Journal of Earth Science, Vol. 29, No. 3, p. 479–491, June 2018 ISSN 1674-487X Printed in China https://doi.org/10.1007/s12583-017-0963-x

Carbon-Isotope Excursions Recorded in the System, South China: Implications for Mass Extinctions and Sea-Level Fluctuations

Jingxun Zuo *1, Shanchi Peng2, Yuping Qi2, Xuejian Zhu2, Gabriella Bagnoli3, Huaibin Fang1 1. Henan Institute of Geological Survey, Zhengzhou 450001, China 2. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China 3. Department of Earth Sciences, University of Pisa, Via S. Maria, 53, Pisa 56126, Italy Jingxun Zuo: https://orcid.org/0000-0002-5477-017X

ABSTRACT: Cambrian carbonates with abundant fossils of agnostoid deposited on the southern slope (Jiangnan slope belt) of the Yangtze Platform and in the Jiangnan deepwater basin are well exposed in the Wangcun Section of western , South China, and in the Duibian A Section of western Zhejiang, southeastern China, respectively. To better understand the response of carbon- isotope excursions to depositional environment changes, mass extinctions and eustatic events, we col- lected 530 carbonate samples in fresh roadcut exposures of the two measured sections for analysis of carbon and oxygen isotopic compositions. Data of δ13C from the Wangcun Section, western Hunan, South China, demonstrate that the Cambrian carbon-isotope profile includes three remarkable positive

excursions CPEwc-1, 2, 3 in the Upper Series 2, in the Lower and in the Middle Series. Three distinctive negative excursions CNEwc-1, 2, 3 were separately tested in the Lower Series, Lower Series 3 and in the Upper Furongian Series. Similarly, in the corresponding horizons in the

Duibian A Section, Zhejiang Province, southeastern China, three positive excursions CPEdb-1, 2, 3 and three negative excursions CNEdb-1, 2, 3 also have been discovered. We interpret these significant carbon-isotope excursions as being associated with enhanced biogenic productivity, mass extinctions and eustatic events. KEY WORDS: carbon-isotope excursion, mass extinction, sea-level change, Cambrian, South China.

1 INTRODUCTION Additionally, carbon-isotope records of the traditional Upper Strong perturbations in carbon cycles in oceans are re- Cambrian were documented by Miller et al. (2011, 2006) for corded as excursions in δ13C values from the traditional Early the Lawson Cove Section in Utah, USA. Surprisingly, large Cambrian to the Late Cambrian periods, such as the first re- scale carbon-isotope excursions always coincide with markable negative excursion that occurs in the lowermost part chronostratigraphical boundaries relate to global paleoclimate, of Cambrian System (Brasier and Sukhov, 1998; Derry et al., mass extinctions, and sea-level fluctuations etc. (Li et al., 2017; 1994; Brasier et al., 1990), the second remarkable negative Wang et al., 2017; Babcock et al., 2015). For example, Zhu et excursion that occurs across the traditional Lower Cambrian to al. (2006) integrated carbon-isotope profile and biological the Middle Cambrian transition (Guo et al., 2010, 2005; Zhao events detected in some stratigraphic intervals within the Cam- et al., 2008; Zuo et al., 2008a), and the third prominent carbon- brian System. However, it remains necessary to conduct further isotope excursion, the Steptoean positive carbon-isotope excur- studies in detail for the entirety of Cambrian carbon-isotope sion (SPICE), which occurs in the lower part of the traditional excursions and their relationships with biological events and Upper Cambrian worldwide (Peng et al., 2016; Bagnoli et al., sea-level fluctuations. To establish Cambrian carbon-isotope 2014; Ng et al., 2014; Dilliard et al., 2007; Glumac and Mutti, profiles with high resolution and correlate them between dif- 2007; Saltzman et al., 2004, 2000, 1998; Zhu et al., 2004; ferent continents, we collected 530 carbonate samples in the Glumac and Spivak-Birndorf, 2002; Glumac, 2001). Wangcun Section and the Duibian A Section in South China for carbon and oxygen isotope analysis. Data of the two sections *Corresponding author: [email protected] will enhance our understanding of the Cambrian carbon-isotope © China University of Geosciences and Springer-Verlag GmbH excursions associated with mass extinctions, eustatic events Germany, Part of Springer Nature 2018 and paleoclimatic changes.

Manuscript received June 13, 2017. 2 GEOLOGICAL SETTINGS Manuscript accepted October 15, 2017. Paleogeographically, South China, a crucial area for Cam-

