Paired Δ13 Ccarb and Δ13 Corg Records of the Ordovician on the Yangtze Platform, South China

Australian Journal of Earth Sciences An International Geoscience Journal of the Geological Society of Australia

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δ13 δ13 Paired Ccarb and Corg records of the Ordovician on the Yangtze platform, South China

B.-Y. Li, D.-W. Zhang, X.-Q. Pang, P. Gao, D.-Y. Zhu, K.-Z. Guo & T.-Y. Zheng

To cite this article: B.-Y. Li, D.-W. Zhang, X.-Q. Pang, P. Gao, D.-Y. Zhu, K.-Z. Guo & T.-Y. Zheng δ13 δ13 (2018) Paired Ccarb and Corg records of the Ordovician on the Yangtze platform, South China, Australian Journal of Earth Sciences, 65:6, 809-822, DOI: 10.1080/08120099.2018.1487468 To link to this article: https://doi.org/10.1080/08120099.2018.1487468

Published online: 08 Oct 2018.

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Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=taje20 AUSTRALIAN JOURNAL OF EARTH SCIENCES, 2018 VOL. 65, NO. 6, 809–822 https://doi.org/10.1080/08120099.2018.1487468

13 13 Paired d Ccarb and d Corg records of the Ordovician on the Yangtze platform, South China

B.-Y. Lia,b, D.-W. Zhangb, X.-Q. Panga, P. Gaob, D.-Y. Zhub, K.-Z. Guoa and T.-Y. Zhenga aState Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China; bPetroleum Exploration and Production Research Institute, Sinopec, Beijing, China

ABSTRACT ARTICLE HISTORY During the Ordovician, huge biological revolutions and environmental changes happened in Received 17 October 2017 Earth’s history, including the Great Ordovician Biodiversification Event, global cooling and so on, Accepted 6 June 2018 but the cause of these events remains controversial. Herein, we conducted a paired carbon iso- 13 13 KEYWORDS topic analysis of carbonate (d Ccarb) and organic matter (d Corg) through the Ordovician in the 13 carbon isotope stratigraphy; Qiliao section on the Yangtze platform of South China. Our results showed that the d Ccarb trend d13 Ordovician; positive of the Qiliao section can be correlated with local and global curves. The Corg trend seems is excursion; global cooling; d13 d13 less clear than the Ccarb trend for stratigraphic correlations, but some Corg positive excursions Yangtze platform in the Middle and Upper Ordovician may be used for correlation studies. These carbon isotopic records may have global significance rather than local significance, revealing several fluctuations to 13 13 the global carbon cycle during the Ordovician. Several known d Ccarb and d Corg negative and positive excursions have been recognised in this study, including the early Floian Negative 13 Inorganic Carbon (d Ccarb) Excursion (EFNICE), as well as the early Floian Positive Organic Carbon 13 13 13 (d Ccarb) Excursion, the mid-Darriwilian Inorganic Carbon (d Ccarb and d Corg) Excursion (MDICE), 13 13 and the early Katian Guttenberg Inorganic Carbon (d Ccarb and d Corg) Excursion (GICE). These 13 positive excursions and a smooth decline trend of d Corg values during the early to mid-Floian may imply multiple episodes of enhanced organic carbon burial that began at the early Floian stage, probably resulting in further decline in atmospheric pCO2 and then global cooling.

Introduction controversial (Marenco et al., 2016; Servais et al., 2009; Wang, Chatterton, & Wang, 1997). Previous studies sug- During the Ordovician, significant changes in the Earth’seco- gested that the GOBE may be controlled by not only the systems (Harper, Zhan, & Jin, 2015;Marenco,Martin,Marenco, intrinsic factors (essentially biological) but also the extrinsic &Barber,2016;Munnecke,Calner,Harper,&Servais,2010; factors (essentially environmental) (Harper et al., 2015; Sepkoski, 1981), including a long-term biodiversification and a Zhan, Jin, & Liu, 2013). Intrinsic factors include the competi- short-term mass extinction at the end of Ordovician, have tion, predation and plankton revolution. More notably, been linked to global cooling (Trotter, Williams, Barnes, extrinsic factors have been widely studied, including ero- Lecuyer, & Nicoll, 2008). Previous studies (Harper et al., 2015; sion, high sea levels, global cooling, oxygenation, tectonic Servais et al., 2009;Webby,Paris,Droser,&Percival,2004) and magmatic activity, and volcanicity (Harper et al., 2015; revealed that the Great Ordovician Biodiversification Event Marenco et al., 2016; Trotter et al., 2008; Zhang, Shen, & (GOBE) initiated at the mid-Ordovician, ca 470 Ma, and pos- Algeo, 2010). These environmental changes provide not sibly lasted into the Late Ordovician. By the end of only suitable water column conditions (e.g. oxygen con- Ordovician, the number and species of most faunal groups tents, temperatures) but also inorganic nutrients for the had risen to more than triple of those in the Early Ordovician growth and metabolism of progressively larger animals. (Harper et al., 2015; Webby et al., 2004). The GOBE ended Among them, climatic cooling may play an important role with sudden and catastrophic extinctions at the terminal in triggering the GOBE (Marenco et al., 2016; Trotter et al., Ordovician, ca 444 Ma (Gradstein, Ogg, & Smith, 2004), which 2008; Zhang et al., 2010). During the Early–Middle was probably associated with rapid ice sheet growth over Ordovician, the atmospheric pCO2 levels were estimated to polar landmasses (Brenchley et al., 1994; Wang et al., 1987). be 14 to 18 times that of the modern atmospheric level Compared with the widely studied Cambrian explosion, (Herrmann, Patkowsky, & Pollard, 2003). That is, the GOBE the GOBE has received less attention (Bottjer, Droser, may have occurred under super greenhouse conditions

Sheehan, & McGhee, 2001) and its cause remains (Gibbs, Barron, & Kump, 1997). Since atmospheric CO2 is

