2. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;
3. School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK;
4. Non-ferrous Metals Geological Exploration Bureau of Zhejiang Province, Shaoxing 312000, China
The largest mass extinction in geological history occurred around the Permian-Triassic boundary (PTB). It has been studied for several decades and various extinction patterns (e.g., Shen et al., 2018, 2011; Jiang et al., 2015; Wignall, 2015; Wang et al., 2014; Song et al., 2013; Yin et al., 2012) noted with numerous causes (e.g., Sun et al., 2018; Baresel et al., 2017; Ernst and Youbi, 2017; Foster et al., 2017; Brand et al., 2016; Chen B et al., 2016; Grasby et al., 2016; Xiang et al., 2016; Chen Z-Q et al., 2015; Clarkson et al., 2015; Jiang et al., 2015; Song et al., 2014; Yin et al., 2014; Joachimski et al., 2012; Sun et al., 2012) proposed, but debate on the timing and nature of the crisis continues. Much of this discussion has been focused on sections in South China, notably the Global Stratotype Section and Point (GSSP) at Meishan and the auxiliary GSSP at Shangsi. Many sections in the Nanpanjiang Basin area have also provided important insights into PTB events, e.g., the persistence of siliceous deposition into the Early Triassic at Gaimao (Yang et al., 2012), the occurrence of delayed extinction in the deep-water location of Bianyang (Jiang et al., 2015) and the age of post-extinction microbialite facies following a hiatus in shallow, carbonate settings (Baresel et al., 2017; Jiang et al., 2014) at the Great Bank of Guizhou (GBG).
Like the GBG, the Napo Platform is also found within the Nanpanjiang Basin, being located near the southwestern margin (Fig. 1). We have undertaken the first study of conodont biostratigraphy and δ13Ccarb fluctuations at the Pojue and Dala sections (Fig. 1), in order to better constrain the mass extinction and environmental changes during the PTB on this little studied platform.1 GEOLOGICAL SETTINGS AND FACIES DESCRIPTION
The Pojue Section is located at Pojue Village, Longhe Town, Napo County (Fig. 1). The Dala Section occurs about 37 km south of Pojue and is located at Dala Village, Nanpo Town, Jingxi County (Fig. 1). These two sections record carbonate- dominated marine sedimentation during the PTB interval and are well exposed and easily accessible.
The Upper Permian strata at Pojue are dominated by bioclastic packstone and algal-laminated bindstone of the Wuchia- ping Formation. The former lithology has a rich biota which includes fusulinids, foraminifers, calcareous algae and ostracods (Fig. 2a). Only calcareous algae and foraminifers occur in the algal-laminated bindstone. The diverse biota indicates an open shallow-marine setting and a low to moderate high-energy environment in the Late Permian.
The Permian-Triassic transition strata of Pojue consist of an 8 cm-thick limestone bed (Fig. 2a), which occurs above the algal-laminated bindstone and is overlain by microbialite. It consists of dolomitized mudstone (Bed 10B, Fig. 2d), foraminiferal cortoid grainstone (Bed 10A, Fig. 2d) and stromatolites with wavy laminae. These lithologies occur as multiple distinct units bounded by uneven or irregular trunction surface shown in Fig. 2b. The lowest Triassic bed is a dolomitized mudstone that only yields H. parvus. It is bounded by truncation surfaces and is overlain by microbialite (Fig. 2b). The dolomite crystals are dirty with inclusions (Fig. 2d, Bed 10B).
Lower Triassic limestones sit on a truncation surface (Fig. 2e) at the top of the dolomitized mudstone (Bed 10B) mentioned above or the stromatolites (Bed 10C) from the Permian-Triassic transition bed. The 6 m-thick, calcimicrobial framestone is composed of thrombolites (characterized by clotted structures), showing occasional domal structures (Fig. 2e). This is in turn overlain by thin-bedded micritic mudstone with ostracods and conodonts. Horizontal bioturbation and thin-shelled fossils are seen in other Earliest Triassic thin-bedded strata around this level (e.g., Zhao et al., 2008; Baud et al., 2007; Wignall and Hallam, 1996) but these features are absent from the Pojue strata.