Zuo, J. X., Peng, S. C., Qi, Y. P., et al., 2018. Carbon-Isotope Excursions Recorded in the Cambrian System, South China: Implications for Mass Extinctions and Sea-Level Fluctuations. Journal of Earth Science, 29(3): 479–491. https://doi.org/10.1007/s12583-017-0963-x. http://en.earth-science.net 480 Jingxun Zuo, Shanchi Peng, Yuping Qi, Xuejian Zhu, Gabriella Bagnoli and Huaibin Fang brian System research was located on the northwestern margin ascending order (Peng et al., 2004; Peng, 2003). Generally, of the Gondwana continent during the Lower–Middle Cam- bases of these regional Cambrian series are consistent, respec- brian (Wotte et al., 2007). The global standard stratotype- tively with those of the global Terreneuvian Series, Series 2 sections and points (GSSP) of the (Peng et (provisional), Series 3 (provisional) and the Furongian Series. al., 2009a), Furongian Series/Paibian Stage, and the Jiangsha- The Wulingian Series embraces the Taijiangian, Wangcunian, nian Stage (Peng et al., 2012, 2009b) were established in this and the Guzhangian Stages; the Furongian Series includes the region (Fig. 1). Moreover, the candidate sections and points Paibian, and the Niuchehean stages. Biostrati- including the Jianhe Section in Guizhou and the Wa’ergang graphically, the GSSP of the Guzhangian Stage was defined by Section in Hunan being researched respectively for the global the first appearance datum (FAD) of the agnostoid Cambrian Stage 5 (provisional) and Stage 10 (provisional) are Lejopyge laevigata (Peng et al., 2009a). The Furongian Series located in the same area (Peng et al., 2014; Gaines et al., 2011). (with the same base of the Paibian Stage) was defined by the Additionally, South China also embraces some classical Cam- FAD of Glyptagnostus reticulatus, and the Jiangshanian Stage brian sections such as the Wangcun Section in western Hunan, was defined by the FAD of the agnostoid trilobite Agnostotes Yankong Section in Guizhou, and the Duibian A Section in orientalis (Peng et al., 2011). A detailed carbon-isotope profile Zhejiang. Therefore, South China has become the most famous around the FAD of Lejopyge laevigata in the global stratotype area for Cambrian GSSPs and is leading the world in Cam- Luoyixi Section has been reported by Zuo et al. (2008b). brian chronostratigraphy studies. The Cambrian System mainly consists of carbonates and has extensive outcrops in 2.1 Wangcun Section, Western Hunan South China. Studies on regional lithofacies indicate that the The Wangcun Section is situated on the north bank of the southwestern part of South China, including eastern Yunnan, Youshui River, 300 m northeast of the GSSP site (Luoyixi Sec- western Sichuan and western and central Guizhou, was the tion) of the Guzhangian Stage, which is situated on the south broad, extensive Yangtze Platform throughout most of the bank of the Youshui River. Both sections are new roadcuts with Cambrian Period. To the southeast, the narrow ocean-facing fresh rock exposures along the Youshui Valley in western Hu- Jiangnan slope belt was in the border area between Guizhou nan. The Cambrian System, underlain by limestone of the and Hunan. Still further eastward, the Jiangnan deepwater Lower Nanjinguan Formation, conformably over- basin was in central Hunan and southeastern Guizhou (Feng et lies chert strata of the Neoproterozoic Liuchapo Formation. al., 2002). Paleogeographically, the GSSPs of the Furongian Cambrian strata in western Hunan are ~1 750 m thick. Series (or the Paibian Stage), the Guzhangian Stage, the poten- Lithologically, relevant strata are assigned to the Niutitang, tial GSSP for Stage 10 (Wa’ergang Section), and the classical Balang, Tsinghsutung, Aoxi, Huaqiao, Shenjiawan and Lou- Wangcun Section in western Hunan are all located on the shankwan formations in ascending order. Jiangnan slope belt. In contrast, the Duibian A Section and the In the Wangcun Section, the Niutitang Formation consists Jiangshanian GSSP Section (Duibian B Section) of western of dark mudstone interbedded with significant Zhejiang are situated in the Jiangnan deepwater basin (Fig. 1). thin-bedded, fine-grained quartz sandstones and medium- This research focused on the records of carbon-isotope excur- bedded limestones that are interpreted to be sediments of tur- sions associated with eustatic changes in different environ- bidity current events during eustatic changes. Analysis of major ments in South China. elements and redox-sensitive trace elements suggest that the Chronostratigraphically, the Cambrian System in South dark mudstone was deposited mostly under anoxic deepwater China is divided into the Diandongian Series, Qiandongian oceanic settings during the Early Cambrian (Zhang et al., 2016; Series, Wulingian Series, and the Furongian Series (Fig. 2), in Wang et al., 2015). The Balang Formation is composed of gray

Hanzhong N (a) North China Nanjing Korea &

Platform 30ºN North China Hefei New Guinea Shanghai Nanjiang Yichang Burma belt2

Chengdu Wuhan Hangzhou IndoChina Asian shelf Asian Yangtze River South China Austrlia Kangding Jinhua Chongqing Zhangjiajie Tarim slope Nanchang º Iran Tibet Yangtze River 4 1 3 0 Changsha India Xichang Zunyi Turkey Arabia Yangtze 5 E-Europe Platform Hengyang C-Europe Sardina Jiangnan Fuzhou Taipei France Antarctica Guiyang Southeastern New- Paleo- Platform Spain Africa European shelf European Zealand uplift Kunming deepwater basin Jiangnan Shaoguan Morocco

Meizhou 30ºS Baise Paleo- Wenshan uplift South America Nanning 1. Wangcun Section, Guzhang County, western Hunan

0 500 km 2. Duibian ASection, Jiangshan County, Zhejiang shelf 3. Wa’ergan Section, Taoyuan County, western Hunan 4. Luyixi Section, Guzhang County, western Hunan (b) 5. Yankong Section, Jinsha County, northern Guizhou American-African

Figure 1. Paleogeographic settings of the Wangcun and the Duibian A sections in South China. (a) Paleogeographic reconstruction of South China during Cambrian modified from Feng et al. (2002). (b) Paleogeographic reconstruction of Gondwana during the Lower–Middle Cambrian modified from Wotte et al. (2007).

Carbon-Isotope Excursions Recorded in the Cambrian System, South China 481

Global South China System Chronostratigraphy First appearance datum Chronostratigraphy Lithostratigraphy (FAD) of the key fossils Series Stage Series Stage Wangcun Section Dubian A Section

Ordovician Nanjinguan Fm. Yinchupu Fm. Iapetognothus fluctivagus Stage 10 Niuchehean Loushankwan Fm. (provisional) Siyangshan Fm. Lotagnostus americanus Shenjiawan Fm. Jiangshanian Jiangshanian Agnostotes orientalis

Furongian

Furongian Paibian Paibian Huayansi Fm. Glyptagnostus reticulatus Guzhangian Guzhangian Huaqiao Fm. Lejopyge laevigata

Drumian Wangcunian Yangliugang Fm.

Series 3 Series Ptychagnostus atavus

Wulingian Cambrian (provisional) Stage 5 Taijiangian Aoxi Fm. (provisional) ? Ocryctocephalus indicus Dachenling Fm. Stage 4 Tsinghustung Fm. (provisional) Duyunian ? Olenellus or Redlichia

Series 2 Series Stage 3 Balang Fm. (provisional) Nangaoan

(provisional) Trilobite Qiandongian Hotang Fm. Stage 2 Meishucunian Niutitang Fm. (provisional) ? SSF species Jiningian

Terreneuvian Trichophycus pedum Diandongian Liuchapo Fm. Tongying Fm.

