A total of 38 samples (each one > 6 kg) were collected from six units in the Shuixiakou Section (Fig. 4). All samples were crushed into 1–2 cm3 fragments, dissolved in 8% acetic acid, and followed by heavy liquid separation. Conodont elements were picked from the residues using a stereoscopic microscope. Scanning electron microscope (SEM) was used for photography.
Conodont oxygen isotope analyses were performed using a TC-EA (high-temperature conversion-elemental analyzer) coupled online to a ThermoFinnigan Delta V Plus mass spectrometer. Conodonts samples of 0.5 to 1 mg were dissolved in HNO3 with the phosphate group precipitated as Ag3PO4 (Joachimski et al., 2009). The Ag3PO4 (0.2 to 0.3 mg) was weighed into silver foil and transferred to the sample carousel of the TC-EA. At 1 450 ℃, the phosphate is reduced and CO formed as the analyzed gas. CO was transferred in a helium stream through a gas chromatograph via a Conflo Ⅲ interface to the mass spectrometer. All δ18Oapatite values were reported in ‰ relative to VSMOW. Samples as well as standards were generally measured in triplicate. The measurements were calibrated by performing a two-point calibration using NBS 120c (21.7‰) and a commercial Ag3PO4 (9.9‰). A laboratory standard was used as a control standard (TueA) and processed together with the samples. External reproducibility, monitored by replicate analyses of samples as well as the laboratory standard, was 16.5‰±0.15‰ (1σ; n=4). Note the gap of 0.9‰ in the δ18Oapatite values between the Shuixiakou and Tieqiao sections, since the δ18O of NBS 120c was reported as 22.6‰ in the published Tieqiao Section (Chen et al., 2011). Paleotemperatures were calculated using the temperature equation for biogenic apatite published by Pucéat et al. (2010).
Figure 5. δ18Oapatite records of the Roadian-Wordian (Middle Permian) interval in the Shuixiakou Section, and its correlation with the Tieqiao Section (Chen et al., 2011). Note that the δ18Oapatite values of the Tieqiao Section (Chen et al., 2011) were corrected by -0.9‰ since δ18O of NBS 120c was reported as 22.6‰ (compared to 21.7‰ in this study).
Sample Thickness (m) Conodont zone/stage Unit/formation δ18Oapatite (-J, ‰, VPDB) δ18Oapatite (-H, ‰, VPDB) T (℃) SXK-2 7.4 J. nankingensis/Roadian Unit I/Wulipo 19.8 19.9 27.1/26.7 SXK-5 52.7 J. nankingensis/Roadian Unit Ⅱ/Wulipo 18.8 18.6 31.3/32.2 SXK-6 91.2 J. aserrata/Wordian Unit Ⅲ/Shuixiakou 19.4 28.8 SXK-10 121.8 J. aserrata/Wordian Unit Ⅲ/Shuixiakou 19.2 29.7 SXK-12 126.2 J. aserrata/Wordian Unit Ⅲ/Shuixiakou 19.1 30.1 SXK-15 134.8 J. aserrata/Wordian Unit Ⅲ/Shuixiakou 19.2 29.7 SXK-16 137.3 J. aserrata/Wordian Unit Ⅲ/Shuixiakou 19.2 29.7 SXK-17 138.6 J. aserrata/Wordian Unit Ⅲ/Shuixiakou 19.4 28.8 SXK-26 174.8 J. aserrata/Wordian Unit Ⅳ/Shuixiakou 20.1 25.9 SXK-27 176.9 J. aserrata/Wordian Unit Ⅳ/Shuixiakou 20.2 25.4 SXK-28 178.2 J. aserrata/Wordian Unit Ⅳ/Shuixiakou 19.9 26.7 SXK-34 189.7 J. aserrata/Wordian Unit Ⅴ/Shuixiakou 19.9 26.7 SXK-36 192.5 J. aserrata/Wordian Unit Ⅴ/Shuixiakou 19.8 27.1 SXK-37 234.8 J. aserrata/Wordian Unit Ⅵ/Shuixiakou 20 26.3 SXK-38 250.3 J. aserrata/Wordian Unit Ⅵ/Shuixiakou 19.8 27.1 -J. Jinogondolella; -H. Hindeodus, Pseudohindeodus and Sweetognathus. The temperatures in shade corresponding to the values of δ18Oapatite in shade.