The lithology of the Dala Section is similar to that of the Pojue Section, but with some minor differences. The Upper Permian Wuchiaping Formation bioclastic packstone contains fusulinids, foraminifers, calcareous algae, ostracods and echinoderms, but the absence of algal-laminated bindstone suggests a slightly deeper water setting. The Wuchiaping Formation is once again capped by a truncation surface. Thus, the Dala Section consists, in ascending order, of cortoid grainstone with fusulinids and foraminifers, dolomitized mudstone with the conodont H. praeparvus, a stromatolite bed and thrombolites. The thrombolites bed in turn is overlain by thin-bedded micritic mudstones that contain ostracods or some thin-shelled fossils. Thin oolitic wackstone beds, 5–10 cm thick, are interleaved with the mudstones. The ooids, 1–2 mm in diameter, are partly recrystallized (Fig. 2f), although some show more than one nucleus and a thick cortex with irregular overlapping micritic laminae reminiscent of oncoids (Fig. 2f). Ooids also occur in GBG sections but as thicker beds of oolitic grainstone (e.g., Li et al., 2015, 2013; Tian et al., 2015; Lehrmann et al., 2012). The co-occurrence of ooids, typically produced in agitated conditions and micrite, suggesting quiet energy, at Dala implies the ooids may have formed in shallower waters and then been transported into the depositional setting.2 MATERIALS AND METHODS
In order to investigate the conodont biostratigraphy, 17 samples (each > 4 kg in weight) were collected from the Pojue Section and 21 samples (each > 4 kg) were collected from the Dala Section (Figs. 3 and 5). All samples were broken into fragments then dissolved in dilute acetic acid (10%) and a 2.80–2.82 g/mL heavy liquid solution (solution of lithium heteropolytungstates in water) was used to separate the conodonts from the residues (method see Yuan et al., 2015). In total 2 626 conodont elements belonging to Hindeodus, Isarcicella and Clarkina were obtained. Among them were 29 well preserved P1 elements from Pojue and 322 well-preserved P1 elements from Dala (Fig. 3).
Both the Pojue and the Dala sections of the Napo Platform in Guangxi Province have been sampled for carbon and oxygen stable isotope investigation. Bulk rock carbonates were sampled by micro drill to produce 2–4 mg powders and 13C/12C and 18O/16O ratios of CO2 were generated by the acid reaction and measurement on a Finnigan MAT-253 at the State Key Laboratory of Biogeology and Environment Geology in Wuhan and converted to δ13Ccarb (VPDB) and δ18O (VPDB) (VPDB, Vienna Pee Dee Belemnite) (Fig. 4).3 RESULTS 3.1 Conodont Biostratigraphy 3.1.1 The Pojue Section
We failed to obtain any conodonts from beds 1 to 10A in our collections. However, abundant fusulinids and foraminifera are seen in thin section such as the fusulinids Nankinella sp. and Pisoina sp. (Figs. 2g and 2h). In addition, the fusulinids, Sphaerulina and the coral Liangshanophyllum have also been reported from these beds at Pojue, and Palaeofusulina and Codonofusiella are known from the same level regionally and indicate a Late Permian (Changhsingian) age (Regional Geological Survey Team of the Guangxi Zhuang Autonomous Region Geological Bureau, 1974).
Two species, Hindeodus parvus and Hindeodus praeparvus are found at Bed 10B (Fig. 5 and Plate 1). Although this is not enough to precisely identify a conodont zone, H. parvus marks the base of Triassic (Yin et al., 2001), and therefore the beds above Bed 10B, including the microbialite bed, at Pojue can be assigned an Early Triassic (Griesbachian) age.3.1.2 The Dala Section
Beds 1–8 also failed to yield conodonts at the Dala Section. However, the lithostratigraphy of this interval is very similar to that at Pojue and so we also assign a Latest Permian age to these strata at Dala. The lithostratigraphic succession from the base of Bed 9 is almost the same as that from beds 10A to 10C to Bed 11 at Pojue. Fortunately, more conodonts were collected from beds 9–17 at the Dala Section, enabling the identification of four conodont zones (Plates 1–4). In ascending order, they are: Hindeodus parvus Zone, Isarcicella staeschei Zone, Isarcicella isarcica Zone and Clarkina planata Zone.