CONTACT B.-Y. Li [email protected] State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China ß 2018 Geological Society of Australia 810 B. LI ET AL. the most important greenhouse gas in the atmosphere, its Furthermore, these records may provide evidence for glo- variation could be an indicator for climate change at vari- bal chemostratigraphic correlations and add new insights ous time scales. It is generally accepted that atmospheric into the causes of global cooling. pCO2 is controlled by the carbon cycle between the ocean and atmosphere system (Edwards & Saltzman, 2016; Royer, 2006; Wang et al., 1997). Thus, the carbon isotopic compo- Geological setting and stratigraphy sitions of carbonate and organic matter (OM) can be Regional geology potentially used for recording any changes in the global During the Ordovician, the South China Block was mainly carbon cycle linked to atmosphere pCO2 changes (Edwards & Saltzman, 2016; Hayes, Strauss, & Kaufman, 1999; Pope & composed of three regions: the Yangtze platform, Jiangnan Steffen, 2003; Young, Saltzman, Bergstrom,€ Leslie, & slope, and Zhujiang basin (Munnecke et al., 2011). These Chen, 2008). three regions had the characteristics of parallel distribution 13 13 and southeast–northwest extension. The sedimentary facies The Ordovician d C trends in carbonates (d Ccarb) have been extensively studied, and the corresponding d13C gradually changed from the shallow-water carbonates on excursions have been extensively recorded in numerous the platform in the northwest to the deep-water shales in areas, such as North America (Edwards & Saltzman, 2014, the basin in the southeast (Figure 1). During the Late Edwards & Saltzman, 2016; Young et al., 2008), South China Ordovician, the South China Block had separated from the (Ma, Wang, Zhang, Song, & Fang, 2015; Munnecke, Zhang, Gondwana supercontinent, which was situated at a paleo- Liu, & Cheng, 2011; Zhang et al., 2010), Baltoscandia latitude of about 20 S (Chen, Rong, Li, & Boucot, 2004). (Ainsaar et al., 2010), and Siberia (Ainsaar et al., 2014). The Yangtze platform, which was submerged by a wide These have been used for global and local chemostrati- epeiric sea during most of the Ordovician, has a continuous graphic correlations (Bergstrom,€ Chen, Gutierrez-Marco, & succession of carbonates and mudstones with abundant Dronov, 2009; Wang et al., 1997; Zhang et al., 2010). Unlike fossils (Chen et al., 2004; Yang, Zhang, Chen, Chen, & Wei, 13 13 the widely studied d Ccarb trends, the d Corg trend of 1975). Rich benthic shelly faunas, such as trilobites, bra- Ordovician carbonate has not been well studied. Moreover, chiopods, and echinoderms, have been found in this 13 13 most studies of paired d Ccarb and d Corg have focused sequence (Fan et al., 2015; Ma et al., 2015). High resolution on narrow time intervals (generally the Late Ordovician or biostratigraphy of the Ordovician has also been well estab- Ordovician–Silurian transition period). The carbon isotope lished using the distribution of graptolites, conodonts, and trends of the entire Ordovician have been reported by only some other shelly faunas (Chen, Zhang, & Fan, 2006). Until a few authors (Edwards & Saltzman, 2016; Hayes et al., now, the studies on the lithostratigraphy, biostratigraphy, 1999), and such trends have not yet been reported in and depositional environments of the Ordovician have South China. Therefore, the purpose of this paper is to been carried out in many sections of South China, such as report a paired carbon isotopic record of carbonate the Wangjiawan, Honghuayuan, and Huangnitang sections 13 13 € (d Ccarb) and organic matter (d Corg) from the Qiliao sec- (Ma et al., 2015; Munnecke et al., 2011; Schmitz, Bergstrom, tion on the Yangtze platform throughout the Ordovician. & Wang, 2010; Wang et al., 1997; Young et al., 2008; Zhang

Figure 1. Geographic position of the Qiliao sections and major facies during the Lower–Middle Ordovician in South China (modified from Munnecke et al., 2011). AUSTRALIAN JOURNAL OF EARTH SCIENCES 811

in fossils and may be a potential reference for the Yangtze platform in South China.

Stratigraphy In the Qiliao section, the Lower Ordovician is conformable with the upper Cambrian Maotian Formation dolomites, and the Upper Ordovician is conformable with the lower Silurian Longmaxi Formation black shales (Figure 2). The total thickness of the Ordovician is 463.8 m. The Ordovician can be subdivided, from bottom to top, into the Nanjinguan, Fenxiang, Honghuayuan, Dawan, Shihtzupu, Pagoda, Linhsiang, and Wufeng formations. The Lower Ordovician includes the Nanjinguan, Fenxiang, Honghuayuan, and lower Dawan formations, the Middle Ordovician consists of the upper Dawan and Shihtzupu formations, and the Upper Ordovician covers Pagoda, Linhsiang, and Wufeng formations. The Qiliao section is paleogeographically located on the same carbonate platform as the widely studied Yichang area (Chen et al., 2000, Chen et al., 2006; Rong, Chen, & Harper, 2002; Yang et al., 1975; Zhan & Jin, 2007; Zhang et al., 2010), which can provide lithostratigraphy, biostratigraphy, and chronology data for improving the stratigraphic division and correlation. The eight formations at the Qiliao section, from oldest to youngest are:

1. The Nanjinguan Formation is 97.37 m thick and is equivalent to ‘Tremadocian’ in age based on the first appearance datum (FAD) of graptolite R. taojiangensis in the bottom (Wang et al., 1987). The lithology is composed of grey to dark-grey, thin- to medium-bedded micrite, bioclastic limestone, and nodular limestone, con- taining some dark-grey mudstone interlayers (Figure 3a). Among them, bioclastic limestones include trilobites (e.g. Tungtzuella, Asaphellus, A. cf. bellus Lu, A. cf. inflatus Lu), and brachiopods (e.g. Nanorthishamburgehsis, Lingulella, Oligorthis)(Yangetal.,1975). 2. The Fenxiang Formation is 100.47 m thick, and the conodont O. communis in its basal parts marks the onset of the Floian (Wang et al., 1987; Wang, Chen, Wang, & Li, 2004). The lithology is mainly composed of Figure 2. Stratigraphic column and major fossils of the Ordovician at the medium- to thick-bedded bioclastic limestones, with Qiliao section on the Yangtze platform. The geological timescale is from the some yellowish green mudstone interlayers (Figure ChronostratChart of Cohen, Finney, Gibbard, and Fan (2014). 3b). The bioclastic limestones and mudstones contain trilobites (e.g. Psilocephalina, P. cf. lubrica, Tungtzuella, et al., 2010), which lay the foundation for the calibration of Psilocephalinalubrica), brachiopods (e.g. Nanorthis, stratigraphic correlations and any geological events Oligorthis) and cephalopods (Hopeioceras). occurred in the Ordovician. 3. The Honghuayuan Formation mainly consists of dark The Shizhu area is situated in the centre of Yangtze grey thick-bedded limestones with the thickness of platform. We collected the samples from the Qiliao section, 40.48 m, and the underlying and overlying strata are which is situated at about 20 km southeast of Shizhu mudstones, probably belonging to ‘middle Floian’ in County to the east of Chongqing City (Figure 1). The Qiliao age. The Honghuayuan limestones contain numerous section is a near complete succession from the upper bioclasts, including trilobites (e.g. Psilocephalina, Cambrian to lower Silurian, and the entire Ordovician is Chenkouella), gastropods (Raphistoma, Morchisonia) basically preserved. It is continuous in stratigraphy and rich and cephalopods (Sinoceras)(Figure 3c). 812 B. LI ET AL.

Figure 3. Outcrop photographs of the Qiliao section. (a) Medium- to thick-bedded bioclastic limestone interbedded with thin-bedded mudstone of the Nanjingguan Formation. (b) Thick-bedded bioclastic limestone interbedded with mudstone of the Fenxiang Formation. (c) A Sinoceras fossil from within thick- bedded bioclastic limestone of the Honghuayuan Formation and is filled with calcite cements. (d) Calcareous mudstone of the Dawan Formation. (e) Medium- to thick-bedded grey nodular limestone of Pagoda Formation. (f) Black siliceous shale of the Wufeng Formation.

4. The Dawan Formation is 166.64 m thick and can be sub- and greyish-green to dark grey mudstones (Figure 3d). divided in to the lower and upper parts according to a These two parts can be separated by the FAD of the limestone member situated at 100mabovethebot- conodont B. triangularis indicating the Floian–Dapingian tom of this formation (Zhang et al., 2010). The lower part boundary (Wang et al., 1987, Wang et al., 2004;Yang is mainly composed of yellowish green mudstones, while et al., 1975). The graptolite U. austrodentatus occurs in the upper part consists of the basal nodular limestones the top of the Dawan mudstones suggesting the ‘early AUSTRALIAN JOURNAL OF EARTH SCIENCES 813

Darriwilian stage’ (Wang et al., 1987, Wang et al., 2004). After removing carbonates with 7% HCl, total organic car- The Dawan mudstones are enriched with fossils. Among bon (TOC) and total sulfur (TS) of studied samples were meas- them, brachiopods are the most abundant fossil group, ured by the total carbon and sulfur Leco CS-230 analyser at including Sinorthis, Yangtzeella, Taphrorthis, Taihungshania the State Key Laboratory of Petroleum Resources and (Yang et al., 1975). Prospecting. The analytical precision was better than 10%. 5. The Shihtzupu Formation is 18.67 m thick and is Carbon isotope analyses of carbonates and organic mat- equivalent to ‘Darriwilian’ in age based on the FAD of ter (OM) were measured by Finnigan MAT 253 isotope graptolite Nemagraptus gracilis in the upper parts mass spectrometer at the CUP and the Analytical (Wang et al., 1987, Wang et al., 2004). The lithology Laboratory of Beijing Research Institute of Uranium consists of medium- to thick-bedded nodular lime- Geology, respectively. Carbonates were removed with 7% stones. Brachiopods (e.g. Tetradontella) and trilobites HCl, for carbon isotope analysis of OM. Carbon and oxygen (e.g. Nileus) are also found in this formation. isotopic values of the carbonates were measured by reac- 6. The Pagoda Formation is 18.49 m thick and is equivalent tion with phosphoric acid widely reported by previous to ‘Sandbian to early Katian’ in age based on the FAD of authors (e.g. Zhang et al., 2010). Replicate analyses of the trilobite Nankinolithus in the basal Linhsiang marls studied samples and laboratory standards were conducted (Wang et al., 1987;Yangetal.,1975; Zhang et al., 2010). with an analytical precision better than 0.1 ‰. These iso- The lithology is composed of medium- to thick-bedded topic results were reported in d notation in per mille (‰) nodular limestones interbedded with grey mudstones relative to the standard VPDB. All geochemical results (TOC, 13 13 18 (Figure 3e). They also contain abundant fossils, including TS, d Corg, d Ccarb, and d Ocarb) are listed in Table 1. trilobites (e.g. Nileus, Paracerausus, Remopleurides)and cephalopods (Sinoceras, Michelinoceras)(Yangetal.,1975). Results and discussion 7. The Linhsiang Formation is 5.84 m thick and is equivalent to ‘middle Katian’ in age based on the FAD of the Petrological features graptolite D. complanatus in the basal black shales The lithologies of the Ordovician sediments at Qiliao (Yang et al., 1975; Wang et al., 1987; Zhang et al., mainly include limestone, mudstone and siliceous shale. 2010). This formation mainly consists of nodular lime- The detailed lithological characteristics are illustrated in stones. Trilobites (e.g. Corrugatagnostus) and cephalo- Figure 4. pods (e.g. Trocholites) are found in the limestones. 8. The Wufeng Formation is 15.86 m thick and is equivalent to ‘late Katian to Hirnatian’ in age based on the Limestone FAD of the graptolite G. persculptus in the basal The limestones are developed throughout the Ordovician, Silurian Longmaxi black shales (Wang et al., 1987; including dolomitic limestone (Figure 4a), oolitic limestone Yang et al., 1975; Zhang et al., 2010). The lithology (Figure 4b), bioclastic limestone (Figure 4c), marl (Figure mainly consists of black siliceous shales (Figure 3f). 4d), and nodular limestone (Figure 4e). Among them, bioclastic limestone is abundant and includes trilobites, brachiopods, and ostracodes (Figure 4c). Bioclasts are also Samples and analytical methods abundant in the nodular limestone (Figure 4e) and include Samples ostracodes, lamellibranches, and nautilus.

