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Paired δ13Ccarb and δ13Corg records of theOrdovician 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(2018) Paired δ13Ccarb and δ13Corg 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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Paired d13Ccarb and d13Corg records of the Ordovician on the Yangtzeplatform, 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 Explorationand Production Research Institute, Sinopec, Beijing, China

ABSTRACTDuring the Ordovician, huge biological revolutions and environmental changes happened inEarth’s history, including the Great Ordovician Biodiversification Event, global cooling and so on,but the cause of these events remains controversial. Herein, we conducted a paired carbon iso-topic analysis of carbonate (d13Ccarb) and organic matter (d13Corg) through the Ordovician in theQiliao section on the Yangtze platform of South China. Our results showed that the d13Ccarb trendof the Qiliao section can be correlated with local and global curves. The d13Corg trend seems isless clear than the d13Ccarb trend for stratigraphic correlations, but some d13Corg positive excursionsin the Middle and Upper Ordovician may be used for correlation studies. These carbon isotopicrecords may have global significance rather than local significance, revealing several fluctuations tothe global carbon cycle during the Ordovician. Several known d13Ccarb and d13Corg negative andpositive excursions have been recognised in this study, including the early Floian NegativeInorganic Carbon (d13Ccarb) Excursion (EFNICE), as well as the early Floian Positive Organic Carbon(d13Ccarb) Excursion, the mid-Darriwilian Inorganic Carbon (d13Ccarb and d13Corg) Excursion (MDICE),and the early Katian Guttenberg Inorganic Carbon (d13Ccarb and d13Corg) Excursion (GICE). Thesepositive excursions and a smooth decline trend of d13Corg values during the early to mid-Floianmay imply multiple episodes of enhanced organic carbon burial that began at the early Floianstage, probably resulting in further decline in atmospheric pCO2 and then global cooling.

ARTICLE HISTORYReceived 17 October 2017Accepted 6 June 2018

KEYWORDScarbon isotope stratigraphy;Ordovician; positiveexcursion; global cooling;Yangtze platform

Introduction

During the Ordovician, significant changes in the Earth’s eco-systems (Harper, Zhan, & Jin, 2015; Marenco, Martin, Marenco,& Barber, 2016; Munnecke, Calner, Harper, & Servais, 2010;Sepkoski, 1981), including a long-term biodiversification and ashort-term mass extinction at the end of Ordovician, havebeen linked to global cooling (Trotter, Williams, Barnes,L�ecuyer, & Nicoll, 2008). Previous studies (Harper et al., 2015;Servais et al., 2009; Webby, Paris, Droser, & Percival, 2004)revealed that the Great Ordovician Biodiversification Event(GOBE) initiated at the mid-Ordovician, ca 470Ma, and pos-sibly lasted into the Late Ordovician. By the end ofOrdovician, the number and species of most faunal groupshad risen to more than triple of those in the Early Ordovician(Harper et al., 2015; Webby et al., 2004). The GOBE endedwith sudden and catastrophic extinctions at the terminalOrdovician, ca 444Ma (Gradstein, Ogg, & Smith, 2004), whichwas probably associated with rapid ice sheet growth overpolar landmasses (Brenchley et al., 1994; Wang et al., 1987).

Compared with the widely studied Cambrian explosion,the GOBE has received less attention (Bottjer, Droser,Sheehan, & McGhee, 2001) and its cause remains

controversial (Marenco et al., 2016; Servais et al., 2009;Wang, Chatterton, & Wang, 1997). Previous studies sug-gested that the GOBE may be controlled by not only theintrinsic factors (essentially biological) but also the extrinsicfactors (essentially environmental) (Harper et al., 2015;Zhan, Jin, & Liu, 2013). Intrinsic factors include the competi-tion, predation and plankton revolution. More notably,extrinsic factors have been widely studied, including ero-sion, high sea levels, global cooling, oxygenation, tectonicand magmatic activity, and volcanicity (Harper et al., 2015;Marenco et al., 2016; Trotter et al., 2008; Zhang, Shen, &Algeo, 2010). These environmental changes provide notonly suitable water column conditions (e.g. oxygen con-tents, temperatures) but also inorganic nutrients for thegrowth and metabolism of progressively larger animals.Among them, climatic cooling may play an important rolein triggering the GOBE (Marenco et al., 2016; Trotter et al.,2008; Zhang et al., 2010). During the Early–MiddleOrdovician, the atmospheric pCO2 levels were estimated tobe 14 to 18 times that of the modern atmospheric level(Herrmann, Patkowsky, & Pollard, 2003). That is, the GOBEmay have occurred under super greenhouse conditions(Gibbs, Barron, & Kump, 1997). Since atmospheric CO2 is

CONTACT B.-Y. Li liboyuancup@163.com State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China� 2018 Geological Society of Australia

https://doi.org/10.1080/08120099.2018.1487468VOL. 65, NO. 6, 809–822AUSTRALIAN JOURNAL OF EARTH SCIENCES, 2018

the most important greenhouse gas in the atmosphere, itsvariation could be an indicator for climate change at vari-ous time scales. It is generally accepted that atmosphericpCO2 is controlled by the carbon cycle between the oceanand atmosphere system (Edwards & Saltzman, 2016; Royer,2006; Wang et al., 1997). Thus, the carbon isotopic compo-sitions of carbonate and organic matter (OM) can bepotentially used for recording any changes in the globalcarbon cycle linked to atmosphere pCO2 changes (Edwards& Saltzman, 2016; Hayes, Strauss, & Kaufman, 1999; Pope &Steffen, 2003; Young, Saltzman, Bergstr€om, Leslie, &Chen, 2008).