Hindeodus parvus Zone: Hindeodus parvus first occurred in sample DAL-11 (Fig. 5). While a specimen of Hindeodus cf. parvus is found from Bed 10 and Hindeodus praeparvus is found from the base of Bed 9 (Fig. 5), based on comparison with the Pojue Section, H. parvus could appear in the base of Bed 9 (Fig. 5) at Dala. Thus, we tentatively place the lower limit of this zone at the base of Bed 9. The upper limit is defined by the first occurrence of Isarcicella staeschei. Associated taxa include Hindeodus praeparvus and Hindeodus cf. parvus. This zone is widely reported in Nanpanjiang Basin area (e.g., Bianyang (Yan et al., 2013), Dajiang (Jiang et al., 2014), Xinmin (Zhang et al., 2014), Dawen (Chen et al., 2009; Liu et al., 2007) and Langpai (Ezaki et al., 2008)).
Isarcicella staeschei Zone: Lower limit: first occurrence of Isarcicella staeschei; upper limit: first occurrence of Isarcicella isarcica. This zone ranges from the base of sample DAL-12 to the base of sample DAL-13 (Fig. 5). Associated taxa: Hindeodus parvus, Hindeodus praeparvus and Isarcicella turgida. The I. staeschei Zone also has been widely reported in Nanpanjiang Basin (e.g., Dawen (Chen et al., 2009; Liu et al., 2007), Langpai (Ezaki et al., 2008) and Heping (Krull et al., 2004; Lehrmann et al., 2003)).
Isarcicella isarcica Zone: Lower limit: first occurrence of Isarcicella isarcica; upper limit: first occurrence of Clarkina planata. Associated taxa: H. parvus, H. praeparvus, H. peculiaris, I. staeschei, I. inflata and I. turgida. This zone ranges from the base of sample DAL-13 to the base of sample DAL-15 in the Dala Section (Fig. 5). The zone is widely reported from the Nanpanjiang Basin (e.g., Langpai (Ezaki et al., 2008), Dawen (Chen et al., 2009; Liu et al., 2007), Gaimao (Yang et al., 2012) and Dajiang (Jiang et al., 2014)).
Clarkina planata Zone: Lower limit: first occurrence of Clarkina planata. The upper limit is undefined. Associated taxa: H. parvus, H. praeparvus, H. peculiaris, I. inflatus, I. prisca, I. turgida, C. lehrmanni, C. carinata and C. taylorae. This zone starts from the base of sample DAL-15 (Fig. 5). Wang (1996) first established the Clarkina planata Zone at Meishan, where it occurs in deep-water facies, and considered that it could correspond to the upper Isarcicella staeschei to Hindeodus postparvus zones in shallower water sections. Zhang et al. (2009) defined a Clarkina tulongensis-Clarkina planata assemblage zone, based on the disappearance of I. isarcica and the first occurrence of Sweetospathodus kummeli from beds 52–72 at Meishan. Yang et al. (2012) also reported Clarkina planata Zone at Gaimao in the Nanpanjiang Basin and defined the first occurrence of Clarkina planata and Clarkina krystyni as its base and top, respectively. The Clarkina planata Zone at the Dala Section is equivalent to that at Gaimao (Yang et al., 2012).
These four conodont zones of the Napo Platform can be readily correlated with sections in the Nanpanjiang Basin area and also with the GSSP at Meishan (Fig. 6).3.2 Carbon Isotopes
The oldest δ13Ccarb values are around +5‰ at the base of the Pojue Section and decline to +3‰ in the uppermost 5 m of the Upper Permian strata. Within the lowest Triassic microbialite bed δ13Ccarb values decline rapidly and then reach stable values between -1‰ to 0‰ in Bed 11 and Bed 12. From Bed 13 upwards the δ13Ccarb values recover to higher values around +2‰ (Fig. 5).