Forty-eight samples, which were collected from the Qiliao Mudstone section (Figure 1), include carbonates, mudstones, and The mudstones are mainly developed in the Middle–Lower shales that were used for petrological and geochemical Ordovician sediments. Bioclasts are also abundant in the analyses. Before the analyses, the samples were washed mudstones (Figure 4f–g) but are difficult to identify. then cut to remove weathered parts. To minimise the impact of diagenetic veins on geochemical analysis, the samples were broken into small chips 5 5 mm in size, Siliceous shale and then rock fragments without carbonate veins were The siliceous shales developed within the top of the selected. Each sample was crushed to 200 mesh powder in Ordovician sequence, that is, the Wufeng Formation. The an agate mortar. shalesarerichinquartzandOM(Figure 4h). The rounded quartz may be intimately related with siliceous organisms, while the polygonal quartz may be ascribed to detrital inputs. Analytical methods A Zeiss Axioskop 40 A pol microscope was used for the Evaluation of primary carbon isotopic values optical observation of the Ordovician thin sections at the Laboratory of Structural and Sedimentological Reservoir Bulk carbonate in sediments has generally been regarded as Geology, Sinopec. a suitable material to estimate the d13C value of seawater in 814 B. LI ET AL.

13 13 18 Table 1. Basic information and geochemical data (TOC, TS, d Corg, d Ccarb, and d Ocarb) of studied samples. 13 13 18 Sample Depth (m) Formation Lithology TOC (wt%) TS (wt%) d Corg & VPDB d Ccarb & VPDB d Ocarb & VPDB QL-04 5.4 Nanjinguan Limestone 0.04 0.13 ND 0.2 8.6 QL-05 6.3 Nanjinguan Limestone 0.08 0.52 25.8 0.4 8.9 QL-06 15.3 Nanjinguan Mudstone 0.13 0.74 27.2 0.1 7.5 QL-07 16.1 Nanjinguan Mudstone 0.2 0.24 27.4 1.1 7.8 QL-08 17.3 Nanjinguan Mudstone 0.19 0.34 27.0 0.9 8.1 QL-09 18.5 Nanjinguan Mudstone 0.18 0.59 28.0 3.1 8.3 QL-10 20.9 Nanjinguan Mudstone 0.18 0.26 27.8 0.4 7.5 QL-13 52.2 Nanjinguan Marl 0.14 0.29 27.4 0.2 9.9 QL-14 54.1 Nanjinguan Marl 0.18 0.23 27.0 0.1 9.7 QL-15 55.9 Nanjinguan Marl 0.14 0.24 25.3 0.3 10.3 QL-16 68.2 Nanjinguan Limestone 0.09 0.05 27.5 0.9 8.5 QL-18 88.6 Nanjinguan Limestone 0.08 0.13 ND 1.2 11.1 QL-19 94.6 Nanjinguan Limestone 0.03 0.07 25.0 2.8 11.0 QL-21 108.9 Fenxiang Limestone 0.05 0.05 25.0 3.6 9.6 QL-22 114.5 Fenxiang Mudstone 0.09 0.002 26.7 4.1 11.1 QL-23 126.4 Fenxiang Limestone 0.04 0.28 24.0 1.1 9.7 QL-29 150.0 Fenxiang Mudstone 0.16 0.02 26.8 1.8 12.2 QL-30 153.3 Fenxiang Mudstone 0.10 0.001 27.1 4.6 11.4 QL-31-01 162.1 Fenxiang Limestone 0.06 0.02 25.8 1.9 10.1 QL-32 184.1 Fenxiang Mudstone 0.14 0.001 28.1 4.8 14.8 QL-34 189.0 Fenxiang Mudstone 0.17 0.001 28.1 2.1 11.2 QL-37 208.2 Honghuayuan Limestone 0.11 0.60 27.0 1.6 9.9 QL-38 211.3 Honghuayuan Limestone 0.22 0.37 ND 1.9 9.9 QL-39-02 214.1 Honghuayuan Limestone 0.07 0.23 26.7 1.6 10.2 QL-40 216.8 Honghuayuan Limestone 0.04 0.09 ND 1.0 9.3 QL-41 219.3 Honghuayuan Limestone 0.06 0.07 26.7 1.7 9.2 QL-42 221.8 Honghuayuan Limestone 0.09 0.08 ND 1.7 9.5 QL-43 223.9 Honghuayuan Marl 0.08 0.17 27.6 1.6 9.6 QL-44 225.8 Honghuayuan Marl 0.11 0.11 28.2 1.5 10.1 QL-50 235.1 Honghuayuan Limestone 0.08 0.08 ND 1.2 9.7 QL-51 237.8 Honghuayuan Limestone 0.05 0.13 ND 1.2 10.1 QL-52 245.2 Dawan Limestone 0.06 0.06 26.8 1.2 9.6 QL-53 279.4 Dawan Mudstone 0.11 0.07 26.2 NA NA QL-57 371.2 Dawan Mudstone 0.10 0.001 26.1 2.9 13.2 QL-59 406.0 Shihtzupu Mudstone 0.12 0.46 25.6 4.8 9.4 QL-60 409.4 Shihtzupu Limestone 0.05 0.79 24.6 0.2 10.7 QL-64 423.2 Shihtzupu Mudstone 0.18 0.18 25.7 2.7 9.8 QL-65-01 426.4 Pagoda Limestone 0.07 0.22 ND 0.2 10.0 QL-65-02 428.8 Pagoda Mudstone 0.15 0.49 ND 3.3 9.4 QL-66 433.2 Pagoda Limestone 0.09 0.007 27.3 0.7 10.3 QL-68 436.9 Pagoda Limestone 0.04 0.59 26.3 2.1 10.5 QL-69 440.7 Pagoda Limestone 0.11 0.15 ND 0.9 10.7 QL-72 451.7 Wufeng Shale 3.47 0.19 30.6 2.5 14.1 QL-73 452.8 Wufeng Shale 5.62 0.04 30.6 2.5 13.6 QL-75 445.0 Wufeng Shale 5.47 0.03 30.7 1.6 15.1 QL-76 456.3 Wufeng Shale 3.87 0.006 30.3 4.3 16.2 QL-77 457.5 Wufeng Shale 3.58 0.37 30.2 7.6 15.0 QL-79 459.3 Wufeng Shale 2.89 0.005 30.0 2.8 13.9 ND: not detected; NA: not available. the absence of diagenetic carbonate veins (Edwards & diagenesis/metamorphism. The absence of terrestrial OM dur- Saltzman, 2016) but assessment of the other diagenetic ing the Ordovician, suggests the biologically mediated OM 13 effects on the primary d Ccarb is required. In the absence of was mostly formed through photoautotrophic and possible 13 18 diagenetic carbonate veins, the use of d Ccarb–d Ocarb cross chemoautotrophic pathways. Since the carbonates in our plots and global correlation (Wu, Calner, Lehnert, Peterffy, & study area were mainly deposited under shallow water and 13 13 Joachimski, 2015) can be used to evaluate primary d Cval- oxygenated conditions, the d Corg records can be regarded 18 ues. Primary d Ocarb values are commonly altered by diagen- as the signatures of primary producers in the surface water at esis during burial and a lack of systematic correlation that time. In most of the studied samples, carbonates are 13 18 between d Ccarb and d Ocarb values in the studied section chemical sedimentary rocks, and thus detrital organic carbon suggests that the effect of secondary alteration on the input should be negligible. With the increasing degree of ther- 13 13 d Ccarb values of carbonates at Qiliao is minor (Figure 5a). mal degradation of OM, preferential removal of C is kinetic- Previous studies (Gao et al., 2016;Jiangetal.,2012) ally favoured, yielding 13C-enriched residual OM (Hayes, showed that numerous factors can alter the d13Cvaluesof Kaplan, & Wedeking, 1983).Yang,Xie,Wang,Wang,andLiu OM, including carbon isotope fractionation during primary (2012) measured the reflectance of solid bitumen in the and secondary production, admixture of terrestrial OM, detrital Wufeng shales at Qiliao, and the corresponding equivalent vit- organic carbon input, and post-depositional alteration via rinite reflectance that ranged from 2.41 to 2.73%, shows that AUSTRALIAN JOURNAL OF EARTH SCIENCES 815

