Complete biotic and sedimentary records of the Permian–Triassic transition from Meishan section, South China: Ecologically assessing mass extinction and its aftermath
Introduction
As the greatest biocrisis of life on Earth (Sepkoski, 1982), the Permian–Triassic mass extinction (PTME) changed Earth's ecosystems fundamentally (Benton and Twitchett, 2003, Erwin, 2006). After they had recovered, the marine ecosystems after the PTME gave rise to the forerunners of modern-day ecosystems, both the Triassic and modern ecosystems being comparable to each other in composition of functioning groups and trophic structure (Chen and Benton, 2012). However, the causes of this enigmatic biocrisis have long been disputed despite intense study, and the same is true of the profoundly delayed recovery following the PTME (Erwin, 2001). Thus, studies of these issues have enjoyed a surge in scientific interest in the past 30 years that shows no sign of abating (Chen et al., 2014).
Although this era-boundary crisis has been widely recognized in Permian–Triassic boundary (PTB) sections around the world, many important hypotheses have been proposed based on paleontological and experimental data sampled from the Meishan section of Changhsing County, Zhejiang Province, east China (Renne et al., 1995, Bowring et al., 1998, Jin et al., 2000, Kaiho et al., 2001, Mundil et al., 2001, Yin et al., 2001, Mundil et al., 2004, Grice et al., 2005, Xie et al., 2005, Kaiho et al., 2006a, Kaiho et al., 2006b, Riccardi et al., 2006, Wang and Visscher, 2007, Xie et al., 2007, Cao et al., 2009, Chen et al., 2009, Song et al., 2009, Chen et al., 2010, Huang et al., 2011, Shen et al., 2011, Yin et al., 2012, Song et al., 2013, Song et al., 2013, Wu et al., 2013, Burgess et al., 2014, Wang et al., 2014; Fig. 1A). This section is the Global Stratotype Section and Point (GSSP) for the PTB (Yin et al., 2001; Fig. 1C) and also well known for the best record of both biotic and geochemical signals demonstrating the PTME all over the world. Here, the exposures of the PTB beds are spectacular, extending about 2 km laterally along the Meishan hill (Fig. 1E). The PTME has been well demonstrated by Jin et al. (2000), whose study based on paleontological data from Meishan reveals that this extinction event was abrupt and dramatic, with most Permian organisms being wiped out within a very short interval, which was precisely calibrated to the base of Bed 25, a white clay bed, in Meishan (Fig. 1B, D), while the PTB is placed at the middle of Bed 27, about 16–20 cm above the base of Bed 25 in the same section (Yin et al., 2001; Fig. 1C). As such, the biocrisis clearly pre-dated the PTB (Fig. 1D). The P–Tr ecologic crisis is also marked by a pronounced negative carbon isotopic excursion (Xu and Yan, 1993, Jin et al., 2000, Kaiho et al., 2001, Cao et al., 2002, Xie et al., 2005, Xie et al., 2007; Fig. 2) and is also associated with an end-Permian sulfur event (Kaiho et al., 2006a, Kaiho et al., 2006b, Riccardi et al., 2006).
After Jin et al.'s (2000) influential study, which was largely based on fossil data obtained in the 1980s (i.e., Zhao et al., 1981, Liao, 1984, Sheng et al., 1984, Sheng et al., 1987, Shi and Chen, 1987), abundant brachiopod and foraminifer faunas have been detected from Beds 25 to 27, immediately above the PTME horizon in Meishan (Chen et al., 2005a, Chen et al., 2006b, Song et al., 2007, Song et al., 2009). Quantitative analysis of the updated foraminifer data from Meishan revealed a two-stage extinction pattern near the P–Tr boundary (Song et al., 2009), which agrees well with two distinct peaks of cyanobacteria, detected by biomarker analysis from the same section, suggesting two extinction events corresponding to Beds 25 and 28 (Xie et al., 2005). The two-stage extinction pattern is also strengthened by extremely abundant benthic fossils obtained from a shallow platform facies of the PTB section at Huangzhishan, about 40 km from Meishan (Chen et al., 2009). However, S.Z. Shen et al. (2011) clarified an abrupt biotic decline in a short interval equivalent to Beds 25–28 of Meishan based on quantitative analysis of fossil records from Meishan and other PTB sections in South China. In contrast, H.J. Song et al. (2013) demonstrated nicely a two-stage extinction pattern for the P–Tr crisis based on quantitative analysis of paleontological data derived from Meishan and a further six PTB sections in South China. Thus, debate on whether the PTME was either a single crisis or episodic extinctions still continues (Shen et al., 2011, Song et al., 2013, Wang et al., 2014). Regardless of whether the extinction was single or a two-phase pattern, an increasing number of faunas have been found in Beds 25–28 of Meishan and its counterparts across all of South China, although this interval may just last 60 kyr (Burgess et al., 2014).
