Events during Early Triassic recovery from the end-Permian extinction

Events during Early Triassic recovery from the end-Permian extinction

Global and Planetary Change 55 (2007) 66 – 80 www.elsevier.com/locate/gloplacha Events during Early Triassic recovery from the end-Permian extinction...

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Global and Planetary Change 55 (2007) 66 – 80 www.elsevier.com/locate/gloplacha

Events during Early Triassic recovery from the end-Permian extinction Jinnan Tong a,⁎, Suxin Zhang a , Jingxun Zuo b , Xinqi Xiong c a

GPMR and BGEG laboratories at China University of Geosciences, Wuhan 430074, China b Henan Institute of Geological Survey, Zhenzhou 450007, China c China University of Geosciences Museum, Wuhan 430074, China Received 5 September 2005; accepted 30 June 2006 Available online 25 September 2006

Abstract The Palaeozoic–Mesozoic transition is characterized not only by the biggest Phanerozoic mass extinction, at the end of Permian, but also a prolonged period of recovery of the biota during the succeeding Early Triassic. The delayed recovery is generally attributed to the effects of extreme environmental conditions on the Early Triassic ecosystem. However, there has been very little study of the cause and mechanism of the environmental conditions that prevailed during the period of extinction and subsequent recovery. Research on the Permian–Triassic boundary and Lower Triassic, especially that on environmental events at the beginning of the Triassic in South China, indicates that the slowness of the recovery may be the result of three factors: (1) extreme environmental conditions that persisted through the transitional period and which were maintained by, for example, intermittent contemporary volcanism; (2) a passive evolutionary and ecologic strategy of the biota, in which r-selection taxa were dominant and K-selection forms insignificant; (3) an immature, poorly functioning ecosystem, which had difficulty in responding to and withstanding extreme environmental changes. According to data from South China, environmental changes were frequent during the Late Permian, and especially serious at the Permian–Triassic boundary. The Late Permian ecosystem was well structured and fully functioning as a result of a long period of steady development during the late Palaeozoic, and was capable of resisting general environmental changes. However, increasingly frequent and probably more extreme environmental events in the latest Permian may have led to a general collapse of this ecosystem and to the mass extinction at the end of the Permian. The Early Triassic ecosystem was immature, functioned poorly, and was unable to respond effectively to environmental changes, so that persisting extreme environmental conditions slowed ecosystem reconstruction considerably, and the recovery of the biota therefore took a relatively long time. The environmental events at the Permian–Triassic boundary might not be significantly different from those at other Phanerozoic transitions, but they consisted of a series of events that occurred at intervals during the transitional period. © 2006 Elsevier B.V. All rights reserved. Keywords: Permian–Triassic transition; ecosystem reconstruction; delayed recovery; environmental changes; mass extinction

1. Introduction

⁎ Corresponding author. Tel.: +86 27 62867036; fax: +86 27 87801763. E-mail address: [email protected] (J. Tong). 0921-8181/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2006.06.015

The most outstanding change in the history of life on Earth happened during the Palaeozoic–Mesozoic transition. Amongst the Phanerozoic transitions it features

J. Tong et al. / Global and Planetary Change 55 (2007) 66–80

the biggest mass extinction, at the end of the Permian, and the longest period of recovery of the biota, during the succeeding Early Triassic (Erwin, 1993, 1994, 2000; McGhee et al., 2004). These events also resulted in a remarkable change in the Earth ecosystem, and laid the foundations of the modern marine ecosystems (Erwin, 1993, 1994; Erwin et al., 2002; McGhee et al., 2004). Numerous studies have shown that the end-Permian mass extinction was caused by unusual and rare terrestrial or extraterrestrial events that killed more than 90% of the species of known protozoans and metazoans and virtually destroyed the terrestrial ecosystems, reducing the marine ecosystems to a status equivalent to that of the cynobacteria-dominated “low-level ecosystems” of the earliest stages of life history (Sepkoski et al., 1991; Erwin, 1995; Xie et al., 2005). However, the recovery that followed the mass extinction, and which took nearly the whole of Early Triassic time (Hallam, 1991; Erwin, 1993; Tong, 1997; Erwin, 1998a,b, 2000; Martin et al., 2001; Mundil et al., 2004), has been inadequately studied and is poorly understood. Though recent SHRIMP dating of the Permian–Triassic boundary (Mundil et al., 2004) and the Lower–Middle Triassic boundary (Martin et al., 2001; Payne et al., 2004) indicate that the Early Triassic might not be as long as previously thought, it had a possible duration of around 6 Ma (Gradstein et al., 2004). Thus the Early Triassic recovery took longer than others in the Phanerozoic, which typically took place over as little as 1 to 2 Ma (Hallam, 1991; Erwin, 1998b). This difference in the rate of recovery is poorly understood. The rate of recovery in the Early Triassic was related to the environmental factors that produced nearly complete ecosystem collapse at the end of the Permian; recovery of the biota from this situation was a long process. However, a study of the Lower Triassic of the Central Oman Mountains (Twitchett et al., 2004) indicates that, where environmental conditions were favourable, recovery in the Early Triassic could also take as little as 1 to 2 Ma. Thus the environment was the leading factor in the global Early Triassic ecosystem reconstruction and biotic recovery, and a prolonged period of extreme conditions or environmental fluctuations was the main cause of the delayed recovery (Hallam, 1991; Erwin, 1993; Wignall and Twitchett, 2002; Payne et al., 2004). Why extreme conditions or environmental fluctuations persisted for several million years is, however, difficult to explain. Events at the Permian–Triassic boundary resulted in a crisis in the ecosystems, but their effect on Early Triassic environments is unlikely to have been so prolonged in comparison with those at other times, for example, around the Cretaceous–Tertiary