Figure 2. Cambrian chronostratigraphic classification in the Wangcun and Duibian A sections, South China. Stages and series defined by ratified GSSPs are indicated on the left. Provisional stages and series, indicated by numbers, do not yet have ratified boundary position. Cambrian chronostratigraphy of South China is based on Peng et al. (2001). Global Cambrian chronostratigraphy is based on Peng et al. (2009a). Fm. Formation. dark calcareous mudstone interbedded with medium-bedded lower part. The Dachenling Formation is composed of dark- limestone layers. The Tsinghsutung Formation is mostly char- grey thick-bedded limestone. The Yangliugang Formation pri- acterized by grey thin- to medium-bedded limestone, dolostone marily consists of dark-grey thin-bedded marlstone with gray and argillaceous dolostone. The Aoxi Formation is dominated thick-bedded ribbon limestone with interlayers of marlstone in by gray medium-bedded dolostone containing gray calcareous the upper part. The Huayansi Formation is composed of gray shale in the lower part, gray thin- to thick- bedded dolostone thick-bedded limestone with ribbon argillaceous limestone, containing sandy dolostone layers in the middle part, and gray lenticular limestone formed during carbonate compaction. The dark argillaceous dolostone in the upper part. The Huaqiao Siyangshan Formation consists of massive limestone contain- Formation consists of gray thin-bedded argillaceous limestone ing limestone conglomerate in the lower part and light gray in the lower part, dark gray thick-bedded marlstone containing thin-bedded limestone with calcareous shale in the upper part. lenticular limestone, conglomerate limestone and calcareous In the Duibian A Section, fossils of polymerid trilobites shale in the upper part. The Shenjiawan Formation consists of appear in the lower part of the Cambrian, whereas agnostoid gray thick- to massive-bedded limestone with dolomitic lime- trilobites are abundant in the middle and the upper part of the stone in the upper part. The uppermost formation of the Cam- Cambrian. A biostratigraphic framework with sixteen biozones brian in western Hunan is the Loushankwan Formation, which for the Duibian A Section was described by Lu and Lin (1989, is composed of thick-bedded pure dolostone. 1983, 1981). Subsequently, the biostratigraphic framework was revised as fifteen biozones. Following detailed studies on bio- 2.2 Duibian A Section, Western Zhejiang stratigraphy combined with carbon-isotope stratigraphy, the The Duibian A Section, 200 m north of the Jiangshanian Duibian B Section was ratified as the GSSP of the Jiangshanian GSSP Section in Jiangshan County, western Zhejiang, was Stage (Peng et al., 2011, 2009b, 2005). situated on the foot of the Jiangnan slope belt during the Cam- brian Period. Lithostratigraphically, the Cambrian System in 3 MATERIALS AND METHODS the Duibian A Section is condensed to 370 m in thickness and To obtain the entire Cambrian carbon-isotope profile, 278 is divided into the Hotang, Dachenling, Yangliugang, Huayansi specimens for carbon and oxygen isotopes analysis were col- and Siyangshan formations, in ascending order. lected at an interval of 5 m from fresh exposures in the new The Hotang Formation consists of dark chert, carbonifer- roadcut in the Wangcun Section of South China, and 252 ous mudstone and calcareous mudstone, with coal seams in the specimens were collected in the Duibian A Section at an inter-

482 Jingxun Zuo, Shanchi Peng, Yuping Qi, Xuejian Zhu, Gabriella Bagnoli and Huaibin Fang val of 1 m. Samples were collected without weathered materi- 4.2 Diagenesis of Cambrian Carbonate Rocks als in homogeneous carbonates. Care was taken to avoid frac- Carbon and oxygen isotopic compositions of marine car- tures, calcite veins, and visible breccias. bonates depend on the δ13C and δ18O of the Cambrian seawater Powder samples for carbon and oxygen analysis were mi- in which carbonate was deposited. Carbon-isotope trend ap- crodrilled from the homogeneous carbonate specimens. Powder pears to be more robust to later alteration, as has been previ- samples of 0.2 mg were reacted with orthophosphoric acid to ously reported (Glumac et al., 2007; Maloof et al., 2005). In produce CO2 by Kiel IV automatically connected to a Finnigan contrast, the oxygen isotope is more susceptible to MAT-253 mass spectrometer. Carbon and oxygen isotopic post-alteration, and diagenesis will change the oxygen-isotope analysis were conducted at the State Key Lab of Geological signatures of original marine carbonates (Han et al., 2016), Process and Mineral Resources in China University of Geo- therefore, oxygen-isotope trends of ancient carbonates are sciences, Wuhan, Hubei Province, China. Isotopic results are rarely used in stratigraphic correlations. In order to determine reported as δ values with reference to the Vienna Peedeebe whether carbonate rocks are subjected to strong diagenetic lemnite standard (VPDB). Precision monitoring by NBS-19 alteration, geochemical proxies were used in this study to con- and an internal standard is better than 0.05‰ for carbon and strain the effects of post-depositional alteration on carbonate oxygen isotope ratios. The results of carbon and oxygen iso- isotopic signatures. topes from the two measured sections are presented in Figs. 3, Literatures published on diagenesis indicate that δ13C and 4, 5, and Tables 1, 2. δ18O of marine carbonates are systematically covariant after isotopes fractionation (Jacobson and Kaufman, 1999; Banner 4. RESULTS and Hanson, 1990). Calculations suggest that correlative coef- 4.1 Carbon- and Oxygen-Isotope Composition Distribution ficients between δ13C and δ18O of carbonates from the Wang- According to Fig. 3, δ13C of the Cambrian carbonates from cun and the Duibian A sections are 0.070 9 and 0.001 7, respec- the Wangcun Section in western Hunan varied in a range of tively, which implies that most carbonates have not been af- approximately 4.4‰ to -6.0‰ (Fig. 3a) with an average of fected by strong diagenesis. Additionally, δ18O< -10.0‰ is also 0.7‰, and δ18O changed in a main range from -11.0‰ to used as an indicator for judgment of carbonate diagenesis -7.0‰ with an average of -9.5‰. In the Duibian A Section of (Kaufman et al., 1993; Derry et al., 1992). δ18O of carbonates western Zhejiang, δ13C values of carbonates are distributed in the Wangcun Section were mainly scattered between -7.0‰ from 3.7‰ to -6.3‰ (Fig. 3b) with an average value of 0.9‰, and -11.0‰, and bulk rock samples with δ18O values of and δ18O values of the Cambrian carbonates in the same section < -10.0‰ were mainly collected in the basal and the Upper are mainly distributed from -11.0‰ to -8.0‰ with an average Cambrian in the Wangcun Section of western Hunan. δ18O of value of -10.1‰. Comparatively, We found that the average carbonates from the Duibian A Section, western Zhejiang, values of δ13C and δ18O from the Wangcun Section of western mainly ranged between -11.0‰ and -6.0‰; and carbonates Hunan were slightly different from those in the Duibian A Sec- with δ18O< -10.0‰ were also primarily affiliated with the basal tion of western Zhejiang, because the Wangcun Section was Cambrian and the upper part of the Siyangshan Formation. situated on the southeastern slope of the Yangtze Platform and Generally, samples with relatively low δ18O values were col- the Duibian A Section was situated in the Jiangnan deepwater lected from dark fine-grained sediments which were deposited basin during the Cambrian Period. Therefore, depositional en- under anoxic deepwater environmental conditions. During vironments resulted in differences in δ13C and δ18O background transgression periods, large plenty of fresh water from melting values. of the continental ice caps led to low δ18O of marine

13 δ C δ13C 6.0 6.0 δδ13C=0.029 2 18 O+ 1.176 8 13 18 + 4.0 δδC=0.310 3 O 3.677 8 R2=0.001 7 4.0 R2=0.070 9 2.0 2.0

18 δ18O δ O -20.0 -16.0 -12.0 -8.0 -4.0 0 (‰) -16.0 -14.0 -12.0 -10.0 -8.0 -6.0 -2.0 0 (‰) -2.0 - -2.0 -4.0 -4.0 -6.0

-6.0 -8.0 (‰)

(b) (a) -8.0 (‰)

Figure 3. Correlation between δ13C and δ18O from Cambrian carbonates, South China. (a) Data from the carbonate platform slope facies Wangcun Section, western Hunan, South China; (b) data from the deepwater basin facies Duibian A Section, western Zhejiang, South China.