The Ordovician d13C trends in carbonates (d13Ccarb) havebeen extensively studied, and the corresponding d13Cexcursions have been extensively recorded in numerousareas, such as North America (Edwards & Saltzman, 2014,Edwards & Saltzman, 2016; Young et al., 2008), South China(Ma, Wang, Zhang, Song, & Fang, 2015; Munnecke, Zhang,Liu, & Cheng, 2011; Zhang et al., 2010), Baltoscandia(Ainsaar et al., 2010), and Siberia (Ainsaar et al., 2014).These have been used for global and local chemostrati-graphic correlations (Bergstr€om, Chen, Guti�errez-Marco, &Dronov, 2009; Wang et al., 1997; Zhang et al., 2010). Unlikethe widely studied d13Ccarb trends, the d13Corg trend ofOrdovician carbonate has not been well studied. Moreover,most studies of paired d13Ccarb and d13Corg have focusedon narrow time intervals (generally the Late Ordovician orOrdovician–Silurian transition period). The carbon isotopetrends of the entire Ordovician have been reported by onlya few authors (Edwards & Saltzman, 2016; Hayes et al.,1999), and such trends have not yet been reported inSouth China. Therefore, the purpose of this paper is toreport a paired carbon isotopic record of carbonate(d13Ccarb) and organic matter (d13Corg) from the Qiliao sec-tion on the Yangtze platform throughout the Ordovician.

Furthermore, these records may provide evidence for glo-bal chemostratigraphic correlations and add new insightsinto the causes of global cooling.

Geological setting and stratigraphy

Regional geology

During the Ordovician, the South China Block was mainlycomposed of three regions: the Yangtze platform, Jiangnanslope, and Zhujiang basin (Munnecke et al., 2011). Thesethree regions had the characteristics of parallel distributionand southeast–northwest extension. The sedimentary faciesgradually changed from the shallow-water carbonates onthe platform in the northwest to the deep-water shales inthe basin in the southeast (Figure 1). During the LateOrdovician, the South China Block had separated from theGondwana supercontinent, which was situated at a paleo-latitude of about 20�S (Chen, Rong, Li, & Boucot, 2004).

The Yangtze platform, which was submerged by a wideepeiric sea during most of the Ordovician, has a continuoussuccession of carbonates and mudstones with abundantfossils (Chen et al., 2004; Yang, Zhang, Chen, Chen, & Wei,1975). Rich benthic shelly faunas, such as trilobites, bra-chiopods, and echinoderms, have been found in thissequence (Fan et al., 2015; Ma et al., 2015). High resolutionbiostratigraphy of the Ordovician has also been well estab-lished using the distribution of graptolites, conodonts, andsome other shelly faunas (Chen, Zhang, & Fan, 2006). Untilnow, the studies on the lithostratigraphy, biostratigraphy,and depositional environments of the Ordovician havebeen carried out in many sections of South China, such asthe Wangjiawan, Honghuayuan, and Huangnitang sections(Ma et al., 2015; Munnecke et al., 2011; Schmitz, Bergstr€om,& 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 Munneckeet al., 2011).

810 B. LI ET AL.

et al., 2010), which lay the foundation for the calibration ofstratigraphic correlations and any geological eventsoccurred in the Ordovician.

The Shizhu area is situated in the centre of Yangtzeplatform. We collected the samples from the Qiliao section,which is situated at about 20 km southeast of ShizhuCounty to the east of Chongqing City (Figure 1). The Qiliaosection is a near complete succession from the upperCambrian to lower Silurian, and the entire Ordovician isbasically preserved. It is continuous in stratigraphy and rich

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

Stratigraphy

In the Qiliao section, the Lower Ordovician is conformablewith the upper Cambrian Maotian Formation dolomites,and the Upper Ordovician is conformable with the lowerSilurian Longmaxi Formation black shales (Figure 2). Thetotal thickness of the Ordovician is 463.8 m. The Ordoviciancan be subdivided, from bottom to top, into theNanjinguan, Fenxiang, Honghuayuan, Dawan, Shihtzupu,Pagoda, Linhsiang, and Wufeng formations. The LowerOrdovician includes the Nanjinguan, Fenxiang,Honghuayuan, and lower Dawan formations, the MiddleOrdovician consists of the upper Dawan and Shihtzupu for-mations, and the Upper Ordovician covers Pagoda,Linhsiang, and Wufeng formations. The Qiliao section ispaleogeographically located on the same carbonate plat-form 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 pro-vide lithostratigraphy, biostratigraphy, and chronology datafor improving the stratigraphic division and correlation. Theeight formations at the Qiliao section, from oldest toyoungest are:

1. The Nanjinguan Formation is 97.37 m thick and isequivalent to ‘Tremadocian’ in age based on the firstappearance datum (FAD) of graptolite R. taojiangensis inthe bottom (Wang et al., 1987). The lithology is com-posed of grey to dark-grey, thin- to medium-beddedmicrite, 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) (Yang et al., 1975).

2. The Fenxiang Formation is 100.47 m thick, and theconodont O. communis in its basal parts marks theonset of the Floian (Wang et al., 1987; Wang, Chen,Wang, & Li, 2004). The lithology is mainly composed ofmedium- to thick-bedded bioclastic limestones, withsome yellowish green mudstone interlayers (Figure3b). The bioclastic limestones and mudstones containtrilobites (e.g. Psilocephalina, P. cf. lubrica, Tungtzuella,Psilocephalinalubrica), brachiopods (e.g. Nanorthis,Oligorthis) and cephalopods (Hopeioceras).