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boundary. Early Triassic sequences have, therefore, been examined for information on ecosystem recovery and development during that time. The majority of the Permian–Triassic boundary and Lower Triassic sections in South China have been investigated or re-examined during the search for a candidate Global Stratotype Section and Point (GSSP) for the Olenekian Stage. These studies have shown that records of extreme and unusual environmental events exist not only at the Permian–Triassic boundary but also within the Upper Permian and the Lower Triassic. Those evident in the Upper Permian and Lower Triassic may be less pronounced than those at the Permian–Triassic boundary, but are relevant to the understanding of the mass extinction and the subsequent prolonged recovery. 2. Events at the Permian–Triassic boundary in the Meishan section The supercontinent Pangea represented a maximum of continental aggregation and an important phase in Earth's palaeogeographic history (Valentine and Moores, 1970). It formed during the late Palaeozoic, and resulted in a considerable reduction in the marine shelf depositional area in the Late Permian (Vail et al., 1977). Continuous marine Permian–Triassic sequences are scarce, and South China, which is composed of several blocks from the eastern Tethyan archipelago (Yin et al., 1999), is one of the few regions where such sequences are well-preserved. It was located in a low latitude zone and the geological and biotic records are relatively abundant and complete. Thus it is an important region for the study of Permian–Triassic stratigraphy and events, and includes the GSSP for the base of the Triassic, at Meishan, Zhejiang (Yin et al., 2001). Many Permian–Triassic boundary sections in South China were extensively studied during the search for the GSSP for the base of the Triassic (Fig. 1). The Meishan section is, as the GSSP for the base of the Triassic, the classic site for the study of events and processes in the Permian–Triassic transition; most other sections in South China are well correlated to this section. Nearly all the rare environmental events have been recognized either directly at the Meishan section or initially at other sections and subsequently at the Meishan section. In particular, the record of the biota across the Permian– Triassic boundary has received much attention. The major change in biodiversity is well documented in the boundary sequence (Jin et al., 2000) and studies on biomarkers reveal unusual changes in microbes and related organic and environmental factors during the transition (Grice et al., 2005; Xie et al., 2005).

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Fig. 1. Early Triassic sedimentary provinces and locations of Permian–Triassic boundary and Lower Triassic sections examined in South China (geographic map from Tong and Yin, 2002). Sections 1 2 3 4 5 6 7 8 and 9, studied for carbon isotopes, correspond to the sections in Fig. 7.

In the Permian–Triassic boundary sequence at Meishan the most apparent and well-studied evidence for unusual events in the marine ecosystems during the transitional period are tuffaceous clay beds and sharp negative shifts in carbon isotope composition (δ13C) (Fig. 2). The clay beds contain not only typical volcanic elements, such as tuffaceous clay minerals, illite–montmorillonite mixed-layer structures and volcaniclastic crystals (He, 1981; He et al., 1987, 1989; Wu et al.,

1990; Yin et al., 1992; Yang et al., 1993; Yu et al., 2005), but also unusual components, such as fullerenes with noble gases (Becker et al., 2001), anomalies of some rare elements (Kaiho et al., 2001), and metal grains and microsphaerules (Yang et al., 1993; Kaiho et al., 2001), though some of these rare components require further study and their origin is debated (Zhou and Kyte, 1988; Yang et al., 1993; Braun et al., 2001; Farley and Mukhopadhyay, 2001; Koeberl et al., 2002). A sharp

Fig. 2. Permian–Triassic boundary sequence and environmental events at Meishan, Zhejiang. Carbon isotope curve A is redrawn from Cao et al. (2002); curve B is redrawn from Nan and Liu (2004).

