Download Events during Early Triassic recovery from the end

Global and Planetary Change 55 (2007) 66 – 80
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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
67
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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J. Tong et al. / Global and Planetary Change 55 (2007) 66–80
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).
J. Tong et al. / Global and Planetary Change 55 (2007) 66–80
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
69
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
70
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
J. Tong et al. / Global and Planetary Change 55 (2007) 66–80
71
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.
72
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
73
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
74
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,
J. Tong et al. / Global and Planetary Change 55 (2007) 66–80
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-
75
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.
76
J. Tong et al. / Global and Planetary Change 55 (2007) 66–80
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
J. Tong et al. / Global and Planetary Change 55 (2007) 66–80
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-
77
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.
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