Figure 4. Optical photographs showing lithological characteristics of Ordovician sediments at the Qiliao section. (Notes: all photographs were imaged under the plane-polarised light. (a–e) were stained by alizarin red; (f–h) were not stained.) (a) QL-04, dolomitic limestone. (b) QL-38, oolitic limestone. (c) QL-39-2, bioclastic limestone, bioclasts mainly brachiopods (indicated by yellow arrow) and trilobites (red arrows). (d) QL-15, marl with several pyrites. (e) QL-69-2, nodular limestone, bioclasts mainly ostracods (red arrow) and nautilus (yellow arrow). (f, g) QL-57, QL-64, mudstones contain large amounts of bioclasts that probably consist of brachiopods. (h) QL-76, siliceous shales are rich in organic matter (OM), rounded quartz of probable biogenic origin (red arrows), irregular- shaped detrital quartz (yellow arrow), and mica (green arrow). 816 B. LI ET AL.

13 18 Figure 5. Cross plots of d Ccarb versus (a) d Ocarb and (b) TOC. the OM was thermally overmature. However, Tocque, Behar, in the upper part of Wufeng Formation (i.e. late Katian Budzinski, and Lorant (2005) found that the d13Cvaluesof stage), but finally return to 2.8 ‰ in the top of residual hydrocarbons (C14þ) only shifted positively within 2‰ Wufeng Formation. 13 during kerogen pyrolysis suggesting thermal alteration might The d Corg chemostratigraphy of the Qiliao section also 13 d13 not severely alter the d Corg values of sedimentary rocks. exhibits several significant changes. The Corg values show 13 In addition, systematic correlation between d Corg and a gradual decreasing trend from 25.8 to 27.8 ‰ in the 13 TOC may be established through diagenesis (Kump et al., basal of Nanjinguan Formation. A negative d Corg shift 1999), but no systematic relationship was observed occurs in the early Tremadocian stage, although there is a 13 between d Corg and TOC in the Qiliao section (Figure 5b). gap in the lower part of Nanjinguan Formation (Figure 6). 13 13 These observations suggest that primary d Corg values at The d Corg values also show several cycles from the late the Qiliao section are largely preserved. Tremadocian to the early–middle Floian (Figure 6). In the upper part of Nanjinguan Formation and lower part of Fenxiang Formation, two cycles can be observed with an Carbon isotope chemostratigraphy overall increasing trend from 27.4 to 24 ‰. However, 13 The Ordovician of the Qiliao section exhibits a broad range the d Corg values show an overall apparent negative trend 13 13 ‰ in d Ccarb values (7.6 to þ2.1‰). The d Ccarb values from 28 to 24 through the early Floian, although remain at around 0 ‰ in the Nanjinguan Formation, with three small cycles occur within it (Figure 6). Above this the 13 one exception in the lower part (3.1 ‰), and then d Corg values smoothly rise to 26 ‰ during the mid- smoothly decrease to 4.1 ‰ in the lower part of Floian. There is a large gap from mid-Floian to early 13 Fenxiang Formation, although a gap occurs in the lower Dapingian, so the d Corg in this interval remains unclear. 13 part of Nanjinguan Formation. The d Ccarb values rapidly The Middle and Upper Ordovician show clear changes in 13 13 rise to 1.1 ‰ and tend to be relatively constant (1.6 d Corg values. A small positive shift of the d Corg values, ‰) in the Fengxiang, Honghuayuan and basal Dawan for- from 26 to 24.6 ‰, occurs in the Shihtzupu Formation mations, with two abnormally low values in Fengxiang (equivalent to ‘mid-Darriwilian’ in age) and then decreases mudstones (4.8 and 4.6 ‰). Although there is a large to 30.7 ‰ in the middle part of Wufeng Formation gap in the middle–lower parts of Dawan Formation, the (equivalent to ‘late Katian’ in age), with a small positive 13 d Ccarb values show a relatively low value in the upper reversal in the Pagoda Formation during the early Katian. At d13 Dawan Formation (2.9 ‰), and then decrease to 4.8 ‰ the top of the Wufeng Formation, the Corg values have a in the basal of Shihtzupu Formation. The first enriched smooth increasing trend from 30.7 to 30 ‰ (Figure 6). 13 d Ccarb values, from 4.8 to 0.2 ‰, can be observed in the lower part of Shihtzupu Formation (Figure 6), and then Carbon isotope excursions and stratigraphic to 2.7 ‰ in the top of Shihtzupu Formation. Within the correlations Pagoda Formation (equivalent to ‘Sandbian to middle 13 Katian’ in age), the d Ccarb values show several large (3 Inorganic carbon isotope excursions 13 ‰) fluctuations with a peak value (þ2.1 ‰) in the upper On global and local scales, the d Ccarb trends have similar 13 part. Then, the d Ccarb values rapidly decrease to 7.6 ‰ characteristics and are widely used for stratigraphic AUSTRALIAN JOURNAL OF EARTH SCIENCES 817