Carbon-Isotope Excursions Recorded in the Cambrian System, South China 483 carbonate deposition. Therefore, factors of post-alteration and 4.3 Cambrian Carbon-Isotope Profiles inputs of freshwater from melting continental ice caps changed 4.3.1 Wangcun Section, western Hunan the original signals of carbonates within which δ18O decreased Carbon-isotope stratigraphy from the uppermost part of to low values. Additionally, an important indicator for judging the Balang Formation to the lower part of the Huaqiao Forma- carbonates diagenesis is whether the carbonates with δ18O< tion in the Wangcun Section has been previously performed, -10.0‰ occur in the corresponding horizons in different paleo- and a distinct positive carbon-isotope excursion was detected in geographic sections. If δ18O of carbonates changes in a similar the Tsinghsutung Formation (Zhu et al., 2004). However, new range in contemporaneous depositional successions in different carbon-isotope data in this paper from the Wangcun Section sections, the marine carbonates may not have experienced revealed more detailed carbon-isotope trends that spanned the strong post-depositional alteration. Carbonates from the two entire Cambrian strata (Fig. 4). According to Fig. 4, three re- measured sections mentioned above are fine-grained, homoge- markable positive carbon-isotope excursions (CPEwc-1, neous, bedded limestone or dolostone with no characteristics of CPEwc-2, CPEwc-3) and three distinct negative carbon-isotope recrystallization, metamorphism, or hydrothermal alterations, excursions (CNEwc-1, CNEwc-2, CNEwc-3) have been identified which indicates that δ13C could reflect the primary signatures within the Cambrian strata, characteristics of these carbon- of the ancient seawater. isotope excursions are as follows.

Figure 4. Cambrian carbon isotope evolution recorded in the slope facies Wangcun Section, western Hunan, and the deepwater facies Duibian A Section, west- ern Zhejiang, South China. The CNEwc-1, 2, 3 and the CPEwc-1, 2, 3 represent the order number of negative and positive carbon isotope excursions in the

Wangcun Section, respectively. The CNEdb-1, 2, 3 and the CPEdb-1, 2, 3 are the numbers of the negative and the positive carbon isotope excursions in the Duibian A Section, respectively. The HERB event was discovered slightly above the Hellnmaria/red top members boundary (HERB) at Lawson Cove, western Utah, USA (Ripperdan and Miller, 1995). Currently the HERB event has been found worldwide. Here is the corresponding horizon, where a carbon isotope presents a shift toward low values in the measured sectios, South China.

484 Jingxun Zuo, Shanchi Peng, Yuping Qi, Xuejian Zhu, Gabriella Bagnoli and Huaibin Fang

In ascending order, the first carbon-isotope positive excur- tion, western Hunan. δ13C varies in a generally increasing pro- sion CPEwc-1 occurs in the Tsinghsutung Formation of the file including several small-scale shifts toward high values. upper part of the Series 2, where δ13C rises to the maximum Moreover, all δ13C values of these small-scale shifts are less value of 2.2‰. The second notable positive excursion CPEwc-2 than 0‰. The minimum value of the negative excursion commences on the first appearance datum (FAD) of the Glyp- CNEwc-1 is -6.0‰ at the base of the Cambrian System. The tagnostus reticulatus, which defines the base of the Paibian second negative excursion CNEwc-2 found in the basal part of Stage of the lower part of the Furongian Series (traditional Series 3 spans the base of the Stage (Babcock et al., Upper Cambrian), and ends in strata above the FAD of the Ag- 2007) where the lowest value of δ13C is -2.2‰. The third ex- nostotes orientalis, a level coinciding with the base of the cursion CNEwc-3 in the provisional Stage 10 is a combination global Jiangshanian Stage where δ13C reaches the maximum of three δ13C negative spike shifts. δ13C drops to -1.8‰ from value of 4.4‰. The positive excursion CPEwc-2, coinciding 1.7‰ in the lower spike shift, drops to -1.7‰ from 0.7‰ in the with the famous SPICE excursion, is a significant global posi- middle shift, and drops to -1.8‰ from 0.6‰ in the top shift. Of tive carbon-isotope excursion recorded in the lower part of the these shifts, the lowest one defines the base of the Stage 10. As traditional Upper Cambrian worldwide (Ng et al., 2014; Dil- we know, the base of the GSSP of the Stage 10 is defined by liard et al., 2007; Glumac and Mutti., 2007; Saltzman et al., the FAD of the agnostoid trilobite Lotagnostus americanus 2004, 2000). The third positive carbon-isotope excursion (Peng et al., 2014). Therefore, the location of the first negative

CPEwc-3 occurs in the upper part of the Jiangshanian Stage carbon-isotope shift in the upper most part of Cambrian would where the maximum vales of δ13C are around 3.0‰. be useful for defining the base of the Stage 10. The second shift,

The first carbon-isotope negative excursion CNEwc-1 oc- which corresponds to the HERB event (Ripperdan and Miller, curs in the basal part of the Cambrian System from the Niuti- 1995; Ripperdan et al., 1992), is defined by Eoconodontus tang Formation to the Balang Formation in the Wangcun Sec- (Bagnoli et al., 2017; Miller et al., 2015).

Figure 5. Relationship between carbon isotope shifts detected in the measured sections and bioevents identified worldwide. Stages and series defined by ratified GSSPs are indicated on the timescale. Provisional stages and series, indicated by numbers, do not yet have ratified boundary position. Mass extinction events are based on Zhu et al. (2006) and Babcock et al. (2015). The HERB event was discovered slightly above the Hellnmaria/Red top Members boundary (HERB) at Lawson Cove, western Utah, USA (Ripperdan and Miller, 1995). Currently the HERB event has been found worldwide. Here is the corresponding horizon, where a carbon isotope presents a shift toward low values in the measured sectios, South China. (a) The deepwater facies Duibian A Section, western Zhejiang.