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negative shift and pronounced anomaly in the carbon isotope composition for carbonate (δ13Ccarb) that is welldocumented at Meishan (Chen et al., 1984; Xu and Yan, 1993; Jin et al., 2000; Cao et al., 2002; Nan and Liu, 2004), has been identified in Permian–Triassic boundary sections throughout the world (Baud et al., 1989; Holser et al., 1989; Magaritz et al., 1992; Twitchett et al., 2001; de Wit et al., 2002; Krull et al., 2004). There are, however, some variations in the results and opinions differ with regard to the magnitude of the negative anomaly, the horizon it occurs at, and the number of anomalies in the boundary strata (Cao et al., 2002; Xie et al., 2005). Explanations of the negative anomaly vary but all concur that it resulted from unusual events, either biotic or environmental (see summary by Berner, 2002). The clay beds and δ13C excursions and anomalies at the Permian–Triassic boundary are the visible and tangible indicators of rare ecological events associated with the end-Permian mass extinction and the crisis in the transitional period. However, similar clay beds and δ13C excursions have been observed in the Lower Triassic of South China during the search for a candidate GSSP for the Olenekian (Tong et al., 2002; Zhang et al., 2005). These observations may be relevant to understanding the prolonged Early Triassic recovery and are presented here. 3. Lower Triassic sequence in South China The Lower Triassic sequences in most Permian– Triassic boundary sections were investigated when the boundary was defined (e.g. Sheng et al., 1984, 1987; Yang et al., 1987, 1993). In many sections events were recognized mainly at the base of the Lower Triassic because only the lower part of that sequence was studied. Palaeogeographic differentiation occurred in South China during the Late Permian but Permian– Triassic boundary sequences there appear conformable and the “Permian–Triassic boundary set” is correlatable throughout that region (Peng et al., 2001). Palaeogeographic redifferentiation occurred from late Induan times (Tong and Yin, 2002) and Lower Triassic sections examined include facies from palaeogeographic situations that range from carbonate platform or buildup to turbidite basin (Fig. 1). Fossils are abundant in many sections in the various facies, and conodonts and ammonoids provide chronostratigraphic resolution to at least substage level. At Chaohu, Anhui, where a candidate GSSP for the base of the Olenekian is located (Tong et al., 2003), the Lower Triassic sequence is well-known from recent studies (Tong et al., 2005b) and may be taken as a reference for correlating Lower Triassic

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sections regionally and even globally, and identifying ecological events that occurred during the recovery period (Fig. 3). At Chaohu the most prominent ecosystem change occurred at the Permian–Triassic boundary. Most Permian taxa did not extend up into the Triassic; exceptions were a few dwarf brachiopods that survived from the latest Permian and some conodonts, such as Hindeodus typicalis and Neogondolella carinata, that are found in the “boundary limestone” and range across the Permian–Triassic boundary. Typical Triassic forms, including the conodont Hindeodus parvus, have not been recorded from the “boundary limestone”. The Permian– Triassic boundary is based upon the regional correlation of the “Permian–Triassic boundary set” (Peng et al., 2001; Tong et al., 2001). The lithology also changes at the “Permian–Triassic boundary clay bed”. The argillaceous component increases progressively upwards through the Dalong Formation and the cherty beds characteristic of that formation disappear just below the “boundary clay bed”; limestone, which is the main lithology in the Triassic, appears immediately above that bed, which therefore marks an important change. There is no change in palaeomagnetic polarity at this boundary (Fig. 3). The δ13Ccarb values are strongly negative at the base of the Yinkeng Formation; however, there is no comparative data from the upper Dalong Formation because CO2 was not recovered from the cherty rocks of that formation (Tong et al., 2002, 2005b). In addition to the “boundary clay bed”, several other clay beds occur in the upper part of the Dalong Formation. The clay minerals and illite–montmorillonite mixed-layer structures indicate a volcanic origin but many of the clay beds contain very few typical volcaniclastic minerals, possibly because deposition was in deep water or the volcanism was remote or weak. In the lowermost Triassic of the Chaohu section very few clay beds are seen above the “boundary clay bed”; however, about 1 m above that bed, there is one clay bed which, though not very distinctive in field exposures, contains volcaniclastic minerals such as hexagonal bipyramidal quartz, apatite and iceland spar (Fig. 4). In the Early Triassic, Chaohu was situated on a deep part of the Lower Yangtze carbonate ramp (Tong and Yin, 1998, 2002), and the succession deposited there at that time contains much mudrock and thin-bedded limestone. With the aggregation of the Pangea blocks that resulted in the suturing of the Lower Yangtze and North China blocks in the late Mid Triassic (Li, 2001), the Lower Yangtze region was gradually uplifted and the depositional basin enclosed from late Early Triassic

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J. Tong et al. / Global and Planetary Change 55 (2007) 66–80

Fig. 3. Lower Triassic succession at Chaohu, Anhui (modified from Tong et al., 2005b). M.Tri. — Middle Triassic, UP — Upper Permian, CX — Changhsingian, DMAS — Dongma'anshan Formation, DL — Dalong Formation, N — normal polarity, R — reverse polarity, B — boundary clay bed.

times onwards. Chaohu is one of the very few areas where the Lower Triassic is in relatively deep-water facies and the fossil and depositional sequences are continuous. The Early Triassic biota was dominated by

groups characteristic of disasters, such as generalists and those with mobile life styles (bivalves, ammonoids and conodonts). The composition and structure of the fauna were very variable and unstable, but evolution was quite

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Fig. 4. Volcaniclastic minerals from a clay bed 1 m above the Permian–Triassic boundary in the North Pingdingshan Section, Chaohu. A — hexagonal bipyramidal quartz, B — apatite, C —Iceland spar. The scale bar is 50 μm.

