi, personal communication).Oceanic upwelling of deep-water enriched with nutri-ents, such as nitrates, phosphates, and silica, mayenhance the growth of test as well as soft tissue (e.g.,Yamashita et al., 2002), and this may explain theputative tendency of larger-sized radiolarians in high-latitude and deep seas. Concerning the Late Permian toEarly Triassic interval, however, sea level was generallyrising due to global warming (Hallam and Wignall,1999). Thus, in order to explain the possible enhance-ment of upwelling coupled with nutrient supply, a short-term cooling is needed during the long-term warmingperiod. On the other hand, the sharp drop in TOC acrossthe Unit C/D boundary may indicate that ventilationbecame more efficient to enlarge the radiolarian size.Although there are still many uncertainties in their


response to environmental factors (Lazarus, 2005),radiolarians did respond to a certain change in theenvironment in a more sensitive way than otherorganisms. It is emphasized here that a certain changein environment started in the late Changhsingianapparently prior to the main extinction event, and thiswill be further discussed in connection with volcanismin the next section.

6.3. Volcanic stress

The concentrated occurrence of many felsic tuff bedsin the PTB interval of Chaotian suggests a possible linkto the causal process of the extinction (Isozaki et al.,2004). In particular, 21 fine-grained tuff beds ofcentimeter scale occur in the 12-m-thick PTB interval.Their distribution is not random but, rather, concentrat-ed, e.g., as in a 3-m-thick interval (Unit D and the lowerhalf of Unit E) in which 11 tuff beds (TD1–7, TE1–4)are recognized (Fig. 4). Nine (TD1–7, TE1, 2) of these11 definitely belong to the Upper Changhsingian,whereas the remaining two (TE3, TE4) are not preciselydated biostratigraphically (Fig. 6). Given the relativelyhigher sedimentation rate of Unit D, the concentration iseven more significant, highlighting the frequency of ashfall events at the end of the Changhsingian (Fig. 8).Around the main extinction horizon, in particular, threetuff beds (TD7, TE1, and TE2) occur within a 10-cm-thick interval (inset in Fig. 4). An apparent accumulatedthickness of these 11 tuff beds in Units D and E attains20 cm. These indicate that rhyodacitic volcanism wasactive nearby, and that northern Sichuan often experi-enced ash falls at the conclusion of the Permian. Thisfurther implies that the frequent eruptions of felsicvolcanoes may have played a certain role in environ-mental change relevant to the mass extinction at the endof Permian. Similar occurrences of felsic tuff bedsaround the PTB horizon were reported from more than50 localities in South China (e.g., Yin et al., 1992).Thus, South China as a whole was likely coveredextensively and frequently by felsic ash beds around thePTB time, although the source volcanic domain has notbeen identified yet. In order to document the extent andderivation of the volcanism, precise horizons andfrequency of the tuff beds in other PTB sections shouldbe checked in detail with reference to the PTB interval inChaotian.

In addition to their frequent occurrence, particularlysignificant in Chaotian is the timing of onset of the seriesof ash fall beds. The tuff bed TD1, marking the start ofthe frequent ash fall events (Fig. 8), occurs at the base ofUnit D, ca. 2.3 m below the main extinction horizon at

the top of Unit D (Fig. 4). This does not necessarily meanthat the volcanism had nothing to do with the extinction;instead, it suggests that a volcanism-related environ-mental change started prior to the extinction event. InUnit D, no apparent change in faunal composition isdetected except for the change in radiolarian size. Thismay indicate that a certain environmental change hadalready appeared and accumulated during the intermit-tent ash falls (TD1–6) but not yet reached to a conditioncritical to maintain the ecological stability (Stage I withprecursory environmental stress; Fig. 8). Among variousorganisms, radiolarians were probably one of theorganisms too sensitive to ignore a subtle change inenvironment. The late Changhsingian felsic volcanism,owing to its highly viscous nature, should have beenquite violent in eruption, much more violent than thoseof basalts in general. This may have driven a short-termcooling event by explosive eruptions and resultantsunlight blocking with aerosols in stratosphere. Whenthe environmental change proceeded to cross the thres-hold at the top of Unit D (Stage II with the strongestenvironmental stress), other organisms finally got in-volved, the whole ecological system collapse, and themain extinction occurred.

The post-extinction delayed recovery also may havebeen related to the prolonged felsic volcanism (TE3–5)after the main extinction. Even in the survival stage, aslight enhancement in environment, that has led the firstoccurrence of Triassic index conodont H. parvus and therevival of carbonate sedimentation, started at the base ofUnit F, clearly after the last ash fall of the event in theupper Unit E (TE5) (Fig. 8). This interval under slightlylesser stress (Stage III) allowed biological carbonatemineralization again. Radiolarians, too vulnerable to acertain stress or too much damaged by the end-Changhsingian crisis, unlikely could recover duringthis stage.

