Journal of Asian Earth Sciences 93 (2014) 113–128

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Journal of Asian Earth Sciences

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Nitrogen isotope chemostratigraphy across the Permian–Triassic boundary at Chaotian, Sichuan, South China ⇑ Masafumi Saitoh a,b, , Yuichiro Ueno a,c, Manabu Nishizawa b, Yukio Isozaki d, Ken Takai b,c,e, Jianxin Yao f, Zhansheng Ji f a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan b Laboratory of Ocean-Earth Life Evolution Research (OELE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Natsushima-cho, Yokosuka 237-0061, Japan c Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan d Department of Earth Science and Astronomy, The University of Tokyo, Meguro, Tokyo 153-8902, Japan e Department of Subsurface Geobiological Analysis and Research (D-SUGAR), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Natsushima-cho, Yokosuka 237-0061, Japan f Geology Institute, Chinese Academy of Geological Science, Beijing 100037, China article info abstract

Article history: Nitrogen isotopic compositions of upper Permian to lowermost Triassic rocks were analyzed at Chaotian Received 30 December 2013 in northern Sichuan, South China, in order to clarify changes in the oceanic nitrogen cycle around the Received in revised form 6 June 2014 Permian–Triassic boundary (P–TB) including the entire Changhsingian (Late Late Permian) prior to the Accepted 25 June 2014 extinction. The analyzed ca. 40 m thick interval across the P–TB at Chaotian consists of three stratigraphic Available online 9 July 2014 units: the upper Wujiaping Formation, the Dalong Formation, and the lowermost Feixianguan Formation, in ascending order. The upper Wujiaping Formation, ca. 10 m thick, is mainly composed of dark gray Keywords: limestone with diverse shallow-marine fossils such as calcareous algae and brachiopods, deposited on The end-Permian extinction the shallow shelf. In contrast, the overlying Dalong Formation, ca. 25 m thick, is mainly composed of Changhsingian Enhanced nitrogen fixation thinly bedded black mudstone and siliceous mudstone containing abundant radiolarians, deposited on Anoxia the relatively deep slope/basin. Absence of bioturbation, substantially high total organic carbon contents Global d15N variation across the P–TB (up to 15%), and abundant occurrence of pyrite framboids in the main part of the Dalong Formation indi- cate deposition under anoxic condition. The lowermost Feixianguan Formation, ca. 5 m thick, is com- posed of thinly bedded gray marl and micritic limestone with minor fossils such as ammonoids and 15 conodonts, deposited on the relatively shallow slope. d NTN values are in positive values around +1 to 15 +2‰ in the upper Wujiaping Formation implying denitrification and/or anammox in the ocean. d NTN values gradually decrease to À1‰ in the lower Dalong Formation and are consistently low (around 15 0‰) in the middle Dalong to lowermost Feixianguan Formation. No clear d NTN shift is recognized across 15 the extinction horizon. The consistently low d NTN values suggest the enhanced nitrogen fixation in the ocean during the Changhsingian at Chaotian. Composite profiles based on previous and the present stud- ies demonstrate the substantial d15N variation on a global scale in the late Permian to earliest Triassic; a systematic d15N difference by low and high latitudes is particularly clarified. Although the enhanced nitrogen fixation throughout the Changhsingian at Chaotian was likely a regional event in northwestern South China, the composite d15N profiles imply that the sea area in which fixed nitrogen is depleted has gradually developed worldwide in the Changhsingian, possibly acting as a prolonged stress to shallow- marine biota. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction was eliminated on a global scale in both the marine and terrestrial realm (e.g., Erwin, 2006; Shen et al., 2011b). Several geologic phe- The end-Permian extinction has been recognized as the greatest nomena such as bolide impact (e.g., Xu et al., 1985; Becker et al., biodiversity crisis in the Phanerozoic and a large variety of biota 2001, 2004; Kaiho et al., 2001), the volcanism of the Siberian Traps (e.g., Renne and Basu, 1991; Campbell et al., 1992; Renne et al., ⇑ 1995; Kamo et al., 2003; Reichow et al., 2009; Svensen et al., Corresponding author. Address: 2-12-1 Ookayama, Meguro, Tokyo 152-8551, 2009), oceanic anoxia (e.g., Wignall and Hallam, 1992, 1993; Japan. Tel.: +81 3 5734 2618; fax: +81 3 5734 3416. Wignall and Twitchett, 1996; Isozaki, 1997; Algeo et al., 2008; E-mail address: [email protected] (M. Saitoh). http://dx.doi.org/10.1016/j.jseaes.2014.06.026 1367-9120/Ó 2014 Elsevier Ltd. All rights reserved. 114 M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128

Shen et al., 2011a), hypercapnia (Knoll et al., 1996, 2007), H2S poi- Permian to lowermost Triassic rocks at Chaotian in northern Sich- soning (e.g., Kump et al., 2005; Kaiho et al., 2006; Riccardi et al., uan, South China, in order to clarify changes in the oceanic nitrogen 2006), and oceanic acidification (e.g., Heydari and Hassanzadeh, cycle during the Changhsingian to early Induan (Early Early Trias- 2003; Payne et al., 2007, 2010; Clapham and Payne, 2011) have sic). On the basis of newly obtained lithological and geochemical been proposed as the cause of the severe biotic crisis. Nonetheless, results, we reconstruct sedimentary environments of the analyzed the ultimate trigger mechanism of the extinction is still in discus- P–TB interval, including secular changes in sea-level and in redox, sion (e.g., Bottjer et al., 2008; Payne and Clapham, 2012). and discuss changes in the oceanic nitrogen cycle in the Changh- Chemostratigraphical study of stable isotope geochemistry is singian to early Induan. We correlate the d15N chemostratigraphy useful to correlate sections in different regions around the world at Chaotian with that in other sections around the world and argue and to investigate environmental changes in the past ocean/atmo- whether the observed isotopic trend at Chaotian represents a sphere. For example, stable carbon isotopic composition of inor- regional or global signature. Moreover, compiling the reported 13 ganic carbon (d Ccarb) of upper Permian to lower Triassic rocks chemostratigraphic data in various sections around the world, we was analyzed in various sections around the world (e.g., Baud discuss d15N variations and changes in the oceanic nitrogen cycle 13 et al., 1989; Holser et al., 1989), and a global d Ccarb trend has been around the P–TB on a global scale. constructed demonstrating a remarkable negative excursion at the Permian–Triassic boundary (P–TB) (Korte and Kozur, 2010; Shen et al., 2013). Several mechanisms, such as release of large amounts 2. Geologic setting and general stratigraphy 13 of CO2 and/or methane of low d C value into the ocean/atmosphere involved in volcanic activity of the Siberian flood basalt and upwell- In the Late Permian (Lopingian) to Early Triassic, South China ing of anoxic deep-waters with isotopically light bicarbonate ions was isolated from other continental blocks and located on the east- into shallow continental shelves, have been considered as a cause ern side of Pangea around the equator (Fig. 1B; Scotese and 13 of the d Ccarb excursion, which was linked to the extinction (e.g., Langford, 1995). Shallow-marine shelf carbonates and terrigenous Renne et al., 1995; Knoll et al., 1996; Retallack and Jahren, 2008). clastics with abundant and diverse fossils such as fusulines, bra- Similar to carbon, assimilation of fixed nitrogen is generally chiopods, and mollusks were thickly accumulated over the craton indispensable for organisms. Moreover, nitrate is an important oxi- to form the Yangtze platform (e.g., Zhao et al., 1981; Yang et al., dant in biogeochemical cycles in the modern oceans particularly 1987; Jin et al., 1998). In northern Sichuan, along the northwestern within the oxygen minimum zone (e.g., Canfield et al., 2010). The edge of South China, carbonate and mudstone of relatively deep- oceanic nitrogen cycle has been drastically changed in Earth’s his- water facies were deposited on a slope/basin during the Lopingian tory (e.g., Jenkyns et al., 2001; Garvin et al., 2009; Kikumoto et al., to Early Triassic, probably facing on the eastern paleo-Tethys 2014), and nitrogen isotopic composition of sedimentary organic (Fig. 1C and D; Zhu et al., 1999; Wang and Jin, 2000). matter is useful to understand it (e.g., Altabet and Francois, The Chaotian section is located nearly 20 km to the north of 1994; Pinti and Hashizume, 2011; Robinson et al., 2012). Previous Guangyuan city in northern Sichuan (Fig. 1A). The studied section studies of nitrogen isotope chemostratigraphy at the end-Paleozoic at Chaotian crops out along the Jialingjiang River in a narrow gorge are, however, relatively few compared to those of carbon. Algeo called Mingyuexia, and Middle Permian to lowermost Triassic et al. (2007) analyzed d15N values of upper Permian to lower Trias- rocks are continuously exposed along the eastern bank of the river sic rocks, for the first time, at Guryul Ravine in Kashmir, India, and on the southern limb of an E–W trending anticline (Fig. 3A). The pointed out a negative d15N anomaly around the extinction hori- overall biostratigraphy of the Chaotian section was originally zon. Cao et al. (2009) constructed d15N chemostratigraphy across described by Zhao et al. (1978) and Yang et al. (1987) on the basis the P–TB at Meishan in Zhejiang, South China, the Global Strato- of fusulines, conodonts, and ammonoids. Isozaki et al. (2004) re- type Section and Point (GSSP) of the P–TB, and revealed a progres- examined stratigraphy of this section in higher resolution empha- sive d15N decline in the Changhsingian (Late Late Permian) and a sizing the Permian double extinction event. As to the P–TB event, negative d15N shift around the P–TB suggesting increased nitrogen Isozaki et al. (2007) analyzed lithostratigraphy across the P–TB fixation in the ocean. Luo et al. (2011) showed that the negative and pointed out the causal relationship between intermittent felsic d15N shift around the P–TB occurred widely in South China. Algeo volcanism and the extinction. Ji et al. (2007) constructed detailed et al. (2012) and Knies et al. (2013) recently performed nitrogen conodont biostratigraphy across the P–TB. Cao et al. (2010) ana- isotopic analysis of upper Permian to lower Triassic clastic lyzed inorganic carbon isotopic composition across the P–TB and sequences in Arctic Canada, and clarified relatively high d15N val- reported a negative d13C shift above the extinction horizon. ues in the late Permian implying denitrification. Furthermore, The Permo-Triassic rocks at Chaotian, over 300 m in total thick- Knies et al. (2013) correlated a negative d15N shift around the ness, are composed of Guadalupian (Middle Permian) Maokou For- extinction horizon at Buchanan Lake to that in Meishan. A negative mation, Lopingian Wujiaping and Dalong formations, and d15N shift across the extinction horizon is apparently well corre- lowermost Triassic Feixianguan Formation, in ascending order lated on a global scale. Nonetheless, more chemostratigraphic (Fig. 2; Isozaki et al., 2004, 2007; Saitoh et al., 2013a,b). The Mao- studies are needed in order to construct a general d15N trend kou Formation, over 150 m thick, is mainly composed of massive around the P–TB and to clarify changes in the oceanic nitrogen dark gray bioclastic limestone with abundant and diverse shal- cycle and their causal relationships to the extinction. low-marine fossils such as calcareous algae, ostracodes, brachio- On the other hand, it has been recently suggested that photic pods and fusulines. The uppermost (ca. 11 m thick) part of the zone euxinia has prevailed in the Changhsingian, significantly Maokou Formation is composed of thinly bedded black mudstone before the main phase of the extinction, in various sections around and chert of deep-water facies containing ammonoids, radiolari- the world including South China, Spitsbergen, and Greenland (e.g., ans, and conodonts. The Wujiaping Formation, ca. 70 m thick, is Cao et al., 2009; Nabbefeld et al., 2010; Hays et al., 2012). Nonethe- mainly composed of massive dark gray bioclastic limestone with less, few previous studies focused on the environmental changes black chert nodules/lenses, containing shallow-marine fossils such during the Changhsingian; the causal relationships between the as fusulines, smaller foraminifer, calcareous algae, and brachiopod. environmental changes in the Changhsingian and the extinction At the base of the Wujiaping Formation, a ca. 2 m thick tuffaceous have not been clarified yet. Wangpo bed occurs. The Dalong Formation, ca. 25 m thick, is In the present study, we described litho- and bio-facies and ana- mainly composed of thinly bedded black mudstone and siliceous lyzed nitrogen and organic carbon isotopic compositions of upper mudstone with abundant radiolarians. The uppermost (ca. 3.5 m M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128 115