CPEdb-1, 2, 3 are the numbers of positive carbon isotope shifts and the CNEdb-1, 2, 3 are the numbers of negative carbon isotope shifts in the Duibain A Section, western Zhejiang. (b) The slope facies Wangcun Section, western Hunan, South China. CPEwc-1, 2, 3 are the numbers of positive carbon isotope shifts and the

CNEwc-1, 2, 3 are the numbers of negative carbon isotope shifts in the Wangcun Section.

Carbon-Isotope Excursions Recorded in the Cambrian System, South China 485

Table 1 Results of carbon and oxygen isotopes from Wangcun Section, western Hunan, China (‰)

Depth (m) Rock δ13C δ18O Depth (m) Rock δ13C δ18ODepth (m)Rockδ13C δ18O Depth (m) Rock δ13C δ18O 1 745 L 0.8 -9.7 1 330 L 3.0 -10.4 975 L 3.1 -8.2 615 L 1.3 -9.7 1 740 L 1.2 -9.4 1 325 L 2.1 -10.1 970 L 2.9 -8.3 610 L 0.8 -9.7 1 735 L 0.9 -9.0 1 320 L 2.8 -9.9 965 L 3.3 -8.1 605 L -0.8 -10.0 1 730 L 1.1 -9.7 1 315 L 2.6 -9.9 960 L 3.5 -8.4 600 L -0.4 -10.7 1 725 L 1.0 -10.0 1 310 L 2.8 -9.5 955 L 3.7 -9.2 595 L -1.1 -6.7 1 720 L 0.8 -9.4 1 305 L 2.8 -10.2 950 L 4.1 -7.8 590 L -0.7 -8.6 1 715 L -0.1 -12.3 1 300 L 2.4 -10.0 945 L 4.0 -8.5 585 CM -1.8 -7.2 1 710 D 0.0 -10.6 1 295 L 2.9 -10.1 940 L 3.7 -7.5 580 CM -1.7 -6.4 1 700 D 0.6 -9.6 1 290 L 2.2 -9.9 935 L 4.2 -7.8 575 L -2.0 -8.6 1 690 D -0.2 -9.5 1 285 L 2.7 -10.2 930 L 4.3 -7.9 570 L -1.9 -5.7 1 680 D -0.8 -9.9 1 280 L 2.1 -8.9 925 L 3.9 -7.6 565 L -1.6 -4.6 1 660 D -0.7 -10.7 1 275 L 1.9 -10.1 920 L 4.4 -7.6 550 D -0.1 -5.3 1 650 D -0.4 -9.0 1 270 L 2.2 -11.3 915 L 3.8 -8.9 545 D -0.9 -7.9 1 645 D -0.5 -5.8 1 265 L 2.2 -9.8 910 L 2.4 -9.2 540 D -0.1 -8.8 1 640 D 0.0 -6.5 1 260 L 2.2 -10.2 905 L 2.9 -8.7 535 D 0.0 -8.7 1 635 D -0.3 -8.1 1 255 L 1.3 -10.1 900 L 3.1 -8.9 530 D 0.3 -8.7 1 630 D -0.2 -8.4 1 250 L 1.6 -10.4 895 L 3.0 -8.7 525 D 0.7 -9.2 1 615 D 0.2 -7.2 1 245 L 2.3 -9.9 885 L 2.6 -8.8 520 D 0.2 -8.7 1 595 D -1.8 -9.9 1 240 L 2.2 -10.0 880 L 2.6 -8.9 515 D 0.4 -8.5 1 590 D 0.6 -8.5 1 235 L 0.7 -11.5 875 L 2.5 -8.9 510 D 0.4 -8.3 1 585 D -0.5 -9.6 1 230 L 1.2 -8.7 870 L 2.1 -8.9 505 D -0.1 -9.2 1 580 D -0.8 -10.6 1 225 L 1.4 -11.1 865 L 2.6 -8.5 500 D 0.3 -8.7 1 575 D -0.2 -13.4 1 220 L 1.8 -12.3 860 L 2.1 -9.0 495 D 0.2 -8.7 1 570 D -0.2 -12.4 1 215 L 1.1 -11.9 855 L 1.4 -9.0 490 D 0.6 -8.7 1 565 D 0.5 -8.9 1 210 L 1.3 -11.0 850 L 1.4 -8.9 485 D 1.8 -9.4 1 560 D 0.3 -10.8 1 205 L 0.6 -11.0 845 L 3.5 -8.7 480 D 1.8 -10.1 1 555 DL -1.7 -9.8 1 200 L 0.9 -11.1 840 L 1.4 -8.4 475 D 2.1 -10.3 1 550 DL 0.7 -11.3 1 195 L 0.0 -9.4 835 L 1.3 -8.9 470 D 2.1 -9.4 1 545 DL 0.2 -11.5 1 190 L 1.0 -10.4 830 L 1.6 -9.4 465 D 2.2 -9.9 1 540 DL 0.5 -10.3 1 185 L 1.4 -10.1 825 L 1.2 -9.7 460 D 1.2 -9.1 1 535 DL 0.1 -9.1 1 180 L 2.9 -10.0 820 L 0.8 -9.5 455 D 2.1 -11.0 1 530 DL 0.8 -7.7 1 175 L 1.4 -9.7 815 L 1.6 -9.1 450 D 1.7 -9.7 1 525 DL 0.7 -10.1 1 170 L 1.0 -9.4 810 L 0.8 -9.2 445 D 1.9 -9.6 1 520 DL -0.9 -11.9 1 165 L 1.2 -10.3 805 L 1.0 -8.9 440 D 1.7 -9.0 1 515 L -1.8 -9.0 1 160 L 1.6 -10.5 800 L 0.4 -9.5 435 D 1.4 -9.4 1 510 L -0.7 -12.2 1 155 L 1.3 -9.6 795 L 0.6 -9.7 430 D 0.0 -9.5 1 505 L 0.5 -11.0 1 150 L 0.8 -9.9 790 L 0.4 -9.5 425 D -1.3 -5.7 1 500 L 1.0 -10.6 1 145 L 0.6 -9.2 785 L 1.1 -9.5 420 D -1.8 -6.8 1 495 L 1.5 -9.9 1 140 L 1.1 -9.2 780 L 0.3 -9.8 415 D -1.9 -5.3 1 490 L 1.7 -10.4 1 135 L 1.2 -8.6 775 L -0.5 -10.0 405 M -4.2 -10.1 1 485 L 1.7 -9.4 1 130 L 0.5 -9.3 770 L -0.7 -9.9 400 L -1.4 -11.9 1 480 L 1.4 -11.7 1 125 L 1.1 -8.7 765 L -0.4 -8.6 395 M -2.3 -11.4 1 475 L 1.3 -11.7 1 120 L 1.1 -8.0 760 L 0.2 -9.7 390 M -3.7 -10.2 1 470 L 1.5 -10.6 1 115 L 0.9 -8.7 755 L 0.0 -9.0 385 M -1.4 -11.1 1 465 L 1.5 -9.9 1 110 L 1.8 -8.3 750 L 0.2 -9.5 380 M -1.7 -11.5 1 460 L 1.2 -12.9 1 105 L 1.4 -8.8 745 L 0.5 -9.4 375 L -0.3 -9.5 1 455 L 1.2 -11.7 1 100 L 1.0 -8.6 740 L 0.2 -9.4 365 M -1.5 -11.9 1 450 L 1.3 -8.6 1 095 L 1.3 -9.1 735 L 0.1 -8.7 360 M -3.6 -11.3