The δ13Ccarb values at Dala show a very similar trend to the Pojue Section. Beginning around +4‰ in the Upper Permian of the Dala Section these values remain stable, except for two lower values around 2 m from the top, before a rapid shift to light values that range from -0.5‰ to 0‰ in the microbialite bed (beds 9 to 16). From the base of Bed 17, the δ13Ccarb values return to higher values (around +1‰) again (Fig. 5).4 DISCUSSION 4.1 The Permian-Triassic Boundary
Defined by the first occurrence of H. parvus from Bed 10B at the Pojue Section, the PTB of this section is placed at the uneven surface between Bed 10B (Early Triassic dolomitized mudstone) and Bed 10A (Late Permian cortoid foraminifera grainstone). This situation is closely similar to that of the GBG sections such as Dajiang where the PTB is placed at the uneven surface between Permian bioclastic packstone and Lower Triassic wackstone (Jiang et al., 2014), and Yangtze Platform section like Gaohua (Wang et al., 2016) where the PTB is also placed at the uneven surface between the Upper Permian limestone to Lower Triassic microbialite. The cause of truncation has been debated with both intense ocean acidification (Payne et al., 2007) and emergence followed by karstification being proposed (Wignall et al., 2009). Lehrmann et al. (2015) concluded that the thickness for anisopachous fibrous cements below the truncation surface is random without a downward orientation which suggests the truncation surface has formed by submarine dissolution and it caused 30–100 kyr hiatus during the C. meishanensis, H. changxingensis and C. taylorae zones at Meishan (21 kyr, Wu et al., 2013). However, their evidence was refuted by Kershaw et al. (2016) who favoured physical erosion as the cause of truncation. Given the shallow-water setting of the Napo Platform sections, we consider it likely that a minor sequence boundary due to regression is seen in the studied sections.
H. parvus is absent from Bed 9 at the Dala Section, but comparing with the nearby Pojue Section (only 37 km away) and considering the similar lithological sequence and the fact that H. pareparvus was found at the base of dolomitized mudstone below the microbialite, we surmise that the PTB is likely to occur at the base of Bed 9 at Dala. Also, as noted above, the Permian- Triassic boundary at Dajiang has also been placed at the base of a wackstone below the microbialite, similar to Gaohua (Wang et al., 2016) and Jianzishan (Bai et al., 2017) sections. This inference is supported by the similar negative shifts seen in the δ13Ccarb curve at these two sections. The Hindeodus changxingensis Zone is also probably missing in these two sections (c.f., Jiang et al., 2014). This was followed by the development of the dolomitized mudstone bed and subsequently the microbialite bed during the H. parvus Zone. The first occurrence of H. parvus in some microbialite sections lead Brosse et al. (2015) to argue that this species is an unreliable marker for the base of the Triassic at the Meishan GSSP. In detail, they found that H. parvus first appears below H. eurypyge and I. turgida in their microbialite-bearing study section at Wuzhuan whereas H. parvus first appears above these species at Meishan (Jiang et al., 2007). These observations lead Brosse et al. to conclude that H. parvus appears "late" at Meishan. Alternatively, we note that both H. eurypyge and I. turgida are very rare at Wuzhuan—the former species is only known from a single element—indicating that they are unlikely to show a range comparable with that at Meishan. Both these species are also rare conodonts in our samples from Dala and they are absent from Pojue. Collection failure of H. eurypyge and I. turgida from microbialites is a more parsimonious interpretation of conodont distributions reported by Brosse et al. (2015).4.2 Implications for the Mass Extinction
Shen et al. (2011) estimated that the mass extinction during the PTB lasted for almost 200 kyr and was accompanied by a negative C isotope excursion. This interval was revised down to about 60±48 kyr by Burgess et al. (2014), and shortened further to ~31±31 kyr at Penglaitan Section (Shen et al., 2018). Song et al. (2013) demonstrated that there were two pulses of extinction straddling the PTB. Wang et al. (2014) attributed Song et al.'s findings to the Signor-Lipps effect and facies-dependent occurrences and concluded that there was a single extinction phase. However, Wang et al. (2014) failed to appreciate that the Song et al.