3. The Honghuayuan Formation mainly consists of darkgrey thick-bedded limestones with the thickness of40.48 m, and the underlying and overlying strata aremudstones, probably belonging to ‘middle Floian’ inage. The Honghuayuan limestones contain numerousbioclasts, including trilobites (e.g. Psilocephalina,Chenkouella), gastropods (Raphistoma, Morchisonia)and cephalopods (Sinoceras) (Figure 3c).

Figure 2. Stratigraphic column and major fossils of the Ordovician at theQiliao section on the Yangtze platform. The geological timescale is from theChronostratChart of Cohen, Finney, Gibbard, and Fan (2014).

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4. The Dawan Formation is 166.64 m thick and can be sub-divided in to the lower and upper parts according to alimestone member situated at �100 m above the bot-tom of this formation (Zhang et al., 2010). The lower partis mainly composed of yellowish green mudstones, whilethe upper part consists of the basal nodular limestones

and greyish-green to dark grey mudstones (Figure 3d).These two parts can be separated by the FAD of theconodont B. triangularis indicating the Floian–Dapingianboundary (Wang et al., 1987, Wang et al., 2004; Yanget al., 1975). The graptolite U. austrodentatus occurs inthe top of the Dawan mudstones suggesting the ‘early

Figure 3. Outcrop photographs of the Qiliao section. (a) Medium- to thick-bedded bioclastic limestone interbedded with thin-bedded mudstone of theNanjingguan 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.

812 B. LI ET AL.

Darriwilian stage’ (Wang et al., 1987, Wang et al., 2004).The Dawan mudstones are enriched with fossils. Amongthem, brachiopods are the most abundant fossil group,including Sinorthis, Yangtzeella, Taphrorthis, Taihungshania(Yang et al., 1975).

5. The Shihtzupu Formation is 18.67 m thick and isequivalent to ‘Darriwilian’ in age based on the FAD ofgraptolite Nemagraptus gracilis in the upper parts(Wang et al., 1987, Wang et al., 2004). The lithologyconsists of medium- to thick-bedded nodular lime-stones. Brachiopods (e.g. Tetradontella) and trilobites(e.g. Nileus) are also found in this formation.

6. The Pagoda Formation is 18.49 m thick and is equivalentto ‘Sandbian to early Katian’ in age based on the FAD ofthe trilobite Nankinolithus in the basal Linhsiang marls(Wang et al., 1987; Yang et al., 1975; Zhang et al., 2010).The lithology is composed of medium- to thick-beddednodular limestones interbedded with grey mudstones(Figure 3e). They also contain abundant fossils, includingtrilobites (e.g. Nileus, Paracerausus, Remopleurides) andcephalopods (Sinoceras, Michelinoceras) (Yang et al., 1975).

7. The Linhsiang Formation is 5.84 m thick and is equiva-lent to ‘middle Katian’ in age based on the FAD of thegraptolite D. complanatus in the basal black shales(Yang et al., 1975; Wang et al., 1987; Zhang et al.,2010). This formation mainly consists of nodular lime-stones. Trilobites (e.g. Corrugatagnostus) and cephalo-pods (e.g. Trocholites) are found in the limestones.

8. The Wufeng Formation is 15.86 m thick and is equiva-lent to ‘late Katian to Hirnatian’ in age based on theFAD of the graptolite G. persculptus in the basalSilurian Longmaxi black shales (Wang et al., 1987;Yang et al., 1975; Zhang et al., 2010). The lithologymainly consists of black siliceous shales (Figure 3f).

Samples and analytical methods

Samples

Forty-eight samples, which were collected from the Qiliaosection (Figure 1), include carbonates, mudstones, andshales that were used for petrological and geochemicalanalyses. Before the analyses, the samples were washedthen cut to remove weathered parts. To minimise theimpact of diagenetic veins on geochemical analysis, thesamples were broken into small chips �5� 5mm in size,and then rock fragments without carbonate veins wereselected. Each sample was crushed to 200 mesh powder inan agate mortar.

Analytical methods

A Zeiss Axioskop 40 A pol microscope was used for theoptical observation of the Ordovician thin sections at theLaboratory of Structural and Sedimentological ReservoirGeology, Sinopec.

After removing carbonates with 7% HCl, total organic car-bon (TOC) and total sulfur (TS) of studied samples were meas-ured by the total carbon and sulfur Leco CS-230 analyser atthe State Key Laboratory of Petroleum Resources andProspecting. The analytical precision was better than 10%.

Carbon isotope analyses of carbonates and organic mat-ter (OM) were measured by Finnigan MAT 253 isotopemass spectrometer at the CUP and the AnalyticalLaboratory of Beijing Research Institute of UraniumGeology, respectively. Carbonates were removed with 7%HCl, for carbon isotope analysis of OM. Carbon and oxygenisotopic values of the carbonates were measured by reac-tion with phosphoric acid widely reported by previousauthors (e.g. Zhang et al., 2010). Replicate analyses ofstudied samples and laboratory standards were conductedwith an analytical precision better than 0.1 ‰. These iso-topic results were reported in d notation in per mille (‰)relative to the standard VPDB. All geochemical results (TOC,

) are listed in Table 1.

Results and discussion

Petrological features

The lithologies of the Ordovician sediments at Qiliaomainly include limestone, mudstone and siliceous shale.The detailed lithological characteristics are illustrated inFigure 4.

LimestoneThe limestones are developed throughout the Ordovician,including dolomitic limestone (Figure 4a), oolitic limestone(Figure 4b), bioclastic limestone (Figure 4c), marl (Figure4d), and nodular limestone (Figure 4e). Among them, bio-clastic limestone is abundant and includes trilobites, bra-chiopods, and ostracodes (Figure 4c). Bioclasts are alsoabundant in the nodular limestone (Figure 4e) and includeostracodes, lamellibranches, and nautilus.