Gansu Shaanxi A B

Chaotian Shangsi Guangyuan 32˚N Panthalassa Jialingjiang River South Tethys China equator Sichuan Meishan, Zhejiang, China Taiping, Guangxi, China Pangea Zuodeng, Guangxi, China Chengdu Bulla, Southern Alps, Italy 30˚N Guryul Ravine, Kashmir, India ChangjiangRiver Opal Creek, Alberta, Canada 100 km Chongqing West Blind Fiord, Arctic Canada 106˚E Buchanan Lake, Arctic Canada

Chaotian deep slope/basin ChaotianChaotian C Guanyuan D

30˚N

Fig. 1D deep-water Chengdu mudstone shallow shelf 25˚N Kangdian Chongping basinal siliceous Land rocks open platform carbonate

Yunkai Land Cathaysia near-shore Land sand-/mudstone 20˚N 200 km land area 100˚E 110˚E 120˚E

Fig. 1. Locality and paleogeographic maps of the Chaotian section. A: locality map of the Chaotian. B: simplified global paleogeographic map during the Late Permian to Early Triassic (modified from Scotese and Langford, 1995). C: sedimentary facies distribution of South China in the late Changhsingian (Late Late Permian) (modified from Wang and Jin (2000)). In (B and C), a colored star represents location of a section of the same color (except for Chaotian). D: detailed sedimentary facies in the square in (C). Note that deep-water mudstone accumulated around the Chaotian in the late Changhsingian (modified from Zhu et al. (1999)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) thick) part of the Dalong Formation mostly consists of bedded gray the unit. The upper Wujiaping carbonates yield diverse shallow- limestone (lime mudstone/wackestone) yielding bivalves, brachio- marine fossils such as calcareous algae, crinoids, brachiopods, pods, and ammonoids. The Feixianguan Formation, over 30 m and radiolarians. Burrows of 5–10 mm diameter frequently occur thick, is mainly composed of thinly bedded light-gray micritic throughout the carbonates. limestone containing few conodonts, ammonoids, and brachio- The Dalong Formation, ca. 25 m thick, is mainly composed of pods. The lowermost (1.4 m thick) part of the Feixianguan Forma- thinly bedded laminated black mudstone, black siliceous mud- tion is composed of characteristic gray marl with little fossils. stone, and dark gray muddy limestone (Figs. 3E, 4B and C), whereas the uppermost (ca. 3.5 m thick) part (‘Unit C and D’ in Isozaki et al., 2007) is mostly composed of bedded gray limestone (lime mud- 3. P–TB interval stone/wackestone) (Figs. 3B and 4D). In the mudstone-dominant main part of the Dalong Formation, dark gray muddy limestone 3.1. Lithostratigraphy beds are intercalated but their frequency gradually declines upward (Fig. 2). The mudstone-dominant part yields abundant In this study, we focused on the ca. 40 m thick P–TB interval at radiolarians with minor amounts of ostracodes. Some burrows of Chaotian and collected fresh rock samples in the interval from out- 5–10 mm in diameter are common in the basal part of the Dalong crop and from drill core (Fig. 2). Isozaki et al. (2007) analyzed lith- Formation, although only few and small (1–5 mm in diameter) ostratigraphy of the ca. 12 m thick carbonates across the P–TB burrows are recognized in the lower part of the formation. Biotur- within the interval. The analyzed P–TB interval is composed of bation is absent in the upper part of the mudstone-dominant main three stratigraphic units: 1) the upper Wujiaping Formation, 2) part of the Dalong Formation. Pyrite framboids of 3–10 lmin Dalong Formation, and 3) the lowermost Feixianguan Formation, diameter abundantly occur throughout the mudstone-dominant in ascending order. The upper Wujiaping Formation, ca. 10 m thick, part. In the uppermost Dalong Formation, gray limestones contain is mainly composed of massive dark gray limestone (wackestone) bivalves, ammonoids, conodonts, and radiolarians with minor with some sandy/muddy limestones (Figs. 3D and 4A). Black chert amounts of brachiopods and ostracodes. Burrows of 2–10 mm in nodules of centimeter to decimeter size occur in the upper part of diameter and pyrite particles (10 lm–1 mm in diameter) are 116 M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128

sea-level ( ) & sedimentary environment Age Fm Un. Age Fm calcareous algaecrinoid brachiopod ostracode radiolarian smaller faraminiferbivalve small gastrolodammonoid conodont bioturbation G