A possible cause-and-effect link between large-scalevolcanism and the PTB mass extinction has beendiscussed by many, but most of the discussion to datehas focused on the apparently synchronous Siberiancontinental flood basalt eruption (e.g., Campbell et al.,1992; Renne et al., 1995; Racki and Wignall, 2005). Onthe other hand, various lines of evidence from SouthChina and Japan suggest that the volcanism associatedwith the mass extinction event was not of mafic but,rather, of felsic nature (e.g., Yin et al., 1992; Kozur,1998; Isozaki, 2001b; Isozaki et al., 2004). Beforemaking interpretations based solely on apparent agecoincidence, we once again need to check the characterof the PTB volcanism per se with special referenceto the frequent felsic ash fall events in the late

Fig. 8. Schematic summary diagram showing the frequent ash fall event and bioproductivity collapse around the P TB at Chaotian. Not to scale.Letters I, II, and III represent stages of environmental stress; I: precursory stage, II: the most stressful stage, and III: survival stage with slightly lesserstress. The boundary marl (Unit E) corresponds to the stage II when all biomineralization of carbonate and silica was suppressed. Note that thevolcanic influence appeared much earlier than the main extinction, and that the extinction occurred in the middle of the intermittent volcanism. Therevival of carbonate production started clearly after the frequent volcanic event ceased.


Changhsingian prior to the main end-Permian extinctionevent. Isozaki (2001b, 2007) discussed mantle plume-related volcanism, not of basaltic but of felsic, alkalinenature, in conjunction with the PTB.

6.4. Correlation with Shangsi and Meishan PTBsections

On the basis of the new stratigraphic data in thisstudy, we try to correlate and compare the Chaotiansection with other two representative PTB sections inSouth China, i.e., Shangsi section in Sichuan (Li et al.,1989; Lai et al., 1996; Nicoll et al., 2002) and theMeishan section in Zhejiang (Jin et al., 2000; Yin et al.,2001) (Fig. 1A). The Shangsi section, located in thevicinity of Chaotian, naturally had a similar depositionalsetting of a lower slope facies in general. The Meishansection, known as the GSSP of the PTB, represents anupper slope facies, relatively shallower than the

Chaotian and Shangsi sections. All three PTB sectionswere properly dated by several index fossils; e.g., upperChanghsingian ammonoids of the Pseudotirolites–Pleuronodoceras Zone and the Hypophiceras Zone,lower Induan conodonts of the H. parvus Zone and theI. isarcica Zone in ascending order.

Fig. 9 shows a correlation diagram of the PTBinterval (between the main extinction horizon of thePermian taxa and the first occurrence of I. isarcica) ofthe three sections. The baseline for correlation is set atthe main extinction horizon that corresponds to the topof the last limestone of the Permian, and some tie-linesbetween columns show the horizons of the last Permianammonoid (Hypophiceras) and the first Triassic indexconodonts (H. parvus and I. isarcica). An obviousdifference in thickness of each fossil zone is easilyrecognized between the sections, in particular betweenthe Meishan and the rest two in Sichuan. Concerningthese three stratigraphic intervals, the Shangsi section

Fig. 9. Correlation of the PTB interval between the deeper lower slope facies (Chaotian, Shangsi sections in Sichuan) and the shallower upper slopefacies (Meishan section, Zhejian). Stratigraphic columns, fossil data, and radiometric ages of the Shangsi and Meishan sections are compiled from Liet al. (1989), Lai et al. (1996), Nicoll et al. (2002), Mundil et al. (2004), and Yin et al. (2001).


always possesses the greatest thickness, whereas theMeishan the smallest. For example, thickness of theHypophiceras Zone varies from 11 cm at Meishan to55 cm at Shangsi; that of the H. parvus Zone (betweenthe first occurrence of H. parvus and that of I. isarcica)from 8 cm at Meishan to 410 cm in Chaotian or to398 cm in Shangsi. The interval between the above-mentioned two zones (between the last occurrence ofHypophiceras and the first occurrence of H. parvus)also varies in thickness from 8 cm at Meishan to 380 cmat Shangsi or to 133 cm at Chaotian. Thus, in thePermian–Triassic interval, the Shangsi and Chaotiansection are apparently 20 to 30 times thicker than theMeishan section.

On the other hand, a similarity exists between theShangsi and Chaotian sections, probably reflecting thesimilar sedimentary setting in the northwestern conti-nental margin of South China. Also in terms of rock type,

a clear contrast exists between the two sections inSichuan and the Meishan; the former is characterized bybedded marl and limestone in the lower Induan, whereasthe latter by monotonous mudstone. Thus the Meishansection is lithostratigraphically distinct, both in rock typeand thickness, from those of the two sections in Sichuan.