MudstoneThe mudstones are mainly developed in the Middle–LowerOrdovician sediments. Bioclasts are also abundant in themudstones (Figure 4f–g) but are difficult to identify.

Siliceous shaleThe siliceous shales developed within the top of theOrdovician sequence, that is, the Wufeng Formation. Theshales are rich in quartz and OM (Figure 4h). The roundedquartz may be intimately related with siliceous organisms,while the polygonal quartz may be ascribed to detrital inputs.

Evaluation of primary carbon isotopic values

Bulk carbonate in sediments has generally been regarded asa suitable material to estimate the d13C value of seawater in

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TS, d 18Corg, d13Ccarb, and d13 Ocarb

the absence of diagenetic carbonate veins (Edwards &Saltzman, 2016) but assessment of the other diageneticeffects on the primary d13Ccarb is required. In the absence ofdiagenetic carbonate veins, the use of d13Ccarb–d

18Ocarb crossplots and global correlation (Wu, Calner, Lehnert, Peterffy, &Joachimski, 2015) can be used to evaluate primary d13C val-ues. Primary d18Ocarb values are commonly altered by diagen-esis during burial and a lack of systematic correlationbetween d13Ccarb and d18Ocarb values in the studied sectionsuggests that the effect of secondary alteration on thed13Ccarb values of carbonates at Qiliao is minor (Figure 5a).

Previous studies (Gao et al., 2016; Jiang et al., 2012)showed that numerous factors can alter the d13C values ofOM, including carbon isotope fractionation during primaryand secondary production, admixture of terrestrial OM, detritalorganic carbon input, and post-depositional alteration via

diagenesis/metamorphism. The absence of terrestrial OM dur-ing the Ordovician, suggests the biologically mediated OMwas mostly formed through photoautotrophic and possiblechemoautotrophic pathways. Since the carbonates in ourstudy area were mainly deposited under shallow water andoxygenated conditions, the d13Corg records can be regardedas the signatures of primary producers in the surface water atthat time. In most of the studied samples, carbonates arechemical sedimentary rocks, and thus detrital organic carboninput should be negligible. With the increasing degree of ther-mal degradation of OM, preferential removal of 13C is kinetic-ally favoured, yielding 13C-enriched residual OM (Hayes,Kaplan, & Wedeking, 1983). Yang, Xie, Wang, Wang, and Liu(2012) measured the reflectance of solid bitumen in theWufeng shales at Qiliao, and the corresponding equivalent vit-rinite reflectance that ranged from 2.41 to 2.73%, shows that

Table 1. Basic information and geochemical data (TOC, TS, d13Corg, d13Ccarb, and d18Ocarb) of studied samples.

Sample Depth (m) Formation Lithology TOC (wt%) TS (wt%) d13Corg & VPDB d13Ccarb & VPDB d18Ocarb & VPDB

QL-04 5.4 Nanjinguan Limestone 0.04 0.13 ND �0.2 �8.6QL-05 6.3 Nanjinguan Limestone 0.08 0.52 �25.8 �0.4 �8.9QL-06 15.3 Nanjinguan Mudstone 0.13 0.74 �27.2 �0.1 �7.5QL-07 16.1 Nanjinguan Mudstone 0.2 0.24 �27.4 �1.1 �7.8QL-08 17.3 Nanjinguan Mudstone 0.19 0.34 �27.0 �0.9 �8.1QL-09 18.5 Nanjinguan Mudstone 0.18 0.59 �28.0 �3.1 �8.3QL-10 20.9 Nanjinguan Mudstone 0.18 0.26 �27.8 �0.4 �7.5QL-13 52.2 Nanjinguan Marl 0.14 0.29 �27.4 0.2 �9.9QL-14 54.1 Nanjinguan Marl 0.18 0.23 �27.0 0.1 �9.7QL-15 55.9 Nanjinguan Marl 0.14 0.24 �25.3 �0.3 �10.3QL-16 68.2 Nanjinguan Limestone 0.09 0.05 �27.5 �0.9 �8.5QL-18 88.6 Nanjinguan Limestone 0.08 0.13 ND �1.2 �11.1QL-19 94.6 Nanjinguan Limestone 0.03 0.07 �25.0 �2.8 �11.0QL-21 108.9 Fenxiang Limestone 0.05 0.05 �25.0 �3.6 �9.6QL-22 114.5 Fenxiang Mudstone 0.09 0.002 �26.7 �4.1 �11.1QL-23 126.4 Fenxiang Limestone 0.04 0.28 �24.0 �1.1 �9.7QL-29 150.0 Fenxiang Mudstone 0.16 0.02 �26.8 �1.8 �12.2QL-30 153.3 Fenxiang Mudstone 0.10 0.001 �27.1 �4.6 �11.4QL-31-01 162.1 Fenxiang Limestone 0.06 0.02 �25.8 �1.9 �10.1QL-32 184.1 Fenxiang Mudstone 0.14 0.001 �28.1 �4.8 �14.8QL-34 189.0 Fenxiang Mudstone 0.17 0.001 �28.1 �2.1 �11.2QL-37 208.2 Honghuayuan Limestone 0.11 0.60 �27.0 �1.6 �9.9QL-38 211.3 Honghuayuan Limestone 0.22 0.37 ND �1.9 �9.9QL-39-02 214.1 Honghuayuan Limestone 0.07 0.23 �26.7 �1.6 �10.2QL-40 216.8 Honghuayuan Limestone 0.04 0.09 ND �1.0 �9.3QL-41 219.3 Honghuayuan Limestone 0.06 0.07 �26.7 �1.7 �9.2QL-42 221.8 Honghuayuan Limestone 0.09 0.08 ND �1.7 �9.5QL-43 223.9 Honghuayuan Marl 0.08 0.17 �27.6 �1.6 �9.6QL-44 225.8 Honghuayuan Marl 0.11 0.11 �28.2 �1.5 �10.1QL-50 235.1 Honghuayuan Limestone 0.08 0.08 ND �1.2 �9.7QL-51 237.8 Honghuayuan Limestone 0.05 0.13 ND �1.2 �10.1QL-52 245.2 Dawan Limestone 0.06 0.06 �26.8 �1.2 �9.6QL-53 279.4 Dawan Mudstone 0.11 0.07 �26.2 NA NAQL-57 371.2 Dawan Mudstone 0.10 0.001 �26.1 �2.9 �13.2QL-59 406.0 Shihtzupu Mudstone 0.12 0.46 �25.6 �4.8 �9.4QL-60 409.4 Shihtzupu Limestone 0.05 0.79 �24.6 �0.2 �10.7QL-64 423.2 Shihtzupu Mudstone 0.18 0.18 �25.7 �2.7 �9.8QL-65-01 426.4 Pagoda Limestone 0.07 0.22 ND 0.2 �10.0QL-65-02 428.8 Pagoda Mudstone 0.15 0.49 ND �3.3 �9.4QL-66 433.2 Pagoda Limestone 0.09 0.007 �27.3 0.7 �10.3QL-68 436.9 Pagoda Limestone 0.04 0.59 �26.3 2.1 �10.5QL-69 440.7 Pagoda Limestone 0.11 0.15 ND 0.9 �10.7QL-72 451.7 Wufeng Shale 3.47 0.19 �30.6 �2.5 �14.1QL-73 452.8 Wufeng Shale 5.62 0.04 �30.6 �2.5 �13.6QL-75 445.0 Wufeng Shale 5.47 0.03 �30.7 �1.6 �15.1QL-76 456.3 Wufeng Shale 3.87 0.006 �30.3 �4.3 �16.2QL-77 457.5 Wufeng Shale 3.58 0.37 �30.2 �7.6 �15.0QL-79 459.3 Wufeng Shale 2.89 0.005 �30.0 �2.8 �13.9