fast, resulting in a relatively fine-scale biostratigraphic zonation. The proportion of limestone to mudstone increases progressively from the Yinkeng Formation, through the Helongshan Formation, into the Nanlinghu Formation, marking the collision of the Lower Yangtze and North China blocks. However, this process was

interrupted in the early Olenekian when carbonatedeficient mudstone of the upper Yinkeng Formation accumulated; this was superseded by limestone deposition that resulted in the first thick Lower Triassic limestone at Chaohu, at the base of the Helongshan Formation. Facies and features such as microbialites,

Fig. 5. Clay beds in the lowest Triassic at Meishan, Changxing, Zhejiang. Clay beds: 1— Bed 25, 2— Bed 26, 3— Bed 28, 4— Bed 31, 5— Bed 33, 6— Bed 34 (middle), 7— Bed 34 (top). Volcaniclastics: A— hexagonal bipyramidal quartz (Bed 34, top); B, C— transparent siliceous microspheres (Bed 34, middle); D— rounded hexagonal bipyramidal quartz (Bed 34, middle); E— hexagonal bipyramidal quartz (Bed 31), F, G— hexagonal bipyramidal quartz (Bed 26, base); H, I— hexagonal bipyramidal quartz (Bed 25, base); J— hexagonal bipyramidal quartz (Bed 25, lower part). The scale bar is 50 μm.

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J. Tong et al. / Global and Planetary Change 55 (2007) 66–80

carbonate seafloor fans, flat-pebble conglomerates and wrinkle structures (Sepkoski et al., 1991; Schubert and Bottjer, 1995; Baud et al., 1997; Pruss et al., 2005), occur in the Lower Triassic, especially in the carbonate facies but, though common in the other areas on the Yangtze carbonate platforms and on isolated carbonate buildups and platforms in the Youjiang Depression (Lehrmann, 1999; Kershaw et al., 1999, 2002; Lehrmann et al., 2003; Kershaw et al., 2004; Wang et al., 2005), these are scarce at Chaohu which was in a deeper basinal situation. Nodular limestone is the most common lithology at Chaohu, where flat-pebble conglomerates and wrinkle structures are scarce and occur only in the upper Lower Triassic Nanlinghu Formation, in carbonates that appear to have formed in an intertidal zone; scarce algal mats or algal-laminated structures at that level may be different from microbialites that are common at the base of the Lower Triassic. In addition to the tuffaceous clay beds in the lowermost Triassic, a bed of pyroclastic flow deposits occurs in the upper Olenekian at Chaohu (Li, 1996). In the West Pingding-

shan section at Chaohu, three beds of bentonite occur in a 2 m interval just above the Induan–Olenekian boundary (Tong et al., 2005a). These beds are not very distinctive at outcrop and were recognized only when the section was examined at the centimetre scale, suggesting that similar work elsewhere in the Lower Triassic may reveal more tuffaceous clays. The study of carbon isotopes at Chaohu indicates that changes in δ13C were relatively frequent and pronounced during the Early Triassic; several negative anomalies, some of them very high, occur between the Permian–Triassic boundary and the middle Spathian. The biota and other features of the Lower Triassic succession at Chaohu, though rather atypical because of the palaeogeographical setting, are representative of the development of the marine ecosystem of South China in the Early Triassic. The Lower Triassic at Chaohu is the best studied so far and is representative of that sequence of South China. The Early Triassic environmental events recorded in Chaohu are also recognizable in Lower Triassic sections elsewhere in South China; in some of

Fig. 6. Results of X-ray analysis of clay rocks from the Meishan section, Zhejiang. Clay bed numbers as in Fig. 5, with the addition of 1a (basal “ferric crust”, Bed 25), 1b (Bed 25, lower), 1c (Bed 25, middle), 1d (Bed 25, upper), 1e (Bed 25, top).

J. Tong et al. / Global and Planetary Change 55 (2007) 66–80 Table 1 Content of clastics in the clay beds of the “Permian–Triassic boundary set” at Meishan, Zhejiang Sample no.

3 2 1e 1d 1c 1b 1a

Bed no.

28 26 25

Clastic content (g) in 400 g sample

Number of bipyramidal quartz grains in 1 g clastics

Size in 10– 100# mesh

Size in 100– 200# mesh

Wellshaped

Broken or rounded

10 40 46 9.5 7.5 36 48

8 20 10 1.5 2.5 4 48

0 22 0 1 6 71 102

1 25 1 1 10 113 123

these the records of such features as tuffaceous clay beds and microbialites are more noticeable, because of the different paleogeography. 4. Lower Triassic tuffaceous clay beds Though tuffaceous clay beds in the lowermost Triassic are difficult to recognize at Chaohu, because of its deep basinal setting, they were commonly observed in shallow platform facies. Unfortunately, little is known at present about these tuffaceous clays. At Meishan at least five such clays occur in the lowermost 10 m of the Triassic succession, in the dominantly argillaceous Yinkeng Formation (Fig. 5). At Meishan this formation shows a distinct lithological cyclicity that is characteristic of Milankovitch cycles (Tong and Yin, 1999). Similar cycles occur in other sections of the Lower Triassic in the Yangtze region, and Milankovitch cyclicity has been supported by a study of magnetic susceptibility (Hansen et al., 2000). Since the tuffaceous clays observed in the Lower Triassic at Meishan occur at