In. anoxic relatively biostratigraphically-defined PTB

E. Tr. F

Induan shallow slope Triassic E. Triassic Feixianguan extinction horizon

Feix. Fm E D Chang- hsingian oxic relatively

Dalong C shallow slope B

A Lopingian Wuchiaping Changhsingian Wujiaping

? anoxic deep slope/basin Dalong Fm Permian Capitanian Maokou Lopingian (Late Permian) Guadalupian

black chert black siliceous mudstone dark gray carbonate gray carbonate Wuchiapingian oxic shallow shelf gray marl black mudstone Wordian tuff black chert nodule carbonate abudant (ii=4,5) siliceous rock 50 m common (ii=3) Wujiaping Fm tuff 5 m rare (ii=2) shallow deep chert nodule unrecognized (ii=1)

Fig. 2. Stratigraphic column at Chaotian and the enlarged Permian–Triassic boundary (P–TB) interval analyzed in this study with ranges of various fossil groups. Reconstructed secular changes in sea-level and in sedimentary environments are also illustrated. Un.: unit in Isozaki et al. (2007), E. Tr.: Early Triassic, In.: Induan, Feix.: Feixianguan, ii: ichnofabric index. commonly observed in these limestones. Also, thin (less than that this part is correlated with the Araxoceras-Konglingites Zone 10 cm thick) acidic tuff layers frequently occur in these limestones of the late Wuchiapingian. In the upper part of the mudstone-dom- in the uppermost Dalong Formation. inant main part of the Dalong Formation, ammonoids (Tapashanites The lowermost Feixianguan Formation, ca. 5 m thick, is com- floriformis, T. chaotianensis, T. costatus, Sinoceltites curvatus, Chang- posed of thinly bedded gray marl and light-gray micritic limestone hsingoceras sichuanese, and Pseudostephanites? sp.), conodonts with some sandy/muddy limestones (Figs. 3C, 4E and F). The low- (Clarkina subcarinata, C. changxingensis, C. deflecta, and C. postwang- ermost 1.4 m thick part (‘Unit E’ in Isozaki et al., 2007) is composed i), and brachiopods (Dictyoclostus gratiosus, Waagenites cf. soochow- of characteristic gray marl. This marl unit is almost barren of fossil, ensis, and Leptodus sp.) occur. These fossils indicate that this part although few ammonoids and bivalves occur from the basal part. belongs to the Pseudostephanites–Tapashanites Zone of the early Pyrite particles of 1–100 lm in diameter abundantly occur in the Changhsingian (Late Lopingian). In the uppermost Dalong Forma- marl. The upper part of the lowermost Feixianguan Formation con- tion, micritic limestones yield ammonoids (Pleuronodoceras sists of light-gray micritic limestone containing few conodonts and mapingensis, Pseudotirolites asiaticus, Chaotianoceras modestum, brachiopods. Little trace fossils are recognized in the lowermost Pentagonocers cf. guizhouensis, Rotodiscoceras sp., Pseudogastrioc- Feixianguan Formation. eras sp.) and conodonts (Clarkina changxingensis, C. deflecta, C. sub- carinata, C. meishanensis zhangi). These fossils indicate that the age 3.2. Fossils and ages of these limestones is the Pseudotirolites–Pleuronodoceras Zone and Rotodiscoceras Zone of the late Changhsingian. According to the previous reports of index fossils such as fusu- In the lowermost Feixianguan Formation, the lowermost 1.4 m lines, conodonts, ammonoids, rugose corals, and brachiopods (Zhao thick marl is almost barren of fossil, although the latest Changhsin- et al., 1978; Yang et al., 1987; Isozaki et al., 2004, 2007), the P–TB gian ammonoid Hypophiceras sp. together with Huananoceras sp. interval at Chaotian analyzed in this study are dated as follows and Pleuronodoceras tenuicostatum occur from the basal mudstone (Fig. 2): The Wujiaping limestones yields several species of con- of the marl unit. Conodont Hindeodus parvus occur from the over- odonts (Clarkina orientalis, C. liangshanensis, and C. guanyuanensis), lying micritic limestone (at the base of the ‘Unit F’ in Isozaki et al., fusulines (Codonofusiella schuberteroides, C. asiatica, Reichelina aff. 2007) in the lowermost Feixianguan Formation (Ji et al., 2007). The pulchra, and Palaeofusulina simplex), and rugose corals (Liangshano- occurrence of H. parvus without accompanying Isarcicella isarcica phyllum wengchengense and Waagenophyllum simplex), and ammo- suggests that these limestones belong to the H. parvus Zone of noid Araxoceratidae gen. et sp. indet. These fossils indicate that the the early Induan. The overlying limestone of the lowermost Feix- upper Wujiaping Formation is correlated with the Wuchiapingian ianguan Formation yields conodonts H. parvus, I. isarcica, and I. sta- (Early Lopingian). eschei, indicating that this part is correlated with the Isarcicella In the lower part of the mudstone-dominant main part of the Zone of the early Induan. Bivalve Claraia wangi and brachiopod Lin- Dalong Formation, ammonoids Konglingites sp. and Jinjiangoceras gula sp. also occurs in the lowermost Feixianguan Formation. The sp. and conodont Clarkina orientalis occur. Konglingites sp. indicates age of the lowermost Feixianguan Formation ranges, therefore, M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128 117

Feixianguan Fm A Wujiaping Fm P-T boundary Dalong Fm

Maokou Fm G-L boundary

B C

extinction horizon (Unit D/E boundary)

Unit C/D boundary

D E

5 mm 5 mm

Fig. 3. A distant view and photographs of outcrops and polished slabs of the P–TB interval at Chaotian. A: a distant view of the Chaotian section (circled car for scale). B and C: outcrops of the uppermost Dalong Formation (circled water bottle for scale in (C)). D and E: polished slabs of bioclastic limestone in the upper Wujiaping Formation (D) and of laminated black mudstone in the Dalong Formation (E). Note a clear contrast in bioturbation between them.

15 from the latest Changhsingian to early Induan. The main extinction Belemnite (V-PDB) standard, respectively, according to d NTN 13 horizon at Chaotian is placed at the base of the Feixianguan Forma- and d Corg =(Rsample/Rstandard À 1) Â 1000, where R is the isotopic tion (‘Unit D/E boundary’ in Isozaki et al., 2007). The biostrati- ratio (15N/14N and 13C/12C, respectively) of sample and standard. 15 13 graphically-defined P–TB is assigned to the horizon of the first The analytical reproducibility of the d NTN and d Corg values occurrence of H. parvus at the base of micritic limestone in the low- determined by replicate analyses of laboratory standards is better ermost Feixianguan Formation (‘Unit E/F boundary’ in Isozaki et al., than 0.4‰ and 0.2‰ (1r), respectively. 2007). Refer to Isozaki et al. (2004, 2007) for more details concern- ing biostratigraphical assignment across the P–TB at Chaotian. 5. Results

15 13 4. Analytical method Table 1 lists all measurements of d NTN and d Corg values, TN and TOC contents, and TOC/TN atomic ratios of samples collected 0.2–5.0 g of powdered sample was treated with 10 M HCl for from the study section. Fig. 5 shows chemostratigraphic profiles 15 13 >24 h and all the carbonate was dissolved. The residue was sepa- of d NTN and d Corg values, TOC and TN contents, and TOC/TN rated by repeated centrifugation adding distilled water; the sample atomic ratios of the analyzed P–TB interval at Chaotian. Figs. 6 was then dried at 70 °C for >12 h. 3–50 mg of the residue was and 7 show geochemical cross-plots of the analyzed samples. 15 placed into a tin cup; total nitrogen and organic carbon isotopic In the upper Wujiaping Formation, d NTN values range mostly values and total nitrogen (TN) and total organic carbon (TOC) con- between 0 and +2‰ although they are rather scattered (Fig. 5). 15 tents were measured by a ThermoFinnigan DELTAplus Advantage In the Dalong Formation, d NTN values gradually decrease from mass spectrometer coupled with an EA 1112 Series FLASH Elemen- +1 to À1‰ in the lower part and mostly range between À1 and 15 tal Analyzer in Japan Agency for Marine-Earth Science and Tech- 0‰ in the middle to upper part. d NTN values are rather constant nology (JAMSTEC). Nitrogen and carbon isotopic compositions are across the extinction horizon and are around 0‰ in the lowermost 13 reported in ‰ relative to atmospheric N2 and to Vienna Peedee Feixianguan Formation although somewhat scattered. d Corg 118 M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128

A B

1 mm 500 µm

C D

500 µm 500 µm

E F

1 mm 1 mm

Fig. 4. Photomicrographs of the P–TB interval at Chaotian. A: bioclastic limestone with common burrows in the upper Wujiaping Formation. B and C: laminated black mudstone (B) and black siliceous mudstone with abundant radiolarians (C) in the Dalong Formation. D: gray limestone with common radiolarians in the uppermost Dalong Formation. E and F: gray marl with abundant pyrite particles (black dots) (E) and gray limestone (F) in the lowermost Feixianguan Formation.