ND: not detected; NA: not available.

814 B. LI ET AL.

Figure 4. Optical photographs showing lithological characteristics of Ordovician sediments at the Qiliao section. (Notes: all photographs were imaged underthe 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 thatprobably 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).

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the OM was thermally overmature. However, Tocqu�e, Behar,Budzinski, and Lorant (2005) found that the d13C values ofresidual hydrocarbons (C14þ) only shifted positively within 2‰during kerogen pyrolysis suggesting thermal alteration mightnot severely alter the d13Corg values of sedimentary rocks.

In addition, systematic correlation between d13Corg andTOC may be established through diagenesis (Kump et al.,1999), but no systematic relationship was observedbetween d13Corg and TOC in the Qiliao section (Figure 5b).These observations suggest that primary d13Corg values atthe Qiliao section are largely preserved.

Carbon isotope chemostratigraphy

The Ordovician of the Qiliao section exhibits a broad rangein d13Ccarb values (�7.6 to þ2.1‰). The d13Ccarb valuesremain at around 0 ‰ in the Nanjinguan Formation, withone exception in the lower part (�3.1 ‰), and thensmoothly decrease to �4.1 ‰ in the lower part ofFenxiang Formation, although a gap occurs in the lowerpart of Nanjinguan Formation. The d13Ccarb values rapidlyrise to �1.1 ‰ and tend to be relatively constant (�1.6‰) in the Fengxiang, Honghuayuan and basal Dawan for-mations, with two abnormally low values in Fengxiangmudstones (�4.8 and �4.6 ‰). Although there is a largegap in the middle–lower parts of Dawan Formation, thed13Ccarb values show a relatively low value in the upperDawan Formation (�2.9 ‰), and then decrease to �4.8 ‰in the basal of Shihtzupu Formation. The first enrichedd13Ccarb values, from �4.8 to �0.2 ‰, can be observed inthe lower part of Shihtzupu Formation (Figure 6), and thento �2.7 ‰ in the top of Shihtzupu Formation. Within thePagoda Formation (equivalent to ‘Sandbian to middleKatian’ in age), the d13Ccarb values show several large (�3‰) fluctuations with a peak value (þ2.1 ‰) in the upperpart. Then, the d13Ccarb values rapidly decrease to �7.6 ‰

in the upper part of Wufeng Formation (i.e. late Katianstage), but finally return to �2.8 ‰ in the top ofWufeng Formation.

The d13Corg chemostratigraphy of the Qiliao section alsoexhibits several significant changes. The d13Corg values showa gradual decreasing trend from �25.8 to �27.8 ‰ in thebasal of Nanjinguan Formation. A negative d13Corg shiftoccurs in the early Tremadocian stage, although there is agap in the lower part of Nanjinguan Formation (Figure 6).The d13Corg values also show several cycles from the lateTremadocian to the early–middle Floian (Figure 6). In theupper part of Nanjinguan Formation and lower part ofFenxiang Formation, two cycles can be observed with anoverall increasing trend from �27.4 to �24 ‰. However,the d13Corg values show an overall apparent negative trendfrom �28 to �24 ‰ through the early Floian, althoughthree small cycles occur within it (Figure 6). Above this thed13Corg values smoothly rise to �26 ‰ during the mid-Floian. There is a large gap from mid-Floian to earlyDapingian, so the d13Corg in this interval remains unclear.The Middle and Upper Ordovician show clear changes ind13Corg values. A small positive shift of the d13Corg values,from �26 to �24.6 ‰, occurs in the Shihtzupu Formation(equivalent to ‘mid-Darriwilian’ in age) and then decreasesto �30.7 ‰ in the middle part of Wufeng Formation(equivalent to ‘late Katian’ in age), with a small positivereversal in the Pagoda Formation during the early Katian. Atthe top of the Wufeng Formation, the d13Corg values have asmooth increasing trend from �30.7 to �30 ‰ (Figure 6).