486 Jingxun Zuo, Shanchi Peng, Yuping Qi, Xuejian Zhu, Gabriella Bagnoli and Huaibin Fang

Table 1 Continued

Depth (m) Rock δ13C δ18O Depth (m) Rock δ13C δ18ODepth (m)Rockδ13C δ18O Depth (m) Rock δ13C δ18O 1 445 L 1.5 -8.6 1 090 L 1.5 -7.7 730 L 0.0 -9.2 355 M -3.7 -10.7 1 440 L 1.4 -10.5 1 085 L 1.5 -8.7 725 L -0.5 -9.9 345 M -1.4 -11.0 1 435 L 1.4 -11.0 1 080 L 0.9 -9.0 720 L -0.5 -8.9 340 M -1.2 -11.7 1 430 L 1.4 -9.9 1 075 L 1.0 -7.9 715 L 0.8 -9.5 335 M -2.0 -12.3 1 425 L 1.8 -9.2 1 070 L 1.5 -8.3 710 L 0.1 -9.6 310 M -3.7 -12.1 1 420 L 1.4 -8.6 1 065 L 1.6 -8.7 705 L 0.3 -10.3 305 M -3.1 -12.7 1 415 L 1.6 -9.6 1 060 L 1.1 -8.7 700 L -0.4 -10.4 290 M -2.9 -13.4 1 410 L 1.5 -9.5 1 055 L 1.1 -9.0 695 L 0.7 -9.5 280 M -3.6 -14.1 1 405 L 1.7 -9.2 1 050 L 0.8 -8.4 690 L -0.4 -9.3 180 L -1.0 -12.5 1 400 L 1.7 -9.3 1 045 L 0.5 -8.8 685 L -0.3 -9.7 176 L -0.5 -12.5 1 395 L 1.2 -9.2 1 040 L 1.1 -8.2 680 L 0.3 -9.6 174 L -1.1 -10.2 1 390 L 2.5 -9.4 1 035 L 1.2 -8.7 675 L 0.0 -9.7 170 L -1.2 -12.0 1 385 L 2.0 -9.3 1 030 L 1.6 -7.5 670 L -0.2 -9.5 165 L -1.4 -12.0 1 380 L 2.1 -9.5 1 025 L 1.0 -9.2 665 L -0.3 -9.5 125 L -0.3 -13.1 1 375 L 1.9 -9.8 1 020 L 1.9 -9.5 660 L -0.4 -8.4 120 L -1.6 -13.7 1 370 L 2.2 -10.0 1 015 L 1.4 -8.7 655 L 0.2 -10.0 115 L -0.4 -12.1 1 365 L 2.2 -8.8 1 010 L 1.5 -9.3 650 L 0.0 -10.0 110 L -3.4 -10.3 1 360 L 2.2 -9.5 1 005 L 2.2 -9.0 645 L 0.5 -6.2 109 L -4.6 -13.3 1 355 L 2.4 -10.3 1 000 L 3.0 -8.6 640 L 0.0 -8.0 3 L -6.0 -10.1 1 350 L 2.0 -10.0 995 L 3.0 -8.3 635 L -0.3 -9.6 0 L -5.6 -11.1 1 345 L 2.2 -8.7 990 L 2.0 -8.4 630 L -0.4 -9.0 1 340 L 2.6 -9.0 985 L 2.9 -8.1 620 M 0.2 -7.9

CM. Calcareous mudstone; D. dolostone; DL. dolomitic limestone; L. limestone; M. marlstone; S. shale.

Table 2 Results of carbon and oxygen isotopes from Duibian A Section, western Zhejiang, China (‰)

Depth (m) Rock δ13C δ18O Depth (m) Rock δ13C δ18ODepth (m)Rockδ13C δ18O Depth (m) Rock δ13C δ18O 333.0 S -0.2 -8.9 239.0 L 0.9 -8.5 174.0 L 1.7 -8.9 100.0 L 0.1 -8.2 332.0 S 0.9 -10.1 238.0 L 1.1 -11.3 173.0 L 0.8 -10.4 99.0 L 0.0 -9.1 331.0 L 1.5 -9.7 237.0 L 0.9 -10.5 172.0 L 1.0 -9.8 98.0 L 0.0 -9.0 330.0 L 1.6 -9.8 236.0 L 0.9 -8.5 171.0 L 1.2 -12.5 97.0 L -0.2 -8.4 329.0 L 1.4 -10.0 235.0 L 1.1 -9.8 170.0 L 0.9 -11.4 96.0 L -0.1 -8.7 328.0 L 1.4 -10.0 234.0 L 1.0 -8.4 169.0 L 1.0 -12.3 95.0 L -0.4 -7.9 326.0 L 1.2 -10.0 233.0 L 0.9 -7.8 168.0 L 1.2 -11.8 94.0 L -0.2 -9.3 325.0 L 1.3 -9.9 232.0 L 0.9 -8.5 167.0 L 1.1 -10.0 93.0 L -0.6 -9.5 324.0 L 0.6 -9.4 231.0 L 1.1 -8.9 166.0 L 1.0 -8.8 92.0 L -0.9 -9.8 323.0 L 2.0 -9.8 230.0 L 1.5 -8.9 165.0 L 1.3 -9.2 91.0 L 0.2 -10.1 321.5 L 1.8 -10.1 229.0 L 1.7 -8.4 164.0 L 1.1 -8.2 90.0 L -0.1 -9.3 320.0 L 1.7 -10.0 228.0 L 1.7 -8.7 163.0 L 1.2 -8.0 88.0 L -0.6 -10.6 319.0 L 2.0 -9.9 227.0 L 1.7 -8.2 162.0 L 1.0 -8.6 87.0 L -1.2 -10.0 318.0 L 1.6 -10.3 225.0 L 2.1 -8.5 161.0 L 0.5 -9.1 86.0 L -0.9 -9.7 316.0 L 1.4 -10.0 224.0 L 2.4 -8.6 160.0 L 1.1 -8.6 85.0 L -0.2 -5.8 315.0 L 1.3 -9.8 223.0 L 2.6 -8.6 159.0 L 1.1 -8.5 84.0 L -0.1 -5.8 312.0 L 1.0 -10.3 222.0 L 2.7 -8.7 158.0 L 1.1 -8.3 83.0 L -1.1 -9.8 311.0 L 1.2 -10.1 221.0 L 2.6 -9.0 157.0 L 1.1 -8.8 82.0 L -1.2 -12.4 310.0 L 1.0 -10.4 220.0 L 2.7 -9.4 156.0 L 1.0 -7.5 81.0 L -0.9 -13.2 309.0 L 0.7 -10.2 219.0 L 2.8 -9.1 155.0 L 0.9 -8.7 80.0 L -0.8 -13.2 308.0 L 1.6 -11.2 218.0 L 3.0 -9.0 154.0 L 0.7 -8.9 78.0 L -0.4 -10.4 306.0 L 1.7 -10.7 217.0 L 3.0 -9.5 153.0 L 0.5 -9.0 77.0 L -0.4 -13.9 305.0 L 1.9 -10.3 216.0 L 3.2 -9.2 152.0 L 0.6 -8.7 76.0 L -0.4 -11.7

