Nitrogen Isotope Chemostratigraphy Across the Permian-Triassic Boundary at Chaotian, Sichuan, South China
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Journal of Asian Earth Sciences 93 (2014) 113–128 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes 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.