Carbon isotope excursions and stratigraphiccorrelations

Inorganic carbon isotope excursionsOn global and local scales, the d13Ccarb trends have similarcharacteristics and are widely used for stratigraphic

Figure 5. Cross plots of d13Ccarb versus (a) d18Ocarb and (b) TOC.

816 B. LI ET AL.

correlations (Ainsaar, Meidla, & Martma, 1999; Bergstr€om,Young, & Schmitz, 2010; Buggisch, Keller, & Lehnert, 2003;Ludvigson et al., 2004; Ma et al., 2015). At least five significantd13Ccarb positive excursions and one d13Ccarb negative excur-sion have been found in the Ordovician strata from morethan one palaeocontinent, including the mid-DarriwilianInorganic Carbon Excursion (MDICE), the early KatianGuttenberg Inorganic Carbon Excursion (GICE) and KopeInorganic Carbon Excursion, the mid-Katian WaynesvilleInorganic Carbon Excursion, the Hirnantian Inorganic CarbonExcursion (HICE), and the Early Floian Negative IsotopicCarbon Excursion (EFNICE). These excursions, as an alternativetool to classical biostratigraphy, can be applied for intercon-tinental correlation (Ainsaar et al., 2010; Bergstr€om et al.,2010). Among them, the MDICE, GICE and HICE have beenwidely found in numerous sections in North America(Edwards & Saltzman, 2014, Edwards & Saltzman, 2016; Younget al., 2008), South China (Ma et al., 2015; Munnecke et al.,2011; Zhang et al., 2010), Siberia (Ainsaar et al., 2014), andBaltica (Wu et al., 2015).

At Qiliao, the positive increase of �4.6 ‰ for d13Ccarb(i.e. �4.8 to �0.2 ‰), which was observed in theShihtzupu Formation (Figure 6), might be correlated to themiddle Darriwilian stage, that is, the MDICE (Bergstr€omet al., 2009; Munnecke et al., 2011; Zhang et al., 2010). TheMDICE has been widely recognised in South China andelsewhere; for example, a first record of the MDICE inSouth China was reported by Schmitz et al. (2010). In theGreat Basin, the d13C values rise from��5 to �1 ‰ bythe middle Darriwilian (Edwards & Saltzman, 2014). In

central Nevada, the d13Ccarb values show a trend from �2to 0 ‰ during the middle Chazyan stage (Saltzman &Young, 2005). In addition, a positive excursion of thed13Ccarb values from �1.7 to �0.3 ‰ was also found in theDarriwilian stage of the Argentine Precordillera (Buggischet al., 2003).

A second positive d13Ccarb excursion from �3.3 to þ2.1‰ was observed in the Pagoda Formation, which mightbe correlated to the late Sandbian to Katian stage (Figure6). This d13Ccarb excursion is readily identified in thePagoda Formation from several sections in South China(Bergstr€om et al., 2009; Fan et al., 2015; Ma et al., 2015;Munnecke et al., 2011; Zhang et al., 2010) and has beensuggested to be GICE. This positive carbon isotopic eventon the global scale has been widely reported in NorthAmerica (Edwards & Saltzman, 2016; Saltzman & Young,2005) and Europe (Ainsaar et al., 1999) but the number ofpeak d13Ccarb values and the d13Ccarb shift in GICE intervalare varied. Several authors found that the GICE curves havesome d13Ccarb fluctuations, for example, 2-peak or 3-peakshapes are widely reported (Fan et al., 2015; Goldmanet al., 2007; Munnecke et al., 2011; Zhang et al., 2010).Among them, some minor peaks may reflect local perturba-tions. The d13Ccarb shift in GICE interval also shows varia-tions from þ1 to þ4 ‰ (Fan et al., 2015; Munnecke et al.,2011; Zhang et al., 2010), but the d13Ccarb peak values aresimilar, with the range of þ2 to þ3 ‰ (Bergstr€om et al.,2009). At the Qiliao section, similar d13Ccarb characteristicsare seen in the Pagoda Formation, probably pointingto GICE.

Figure 6. Carbon isotopic chemostratigraphy of the Qiliao section in South China and its correlation to that in the Honghuayuan section (Munnecke et al.,2011; Zhang et al., 2010) and the Huangnitang section (Munnecke et al., 2011), as well as the global d13Ccarb curve (Bergstr€om et al., 2009) and sea-level vari-ation of the Yangtze Platform (Su, 2007).

AUSTRALIAN JOURNAL OF EARTH SCIENCES 817

The largest positive d13Ccarb excursion of up to 8 ‰,known as the HICE, is one of the most significant andextensively recorded excursions in the Upper Ordovician(Schmitz & Bergstr€om, 2007; Bergstr€om et al., 2009) withseveral studies suggesting the peak values of the HICE liewithin the Hirnantian stage (Bergstr€om et al., 2009; Fan,Peng, & Melchin, 2009). In South China, the Kuanyinqiaomember limestones at the top of the Wufeng Formationcorrespond to the Hirnantian stage (Fan et al., 2009; Wanget al., 1997). However, the limestone bed may be erodedor not be deposited in Qiliao at that time as the largestpositive d13Ccarb excursion is not recorded in our study.Nevertheless, a trend of increasing d13Ccarb values from�7.6 to �2.8 ‰ was observed in the top of theWufeng Formation.