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corresponding horizons in many other sections in South China and have been related to Milankovitch cycles, they are regarded as ideal markers for high-resolution correlation. However, direct correlation is not possible because the numbers of clay beds differ between sections. Correlation would be possible only if particular clay beds were shown to have distinctive features; this has not yet been demonstrated by either field or laboratory studies. Among the tuffaceous clays around the Permian– Triassic boundary the “boundary clay bed” appears the easiest to recognize because of its association with the “boundary limestone” and the first Triassic clay bed to form the “Permian–Triassic boundary set”, which has been recognized and correlated throughout South China (Peng et al., 2001). The lithology of this bed is, however, not distinctive, and there is no difference from the other clay beds either in clay mineral content and composition or in volcaniclastic minerals and their morphology (Figs. 5 and 6). Repeated sampling and study of material from fresh outcrops, deep excavations and boreholes at this locality has yielded varying results, because of diagenesis and, probably, weathering. Detailed sampling of the “boundary clay bed” (Bed 25) at Meishan also shows that the composition of the clay and clastic minerals varies considerably within that bed (Fig. 6, Table 1). The anomalies of some elements and components mostly occur in the lower samples and this may explain why other studies that employed different, or less detailed, sampling produced different results. With the exception of some unusual components and chemical anomalies in the “boundary clay bed”, the clay beds show virtually no distinctive compositional or structural characters (Fig. 6, Tables 2 and 3). This situation exists elsewhere in South China; for example, in the Daxiakou section, Yichang, Hubei (Zhang et al., 2005) (Table 4).

Table 2 Clay mineral composition and content of clay rocks in the Meishan section, Zhejiang Sample no.

Bed no.

Clay minerals

Content of clay minerals (%)

Content in the mixed-layers (%) Illite layer

Montmorillonite layer

7 6 5 4 3 2 1e 1d 1c 1b 1a

34

I–M mixed-layer I–M mixed-layer + chlorite + illite I–M mixed-layer I–M mixed-layer + illite + kaolinite I–M mixed-layer I–M mixed-layer I–M mixed-layer + illite I–M mixed-layer I–M mixed-layer Kaolinite + illite Kaolinite + illite

100 95 55 95 70 25 25 95 20 10 10

Irregular mixed-layer Irregular mixed-layer 51 50 55 53 47 42 5 No No

49 50 45 47 53 58 50

33 31 28 26 25

I–M — illite–montmorillonite.

1.14 1.15 1.24 0.82 0.81 1.00 1.01

δCe ∑

241.96 417.11 183.43 158.69 292.90 378.14 380.92 41.69 56.69 23.46 33.98 36.10 52.80 50.30

Y

0.61 0.83 0.43 0.51 0.74 0.95 0.94

Lu

79

4

14

45

12

32

10 9

0 − 35

2 1

I–M— illite–montmorillonite. Depths are relatively the “boundary clay bed” (Bed 10, depth 0); positive values above that level, negative below.

Tm Dy Tb

1.35 1.83 0.71 0.81 1.11 1.86 1.78 8.05 11.39 4.58 4.41 6.15 9.39 9.18

Gd Eu Nd

41.66 78.90 32.33 20.23 44.10 61.70 61.30

Pr

11.43 19.63 8.92 5.57 13.20 17.50 17.60

Sm

1.62 1.94 0.72 0.77 1.13 1.35 1.39

Yb

3

8.92 11.45 4.33 5.38 6.65 11.20 11.20

4.37 6.25 3.01 3.44 5.05 6.90 6.93

18

0.66 0.94 0.43 0.54 0.72 1.04 1.00

5

4.55 6.47 2.53 3.54 4.12 6.37 6.28

154 139

Er

23 21

1.65 2.23 0.86 1.12 1.34 2.18 2.12

7 6

I–M mixed-layer mineral (60) + gypsum (30) + quartz (5) + calcite (5) I–M mixed-layer mineral (80) + gypsum (20) I–M mixed-layer mineral (80) + gypsum (0) + quartz (5) + calcite (5) + kaolinite (10) I–M mixed-layer mineral (20) + gypsum (45) + quartz (5) + calcite (30) I–M mixed-layer mineral (80) + gypsum (15) + quartz (5) I–M mixed-layer mineral (30) + gypsum (40) + quartz (10) + illite (20) I–M mixed-layer mineral (80) + gypsum (20) I–M mixed-layer mineral (80) + gypsum (15) + quartz (5)

Ho

302

7.56 13.84 6.02 3.95 7.49 11.80 12.10

180.40 142.20 110.30 90.10 149.00 114.00 114.00 57.00 56.00 59.00 38.70 134.00 40.20 38.10