Figure 6. Carbon isotopic chemostratigraphy of the Qiliao section in South China and its correlation to that in the Honghuayuan section (Munnecke et al., 13 € 2011; Zhang et al., 2010) and the Huangnitang section (Munnecke et al., 2011), as well as the global d Ccarb curve (Bergstrom et al., 2009) and sea-level variation of the Yangtze Platform (Su, 2007).

€ 13 correlations (Ainsaar, Meidla, & Martma, 1999;Bergstrom, central Nevada, the d Ccarb values show a trend from 2 Young, & Schmitz, 2010; Buggisch, Keller, & Lehnert, 2003; to 0 ‰ during the middle Chazyan stage (Saltzman & Ludvigson et al., 2004;Maetal.,2015). At least five significant Young, 2005). In addition, a positive excursion of the 13 13 13 d Ccarb positive excursions and one d Ccarb negative excur- d Ccarb values from 1.7 to 0.3 ‰ was also found in the sion have been found in the Ordovician strata from more Darriwilian stage of the Argentine Precordillera (Buggisch than one palaeocontinent, including the mid-Darriwilian et al., 2003). 13 Inorganic Carbon Excursion (MDICE), the early Katian A second positive d Ccarb excursion from 3.3 to þ2.1 Guttenberg Inorganic Carbon Excursion (GICE) and Kope ‰ was observed in the Pagoda Formation, which might Inorganic Carbon Excursion, the mid-Katian Waynesville be correlated to the late Sandbian to Katian stage (Figure 13 Inorganic Carbon Excursion, the Hirnantian Inorganic Carbon 6). This d Ccarb excursion is readily identified in the Excursion (HICE), and the Early Floian Negative Isotopic Pagoda Formation from several sections in South China Carbon Excursion (EFNICE). These excursions, as an alternative (Bergstrom€ et al., 2009; Fan et al., 2015; Ma et al., 2015; tool to classical biostratigraphy, can be applied for intercon- Munnecke et al., 2011; Zhang et al., 2010) and has been tinental correlation (Ainsaar et al., 2010;Bergstrom€ et al., suggested to be GICE. This positive carbon isotopic event 2010). Among them, the MDICE, GICE and HICE have been on the global scale has been widely reported in North widely found in numerous sections in North America America (Edwards & Saltzman, 2016; Saltzman & Young, (Edwards & Saltzman, 2014, Edwards & Saltzman, 2016;Young 2005) and Europe (Ainsaar et al., 1999) but the number of 13 13 et al., 2008), South China (Ma et al., 2015;Munneckeetal., peak d Ccarb values and the d Ccarb shift in GICE interval 2011; Zhang et al., 2010), Siberia (Ainsaar et al., 2014), and are varied. Several authors found that the GICE curves have 13 Baltica (Wu et al., 2015). some d Ccarb fluctuations, for example, 2-peak or 3-peak 13 At Qiliao, the positive increase of 4.6 ‰ for d Ccarb shapes are widely reported (Fan et al., 2015; Goldman (i.e. 4.8 to 0.2 ‰), which was observed in the et al., 2007; Munnecke et al., 2011; Zhang et al., 2010). Shihtzupu Formation (Figure 6), might be correlated to the Among them, some minor peaks may reflect local perturba- € 13 middle Darriwilian stage, that is, the MDICE (Bergstrom tions. The d Ccarb shift in GICE interval also shows varia- et al., 2009; Munnecke et al., 2011; Zhang et al., 2010). The tions from þ1toþ4 ‰ (Fan et al., 2015; Munnecke et al., 13 MDICE has been widely recognised in South China and 2011; Zhang et al., 2010), but the d Ccarb peak values are elsewhere; for example, a first record of the MDICE in similar, with the range of þ2toþ3 ‰ (Bergstrom€ et al., 13 South China was reported by Schmitz et al. (2010). In the 2009). At the Qiliao section, similar d Ccarb characteristics Great Basin, the d13C values rise from 5to1 ‰ by are seen in the Pagoda Formation, probably pointing the middle Darriwilian (Edwards & Saltzman, 2014). In to GICE. 818 B. LI ET AL.