In addition, a significant negative d13Ccarb excursionfrom �1 to �4 ‰ was observed at the boundary betweenthe Nanjinguan and Feixiang formations (Figure 6). Thismight be correlated to the Tremadocian/Floian boundary,and could be defined as the EFNICE, which is widelyreported, such as the general curve (Bergstr€om et al.,2009), composite curves from the Great Basin (Saltzman,Edwards, Adrain, & Westrop, 2015) and South China(Munnecke et al., 2011). In general, the d13Ccarb values dis-play a smooth decreasing trend from �1 to 0 ‰ duringthe Tremadocian to mid-Floian. Several single-point minimain the early Tremadocian and early Floian may reveal smallperturbations to local carbon pools.

Organic carbon isotope excursionsCompared with d13Ccarb trends, Ordovician d13Corg varia-tions are not clearly related to global correlations (e.g. acompilation of d13Corg data in Edwards & Saltzman, 2016).However, some d13Corg excursions, such as the GICE andHICE (Fan et al., 2009; Wang et al., 1997; Young et al.,2008), may be used for stratigraphic correlations. At Qiliao,the first negative d13Corg shift (from �25.8 to �28 ‰) canbe clearly observed in the early Tremadocian. Edwards andSaltzman (2016) also reported a decreasing a d13Corg trendfrom �26.5 to �29.5 ‰ throughout the Tremadocianbased on composite data. At least eight positive d13Corgexcursions are recorded in the Qiliao section. During theTremadocian to mid-Floian, the d13Corg curves show severalfluctuations, including six small cycles (Figure 6) while thed13Corg values exhibit an increase trend from �27 to �25‰ during the late Tremadocian and a decline trend from�24 to �27 ‰ during the early to mid-Floian, with thepeak value (�24 ‰) in the early Floian (Figure 6). A largerpositive excursion (8 ‰) in the early to mid-Floian inSouth China was also presented by Zhang et al. (2010).Although both curves have different structures, a decreas-ing trend of d13Corg values occurs during the early to mid-Floian. Two small positive excursions (1 ‰ for the both)occur in the middle Darriwilian and early Katian (Figure 6),which may be correlated to the MDICE and GICE events,respectively. However, the d13Corg excursions show

significantly lower amplitudes than the d13Ccarb excursions (cf.,Munnecke et al., 2008; Young et al., 2010). In addition, a posi-tive d13Corg excursion is reported in the Hirnantian (Fan et al.,2009; Wang et al., 1997), and an increased d13Ccarb trend from�7.6 to �2.8 ‰ was observed in the top of the Wufengshales. A similar increasing trend and positive excursion ind13Corg values were not shown in our study (Figure 6).Munnecke et al. (2011) observed that the d13Corg excursiondid not precisely correspond to the d13Ccarb excursion, andthe latter may precede the former. The d13Corg values aremore easily altered by primary (local environment changesand organic matter heterogeneity; Gao et al., 2016; Meyers,1997) and secondary processes (e.g. thermal alteration andhydrocarbon migration; Jiang et al., 2012; Munnecke et al.,2010) than d13Ccarb values and d13Corg values are commonlyhighly variable so their stratigraphic use is limited.

Cause and consequence of carbon isotope excursions

The causes of the early Ordovician negative d13C excursionhave received little attention. The EFNICE is widelyrecorded in many areas (Bergstr€om et al., 2009; Munneckeet al., 2011; Saltzman et al., 2015). During theTremadocian–Floian transition, the global ocean environ-

Compared with negative d13C excursions, the Ordovicianpositive d13C excursions have received much attention, buttheir causes also remain unclear. Several potential explana-tions have been proposed, such as sea-level change, globalcooling, and enhanced burial of OM (Buggisch et al., 2003;Fan et al., 2015; Saltzman et al., 2015; Zhang et al., 2010)with some authors suggesting that sea-level variations arethe major driving force for the positive excursions(Buggisch et al., 2003; Fanton & Holmden, 2007; Kump &Arthur, 1999). Sea-level rise introduces a cooler and nutri-ent-rich water mass onto the carbonate platform, when inconjunction with increased primary productivity and theburial of 13C-enriched OM, increases in d13C values pre-served in authigenic carbonate sediments. However,

818 B. LI ET AL.

ment may have changed. Marenco et al. ( ) showed thatthe shallow water carbonates of the late Tremadocian fromwestern Utah have higher Th/U values due to U sequestra-tion in anoxic black shales in deep water, indicatingincreased global ocean anoxia during that time. However,an overall decrease trend in Th/U ratios throughout theTremadocian to Floian interval implies an increasing globalseawater uranium concentration, that is, increasing globalocean oxygenation. Saltzman et al. ( ) also found thatseveral episodes of extinction in the late Tremadocian wereassociated with the upwelling of deep anoxic water ontothe shelf. At the Qiliao section, the lower d13Corg values inthe Nanjinguan Formation may reflect a relatively reducingenvironment, since the anoxic water within stratifiedwater column is commonly enriched in recycled 13C (Gaoet al., ; Jiang et al., ). Thus, we infer that theEFNICE may be caused by an anoxic event during theTremadocian–Floian transition.

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Bergstr€om et al. (2010) reported several positive excursionsin transgressive strata yet others occurred in regressivestrata. A direct relationship between the sea level and car-bon isotopic excursions has not been illustrated on theYangtze platform (Su, 2007; Figure 6).