values are relatively high and mostly around À25‰ in the upper observed between black mudstone and dark gray muddy carbon- Wujiaping Formation. Across the Wujiaping/Dalong formation ates in the Dalong Formation. In the uppermost Dalong Formation, 13 boundary, d Corg values slightly decrease to À27‰ and then grad- C/N ratios decrease below 10. Around the P–TB, C/N ratios are ually increase to À25‰ upward in the Dalong Formation. In the mostly below 6.6 (Redfield value) reaching the minimum value lowermost Feixianguan Formation, immediately above the extinc- of 0.6 immediately above the extinction horizon. In the upper part 13 tion horizon, d Corg values sharply drop to À32‰. of the lowermost Feixianguan Formation, C/N ratios are around 10 TN contents are relatively low (105 ppm on average) in the but somewhat scattered. upper Wujiaping limestone while they are considerably high in the overlying Dalong Formation (Fig. 5). TN contents of black mud- stone (4547 ppm on average) are systematically higher than those 6. Discussion of muddy carbonate (821 ppm on average) in the Dalong Forma- tion. In the lowermost Feixianguan Formation, TN contents are rel- 6.1. Reconstruction of sedimentary environment atively low (289 ppm on average). The stratigraphic trend of TOC contents is similar to that of TN contents. TOC contents are rela- 6.1.1. Sea-level change tively low (0.2% on average) in the upper Wujiaping limestone On the basis of litho- and bio-facies characteristics, we recon- although they are substantially high in the overlying Dalong For- struct the depositional environments of the analyzed P–TB interval mation. TOC contents of black mudstone (8.2% on average) are sys- at Chaotian (Fig. 2). In the upper Wujiaping Formation, fine- tematically higher than those of muddy carbonate (1.4% on grained bioclastic limestone with shallow-marine fossils such as average) in the Dalong Formation. In the lowermost Feixianguan calcareous algae and crinoids suggests that this part was deposited Formation, TOC contents are relatively low (0.1% on average). C/N on the shallow shelf. Dominance of lime-mud indicates that these atomic ratios are rather scattered (between 10 to 30) but consis- limestones were deposited below the storm wave base (generally tently around 20 in the upper Wujiaping and lower to middle 50–80 m deep). In contrast, the overlying mudstone-dominant main Dalong formations. No systematic difference of C/N ratios is part of the Dalong Formation is composed of very fine-grained M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128 119

Fig. 5. Chemostratigraphy of the P–TB interval at Chaotian. TN: total nitrogen.

black mudstone and siliceous mudstone containing abundant radi- Chaotian, suggested by the sharp lithofacies change across the olarians, indicating that this part was deposited on the relatively Wujiaping/Dalong formation boundary, looks too abrupt to be deep slope/basin. Lithofacies change across the Wujiaping/Dalong attributed solely to the gradual eustatic transgression during the formation boundary is clear though no evidence for erosion is rec- Lopingian. The abrupt deepening at Chaotian possibly records a ognized at this lithofacies boundary. Frequency of intercalation of local subsidence of the sedimentary basin in northwestern South muddy carbonate beds, possibly carbonate debris flow deposits, China, in addition to the eustacy. On the other hand, a widespread gradually declines upward in the mudstone-dominant main part regression occurred in the latest Changhsingian in South China, fol- of the Dalong Formation; the sedimentary environment probably lowing the long-term Lopingian transgression (Zhang et al., 1997; became deeper through this mudstone-dominant part. The upper- Yin et al., in press). The shallowing observed in the upper Dalong most Dalong and lowermost Feixianguan formations mainly con- Formation at Chaotian is apparently correlated with it. Biostrati- sist of micritic limestone and gray marl. Although few shallow- graphic records of ammonoid and conodont demonstrate, however, marine fossils occur, carbonate deposition implies that these inter- that the shallowing at Chaotian in the early to middle Changhsin- vals were deposited on a relatively shallow environment. Domi- gian clearly precedes the widespread regression in South China in nance of lime-mud and very fine clay-sized particles without the latest Changhsingian (Clarkina meishanensis and Hindeodus cross lamination suggests that these carbonates were deposited changxingensis zones). Overall, the remarkable transgression– below the storm wave base. We interpret the uppermost Dalong regression cycle in the Dalong Formation at Chaotian mainly and lowermost Feixianguan formations were deposited on the rel- reflects a local subsidence and subsequent rise of the sedimentary atively shallow slope. Lack of fossil occurrence in the lowermost basin possibly controlled by regional tectonics in northwestern Feixianguan Formation is attributed to the global extinction at South China. the base of the formation and its aftermath. Litho- and bio-facies changes document that the sea-level lar- gely fluctuated in the analyzed P–TB interval at Chaotian as follows 6.1.2. Redox change (Fig. 2): 1) a transgression shifted the sedimentary environment Corresponding to the sea-level fluctuations, redox condition of from the shallow shelf to relatively deep slope/basin in the middle the sedimentary environments remarkably changed in the P–TB to late Wuchiapingian, and 2) the sea-level rapidly dropped and interval at Chaotian (Fig. 2). In the upper Wujiaping Formation, the sedimentary environment shifted to the relatively shallow bioturbation is commonly observed showing oxic condition on slope in the early to middle Changhsingian. In general, after the the shallow shelf. In contrast, in the lower Dalong Formation, global regression at the end-Guadalupian, the Lopingian sequences degree of bioturbation abruptly declines accompanied with bur- represent a transgressive system worldwide (e.g., Jin et al., 1998; row-size decreasing. Trace fossils are absent in the main part of Haq and Schutter, 2008), including South China (Jin et al., 1994). the Dalong Formation of deep-water facies. The sedimentary envi- The observed deepening in the Dalong Formation at Chaotian is ronment likely shifted from oxic shallow shelf to anoxic deeper concordant with this eustatic trend. However, the deepening at slope/basin by the deepening. Substantially high TOC contents 120 M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128

A B 1500

10000

1000

Redfield ratio (C/N = 6.6) 5000 TN [ppm] TN [ppm] 500

Redfield ratio (C/N = 6.6)

0 0 0481216 0 0.1 0.2 0.3 0.4 TOC [%] TOC [%]

40 C D dark gray ls. in the Feixianguan Fm 1.6 gray marl in the Feixianguan Fm

30 gray ls. in the Dalong & Feixianguan Fm 1.2 black ms. in the Dalong Fm

dark gray dol. in the Dalong Fm 20 0.8 dark gray muddy ls. in the Dalong Fm N/C atomic C/N atomic gray ms. in the Dalong Fm 10 0.4 dark gray ls. in the Wujiaping Fm

Redfield ratio (C/N = 6.6) Redfield ratio (N/C = 0.152) sample of low C/N ratio (< 6.6: Redfield value) 0 0 0 4 8 12 16 0 4000 8000 12000 TOC [%] TN [ppm]