13 13 The largest positive d Ccarb excursion of up to 8 ‰, significantly lower amplitudes than the d Ccarb excursions (cf., known as the HICE, is one of the most significant and Munnecke et al., 2008; Young et al., 2010). In addition, a posi- 13 extensively recorded excursions in the Upper Ordovician tive d Corg excursion is reported in the Hirnantian (Fan et al., € € 13 (Schmitz & Bergstrom, 2007; Bergstrom et al., 2009) with 2009; Wang et al., 1997), and an increased d Ccarb trend from several studies suggesting the peak values of the HICE lie 7.6 to 2.8 ‰ was observed in the top of the Wufeng within the Hirnantian stage (Bergstrom€ et al., 2009; Fan, shales. A similar increasing trend and positive excursion in 13 Peng, & Melchin, 2009). In South China, the Kuanyinqiao d Corg values were not shown in our study (Figure 6). 13 member limestones at the top of the Wufeng Formation Munnecke et al. (2011) observed that the d Corg excursion 13 correspond to the Hirnantian stage (Fan et al., 2009; Wang did not precisely correspond to the d Ccarb excursion, and 13 et al., 1997). However, the limestone bed may be eroded the latter may precede the former. The d Corg values are or not be deposited in Qiliao at that time as the largest more easily altered by primary (local environment changes 13 positive d Ccarb excursion is not recorded in our study. and organic matter heterogeneity; Gao et al., 2016;Meyers, d13 Nevertheless, a trend of increasing Ccarb values from 1997) and secondary processes (e.g. thermal alteration and 7.6 to 2.8 ‰ was observed in the top of the hydrocarbon migration; Jiang et al., 2012;Munneckeetal., 13 13 Wufeng Formation. 2010)thand Ccarb values and d Corg values are commonly d13 In addition, a significant negative Ccarb excursion highly variable so their stratigraphic use is limited. from 1to4 ‰ was observed at the boundary between the Nanjinguan and Feixiang formations (Figure 6). This might be correlated to the Tremadocian/Floian boundary, Cause and consequence of carbon isotope excursions and could be defined as the EFNICE, which is widely The causes of the early Ordovician negative d13C excursion € reported, such as the general curve (Bergstrom et al., have received little attention. The EFNICE is widely 2009), composite curves from the Great Basin (Saltzman, recorded in many areas (Bergstrom€ et al., 2009; Munnecke Edwards, Adrain, & Westrop, 2015) and South China et al., 2011; Saltzman et al., 2015). During the d13 (Munnecke et al., 2011). In general, the Ccarb values dis- Tremadocian–Floian transition, the global ocean environ- ‰ play a smooth decreasing trend from 1to0 during ment may have changed. Marenco et al. (2016) showed that the Tremadocian to mid-Floian. Several single-point minima the shallow water carbonates of the late Tremadocian from in the early Tremadocian and early Floian may reveal small western Utah have higher Th/U values due to U sequestra- perturbations to local carbon pools. tion in anoxic black shales in deep water, indicating increased global ocean anoxia during that time. However, Organic carbon isotope excursions an overall decrease trend in Th/U ratios throughout the 13 13 Tremadocian to Floian interval implies an increasing global Compared with d Ccarb trends, Ordovician d Corg variations are not clearly related to global correlations (e.g. a seawater uranium concentration, that is, increasing global 13 ocean oxygenation. Saltzman et al. (2015) also found that compilation of d Corg data in Edwards & Saltzman, 2016). 13 several episodes of extinction in the late Tremadocian were However, some d Corg excursions, such as the GICE and HICE (Fan et al., 2009; Wang et al., 1997; Young et al., associated with the upwelling of deep anoxic water onto d13 2008), may be used for stratigraphic correlations. At Qiliao, the shelf. At the Qiliao section, the lower Corg values in 13 the Nanjinguan Formation may reflect a relatively reducing the first negative d Corg shift (from 25.8 to 28 ‰) can be clearly observed in the early Tremadocian. Edwards and environment, since the anoxic water within stratified 13 13 Saltzman (2016) also reported a decreasing a d Corg trend water column is commonly enriched in recycled C (Gao from 26.5 to 29.5 ‰ throughout the Tremadocian et al., 2016 ; Jiang et al., 2012). Thus, we infer that the 13 based on composite data. At least eight positive d Corg EFNICE may be caused by an anoxic event during the excursions are recorded in the Qiliao section. During the Tremadocian–Floian transition. 13 d13 Tremadocian to mid-Floian, the d Corg curves show several Compared with negative C excursions, the Ordovician fluctuations, including six small cycles (Figure 6) while the positive d13C excursions have received much attention, but 13 d Corg values exhibit an increase trend from 27 to 25 their causes also remain unclear. Several potential explana- ‰ during the late Tremadocian and a decline trend from tions have been proposed, such as sea-level change, global 24 to 27 ‰ during the early to mid-Floian, with the cooling, and enhanced burial of OM (Buggisch et al., 2003; peak value (24 ‰) in the early Floian (Figure 6). A larger Fan et al., 2015; Saltzman et al., 2015; Zhang et al., 2010) positive excursion (8 ‰) in the early to mid-Floian in with some authors suggesting that sea-level variations are South China was also presented by Zhang et al. (2010). the major driving force for the positive excursions Although both curves have different structures, a decreas- (Buggisch et al., 2003; Fanton & Holmden, 2007; Kump & 13 ing trend of d Corg values occurs during the early to mid- Arthur, 1999). Sea-level rise introduces a cooler and nutri- Floian. Two small positive excursions (1 ‰ for the both) ent-rich water mass onto the carbonate platform, when in occur in the middle Darriwilian and early Katian (Figure 6), conjunction with increased primary productivity and the which may be correlated to the MDICE and GICE events, burial of 13C-enriched OM, increases in d13C values pre- 13 respectively. However, the d Corg excursions show served in authigenic carbonate sediments. However, AUSTRALIAN JOURNAL OF EARTH SCIENCES 819

Bergstrom€ et al. (2010) reported several positive excursions subsequent enhanced burial of 12C-enriched OM, causes the in transgressive strata yet others occurred in regressive dissolved inorganic carbon (DIC) reservoir of seawater that is strata. A direct relationship between the sea level and car- enriched in 13C, and leads to positive carbon isotopic excur- bon isotopic excursions has not been illustrated on the sions. Several authors inferred that large positive carbon iso- Yangtze platform (Su, 2007; Figure 6). topic excursions may indicate major events of enhanced