In addition, a further extinction event resulting in depletion of Permian reefs in South China was calibrated to the base of Bed 24e at Meishan (Yang et al., 1993). Yin et al. (2007) re-documented biotic and geochemical signal changes across this horizon, which is reinforced by several lines of evidence, including reduction in conodont sizes (Luo et al., 2006), possible extinction of radiolarians in deep habitats and a negative shift in organic carbon isotope values (Cao et al., 2009). To sum up, biotic variations based on sound paleontology over the P–Tr transition have been far less studied in comparison with the intense geochemical studies of this catastrophe in most PTB sections. Current, updated fossil records from extensive PTB sections are crucial to reveal the true biotic responses to these environmental crises.
As briefly summarized above, there have been great advances in research on the PTME at Meishan in recent years. Multiple scenarios interpreting the causes of the P–Tr biocrisis have been proposed based on experimental data sampled from this section. Nevertheless, any reasonable models interpreting the P–Tr crisis need to be tested by analysis of precise biotic extinction patterns and physiological reactions of victims and survivors (Knoll et al., 2007). As a result, we herein document the updated, complete fossil and sedimentary records, including microfacies, microfossils, body and trace fossils, and pyrite framboids, throughout the P–Tr transition and attempt to test biotic responses to various environmental and climatic catastrophes from the GSSP Meishan.
Section snippets
Biostratigraphy and correlations
After Yin et al.'s (2001) placement of the PTB at the base of Bed 27c, marked by the first appearance datum (FAD) of the conodont Hindeodus parvus, Jiang et al. (2007) established gondolellid and hindeodid conodont zones across the PTB in Meishan. The former include the Clarkina yini, Clarkina meishanensis and Clarkina taylorae Zones, while the latter comprise the Hindeodus latidentatus, Hindeodus praeparvus, Hindeodus changxingensis, H. parvus, Isarcicella staeschei, and Isarcicella isarcica
Microstratigraphy, fossil fragment contents and paleoenvironmental analysis of the P–Tr transition
At Meishan, the P–Tr succession comprises the Changhsing and Yinkeng Formations below and above. The former unit is a 41-m-thick carbonate succession consisting of medium- to thin-bedded limestone, while the Yinkeng Formation is about 15 m thick and dominated by mudstone and muddy limestone in the lower part and characterized by thin-bedded limestone in the upper part (Fig. 3). These two formations have been frequently described (Zhao et al., 1981, Yang and Jiang, 1981, Sheng et al., 1984, Sheng
Biodiversity variations over the P–Tr transition
Comprehensive paleontological studies of the Meishan section were undertaken in the 1980s (Zhao et al., 1981, Sheng et al., 1984, Shi and Chen, 1987, Yang et al., 1987). The fossil record employed by Jin et al. (2000) to document the PTME pattern, which shows an abrupt extinction calibrated to the base of Bed 25, was sourced mainly from these studies. Since then, more diverse faunas and floras have been documented from Meishan, including foraminifers (Song et al., 2007, Song et al., 2009),
Trace fossils and bioturbation
At Meishan, Bottjer et al. (1988) made the first attempt to ecologically test the PTME based on trace-fossil assemblages. These authors, however, could not collect sufficient trace fossils because of restricted exposure at that time, but they noted that ichnotaxa from the PTB beds are dominated by Planolites and Chondrites, which indicate generally a poorly oxygenated environment (Bottjer et al., 1988). Later, Cao and Shang (1998) reported a few ichnotaxa such as Thalassinoides, Planolites and
Size variations of pyrite framboids and redox conditions over the P–Tr transition
Pyrite is commonly present in the latest Changhsingian to Griesbachian rocks at Meishan (Wignall and Hallam, 1993), which is also confirmed by our observations of thin sections through the P–Tr transition at Meishan. Several pyrite-enriched beds have been treated as indications of anoxic conditions at Meishan (Wignall and Hallam, 1993). In particular, pyrite framboids, which are spherical aggregates of pyrite microcrystals, are rather abundant in these pyrite-enriched beds near the PTB at
Ecologic testing of extinction patterns
The updated fossil record from Meishan shows two pronounced declines of species richness at the bases of Beds 25 and 28 (H.J. Song et al., 2013; Fig. 26). Similarly, fossil fragment contents recorded in thin sections also show two distinct drops in both abundance and diversity corresponding to the top of Bed 24e and base of Bed 28 (Fig. 6, Fig. 14). Further, ichnodiversity also declined within Beds 24 and 27. In Bed 24, trace fossils are rather abundant and comprise four distinct ichnogenera:
Conclusions
Updated conodont biostratigraphy allows the establishment of eight conodont zones from the latest Changhsingian to early Griesbachian at Meishan, the C. yini, C. meishanensis, H. changxingensis, C. taylorae, H. parvus, I. staeschei, I. isarcica, and C. planata zones. Microstratigraphic analysis shows that a major turnover in fossil fragment contents and ichnodiversity occurs across the boundary between Beds 24e-5 and 24e-6, suggesting the actual mass extinction horizon in thin section. Bed 27
Acknowledgments
We thank J. Tong for help in the field and H.J. Song for discussion on foraminiferal taxonomy of collections from the PTB beds in Meishan. We are also grateful to the Guest Editor Thomas Algeo and two anonymous reviewers for their critical comments and constructive suggestions, which have improved greatly the quality of the paper. This work was supported by the 973 Program of China (2011CB808800), the 111 Program of China, Ministry of Education of China, a NSFC grant (No 41272023), and two
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