Fig. 6. Relationships between TN and TOC contents of the P–TB interval at Chaotian. In (D), a positive correlation between N/C ratios and TN contents is recognized in carbonate samples of exceptionally low C/N ratio (<6.6) in the uppermost Dalong to lowermost Feixianguan Formation. y-Intercept of the approximation line is 0.152 (Redfield value). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(up to 15%) and abundant occurrence of pyrite framboids in the sinking in the water column and/or after deposition because C/N mudstone-dominant part of the Dalong Formation support this. ratios of almost all samples are higher than 6.6, the Redfield value. On the other hand, in the uppermost Dalong Formation, burrows It is noteworthy that, in some gray marl and muddy carbonates are commonly observed in the limestones of shallow-water facies. around the P–TB (outline symbols in Figs. 5–7), nitrogen is Rapid sea-level fall probably returned the sedimentary environ- enriched and C/N ratios are exceptionally low (<6.6). In these sam- ment to the oxic slope. It is noteworthy that trace fossils disappear ples, their nitrogen enrichment looks to be irrespective of TOC con- across the extinction horizon and little burrowing is recognized in tents that are mostly around 0.1% (Fig. 6B). The nitrogen the lowermost Feixianguan Formation. Absence of bioturbation enrichment in these marl and carbonates suggests addition of implies elimination of burrow producers at the extinction horizon nitrogen from another source except for organic matter, because and/or an emergence of anoxia in the earliest Induan. Abundant C/N ratios of living marine organisms are generally around the occurrence of pyrites in the lowermost Feixianguan carbonates Redfield value. It is unlikely that the exceptionally low C/N ratios suggests that anoxic waters developed onto the relatively shallow were caused by preferential loss of organic carbon after burial, slope at Chaotian in the aftermath of the extinction. It is consistent because TOC contents of the samples of exceptionally low C/N ratio with the widely recognized shallow-marine anoxia in the earliest are almost the same as those of other carbonates in the uppermost Triassic around the world (Wignall and Hallam, 1992, 1993; Dalong and lowermost Feixianguan formations (Fig. 6B). Instead, Wignall and Twitchett, 1996). TN contents of the samples of exceptionally low C/N ratio are clearly higher than those of other carbonates around the P–TB, 6.2. Preservation of the original isotopic composition showing an additional source of nitrogen. The samples of which C/N ratio is exceptionally low (<6.6) exhibit a strong positive cor- Post-depositional processes may have changed original isotopic relation between TN contents and N/C ratios, also supporting compositions in the analyzed rocks at Chaotian. In order to evalu- excess addition of nitrogen (Fig. 6D; Kikumoto et al., 2014). We ate diagenetic imprints on original isotopic compositions, we infer that ammonium-bearing clay minerals/micas in the rocks examined the relationships between TN and TOC contents of the are another source of nitrogen (e.g., Müller, 1977; Juster et al., analyzed samples and their correlations with observed isotopic 1987). Although clay minerals/micas may also have been effective compositions (Figs. 6 and 7). to keep ammonium ion released from organic matter into the sed- iments by adsorption, the observed exceptionally low C/N ratios at 6.2.1. Variability of TN and TOC contents Chaotian postulate them not only as a reservoir, but a source of In general, TN and TOC contents of analyzed samples are posi- nitrogen. The C/N records at Chaotian suggest that, around the tively correlated throughout the section (Fig. 6A). It indicates that P–TB, ammonium-bearing clay minerals/micas may have been the main source of nitrogen in the rocks is sedimentary organic supplied from the continent to the relatively shallow slope in matter, although some organic nitrogen may have been lost during northwestern South China. M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128 121

A B C

2 2 2 (C/N = 6.6) Redfield ratio Redfield ratio 1 1 1 (N/C = 0.152)

0 0 0

-1 -1 -1

-2 -2 -2

0 5000 10000 010 20 30 40 0 0.4 0.8 1.2 1.6 TN [ppm] C/N atomic N/C atomic D E F -22 -22 2

-24 -24 (C/N = 6.6) Redfield ratio 1

-26 -26 0

-28 -28 -1

-30 -30 -2

-32 -32 0 4 8 12 16 0 10 20 30 40 -32 -30 -28 -26 -24 -22 TOC [%] C/N atomic

dark gray muddy limestone in the Dalong Fm gray limestone in the Dalong & Feixianguan Fm sample of low C/N ratio (< 6.6: Redfield value) gray mudstone in the Dalong Fm black mudstone in the Dalong Fm dark gray limestone in the Feixianguan Fm dark gray limestone in the Wujiaping Fm dark gray dolostone in the Dalong Fm gray marl in the Feixianguan Fm

15 Fig. 7. Nitrogen and organic carbon isotopic compositions and their relations to TN and TOC contents. In C, upper and lower curves represent d NTN changes according to addition of nitrogen from clay minerals/micas to sedimentary organic nitrogen of which d15N value is +0.2‰ and À2.0‰, respectively. We assumed that a C/N ratio of original organic matter in sediments is 6.6 (Redfield value) and that the d15N value of added nitrogen from clay minerals/micas is À0.15‰. Note that samples of exceptionally low C/N ratio (<6.6) are generally within the area between the two curves, suggesting that their apparent variations in d15N value and N/C ratio can be explained by addition of nitrogen from clay minerals/micas. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