Therefore, it is difficult to assess how much the events indicated by the “boundary clay bed” differ from those in the Lower Triassic. Mid and late Early Triassic volcanism, such as that observed in the Lower Triassic at Chaohu (Li, 1996), has received little study and is poorly understood at present. Volcanic rocks have also been reported from the upper Lower Triassic at Tongling, Anhui (Hou, 1987) and Zhenjiang, Jiangsu (Metcalfe et al., 2005), and have been observed by the writers in the middle and upper Lower Triassic in sections such as Ziyun and Wanmo, in southern Guizhou, and Fengshan and Nandang, in northwestern Guangxi. In addition, tuffaceous beds occur widely in southwestern China at the Lower and Middle Triassic boundary (Zhu, 1994, 1995; Hu et al., 1996; Chen et al., 1999; Payne et al., 2004). The local tectonic setting of southern Guangxi resulted in considerable volcanic activity that extended from the late Palaeozoic into the Early Triassic (Liu et al., 1993; Liang et al., 2001; Newkirk et al., 2002). 5. Lower Triassic carbon isotope excursion

Bed

34 33 31 28 26 25

Sample

7 5 4 3 2 1d 1b

La

Ce

60.70 16.10 13.10 31.70 40.50 52.30 51.40 3.80 12.20 11.40 31.70 14.30 12.50 11.50 1.10 5.00 4.70 14.00 18.00 8.36 8.10 34 33 31 28 26 25 7 5 4 3 2 1d 1b

Sample Bed Depth Minerals and contents (%) no. no. (cm) 44

82.88 154.50 73.41 46.18 103.00 136.00 140.00

18.60 17.40 8.90 5.20 9.83 21.70 21.10 541.10 495.30 212.50 217.10 261.00 513.00 505.00 46.00 39.00 75.00 172.00 310.00 88.30 91.70

51.80 63.60 62.10 16.60 23.30 57.20 58.20

Hf Zr Th Ba Rb Sr Cu Ni Co Bed Sample

Table 3 Distribution of minor elements and REE in clay beds at Meishan, Zhejiang

Table 4 Mineral composition of clay beds at Daxiakou, Xingshan, Hubei

8

24.96 50.22 21.69 28.26 62.00 57.10 58.80

4.60 4.30 7.00 0.50 1.65 2.73 2.66 20.20 17.00 25.30 13.60 21.40 19.50 19.80

Ta Nb

4.30 22.90 15.60 39.80 74.10 49.40 47.30

J. Tong et al. / Global and Planetary Change 55 (2007) 66–80

Pb

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A major negative δ13C excursion has been observed worldwide at the Permian–Triassic boundary. Another negative excursion with values even lower than those at that boundary occurs in the Lower Olenekian (Smithian) (Fig. 3) and is followed by high values at the beginning of the Upper Olenekian (Spathian). This δ13C excursion is thought to be closely related to the process of ecosystem reconstruction and biotic recovery at the beginning of the Triassic (Tong et al., 2002). It was recognized in the North Pingdingshan section, Chaohu,

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in 2001. In the following year two other sections at Chaohu were sampled and the North Pingdingshan section was re-sampled. The results from this additional sampling substantiated and refined the character of the Lower Triassic δ13Ccarb excursion (see Fig. 3) (Tong et al., 2005b). Subsequently, carbon isotopes were studied from other Lower Triassic sections representing various palaeogeographic settings in different parts of South China (Fig. 7). Though the δ13C values vary from section to section, the excursions show a very similar pattern and more or less follow that recorded at Chaohu, except for a small positive shift (peak) in the early Induan (late Griesbachian) in some sections. The late Spathian negative shift at Chaohu (Fig. 3) may have resulted from local tectonism. There was therefore, as noted by Payne et al. (2004), a large perturbation in the carbon isotope composition for carbonate during the Early Triassic. The first major change in this composition occurred in the latest Permian and resulted from events connected with the endPermian mass extinction. Carbon isotope composition was very unstable across the Permian–Triassic boundary but recovery occurred immediately in the early Induan (Griesbachian). This recovery was not, apparently, sus-

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tained; further change occurred later in the Induan (early Dienerian) before recovery resumed late in the Dienerian and continued into the earliest Olenekian (early Smithian) when δ13C values were very high in some areas. However, in most sections δ13C values are very negative in the Smithian, and in some sections are even lower than those around the Permian–Triassic boundary (Fig. 7). This negative phase persisted for a relatively long period, until a rapid positive shift occurred at the Smithian– Spathian boundary. High positive δ13C values persisted through the early Spathian and were followed by a negative shift in the late Spathian. A positive shift in δ13C values occurs at the boundary of the Lower and Middle Triassic (Figs. 3 and 7). 6. Discussion and conclusion Environmental change at the end of the Palaeozoic resulted in a mass extinction, the recovery from which was prolonged because of harsh environmental conditions during the Early Triassic (Woods et al., 1999; Payne et al., 2004; Pruss and Bottjer, 2004); such conditions usually reflect abnormal environmental events and changes.

Fig. 7. Lower Triassic carbon isotope excursions in sections in South China. See Fig. 1 for locations.