If this apparent correlation is valid, the thickness ofthe PTB interval between the horizons of mainextinction and that of the first Triassic taxon differsconsiderably between the Meishan and Chaotian/Shangsi sections; the former (19 cm) is thinner thanthe latter (145, 435 cm) by one order of magnitude, aspreliminarily emphasized by Isozaki et al. (2004). Thiscontrast may suggest that the Meishan section is highlycondensed or even bearing unrecognized hiatus acrossthe PTB (although it is currently accepted as GSSP), andthat the PTB interval is likely preserved in a greaterthickness thus potentially more complete at Chaotian/


Shangsi. The assignment of this apparent contrast inthickness is highly dependent on the identification ofhorizons of the first occurrence of the lowermost Induanindex conodonts, such as H. parvus and I. isarcica,although both taxa occur rarely from the Chaotian andShangsi sections (Nicoll et al., 2002; Ji et al., 2007-thisissue). If the first occurrences of these two taxa do notnecessarily correspond to their first appearance owing tothe Signor–Lipps effect, the above-mentioned correla-tion and thickness contrast needs considerable amend-ment. Rare and isolated occurrence of “Ophiceras” fromBed 28c at Shangsi (Lai et al. (1996) below the FO of H.parvus also needs careful re-evaluation. Nonetheless,between the two sections in Sichuan, the thickness of thethree intervals (Hypophiceras Zone, H. parvus Zone,and the interval between them) appears close toproportional (Fig. 9), supporting the essentially greaterthickness in northern Sichuan. In addition, Ji et al.(2007-this issue) emphasized the predominant occur-rence of conodont C. taylorae from Unit D, whereas it isabsent in the Changxing Formation (Bed 24 and below)at Meishan. This further suggests that a missing intervalmay exist also in the uppermost Changxing limestoneimmediately below the PTB clay/marl units at Meishan.

This kind of conundrum in biostratigraphic correla-tion cannot be solved solely based on the occurrence ofrare fossils per se, and needs independent checkutilizing other approaches. The preservation of possiblythick PTB interval in northern Sichuan offers anopportunity for high-resolution chemostratigraphicanalysis focused on examining detailed correlation anddetecting subtle environmental changes across the PTB.We are currently analyzing stable carbon isotopestratigraphy of the two sections in Sichuan, and theresults will be reported elsewhere.

7. Conclusions

The present stratigraphical study at Chaotian innorthern Sichuan identified a PTB interval (ca. 1.4-m-thick marl) between the main extinction horizon of theChanghsingian taxa and the first occurrence horizon of theInduan index conodont. Radiolarians disappeared abrupt-ly together with ammonoids, bivalves, and conodonts atthe top of the youngest Permian limestone. This indicatesthat the PTB extinction terminated not only various LatePermian benthic/nektonic metazoans forming calcareousskeletons but also silica-stabilizing planktonic protozo-ans, and that marine productivity in total declined sharply.Radiolarians reacted most sensitively to the environmen-tal change that already started in the late Changhsingianwell before the final extinction event. The unique

boundary marl (Unit E) probably represents a period ofthe strongest environmental stresses that suppressedproductivity both of silica- and carbonate-secretingorganisms. The unusually concentrated occurrence of 21felsic tuff beds, seven of them particularly in theuppermost Changhsingian immediately below the mainextinction horizon, suggests that the frequent felsicvolcanisms may have been responsible for the greatbiodiversity loss at the end of Permian and prolongedpost-extinction lag before the initial Triassic recovery. ThePTB boundary interval at Chaotian is apparently thickerthan the equivalent section in the Meishan GSSP byalmost one order of magnitude; therefore, it is likely to besuperior for detailed chemostratigraphic analyses that willallow detection of subtle environmental changes acrossthe PTB and high-resolution correlations with other PTBsections.

Acknowledgments

We greatly appreciate Harutaka Sakai, TomomiKubo, Hiroshi Nishi, Hodaka Kawahata, and MasaoTakano for their help in fieldwork, Hisayoshi Igo,Guifang Liu, Kozo Takahashi, and Lipei Zhan forcomments on fusulines, ammonoids, radiolarians, andbrachiopods, Akihito Kuno, Motoyuki Matsuo, andHideyoshi Yoshioka for geochemical measurements.Thomas Algeo, Ian Metcalfe, Micha Horacek, andChunjiang Wang provided constructive reviews to themanuscript. Brian F. Windley checked the language.This research was supported by Japan Society for thePromotion of Science (project nos. 12573011 and16204040) and by National Nature Science Foundationof China (project nos. 40502004 and 49972014).

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