15 Except for these samples of low C/N ratio (<6.6), no linear cor- of nitrogen from clay minerals/micas on bulk d NTN value of a rock relation between TN contents and N/C ratios is recognized in each sample by simple calculations. The results suggest that the 15 rock type or stratigraphic unit in the analyzed P–TB interval observed variations of d NTN values of marl and carbonates of (Fig. 6D). It implies that addition of nitrogen from clay minerals/ exceptionally low C/N ratio (<6.6) can be explained by a mixing micas is not significant in these samples. No linear correlation of organic nitrogen and nitrogen from clay minerals/micas, assum- between C/N ratio and TOC content is observed in each rock type ing that the d15N value of nitrogen from clay minerals/micas is or stratigraphic unit (Fig. 6C). If selective addition or loss of carbon À0.15‰ (Fig. 7C). We do not consider these carbonates of excep- occurs during diagenesis, a positive correlation between C/N ratio tionally low C/N ratio to reconstruct the oceanic nitrogen cycle in and TOC content would be expected (e.g., Calvert, 2004). On the the later discussion. 15 other hand, if the sediments suffer significant thermal maturation No linear correlation between d NTN value and TN content is after deposition, a negative correlation between C/N ratio and TOC observed in the analyzed samples (Fig. 7A). d15N value of organic content accompanied by increased C/N ratio and decreased TOC matter in sediments would increase due to devolatilization and a content would be expected (Bristow et al., 2009). The lack of any preferential loss of 14N during early diagenesis and thermal matu- correlation in the analyzed samples suggests that those are not ration (e.g., Altabet and Francois, 1994; Williams et al., 1995; Jia the case at Chaotian. and Kerrich, 2004; Robinson et al., 2012), and in that case, negative 15 correlation between d NTN value and TN content would be 6.2.2. Variability of nitrogen and carbon isotopic compositions expected (Cremonese et al., 2013). No linear correlation at Chao- Addition of nitrogen from clay minerals/micas in some marl and tian indicates that selective loss of 14N after burial is not signifi- 15 muddy carbonates around the P–TB discussed above is compatible cant. Similarly, no linear correlation between d NTN values and 15 15 with observed d NTN values of these rocks (Fig. 7C). The d NTN C/N ratios is recognized at Chaotian (Fig. 7B). Increasing C/N ratios values of sample of especially high N/C ratio (>1) are consistently may reflect thermal maturation after deposition (Bristow et al., 15 around À0.3‰. On the basis of the observed relationship between 2009). No linear correlation between d NTN value and C/N ratio 15 15 d NTN value and N/C ratio, we estimated the influence of addition at Chaotian implies that the influence of maturation on d NTN 122 M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128 value is negligible. On the other hand, no linear correlation as biological nitrate assimilation generally exhibits kinetic isotopic 13 between d Corg value and TOC content is observed in each rock fractionation (ca. 5‰) (e.g., Sigman et al., 2009). In a nitrate-limited type or stratigraphic unit (Fig. 7D), suggesting little preferential condition, oceanic nitrate is completely assimilated and d15N value 12C loss during degradation (e.g., Hayes et al., 1983; Ueno et al., of generated bulk organic nitrogen is equal to that of the original 2004). Hydrothermal alteration and/or metamorphic devolatiliza- nitrate. To the contrary, in a nitrate-enriched condition, partial 15 13 15 tion can lead to a positive correlation between d NTN and d Corg assimilation of nitrate results in lower d N value of organic nitro- values due to preferential loss of 14N and 12C(Pinti et al., 2009). gen compared to that of the substrate. It is noteworthy, however, 15 13 No correlation between d NTN and d Corg values at Chaotian sug- that the mudstone-dominant part of the Dalong Formation at gests that 14N and 12C loss after burial may have had only a minor Chaotian was probably deposited under anoxic condition. The influence on the original isotopic compositions (Fig. 7F). anoxic condition would have increased recycling of phosphorus 15 It is noteworthy that, even though somewhat scattered, d NTN from sediments to the ocean (Ingall et al., 1993). Moreover, high 13 and d Corg records represent stratigraphic trends in the analyzed TN contents (up to 10,000 ppm) in the Dalong Formation show P–TB interval at Chaotian irrespective of lithological difference extensive burial of organic nitrogen, and thus removal of fixed (Fig. 5). Post-depositional changes may have not been large and/ nitrogen from the ocean, under anoxic condition (Fig. 5). Under or evenly occurred irrespective of lithological character and strati- the circumstances, primary productivity in the surface ocean may graphic unit. Also, deposition under anoxic condition in the main have been limited by nitrogen rather than phosphorus in the part of the Dalong Formation and the lowermost Feixianguan For- Dalong Formation at Chaotian; it is unlikely that the surface ocean 15 mation would have been suitable to preserve the original d NTN was enriched in nitrate. Incomplete nitrate utilization may not be 15 signatures (Altabet et al., 1999; Thomazo et al., 2011). In summary, responsible for the d NTN decrease at Chaotian. On the other hand, even though some post-depositional processes may have slightly a decline of denitrification in the water column is also unlikely to 15 13 15 changed d NTN and d Corg values of the analyzed rock samples, explain the observed d NTN decrease at Chaotian, because the it is suggested that the observed isotopic values basically represent Dalong Formation was probably deposited under anoxic condition their original compositions. (Fig. 2), and nitrate is generally a major oxidant in the oceanic bio- geochemical cycles under oxygen-depleted condition (e.g., 6.3. Chemostratigraphy at Chaotian Codispoti and Christensen, 1985). Increased flux of terrestrial plants to the sedimentary environ- 15 6.3.1. Enhanced nitrogen fixation during the Changhsingian ment could be an additional mechanism for the d NTN decrease 15 15 In the upper Wujiaping Formation at Chaotian, d NTN values at Chaotian, as land plants generally exhibit relatively low d N range mostly between 0 and +2‰ although they are rather scat- values compared to marine algae (Peters et al., 1978). However, tered (Fig. 5). Positive d15N value is normal in modern shelf sedi- the reconstructed sea-level changes at Chaotian indicate a trans- ments and in past geologic records. In modern oceans, nitrate is gression in the lower Dalong Formation. Increasing contribution partially reduced to N2 by microbial denitrification and removed of terrestrial plants to sedimentary organic matter is not concor- from the oceanic biogeochemical cycles (e.g., Wada and Hattori, dant with the deepening of the sedimentary environment. More- 1991; Sigman et al., 2009). Denitrification in the water column over, no clear stratigraphic change of C/N ratio is recognized in generally exhibits large isotopic fractionation (ca. 25‰), in contrast the lower Dalong Formation. If flux of land plants increase, C/N to denitrification within the sediments with little isotope fraction- ratio would rise because C/N ratios of terrestrial plants are gener- ation (Brandes and Devol, 2002). On the other hand, anaerobic ally higher (20–40; Meyers, 1994) than those of marine plankton ammonium oxidation (anammox) is another major pathway for (4–7; Müller, 1977) due to nitrogen-poor cellulose tissue. No clear removal of fixed nitrogen from the ocean converting nitrite and C/N change suggests that increased input of terrestrial organic ammonium to N2, particularly in the oxygen minimum zone (van matter is unlikely in the lower Dalong Formation at Chaotian. In 15 de Graaf et al., 1995; Dalsgaard et al., 2003). Brunner et al. summary, the observed d NTN decrease at Chaotian indicates (2013) recently reported large isotopic fractionations in this reac- enhanced nitrogen fixation in the ocean along the northwestern 15 tion (enitrite?N2  +16‰, eammonium?N2  +24–29‰). Residual N- margin of South China. enriched nitrate in the ocean is assimilated into biomass and In the overlying middle–upper Dalong and lowermost Feixian- 15 finally preserved into sedimentary organic matter. The positive guan formations at Chaotian, d NTN values range mostly from 15 d NTN values in the upper Wujiaping Formation at Chaotian sug- À1to0‰ (Fig. 5). Age constraints indicate that enhanced nitrogen gest denitrification and/or anammox in the Wuchiapingian ocean fixation continued during the entire Changhsingian and early Ind- 15 in northwestern South China. uan. d NTN values are rather constant and no clear stratigraphic 15 In the overlying lower Dalong Formation at Chaotian, d NTN change is recognized around the extinction horizon at Chaotian, 15 values gradually decrease from +2 to À1‰ (Fig. 5). Four potential although d NTN values of some gray marl and muddy carbonates 15 mechanisms can explain the d NTN decrease in the lower Dalong are probably influenced by addition of nitrogen from clay miner- Formation: 1) enhanced nitrogen fixation, 2) partial assimilation als/micas and should be excluded from consideration. It is sug- of nitrate, 3) decline of water-mass denitrification, and 4) increased gested that the oceanic nitrogen cycle was not disturbed flux of terrestrial plants to the sedimentary environment. Microbial significantly during the extinction event. nitrogen fixation, a major pathway for input of fixed nitrogen to the ocean, exhibits small isotope fractionation (0 to À2‰; 6.3.2. Non-parallel organic and carbonate d13C changes in the Minagawa and Wada, 1986), and d15N value of generated ammo- aftermath of the extinction 13 nium is similar to that of atmospheric N2. Enhanced nitrogen fixa- In the upper Wujiaping Formation at Chaotian, d Corg values 15 13 tion increases input of fixed nitrogen of relatively low d N value are mostly around À25‰ (Fig. 5). Although d Corg values slightly (0‰) to the ocean as nitrogen isotopic composition of atmo- decrease to À27‰ across the Wujiaping/Dalong formation bound- spheric N2 has not been changed significantly in Earth’s history ary, they are rather consistent in the overlying Dalong Formation. 15 13 (Nishizawa et al., 2007), and consequently lowers the d N value d Corg values abruptly drop for 7‰ from À25 to À32‰ in the low- of oceanic nitrate; this scenario may have occurred in the lower ermost Feixianguan Formation, immediately above the extinction 13 Dalong Formation at Chaotian. horizon. Four potential mechanisms can explain the d Corg drop Partial assimilation of nitrate in a nitrate-enriched condition (Fig. 5): 1) a negative d13C shift of dissolved inorganic carbon 15 could also have decreased the d NTN values in the analyzed rocks, (DIC) in the ocean, 2) an increase of carbon isotopic fractionation M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128 123

Table 1 The present analytical results of the P–TB interval at Chaotian.

15 13 Formation Sample ID Lithology Thickness d N d Corg TN TOC C/N (m) vs air (‰) vs VPDB (‰) (ppm) (%) atomic