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The Earth ecosystem is capable of self-adjustment to maintain an ecological balance. The effects of environmental events on the ecosystems depend upon the maturity of the ecosystem structure. The stability of the environment, and the ability of biota to reorganize determine the maturity of the ecosystem structure (Shen and Shi, 2002). After an extended period of development through the Carboniferous and Permian, the ecosystems in the latest Palaeozoic were mature and had the ability to survive environmental changes. Therefore, the great environmental changes at the end of the Mid Permian (Guadalupian), though causing a significant mass extinction (Jin et al., 1994; Stanley and Yang, 1994), did not destroy the Palaeozoic marine ecosystems completely. The Late Permian (Lopingian) marine ecosystem maintained the late Palaeozoic biota and ecosystem structure though the composition of the biota clearly differed from that in earlier times (see the compositions of various taxa during the Changhsingian in Yang et al., 1987, 1993; Yin et al., 2000). However, environmental events occurred with increasing frequency during the Late Permian. From latest Mid Permian time the extent of shallow shelf environments was rapidly reduced globally, probably because of a rapid global sea-level fall or the maximum of Pangea aggregation, and thus the stable evolution of marine ecosystems was severely impeded. Also, volcanic events were frequent and such activity appears to have reached a maximum in the latest Changhsingian. Probably superimposed on, or initiating, some other unusual environmental events, the full collapse of the Palaeozoic ecosystems and the biggest mass extinction in the Phanerozoic, finally occurred and is now used to mark the Permian–Triassic transition. The reconstruction of the ecosystem following the end-Permian mass extinction was a prolonged process and full biotic recovery took nearly the entire Early Triassic. At the beginning of the Triassic the ecosystem was very fragile because the biota, heavily depleted by the mass extinction, was in a stagnant state, with rselection taxa dominant and K-selection forms barely surviving (MacArthur and Wilson, 1946); unfavourable environmental conditions, such as anoxia, impeded the reconstruction of the ecosystem. In natural ecosystems the growth and evolution of populations follow two strategies: r-selection and K-selection. Organisms in the former category are subject to rapidly changing environments with highly fluctuating food resources and are characterized by high birthrate, short life-span, small body-size and little care for offspring; the K-selected forms live in a more uniform or predictable environment with population sizes close to the environmental

carrying capacity. Theoretically, r-selection emphasizes adaptations for rapid population growth and K-selection emphasizes competitive ability (Strickberger, 2000). It appears that, immediately following the endPermian mass extinction, the marine ecosystem had returned to a condition similar to that dominated by cynobacterians at the beginning of the Phanerozoic (Sepkoski et al., 1991; Pruss et al., 2004; Xie et al., 2005). The biota played a leading role in the development of such a “low-level” ecosystem. At the beginning of the Mesozoic the metazoans in particular would have had the potential to promote the re-organization of the ecosystem when life had evolved to a relatively high level. If there were no further environmental changes, severe environmental conditions could not have lasted for several million years because they would have been modified by the Early Triassic biota. Therefore, environmental changes and events that maintained unfavourable conditions must have continued spasmodically through the Early Triassic to delay progress in ecosystem reconstruction and biotic recovery. A possible cause of such changes is, according to the evidence from South China, volcanism that may have been related to the Siberian Traps (Campbell et al., 1992; Renne et al., 1995). The involvement of other events, such as bolide impact or mass release of methane hydrate, requires further study. Anoxia appears to be one of the direct indicators of a harsh environment affecting Early Triassic marine ecosystems (Wignall and Twitchett, 1996; Isozaki, 1997; Wignall and Twitchett, 2002), though whether this was widespread in the oceans and persisted throughout that time is uncertain. For example, anoxia did not occur on carbonate buildups and isolated platforms (Lehrmann, 1999; Lehrmann et al., 2003), but the extremely shallow water in those areas prevented the normal reconstruction of the ecosystem during the Early Triassic. A study of the ferruginous (Fe2+ and Fe3+) phases in the Lower Triassic at Chaohu indicates that a period of increased availability of free oxygen occurred during the early Olenekian. As Chaohu was a relatively deep basinal area at that time, this is attributed to an interruption of “stratified ocean” conditions “by a short period of circulation, bringing oxygen rich water to the seafloor” (Horacek et al., 2005). Thus it can be seen that fluctuations caused by intermittent environmental events were the main factor in the delayed reconstruction of marine ecosystems in the early Mesozoic. These events may be virtually indistinguishable from those that occurred in the Late Permian and even around the Permian–Triassic boundary. The significant difference is, however, that those in the Late Permian affected a mature, established