Carbon-Isotope Excursions Recorded in the Cambrian System, South China 487

Table 2 Continued

Depth (m) Rock δ13C δ18O Depth (m) Rock δ13C δ18ODepth (m)Rockδ13C δ18O Depth (m) Rock δ13C δ18O 305.0 L 1.9 -10.3 216.0 L 3.2 -9.2 152.0 L 0.6 -8.7 76.0 L -0.4 -11.7 304.0 L 1.8 -11.0 215.0 L 3.2 -9.1 151.0 L 0.5 -9.1 75.0 L -0.4 -10.2 303.0 L 1.3 -10.9 214.0 L 3.5 -10.0 150.0 L 0.7 -8.2 74.0 L -0.4 -11.5 302.0 L 1.4 -10.6 213.0 L 3.4 -12.2 149.0 L 0.5 -9.3 73.0 L -0.5 -11.1 301.0 L 1.4 -10.8 211.0 L 3.6 -11.8 148.0 L 0.3 -9.4 72.0 L -0.7 -12.3 300.0 L 1.2 -11.0 210.0 L 3.5 -13.1 147.0 L 0.1 -8.6 71.0 L -0.7 -10.4 299.0 L 1.0 -10.9 209.0 L 3.1 -13.0 141.0 L -0.7 -9.2 70.0 L -0.6 -10.3 298.0 L 1.2 -10.8 208.0 L 3.0 -10.8 140.0 L -0.5 -9.6 69.0 L 0.4 -8.2 296.0 L 1.2 -9.5 207.0 L 2.5 -10.1 139.0 L 0.2 -6.8 68.0 L -0.3 -7.7 295.0 L 1.5 -10.4 206.0 L 3.3 -10.3 138.0 L -0.4 -10.1 67.0 L -1.3 -14.0 294.0 L 1.5 -10.1 205.0 L 3.1 -11.0 137.0 L -0.4 -9.4 66.0 L -1.6 -8.7 293.0 L 1.4 -10.1 204.0 L 3.4 -10.2 136.0 L -0.1 -8.7 65.0 L -1.0 -15.9 292.0 L 1.3 -9.9 203.0 L 3.3 -12.0 135.0 L -0.2 -8.9 64.0 L 0.1 -9.4 291.0 L 1.5 -10.9 202.0 L 3.3 -12.9 134.0 L -0.3 -9.6 63.0 L 0.0 -11.1 290.0 L 1.7 -11.1 201.0 L 3.5 -12.1 133.0 L -0.3 -9.7 62.0 DL 0.8 -8.4 289.0 L 1.4 -11.8 200.0 L 3.0 -14.2 132.0 L -0.4 -9.7 61.0 DL 0.2 -6.9 288.0 L 2.0 -10.4 199.0 L 3.3 -14.4 131.0 L -0.3 -9.4 60.0 DL 0.2 -10.8 287.0 L 1.9 -10.6 198.0 L 3.4 -12.6 130.0 L -0.6 -9.2 59.0 DL 0.2 -8.2 286.0 L 1.8 -10.7 197.0 L 3.7 -12.8 129.0 L 0.1 -8.6 58.0 DL 0.7 -12.8 278.0 L 1.9 -11.7 196.0 L 3.3 -11.6 128.0 L 0.1 -7.8 57.0 DL 0.4 -12.9 276.0 L 0.9 -10.8 195.0 L 3.3 -13.2 126.0 L -0.3 -8.9 56.0 DL 0.9 -13.2 273.0 L 0.7 -11.5 194.0 L 3.4 -13.1 125.0 L -0.2 -9.1 55.0 DL 0.5 -8.5 272.0 L 0.4 -11.0 193.0 L 3.2 -13.1 124.0 L -0.3 -9.0 54.0 DL 0.5 -10.9 271.0 L 0.8 -11.0 192.0 L 1.9 -12.7 123.0 L -0.3 -9.9 53.0 DL 0.7 -10.6 270.0 L 0.7 -9.5 191.0 L 2.9 -8.6 122.0 L -0.6 -9.8 52.0 DL 0.4 -18.6 269.0 L 0.7 -9.7 190.0 L 2.9 -8.7 121.0 L -0.4 -9.3 51.0 DL 0.6 -9.6 267.0 L 0.6 -10.0 189.0 L 2.7 -9.2 120.0 L -0.3 -9.7 50.0 DL 0.6 -14.7 264.0 L 0.4 -10.4 188.0 L 3.1 -13.0 119.0 L -0.3 -9.5 49.0 DL 0.1 -7.6 261.0 L 0.3 -9.5 187.0 L 2.5 -9.2 118.0 L -0.4 -9.8 48.0 DL -0.2 -8.2 259.5 L 0.5 -10.3 186.0 L 2.0 -8.7 117.0 L -0.4 -8.7 47.0 DL 0.2 -8.6 258.0 L 0.2 -9.9 185.0 L 2.4 -8.7 116.0 L -0.3 -10.1 46.0 DL -1.8 -14.2 252.0 L 0.7 -9.6 184.0 L 2.4 -8.8 110.0 L -0.4 -9.7 45.5 DL -2.7 -11.7 251.0 L 1.1 -8.2 183.0 L 2.3 -9.2 109.0 L 0.0 -10.4 45.0 L -1.3 -14.7 250.0 L 0.9 -9.5 182.0 L 2.5 -10.9 108.0 L 0.2 -10.1 44.0 L -1.6 -14.7 249.0 L 0.8 -9.7 181.0 L 2.3 -12.0 107.0 L 0.0 -10.1 43.0 L -2.1 -7.1 247.0 L 1.1 -8.7 180.0 L 2.1 -11.1 106.0 L -0.4 -11.1 42.0 L -1.7 -12.9 244.0 L 1.0 -9.3 179.0 L 2.3 -9.8 105.0 L 0.0 -12.0 41.0 L -2.1 -6.7 243.0 L 1.0 -9.6 178.0 L 2.1 -8.9 104.0 L 0.0 -11.9 25.0 S -6.3 -18.8 242.0 L 1.2 -8.8 177.0 L 2.0 -9.3 103.0 L -0.2 -13.1 5.0 D 0.5 -6.6 241.0 L 1.0 -8.9 176.0 L 1.8 -9.7 102.0 L -0.3 -12.7 2.5 D 1.6 -6.7 240.0 L 1.1 -9.0 175.0 L 2.0 -8.3 101.0 L -0.4 -8.9 0.0 D 2.2 -3.3