Table 1. The δ18Oapatite records of the Roadian-Wordian interval in the Shuixiakou Section
2.1. Conodont Samples
2.2. Oxygen Isotope Analysis of Conodont Apatite
In ascending order, the six units of the Middle Permian strata at the Shuixiakou Section can be described. Unit Ⅰ is dominated by grey, thin to thick, bioclastic limestones intercalated with mudstones containing shallow-water fossils including ostracods, gastropods and fusulinids, sponge spicules and burrows. Unit Ⅱ is dominated by mudstones intercalated with shale, chert and thin, bioclastic marly limestone, that contains shallow-water fossils including brachiopods, ostracods, gastropods and sponge spicules—the latter suggesting deeper waters. Unit Ⅲ is dominated by mudstones intercalated with shale and thin to medium bedded, bioclastic marly limestones containing a deep-marine fauna including abundant sponge spicules and rarer brachiopods in the lower part and a shallow water assemblage, including brachiopods, bivalves, solitary rugose corals, ostracods and echinoids, in the upper part. Unit Ⅳ is dominated by grey, thin to thick-bedded bioclastic limestone that contains brachiopods, ostracods, sponge spicules and crinoids. The lower part of Unit Ⅴ consists of black cherts with abundant sponge spicules, and the upper part of Unit Ⅴ is dominated by cherts intercalated with marly limestone. Unit Ⅵ is composed of marly limestone-shale intercalations containing shallow-marine brachiopods, crinoids, trilobites, ostracods, gastropods and sponge spicules (Figs. 3, 4).
Burrows are observed in samples SXK-1, 2 of Unit I. Hummocky cross bedding appears in the thin-bedded, limestone from samples SXK-8 and SXK-13 within Unit Ⅲ and Sample SXK-25 within Unit Ⅳ (Fig. 3c), whilst planar lamination appears in thin-bedded, marly limestone of Sample SXK-6 in Unit Ⅲ and cherts from samples SXK-32 to 36 within Unit Ⅴ (Fig. 3e). In the thin to medium-bedded, bioclastic, marly limestones of units Ⅱ and Ⅲ, the carbonate matrix is a calcisiltite in samples SXK-4, 5, 15 to 17, microspar in samples SXK-8, 13, 14 and micrite in samples SXK-6, 7, 9 to 12. Abundant fossil shells that are embraced by fine bioclastic grains are well preserved in Sample SXK-9, 14, 28 (Fig. 3).
The parallel bedding changes gradually into hummocky cross stratification and then into graded bedding (carbonate matrixes: from micrite/microspar to calcisiltite in upward) in the thin, bioclastic marly limestone beds of Unit Ⅲ. Ripple marks were reported in the limestone beds of the Wulipo and Shuixiakou formations in the Xikou area (Ding et al., 1989, 1983). These structures appear in units Ⅰ, Ⅳ and Ⅵ in the Shuixiakou Section. This range of evidences suggests deposition in the Shuixiakou Section was under storm influence. Unit Ⅰ and Unit Ⅳ are interpreted as mid-ramp facies while units Ⅱ, Ⅲ, Ⅴ and Ⅵ are interpreted as outer ramp or basinal facies.
1 2 5 6 9 10 11 12 13 14 15 16 17 18 21 24 26 27 28 29 31 34 37 38 H. excavatus 1 19 1 2 2 3 1 9 3 1 1 H. gulloides 1 H. minutus 1 2 2 2 1 H. wordensis 6 1 1 1 3 3 J. nankingensis nankingensis 1 15 1 4 4 3 6 1 J. nankingensis tenuis 199 4 J. aserrata 5 3 7 2 9 1 1 3 5 18 96 1 2 1 1 J. bitteri 13 M. cf. lamberti 4 Ps. cf. elliptica 2 1 Ps. ramovsi 8 19 1 13 7 8 1 Pu. vigilans 1 Sw. hanzhongensis 3 2 6 3 3 Sw. cf. subsymmetricus 1
Table 2. Statistics and distribution of conodont species (P1 element) of the Shuixiakou Section. H. Hindeodus; J. Jinogondolella; M. Mesogondolella; Ps. Pseudohindeodus; Pu. Pustulognathus; Sw. Sweetognathus.