Feixianguan G8 Dark gray limestone 40.3 0.8 À29.6 48 0.0 6.8 Feixianguan G6 Dark gray limestone 40.0 À0.6 À31.3 26 0.0 14.7 Feixianguan G4 Gray limestone 39.7 0.1 À31.4 49 0.1 12.2 Feixianguan G1 Dark gray limestone 39.3 À0.4 À31.8 90 0.1 8.6 Feixianguan F11 Gray limestone 38.8 À1.0 À31.6 42 0.1 14.3 Feixianguan F7 Dark gray limestone 38.5 À1.4 À31.7 236 0.1 4.0 Feixianguan F5 Gray marl 38.3 À1.5 À28.2 81 0.0 5.1 Feixianguan F3 Gray limestone 38.0 À0.8 À30.8 22 0.0 20.0 Feixianguan F1 Gray limestone 37.7 0.9 À30.1 74 0.1 10.3 Feixianguan E7 Gray marl 36.8 À0.4 À25.6 1104 0.1 0.7 Feixianguan E2 Gray marl 36.4 À0.2 À25.0 1401 0.1 0.6 Dalong D19 Gray limestone 35.7 À28.2 32 0.0 8.7 Dalong D15 Gray limestone 35.3 À0.3 À25.6 1035 0.1 0.8 Dalong D13 Gray limestone 35.1 À0.7 À25.2 648 0.2 2.7 Dalong D9 Gray limestone 34.8 À0.8 À25.2 590 0.1 1.4 Dalong D3 Gray limestone 34.2 0.5 À25.7 13 0.0 14.6 Dalong C8 Black mudstone 33.6 0.2 À26.0 1639 0.9 6.4 Dalong C6 Black mudstone 33.2 À1.1 À26.7 3224 2.0 7.1 Dalong B33 Black mudstone 32.3 À0.2 À27.1 1225 1.6 15.5 Dalong B26 Black mudstone 31.7 À0.1 À27.2 1288 1.5 14.0 Dalong B20 Dark gray muddy limestone 31.2 À0.5 À26.0 657 0.7 12.2 Dalong B10 Dark gray muddy limestone 30.5 0.6 À27.2 245 0.7 34.1 Dalong B3 Dark gray muddy limestone 30.0 0.8 À26.5 261 0.7 32.9 Dalong A11 Black mudstone 29.4 À0.3 À26.6 2286 4.8 24.4 Dalong Dalong49 Black mudstone 27.5 À0.3 À26.8 5727 10.1 20.6 Dalong Dalong48 Black mudstone 27.1 À0.1 À26.8 6590 10.4 18.4 Dalong Dalong47 Black mudstone 26.6 À1.3 À26.6 5547 10.0 21.0 Dalong Dalong46 Black mudstone 26.0 À0.7 À26.7 4762 8.1 19.9 Dalong Dalong45 Black mudstone 25.1 À0.2 À26.6 5095 10.0 22.8 Dalong Dalong44 Black mudstone 24.9 À0.5 À26.8 7432 9.9 15.6 Dalong Dalong43 Black mudstone 24.4 À1.4 À26.7 6422 12.3 22.3 Dalong Dalong42 Black mudstone 23.9 À0.2 À26.2 6174 9.0 17.1 Dalong Dalong41 Black mudstone 22.3 À1.1 À27.2 4790 7.5 18.3 Dalong Dalong40 Black mudstone 22.0 À1.0 À27.2 6029 9.2 17.7 Dalong Dalong39 Black mudstone 21.8 À0.3 À27.1 5553 13.2 27.6 Dalong Dalong38 Black mudstone 21.3 À0.9 À27.0 4399 9.4 24.8 Dalong Dalong37 Black mudstone 21.0 À1.2 À27.1 5093 8.7 19.9 Dalong Dalong36 Black mudstone 20.5 À0.9 À27.3 5680 12.3 25.3 Dalong Dalong35 Dark gray dolostone 20.3 À0.6 À27.7 1021 1.5 16.7 Dalong Dalong34 Black mudstone 20.2 À0.8 À27.0 5139 9.4 21.2 Dalong Dalong33 Black mudstone 19.9 À0.9 À27.3 6412 11.3 20.6 Dalong Dalong32 Dark gray muddy limestone 19.9 À2.0 À27.8 328 0.7 24.0 Dalong Dalong31 Black mudstone 19.7 À1.0 À27.6 2916 5.9 23.5 Dalong Dalong30 Dark gray muddy limestone 19.6 À0.4 À28.5 1471 2.5 20.1 Dalong Dalong29 Black mudstone 19.2 À0.8 À27.2 4955 10.0 23.5 Dalong Dalong28 Black mudstone 18.6 À0.1 À27.2 3319 3.9 13.7 Dalong Dalong27 Black mudstone 18.3 0.2 À27.4 4308 8.3 22.6 Dalong Dalong26 Dark gray muddy limestone 18.1 À0.2 À27.6 818 1.4 19.7 Dalong Dalong25 Dark gray bioclastic limestone 17.6 À0.3 À27.3 3095 5.8 21.9 Dalong Dalong24 Dark gray muddy limestone 17.6 À0.1 À28.1 674 1.0 16.5 Dalong Dalong23 Black mudstone 17.5 0.0 À26.8 10239 15.1 17.2 Dalong Dalong22 Dark gray muddy limestone 17.4 À0.9 À28.2 1505 1.5 11.9 Dalong Dalong21 Black mudstone 17.2 0.3 À27.1 6121 10.3 19.6 Dalong Dalong20 Black mudstone 16.8 0.6 À27.4 7529 11.9 18.5 Dalong Dalong19 Dark gray muddy limestone 16.5 À1.4 À28.1 526 0.7 16.6 Dalong Dalong18 Black mudstone 16.3 À0.7 À27.0 5855 12.4 24.7 Dalong Dalong17 Black mudstone 16.0 0.2 À27.2 5415 11.6 25.0 Dalong Dalong16 Dark gray muddy limestone 15.7 0.3 À27.8 981 1.8 21.2 Dalong Dalong15 Dark gray muddy limestone 15.6 0.0 À28.0 942 1.9 24.0 Dalong Dalong14 Dark gray muddy limestone 15.6 0.2 À27.8 1439 3.1 25.3 Dalong Dalong13 Black mudstone 15.4 À0.1 À27.0 6646 13.4 23.6 Dalong Dalong12 Dark gray bioclastic limestone 15.2 À0.8 À28.2 113 0.2 21.6 Dalong Dalong11 Black mudstone 15.0 0.8 À27.2 3893 8.7 26.2 Dalong Dalong10 Black mudstone 14.8 0.2 À27.0 2327 3.9 19.6 Dalong Dalong9 Black mudstone 14.5 À0.1 À26.4 4586 11.9 30.2 Dalong Dalong8 Black mudstone 14.0 0.0 À26.4 2020 5.6 32.5 Dalong Dalong7 Dark gray muddy limestone 14.0 0.1 À27.1 833 2.3 32.2 Dalong Dalong6 Black mudstone 13.7 0.0 À26.3 2737 7.1 30.2 Dalong Dalong5 Black mudstone 13.2 0.6 À26.2 1720 3.6 24.4 Dalong Dalong4 Dark gray muddy limestone 12.9 0.4 À26.4 615 0.7 14.1 Dalong Dalong3 Black mudstone 12.8 À0.1 À26.2 1629 4.1 29.6 Dalong Dalong2 Black mudstone 11.9 0.8 À24.8 612 0.8 15.6 Dalong Dalong1 Gray mudstone 11.4 0.6 À24.9 194 0.4 21.5

(continued on next page) 124 M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128

Table 1 (continued)

15 13 Formation Sample ID Lithology Thickness d N d Corg TN TOC C/N (m) vs air (‰) vs VPDB (‰) (ppm) (%) atomic

Wujiaping Wujiaping20 Dark gray limestone 11.0 À0.4 À26.8 65 0.1 14.1 Wujiaping Wujiaping19 Dark gray limestone 10.6 À1.6 À25.1 55 0.1 17.9 Wujiaping Wujiaping18 Dark gray limestone 10.2 0.1 À24.9 60 0.1 12.8 Wujiaping Wujiaping17 Dark gray limestone 9.6 0.5 À24.6 288 0.3 13.9 Wujiaping Wujiaping16 Dark gray limestone 9.2 0.5 À22.6 102 0.1 10.9 Wujiaping Wujiaping15 Dark gray limestone 8.6 1.1 À24.9 19 0.0 23.8 Wujiaping Wujiaping14 Dark gray limestone 8.0 2.1 À24.5 73 0.1 22.0 Wujiaping Wujiaping13 Dark gray limestone 7.2 2.2 À24.7 32 0.1 19.1 Wujiaping Wujiaping12 Dark gray limestone 6.6 1.2 À24.7 123 0.2 18.3 Wujiaping Wujiaping11 Dark gray limestone 6.1 1.8 À25.0 28 0.1 23.5 Wujiaping Wujiaping10 Dark gray limestone 5.6 À25.0 76 0.1 18.8 Wujiaping Wujiaping9 Dark gray limestone 5.1 0.8 À24.7 17 0.0 30.0 Wujiaping Wujiaping8 Dark gray limestone 4.6 À0.3 À23.3 293 0.3 11.9 Wujiaping Wujiaping7 Dark gray limestone 4.0 À1.8 À24.3 121 0.2 21.6 Wujiaping Wujiaping6 Dark gray limestone 3.3 À2.2 À24.3 200 0.4 21.3 Wujiaping Wujiaping5 Dark gray limestone 2.8 À1.9 À25.1 129 0.2 21.9 Wujiaping Wujiaping4 Dark gray limestone 2.0 1.4 À24.8 291 0.8 33.4 Wujiaping Wujiaping3 Dark gray limestone 1.1 1.5 À28.4 63 0.2 30.1 Wujiaping Wujiaping2 Dark gray limestone 0.7 1.7 À27.1 21 0.0 28.0 Wujiaping Wujiaping1 Dark gray limestone 0.0 1.7 À26.2 52 0.1 14.3