ʼs study placed the second phase of extinction in the Early Triassic whereas the Signor-Lipps effect is only capable of making a mass extinction appear older not younger in time. In other words, implicit in the conclusion of Wang et al. is that the PTB mass extinction is an Earliest Triassic event even though they favored a single, Latest Permian extinction event. Jiang et al. (2015) reported an End-Permian foraminifera extinction at Bianyang, a deep-water location, which was later than that seen at Meishan. In contrast, He et al. (2015) suggested the deep-water brachiopods disappeared earlier than shallow-water brachiopods. Thus, extinction patterns clearly vary in different depositional settings in South China and span from the Latest Permian to the Earliest Triassic (see also Song et al., 2014). Biostratigraphically, the maximum extinction interval ranges from Bed 24e to Bed 28 at Meishan (Shen et al., 2011) and thus started in the C. mei- shanensis Zone and ended in the I. staeschei Zone (Chen Z-Q et al., 2015; Jiang et al., 2007). Although the H. changxingensis Zone is likely absent at Dala and Pojue, these sections still record most of the PTB interval. It is interesting that the δ13Ccarb record at the two sections also shows lower values during the extinction interval (Fig. 7). This correspondence is also seen at the Dajiang Section in GBG area (Luo et al., 2010) and at many other sections such as the Meishan GSSP (Shen et al., 2013; Fig. 7). Pojue and Dajiang sections both show a single negative shift beginning at the end of the Permian and culminating in an Earliest Triassic low of about 0 to -1‰ (Yin et al., 2014). Considering the C. mei- shanensis (±H. changxingensis) missing zone(s) (Yin et al., 2014), the shallow platform facies sections also can be correlated with the Meishan Section through the δ13Ccarb excursion, despite only one negative excursion observed at shallow water sections. Pojue, Dala, and Dajiang (shallow water) sections have higher carbon isotope values in the extinction interval, of about 0 to -1‰, than deeper water sections like Meishan which have negative values of about -2‰ to -5‰ (Song et al., 2012).
The Permian-Triassic mass extinction was not a sudden event in South China, but a phase of progressive environmental deterioration, spanning a few hundred of thousand years, linked to rapid global warming and the widespread development of marine anoxia (e.g., Jiang et al., 2015; Song et al., 2014). The microbialites of the Napo Platform developed on carbonate platforms during the PTB extinction and represent a simple microbial ecosystem (Wang et al., 2011). The abundant microbes could have served as a major food source and also provided a local oxygen supply, thereby enhancing the hospitality of the ecosystem and providing a refuge for ostracods and other organisms, although the community lacked a well-developed trophic hierarchy (Forel et al., 2013). The organisms present in the microbialite ecosystems (bivalves, ostracods and microgastropods) belong to groups with exceptional tolerance to high temperatures (Song et al., 2014), a factor that may also have been important in their occurrence. Unusually diverse bivalve and gastropod assemblages have also been reported from the contemporary microbialites at the Shanggan Section, Leye County, Guangxi, South China (Hautmann et al., 2011). Thus, like deep-water settings, microbialites offered a temporary refuge during the PTB mass extinction.5 CONCLUSIONS
The conodont biostratigraphy and carbon isotope record across the PTB at Pojue and Dala on the Napo Platform have been documented. Four conodont zones: H. parvus Zone, I. staeschei Zone, I. isarcica Zone and C. planata Zone are identified at Dala. Defined by the first occurrence of H. parvus, the PTB is placed at the base of a dolomitized bed (Bed 10B) at Pojue. Correlating, using lithofacies development and carbon isotope stratigraphy, the PTB at Dala is also placed at the base of a similar dolomitized bed (base of Bed 9 Fig. 5, B). The idea that H. parvus appears at a lower level in microbialite sections than in the Meishan Stratotype is based on the late occurrence of the rare species H. eurypyge and I. turgida in the microbialites and so we suggest that this notion is unsafe.
Both carbon isotope curves at the two sections show lower values during the mass extinction interval implying that the mass extinction was not a sudden event. The Napo Platform sections show a sequence of events that provide much detail on the cause of the mass extinction. Thus, Latest Permian times saw the development of shallow-water packstone facies with a high-diversity fauna. These strata are truncated and a hiatus spanning the C. meishanensis and H. changxingensis Zone is developed. The mass extinction began during this unrecorded interval on the Napo Platform.ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (No. 41572324), the Special Project on Basic Work of Science and Technology from the National Ministry of Science and Technology of China (No. 2015FY310100-11), and the China Geological Survey (No. DD20160120-04). SEM pictures and carbon isotopes data were undertaken at the State Key Laboratory of Biogeology and Environmental Geology (China). We thank Suxin Zhang, Yuheng Fang for their assistance in SEM. Thanks also go to Huyue Song for the helps in processing the carbon isotopes data. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0884-3.
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