Jinogondolella nankingensis Zone (Unit Ⅰ–lower Unit Ⅲ)
The upper limit of this zone is defined by the first occurrence of Jinogondolella aserrata (Clark and Behnken, 1979) in the Sample SXK-6 of Unit Ⅲ. Jinogondolella nankingensis nankingensis, Hindeodus excavatus, H. gulloides, H. minutus, H. wordensis, Mesogondolella cf. lamberti, Pseudohindeodus ramovsi and Sweetognathus hanzhongensis co-occur in this interval (Fig. 4; Table 2). The lower limit of this zone has not been defined. Jinogondolella nankingensis nankingensis is the marker of the Roadian strata, and it has been reported from USA (e.g., Mei and Henderson, 2002; Kozur, 1995; Behnken et al., 1986; Clark and Behnken, 1979; Behnken, 1975; Clark and Mosher, 1966), Canada (Orchard et al., 2001; Orchard, 1987), Tajikistan (Angiolini et al., 2015), Japan (Nishikane et al, 2014), Norway (Nakrem, 1991), South China (e.g., Sun et al., 2017; Wu et al., 2017; Zhang et al., 2010; Ching, 1960) and southeastern Qinling region (Cheng et al., 2017). Both H. excavatus and H. minutus are cosmopolitan species and are common in the Middle Permian strata (e.g., Sun et al., 2017; Wang et al., 2016; Lai et al., 2008; Behnken, 1975). Hindeodus gulloides has been reported from the late Roadian strata in West Texas (Kozur and Mostler, 1995), the Kungurian strata in Northeast Thailand (Burrett et al., 2014) and the late Kungurian strata in South China (Sun et al., 2017). Hindeodus wordensis is known from the middle Roadian to early Capitanian strata in West Texas (e.g., Wardlaw and Nestell, 2015; Nicora et al., 2009; Wardlaw, 2000), the Wordian to Capitanian strata in Tajikistan (Angiolini et al., 2015), the Wordian strata in Japan (Nishikane et al., 2014). Pseudohindeodus ramovsi has been reported from the Kungurian to Capitanian strata in South China (e.g., Sun et al., 2017, 2010, 2008; Wang et al., 2016; Lai et al., 2008) and West Texas (e.g., Lambert et al., 2010; Wardlaw, 2000), the Roadian to Capitanian strata in Japan (Nishikane et al., 2014), the late Kungurian to early Roadian strata in Oman (Henderson and Mei, 2003) and the Kungurian strata in Tajikistan (Angiolini et al., 2015). Sweetognathus hanzhongensis has been reported from the Roadian to Wordian strata in Hanzhong area (Shaanxi Province, China; Wang, 1978), the late Kungurian to Capitanian strata in South China (e.g., Sun et al., 2017; Ma et al., 2016).
Jinogondolella aserrata Zone (upper Unit Ⅲ–Unit Ⅵ)
The upper limit of this zone has not been recognized.
The lower limit is defined by the first occurrence of Jinogondolella aserrata in Sample SXK-6 of Unit Ⅲ (Fig. 4). Jinogondolella aserrata, J. bitteri, J. nankingensis nankingensis, J. nankingensis tenuis, Hindeodus excavatus, H. minutus, H. wordensis, Pseudohindeodus cf. elliptica, Ps. ramovsi, Pustulognathus vigilans, Sweetognathus hanzhongensis and Sw. cf. subsymmetricus all co-occur in this interval (Fig. 4; Table 2). Jinogondolella aserrata is the most cosmopolitan conodont species of the Wordian Stage, and is considered as the good marker for this interval. It has been reported from Canada (Sano and Orchard, 2004), USA (e.g., Wardlaw and Nestell, 2010; Clark and Behnken, 1979), Oman (Kozur and Wardlaw, 2010), South China (e.g., Sun et al., 2017; Wang et al., 2016; Mei et al., 1994), North China (Wang et al., 2004), Tajikistan (Angiolini et al., 2015) and Russia (Rybalka, 1989). Jinogondolella bitteri was found in the late Wordian to Capitanian strata in USA (Wardlaw and Collinson, 1979), Canada (Henderson, 1981), and Oman (Henderson, 2018; Kozur and Wardlaw, 2010). Jinogondolella nankingensis tenuis is known from the Roadian to Wordian strata in West Texas (Wardlaw, 2015; Wardlaw and Nestell, 2015), and it is the first described outside of Texas here. Pustulognathus vigilans was reported from the Wordian to Wuchiapingian strata in Canada (Golding, 2018).
The Global Stratotype Section and Point (GSSP) for the boundary between the Roadian and Wordian stages is defined by the first appearance datum (FAD) of the conodont Jinogondolella aserrata at Getaway Ledge in Texas (Glenister et al., 1999). The Roadian-Wordian boundary in the Tieqiao Section was tentatively placed in Bed 112 of the Maokou Formation (Sun et al., 2017). In the Shuixiakou Section, the conodont J. nankingensis Zone and fusulinid Pseudodoliolina-Cancellina Zone co-occur in the former formation (Cheng et al., 2017; Ding et al., 1989; this study). The fusulinid Neomisellina-Yabeina Zone was recognized in the Shuixiakou Formation and the Roadian-Wordian boundary is placed at the FAD of the conodont Jinogondolella aserrata (at ~51 m below the base of lower Unit Ⅳ) in the lower part of Unit Ⅲ (Fig. 4).