It is widely reported that the drop of pCO2 may have burial of organic carbon during the Ordovician and a corre- contributed to a global cooling that began as early as the sponding drop in atmospheric pCO2 (Saltzman, 2005; Zhang 13 Katian (Saltzman & Young, 2005; Young et al., 2008). An et al., 2010). Therefore, at Qiliao, several small d Corg excur- important response to global cooling is a positive d13C sion in the late Tremadocian to early Floian stage, as well as 13 13 excursion in marine carbonates of the early Katian (Ainsaar several d Ccarb and d Corg excursions in the mid- et al., 1999; Ludvigson, Jacobson, Witzke, & Gonzalez, 1996; Darriwilian to Katian stages might also suggest multiple epi- Young, Saltzman, & Bergstrom,€ 2005, Young et al., 2008). sodes of enhanced organic carbon burial (Figure 6), prob-

Thus, this global cooling event might be reflected in the ably resulting in a further decline in atmospheric pCO2 and carbon isotope record, at least for the GICE. In addition, then global cooling. Middle–Upper Ordovician nodular lime- Fan et al. (2015) proposed burial of methane hydrates dur- stones (Figure 3e and 4e) represent a relatively deep, cold- ing the global cooling reduced the amount of 12C in the water depositional environment (Sweet & Bergstrom,€ 1984; marine carbonates, resulting in a positive d13C excursion. Zhan, Jin, Liu, Corcoran, Luan, & Wei, 2016) and suggest However, direct evidence is lacking. cooling from the early to late Ordovician. Oxygen isotope The most common interpretation for the Ordovician car- values of conodonts also suggest a decreased global tem- bon isotopic excursions is that they reflect the enhanced perature from 41 C in the early Tremadocian stage, to 29 C burial ratio of 13C-depleted OM on a global scale in the middle Darriwilian stage, and then to 23 C in the (Patzkowsky, Slupik, Arthur, Pancost, & Freeman, 1997; Hirnantian stage (Trotter et al., 2008). During the 18 Saltzman et al., 2015; Wang et al., 1997; Young et al., 2008; Early–Middle Ordovician, the increasing d Ocarb trends in Zhang et al., 2010). As 12C is preferentially assimilated via the Great Basin (USA) and Argentina are similar to d18O var- photosynthesis relative to 13C, higher primary productivity iations measured from well-preserved brachiopod calcite possibly driven by elevated availability of nutrients and from various sections worldwide (Figure 7; Buggisch et al.,

13 18 Figure 7. Comparison of the d Ccarb curve at the Qiliao section with the d O curves from Shingle Pass, Ibex area (Edwards & Saltzman, 2014), Argentine Precordillera (Buggisch et al., 2003), and calcitic brachiopods (Shields et al., 2003). 820 B. LI ET AL.

2003; Shields et al., 2003), also representing global progres- Disclosure Statement sive cooling. Previous paleogeographic reconstructions of the No potential conflict of interest was reported by the author(s). Middle Ordovician (ca 470 Ma) world showed that the Great Basin was located at the same latitude (10S) as the South China Block (Scotese & McKerrow, 1991), while the Argentine Funding Precordillera was situated at a higher latitude (40 S). Thus, This work was funded by the National Natural Science Foundation of South China may share similar paleotemperature conditions as China (91755211) and the Ministry of Science and Technology Project the Great Basin, and the general paleotemperature of the both of Sinopec (P14039). should be higher than that of the Argentine Precordillera, 18 which has higher d Ocarb values (Figure 7). In addition, an 13 References overall downward trend in d Corg values from 27 to 24 ‰ € ~ is also recorded in the early to mid-Floian stage (Figure 6), Ainsaar, L., Kaljo, D., Martma, T., Meidla, T., Mannik, P., Nolvak, J., & Tinn, O. (2010). Middle and Upper Ordovician carbon isotope che- which is clearly shown in the Honghuayuan section (Zhang mostratigraphy in Baltoscandia: A correlation standard and clues to et al., 2010). Such a declining trend may also imply increasing environmental history. Palaeogeography, Palaeoclimatology, carbon sequestration of OM, thus leading to gradual cooling. Palaeoecology, 294, 189–201. The enhanced burial ratio of organic carbon may begin at the Ainsaar, L., M€annik, P., Dronov, A. V., Izokh, O. P., Meilda, T., & Tinn, O. earlier Floian stage, and continued into the Katian or even (2014). Carbon isotope chemostratigraphy and conodonts of the Middle–Upper Ordovician succession in Tungus Basin, Siberian Hirnatian stage, finally leading to the Hirnantian glaciations. Craton. In R. B. Zhan & B. Huang (Eds.), Extended Summary of the IGCP Project 591 Workshop 2014 (pp. 1–4). Nanjing, China: Nanjing University Press. Ainsaar, L., Meidla, T., & Martma, T. (1999). Evidence for a widespread Conclusions carbon isotopic event associated with late Middle Ordovician sedimentological and faunal changes in Estonia. Geological Magazine, 13 13 Paired d Ccarb and d Corg compositions of the Qiliao sec- 136,49–62. tion in South China were studied, and the following con- Bergstrom,€ S. M., Chen, X., Gutierrez-Marco, J. C., & Dronov, A. (2009). clusions can be drawn. The new chronostratigraphic classification of the Ordovician System and its relations to major regional series and stages and to d13C chemostratigraphy. Lethaia, 42,97–107. d13 1. The Ccarb values from the Qiliao section can be corre- Bergstrom,€ S. M., Young, S., & Schmitz, B. (2010). Katian (Upper lated with local and global curves, which can be used for Ordovician) d13C chemostratigraphy and sequence stratigraphy in 13 the United States and Baltoscandia: a regional comparison. stratigraphic correlations. The d Corg values of organic- – poor carbonates are more easily altered by primary and Palaeogeography, Palaeoclimatology, Palaeoecology, 296, 217 234. Bottjer, D. J., Droser, M. L., Sheehan, P. M., & McGhee, G. R. (2001). The secondary processes, so their stratigraphic use is limited ecological architecture of major events in the Phanerozoic history 13 13 compared to d Ccarb values. However, some d Corg of marine invertebrate life: The ecological context of macroevolu- positive excursions in the Middle and Upper Ordovician tionary change. In W. D. Allmon & D. J. Bottjer (Eds.), Evolutionary – can still be used for correlation studies. 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