in biological carbon fixation, 3) increased contribution of methan- 6.4. Global variation in d15N records around the P–TB otrophs to sedimentary organic matter, and 4) increased input of terrestrial plants. Cao et al. (2010) analyzed carbon isotopic com- Several previous studies on d15N chemostratigraphic analysis 13 positions of carbonate (d Ccarb) across the P–TB at Chaotian and across the P–TB in various sections around the world reported a 13 reported a negative d Ccarb shift from 1 to À2‰ above the extinc- negative shift across the extinction horizon (Figs. 1B, C and 8). This 13 15 tion horizon. The negative d Corg shift detected in this study is d N shift is well correlated on a global scale and is interpreted to 13 13 apparently concordant with this d Ccarb shift, although the d Ccarb record enhanced nitrogen fixation in the oceans in the aftermath of 13 decrease for 3‰ cannot explain solely the larger d Corg decrease the extinction (Luo et al., 2011; Knies et al., 2013). In contrast, at 15 for 7‰ and an additional mechanism is needed. Chaotian, d NTN values began to decrease in the late Wuchiapin- Ca. 26‰ of apparent isotopic fractionation in carbon fixation gian and are consistently low (mostly ranging between À1 and 13 15 immediately before the d Corg drop at Chaotian (estimated by 0‰) during the Changhsingian. Across the P–TB, d NTN values 13 13 d Corg and d Ccarb records) is consistent with the typical Calvin are rather consistent and no clear shift is recognized. The global cycle. The apparent isotopic fractionation increases to ca. 30‰ at correlation implies that the suggested enhanced nitrogen fixation 13 the stratigraphic horizon of the d Corg minimum above the bio- during the entire Changhsingian was a regional event at Chaotian stratigraphically-defined P–TB (Fig. 5). The reductive acetyl-CoA and not a global phenomenon. pathway can yield larger carbon isotopic fractionation than the Although previous studies reported consistently a negative d15N Calvin cycle (Preuss et al., 1989). In the aftermath of the extinction, shift across the extinction horizon, a composite chemostratigraphic the isotopic fractionation factor in the Calvin cycle and/or flux of d15N trend in the late Permian to earliest Triassic documents that organic carbon from organisms that fixed inorganic carbon via 1) the d15N value and 2) the magnitude of the negative d15N shift reductive acetyl-CoA pathway (such as methanogens) into across the extinction horizon are substantially variable on a global 13 15 sediments may have increased, causing the large d Corg drop at scale (Fig. 9). In general, d N values are relatively high in the clas- Chaotian. On the other hand, the observed difference in d13C tic sequences in high latitudes compared to those in the carbonate decrease between organic and inorganic carbon (ca. 4‰) at Chao- sequences in low latitudes (including Chaotian) (Fig. 8). In high- tian is too small to take organic compounds from methanotrophs latitude regions, upwelling of denitrified 15N-enriched waters from into account (Collister et al., 1992; Hayes, 1994; Summons et al., the deeper oxygen minimum zone would have contributed to the 13 15 1994). Terrestrial plants generally exhibit relatively low d Corg relatively high d N values of nitrate in the surface oceans in the values compared to marine plankton and increased continental late Permian (Schoepfer et al., 2012; Knies et al., 2013). Such a lat- input could lower d13C value of bulk organic matter in sediments itudinal variation in d15N value of sedimentary organic matter is (Sackett and Thompson, 1963; Peters et al., 1978). However, except not observed in the modern oceans (e.g., Tesdal et al., 2012), and for some remarkably low ratios (<6.6) due to addition of nitrogen may have been unique at the end-Paleozoic. from clay minerals/micas, C/N ratios are rather consistent around It is also noteworthy that the magnitude of a negative d15N shift 13 10 and no systematic increase is recognized during the d Corg drop across the extinction horizon is relatively small (<2‰) at the West at Chaotian; it possibly discard a contribution of land plants to the Blind Fiord and Buchanan Lake sections in Arctic Canada, whereas 13 d Corg drop. that at several sections in South China around the equator (except 13 In summary, the large d Corg drop for 7‰ immediately after the for Chaotian) is relatively large (>2‰)(Figs. 8 and 9). Although a 13 15 extinction at Chaotian cannot be explained solely by the d CDIC remarkably large d N drop for 7‰ was reported at Opal Creek, decrease for 3‰ and an additional mechanism is required. In other Alberta, Canada, it occurred significantly before the extinction words, the observed isotopic records document that organic and and is not correlated to negative d15N shifts across the extinction carbonate d13C changes are not paralleled in the aftermath of the horizon in other sections (Schoepfer et al., 2012). In general, extinction. The isotopic fractionation factor in the Calvin cycle when a constant amount of nitrate of low d15N value (0‰) gener- and/or flux of organic carbon from organisms that fixed carbon ated by nitrogen fixation is added to the regional nitrate pool in the via reductive acetyl-CoA pathway into sediments may have ocean, the magnitude of a resultant negative d15N shift in the pool 13 15 increased, resulting in the large d Corg drop. is larger if original d N value of the pool is higher. This is because .Sio ta./Junlo sa at cecs9 21)113–128 (2014) 93 Sciences Earth Asian of Journal / al. et Saitoh M.

Fig. 8. Comparison of nitrogen isotope chemostratigraphies across the P–TB in various sections around the world. Note that d 15 N values are relatively high in high latitudes whereas they are relatively low in South China in low latitudes. Data are from Algeo et al. (2007, 2012), Cao et al. (2009), Luo et al. (2011), Jia et al. (2012), Schoepfer et al. (2012), Knies et al. (2013), and this study. 125 126 M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128

Fig. 9. Composite d15N profiles in the late Permian to earliest Triassic. A color of each section is the same as that in Fig. 1. Chemostratigraphic d15N profiles shown in Fig. 8 are correlated to that in Meishan, the GSSP of the P–TB, particularly focusing on the Wuchiapingian/Changhsingian boundary, the extinction horizon, and the biostratigraphically- defined P–TB. In each section, a sediment accumulation rate is supposed to be constant within the interval between the extinction horizon and the biostratigraphically- defined P–TB, and this accumulation rate is extrapolated to the upper Triassic strata. A similar extrapolation is applied to the Permian strata utilizing the Wuchiapingian/ Changhsingian boundary and the extinction horizon. When the Wuchiapingian/Changhsingian boundary is not clear, a substitute correlatable conodont zone is utilized or an estimated sediment accumulation rate in the Triassic strata is extrapolated to the Permian strata. The composite profiles demonstrate considerable d15N variations around the P–TB on a global scale. Note that, in high latitudes, d15N values are relatively high and the magnitude of a negative d15N shift across the extinction horizon is relatively small (<2‰), compared to those in low latitudes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of an isotopic scale effect; nitrogen isotopic composition of a significantly preceding the main phase of the extinction (Cao et al., nitrate pool of higher d15N value is more susceptible to mixing of 2009; Nabbefeld et al., 2010; Hays et al., 2012). A long-term environ- nitrate of low d15N value. Hence, the observed smaller negative mental deterioration including oceanic nitrogen depletion associ- shifts in higher d15N values across the extinction horizon in high ated with anoxia during the Changhsingian may have acted as a latitudes compared to those in low-latitudes suggest that, in prolonged stress to shallow-marine biota (Isozaki, 1997; Powers high-latitude regions, a smaller amount of fixed nitrogen may have and Bottjer, 2007). In order to clarify fluctuations in the oceanic been added to the regional nitrate pool and/or the size of a regional nitrogen cycle and its relationships to the biotic crisis at the end- nitrate pool in the ocean may have been larger than that in low-lat- Permian, further chemostratigraphic study is needed particularly itude regions. On the other hand, enhanced upwelling of denitrified focusing on not only the P–TB but also the entire Changhsingian. deep-waters may have brought 15N-enriched nitrate to the surface ocean and offset the isotopic effect of addition of fixed nitrogen 7. Conclusions during the extinction event. Although global chemostratigraphic comparisons imply that the In order to clarify changes in the oceanic nitrogen cycle around 15 consistently low d NTN value through the Changhsingian at Chao- the Permian–Triassic boundary (P–TB) including the entire tian was possibly a regional isotopic signature (Figs. 8 and 9), the Changhsingian (Late Late Permian) preceding the extinction, nitro- present results suggest that, at least locally in northwestern South gen isotopic compositions of upper Permian to lowermost Triassic China, nitrogen fixation was characteristically intensified for ca. 2 rocks were analyzed in the Chaotian section, northern Sichuan, million years prior to the extinction (Gradstein et al., 2012). It sug- South China. The following new results were obtained: gests that the ocean was under prolonged nitrogen depletion asso- 15 ciated with anoxia because nitrogen fixation generally occurs 1. The d NTN values are consistently low (around 0‰) in the under nitrogen-limited condition due to its high-energy demand upper Permian Dalong and lowermost Triassic Feixianguan for- 15 to break a triple bond of N2 (Tyrrell, 1999). Cao et al. (2009) also mations at Chaotian. No clear d NTN shift is recognized across 15 15 recognized a progressive d N decrease during the Changhsingian the extinction horizon. The consistently low d NTN values indi- at Meishan and attributed it to increased nitrogen fixation. The cate enhanced nitrogen fixation in the ocean along the north- temporal difference in d15N decrease between Chaotian, Meishan, western margin of South China during the Changhsingian, and other sections around the world implies that the sea area in associated with anoxia. which fixed nitrogen is depleted has gradually developed world- 2. Composite chemostratigraphic profiles on the basis of previous wide in the Changhsingian. A development of such an unusual oce- and the present studies reveal the substantial d15N variation on anic condition in the Changhsingian is consistent with the a global scale in the late Permian to earliest Triassic; in partic- existence of euxinic condition spanning the entire Changhsingian, ular, a latitudinal d15N difference is clearly recognized. Although M. Saitoh et al. / Journal of Asian Earth Sciences 93 (2014) 113–128 127

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