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ecosystem and had no immediate results, though they may have contributed ultimately to the latest Permian extinction, whereas those in the Early Triassic affected a fragile ecosystem undergoing recovery and had immediate impact. The end-Permian mass extinction may have resulted from the influence of additional rare events or simply from the accumulated effect of those that occurred in the late Changhsingian. Current explanations of the carbon isotope anomaly include volcanism and mass release of methane hydrates (see review by Berner, 2002). However, the δ13C record in the Changhsingian and Lower Triassic appears enigmatic and is inadequately explained by these hypotheses. For example, though there were many volcanic events in the Changhsingian, the δ13C record does not show corresponding large negative excursions (Li, 1998; Cao et al., 2002; Nan and Liu, 2004). The Early Triassic δ13 C record shows several large negative excursions after that at the Permian–Triassic boundary (also see Payne et al., 2004). If the carbon isotope anomaly is to be explained by methane hydrate release, we must first understand the mechanism of release that would result in multiple negative carbon isotope excursions in such a short period. Considering the history of the ecosystem and biotic evolution through the transitional time, it appears more likely that the δ13C excursion was related to the development of bio-productivity and that it directly reflects the effect of environmental events on the ecosystem, as well as the relationship between life and environments in the ecosystem. In conclusion, the extensive and prolonged ecosystem and biotic crisis at the beginning of Triassic may have resulted both from events at the Permian–Triassic boundary and subsequent events that contributed to severe environmental conditions, such as anoxia, during the Early Triassic and impeded the “normal” (metazoan) ecosystem reconstruction and biotic recovery. This may be the main difference between the Permian–Triassic biotic transition and others in the Phanerozoic. A series of environmental events and changes that began in the Late Permian resulted in the biggest mass extinction and longest delayed recovery in the Phanerozoic, but the nature of those events and changes may not have differed significantly from those associated with other mass extinctions. Acknowledgements This study is one of a series carried out by the GeoTurn Group at China University of Geosciences. It was supported by the National Natural Science Foun-

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dation of China (Grant Nos. 40232025, 40325004), the Ministry of Education (Grant No. 03033), and the Chinese “973 Program” (Grant No. G2000077705). We thank Drs. Yin Hongfu, Geoffrey Warrington and Mike Orchard for thorough reviewing and comments, and especially Dr. Warrington for a careful revision of the manuscript with great patience. References Baud, A., Magaritz, M., Holser, W.T., 1989. Permian–Triassic of the Tethys: carbon isotope studies. Geologische Rundschau 78, 649–677. Baud, A., Cirilli, S., Marcoux, J., 1997. Biotic response to mass extinction: the lowermost Triassic microbialites. Facies 36, 238–242. Becker, L., Poreda, R.J., Hunt, A.G., Bunch, T.E., Rampino, M., 2001. Impact event at the Permian–Triassic boundary: evidence from extraterrestrial noble gases in fullerenes. Science 291, 1530–1533. Berner, R.A., 2002. Examination of hypotheses for the Permo–Triassic boundary extinction by carbon cycle modeling. Proceedings of the National Academy of Sciences of the United States of America 99, 4172–4177. Braun, T., Osawa, E., Detre, C., Toth, I., 2001. On some analytical aspects of the determination of fullerenes in samples from the Permian/Triassic boundary layers. Chemical Physics Letters 348, 361–362. Campbell, I.H., Czamanske, G.K., Fedorenko, V.A., Hill, R.I., Stepanov, V., 1992. Synchronism of the Siberian Traps and the Permian–Triassic boundary. Science 258, 1760–1763. Cao, C.Q., Wang, W., Jin, Y.G., 2002. Carbon isotope excursions across the Permian–Triassic boundary in the Meishan section, Zhejiang Province, China. Chinese Science Bulletin 47, 1125–1129. Chen, Jinshi, Shao, Maorong, Huo, Weiguo, Yao, Yuyuan, 1984. Carbon isotope of carbonate strata at Permian–Triassic boundary in Changxing, Zhejiang. Scientia Geologica Sinica 19, 88–93 (in Chinese, with English Abstract). Chen, Zhong, Shen, Mingdao, Zhao, Jingsong, Tang, Hongming, 1999. New analysis on the composition of “Mung Bean Rock”. Journal of Southwestern Petroleum Institute 21, 39–42 (in Chinese, with English Abstract). de Wit, M.J., Ghosh, J.G., de Villiers, S., Rakotosolofo, N., Alexander, J., Tripathi, A., Looy, C., 2002. Multiple organic carbon isotope reversals across the Permo–Triassic boundary of terrestrial Gondwana sequences: clues to extinction patterns and delayed ecosystem recovery. The Journal of Geology 110, 227–240. Erwin, D.H., 1993. The great Paleozoic crisis: life and death in the Permian. Columbia University Press, New York. Erwin, D.H., 1994. The Permo–Triassic extinction. Nature 367, 231–236. Erwin, D.H., 1995. Diversity crisis in the geologic past. In: Berg, N. (Ed.), Encyclopedia of Environmental Biology, vol. 1. Academic Press, New York, pp. 507–516. Erwin, D.H., 1998a. After the end: recovery from extinction. Science 279, 1324–1325. Erwin, D.H., 1998b. The end and the beginning: recoveries from mass extinction. Trends in Ecology & Evolution 13, 344–349. Erwin, D.H., 2000. Life's downs and ups. Nature 404, 129–130. Erwin, D.H., Bowring, S.A., Jin, Yugan, 2002. End-Permian mass extinctions: a review. In: Koeber, C. (Ed.), Special Paper 356: Catastrophic Events and Mass Extinctions: Impacts and Beyond, pp. 363–383.

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