It is widely reported that the drop of pCO2 may havecontributed to a global cooling that began as early as theKatian (Saltzman & Young, 2005; Young et al., 2008). Animportant response to global cooling is a positive d13Cexcursion in marine carbonates of the early Katian (Ainsaaret al., 1999; Ludvigson, Jacobson, Witzke, & Gonz�alez, 1996;Young, Saltzman, & Bergstr€om, 2005, Young et al., 2008).Thus, this global cooling event might be reflected in thecarbon isotope record, at least for the GICE. In addition,Fan et al. (2015) proposed burial of methane hydrates dur-ing the global cooling reduced the amount of 12C in themarine carbonates, resulting in a positive d13C excursion.However, direct evidence is lacking.

The most common interpretation for the Ordovician car-bon isotopic excursions is that they reflect the enhancedburial ratio of 13C-depleted OM on a global scale(Patzkowsky, Slupik, Arthur, Pancost, & Freeman, 1997;Saltzman et al., 2015; Wang et al., 1997; Young et al., 2008;Zhang et al., 2010). As 12C is preferentially assimilated viaphotosynthesis relative to 13C, higher primary productivitypossibly driven by elevated availability of nutrients and

subsequent enhanced burial of 12C-enriched OM, causes thedissolved inorganic carbon (DIC) reservoir of seawater that isenriched in 13C, and leads to positive carbon isotopic excur-sions. Several authors inferred that large positive carbon iso-topic excursions may indicate major events of enhancedburial of organic carbon during the Ordovician and a corre-sponding drop in atmospheric pCO2 (Saltzman, 2005; Zhanget al., 2010). Therefore, at Qiliao, several small d13Corg excur-sion in the late Tremadocian to early Floian stage, as well asseveral d13Ccarb and d13Corg excursions in the mid-Darriwilian to Katian stages might also suggest multiple epi-sodes of enhanced organic carbon burial (Figure 6), prob-ably resulting in a further decline in atmospheric pCO2 andthen global cooling. Middle–Upper Ordovician nodular lime-stones (Figure 3e and 4e) represent a relatively deep, cold-water depositional environment (Sweet & Bergstr€om, 1984;Zhan, Jin, Liu, Corcoran, Luan, & Wei, 2016) and suggestcooling from the early to late Ordovician. Oxygen isotopevalues of conodonts also suggest a decreased global tem-perature from 41 �C in the early Tremadocian stage, to 29 �Cin the middle Darriwilian stage, and then to 23 �C in theHirnantian stage (Trotter et al., 2008). During theEarly–Middle Ordovician, the increasing d18Ocarb trends inthe Great Basin (USA) and Argentina are similar to d18O var-iations measured from well-preserved brachiopod calcitefrom various sections worldwide (Figure 7; Buggisch et al.,

Figure 7. Comparison of the d13Ccarb curve at the Qiliao section with the d18O curves from Shingle Pass, Ibex area (Edwards & Saltzman, 2014), ArgentinePrecordillera (Buggisch et al., 2003), and calcitic brachiopods (Shields et al., 2003).

AUSTRALIAN JOURNAL OF EARTH SCIENCES 819

2003; Shields et al., 2003), also representing global progres-sive cooling. Previous paleogeographic reconstructions of theMiddle Ordovician (ca 470Ma) world showed that the GreatBasin was located at the same latitude (10�S) as the SouthChina Block (Scotese & McKerrow, 1991), while the ArgentinePrecordillera was situated at a higher latitude (40�S). Thus,South China may share similar paleotemperature conditions asthe Great Basin, and the general paleotemperature of the bothshould be higher than that of the Argentine Precordillera,which has higher d18Ocarb values (Figure 7). In addition, anoverall downward trend in d13Corg values from �27 to �24 ‰is also recorded in the early to mid-Floian stage (Figure 6),which is clearly shown in the Honghuayuan section (Zhanget al., 2010). Such a declining trend may also imply increasingcarbon sequestration of OM, thus leading to gradual cooling.The enhanced burial ratio of organic carbon may begin at theearlier Floian stage, and continued into the Katian or evenHirnatian stage, finally leading to the Hirnantian glaciations.

Conclusions

Paired d13Ccarb and d13Corg compositions of the Qiliao sec-tion in South China were studied, and the following con-clusions can be drawn.

1. The d13Ccarb values from the Qiliao section can be corre-lated with local and global curves, which can be used forstratigraphic correlations. The d13Corg values of organic-poor carbonates are more easily altered by primary andsecondary processes, so their stratigraphic use is limitedcompared to d13Ccarb values. However, some d13Corgpositive excursions in the Middle and Upper Ordoviciancan still be used for correlation studies. These carbon iso-topic records may reveal several fluctuations to globalcarbon cycle during the Ordovician.

2. Several positive and negative carbon isotope excur-sions have been recognised in this study, including atleast two d13Ccarb positive excursions (i.e. the MDICEand GICE), one d13Ccarb negative excursions (i.e.EFNICE), and two small d13Corg positive excursions (i.e.MDICE and GICE). The EFNICE might be caused by theanoxic event during the Tremadocian–Floian transition.These positive excursions and a smooth decline trendof d13Corg values during the early to mid-Floian mayimply multiple episodes of enhanced organic carbonburial that began at the early Floian stage, probablyresulting in further decline in atmospheric pCO2 andthen global cooling.

Acknowledgments

We are grateful to two anonymous reviewers for their constructivecomments that greatly improved the manuscript.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Funding

This work was funded by the National Natural Science Foundation ofChina (91755211) and the Ministry of Science and Technology Projectof Sinopec (P14039).

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