The relative sea-level changes in the Shuixiakou Section (Xikou area, southeastern Qinling region) appear similar to those recorded in the Tieqiao Section (Laibin area, South China; Fig. 4; Yao et al., 2012; Chen et al., 2009, 1998). During the Jinogondolella nankingensis Zone, the deepening from Unit Ⅰ mid-ramp limestones upwards into the outer ramp facies of units Ⅱ to Ⅲ in the Shuixiakou Section is similar to the synchronous sea-level changes from Bed 111 to beds 112 and 113 in Tieqiao Section (Yao et al., 2012; Chen et al., 2009, 1998). Though the upper limit of J. aserrata Zone in the Shuixiakou Section has not been recognized, similar relative sea-level changes in J. aserrata Zone appeared in both the Shuixiakou and Tieqiao sections. In the J. aserrata Zone, the water depths underwent a shift from the deep outer ramp facies of Unit Ⅲ to shallow mid-ramp facies of Unit Ⅳ, then to deep basin or outer ramp facies of units Ⅴ to Ⅵ again, which is similar to the sea-level changes from beds 112 and 113 to Bed 114, then to Bed 115 in Tieqiao Section (Yao et al., 2012; Chen et al., 2009, 1998).
The conspicuous 40 m-thick bioclastic limestone of Unit Ⅳ in the Shuixiakou Section (Xikou area, southeastern Qinling) and the synchronous reef limestone of Bed 114 in Tieqiao Section (Laibin area, South China) indicate a sea-level drop. Coincidentally, the carbonate buildup that heavily recrystallized and dolomitized of the upper part of the Appel Ranch Menber in Glass Mountains, West Texas, USA suggests subaerial exposure and shallow deposition (Wardlaw, 2000). It seems like a global Wordian regression.
The δ18Oapatite values from the Roadian-Wordian stages of the Shuixiakou Section fluctuate around 19.5‰±0.5‰ (1σ, n=17) which gives calculated seawater temperatures around 28.2± 2.0 ℃ (1σ, n=17; Fig. 5, Table 1). In units Ⅰ, Ⅱ and Ⅲ, the δ18Oapatite values from Jinogondolella (19.3‰±0.4‰; 1σ, n=5) are similar to the δ18Oapatite values from the mixture of Hindeodus, Pseudohindeodus and Sweetognathus (19.2‰±0.5‰; 1σ, n=5). The average paleotemperature estimate of 26.8 ℃ (δ18Oapatite: 19.9‰±0.1‰, 1σ, n=4; Fig. 5, Table 1) in units Ⅴ to Ⅵ is slightly lower than the sea water temperatures around 27.5 ℃ from Bed 115 of the equatorial Tieqiao Section (Laibin area, South China; Chen et al., 2013, 2011). The δ18Oapatite values increase from 19.3‰±0.4‰ (1σ, n=10) in units Ⅰ, Ⅱ and Ⅲ to 20.0‰±0.2‰ (1σ, n=7) in units Ⅳ, Ⅴ and Ⅵ. As the appearance of the 40 m-thick bioclastic limestone of Unit Ⅳ, the seawater temperature decreased from 29.4 to 26.5 ℃ in the Jinogondolella aserrata Zone (Fig. 5, Table 1). It suggests that the Wordian regression may be related to a temperature decreasing.
According to the diversity and taxonomic composition of bivalve and lack of ammonoid, the P3 glacial event began in Roadian and ended in the late Wordian (Davydov et al., 2018; Biakov, 2015; Kutygin, 2015). Due to the extreme cooling, the P3 event corresponds with the disappearance of ammonoids and conodonts in the upper Roadian and Wordian in high- latitudes regions, i.e., N-E Russia, Russian and Canadian Arctic and Australia (Davydov et al., 2018). Furthermore, the calibration of Braxton Formation and Muree Sanstone linked to the P3 glaciation in the eastern Australia was in Wordian (Davydov et al., 2018; Mory et al., 2008). Hence, the temporal coincidence among the P3 glaciation in the eastern Australia, the Wordian regression and the seawater temperature decreasing in Shui- xiakou Section suggests it possible that the temperature decreasing in Shuixiakou Section and the Wordian regression regions may be related to the P3 glacial event.