S. Shale; L. limestone; DL. dolomitic limestone; D. dolostone.

4.3.2 Duibian A Section, western Zhejiang tion; therefore, the detailed carbon-isotope profile of the Early Characteristics of the carbon-isotope profile of the Cam- Cambrian cannot be established in this paper. However, brian Duibian A Section suggest that δ13C values from carbon- dark-grey mudstone, shale and coal seams in the Hotang For- ates of the uppermost Neoproterozoic are approximately 2.2‰, mation indicate that the strata are rich in organic matter buried followed by a sharp drop in which δ13C falls to -6.3‰ in the in anoxic deepwater paleoenvironments in the Early Cambrian. basal part of the Cambrian (Fig. 4), indicating a remarkable From the lower part to the middle part of the Dachenling For- 13 negative excursion (CNEdb-1) occurs in the middle part of the mation, δ C quickly rises to approximately 0.5‰, implying a 13 Hotang Formation. Lithostratigraphically, very few carbonate δ C excursion towards positive values (CPEdb-1). However, layers are interbedded in the black shale of the Hotang Forma- this trend was constrained by a sharp decline in δ13C values in

488 Jingxun Zuo, Shanchi Peng, Yuping Qi, Xuejian Zhu, Gabriella Bagnoli and Huaibin Fang an interval composed of sandy limestone and calcareous shale, cosmopolitan negative carbon-isotope implies that the anomaly which reflects the sea-level falling in the late Epoch 2. The of carbon cycle occurred worldwide in the Early Cambrian minimum δ13C value of -2.7‰ occurs at the beginning of the ocean. The strong negative carbon-isotope excursion was at- wide negative carbon-isotope excursion (CNEdb-2) spanning tributed to geological events that resulted in a succession of the late Stage 4, Stage 5 and the early Drumian Stage. Addi- chemo-, physical- and biotic crisis events, such as the trace and tionally, a compound negative excursion (CNEdb-3) made of rare element anomalies (Zhang J P et al., 2016; Han et al., 2015; three spike shifts in the uppermost strata of the Duibian A Sec- Wang et al., 2015; Zhang W H et al., 2015), extinction of the tion. δ13C falls to 0.7‰, 0.6‰ and 0‰, in the lowest one, the Ediacaran fauna (Babcock et al., 2015; Zhu et al., 2006), and middle one and the top shift, respectively. sea-level rising (Mei et al., 2007) in the Early Cambrian.

Around the FAD of the Ptychagnostus atavus, which defines The positive carbon-isotope excursion CPEwc-1, with a the base of the Drumian Stage in the Duibian A Section, δ13C maximum value of 2.2‰ and amplitude of 5.0‰, occurs in the oscillates between -1.6‰ and 0‰. Upsection, δ13C varies be- in the Wangcun Section (Fig. 5). Strati- tween -1.2‰ and 0.2‰ in the lower part of Yangliugang Forma- graphically, this positive excursion in the slope facies at the tion, between -0.4‰ and 0.2‰ in the middle part of this unit, and Wangcun Section coincides with the CPEdb-1 in the deepwater rises to 1.2‰ at the top of this unit. Biostratigraphically, the FAD facies at the Duibian A Section of western Zhejiang, South of the Lejopyge laevigata in the Duibian A Section is the base of China. However, in corresponding horizons in the Duibian A the Guzhangian Stage, and the FAD of the Glyptagnostus reticu- Section, the amplitude of this excursion is less than that in the latus defines the base of the Paibian Stage. The remarkable posi- Wangcun Section, and all δ13C values of this excursion are not tive carbon-isotope excursion (CPEdb-2 or SPICE) starts in the beyond the average of 0.9‰. With the improvement of oceanic upper part of the Guzhangian Stage, and the onset of this excur- ecosystem environments, the development of marine faunas sion occurs earlier than the first occurrence of the agnostoid trilo- emerged and diversified during the Cambrian biotic explosion bite Glyptagnostus reticulates in the Duibian A Section. During (Debrenne, 1991) accompanied by an increased carbon-isotope this excursion, δ13C rises to a maximum value of 4.0‰ in the profile in the early stage of the Cambrian Period (Maloof et al., middle part of the Huayansi Formation and falls back to 2.5‰ 2005; Kouchinsky et al., 2001; Brasier et al., 1994). near the FAD of the Agnostotes orientalis, which defines the base The negative carbon-isotope excursion CNEwc-2 occurs in of the Jiangshanian Stage. Above the FAD of the Agnostotes ori- the transition from the upper Series 2 to the lower Series 3 (Zhao entalis, δ13C continues to fall to approximately 1.0‰ and oscil- et al., 2008; Dilliard et al., 2007). This negative excursion is lates for 30 m stratigraphic interval in the upper part of the Huay- distinctively identified in the slope facies at the Wangcun Section. 13 ansi Formation. δ C falls to approximately 0‰ across the transi- However, the corresponding excursion (CNEdb-2) contains sev- tion from the uppermost Huayansi Formation to the lowermost eral shifts with low δ13C background values in the condensed Siyangshan Formation. After this carbon-isotope trend, δ13C re- deepwater facies at the Duibian A Section (Fig. 5). This negative Sr 86 turns to approximately 2.0‰ (CPEdb-3) in the upper part of excursion together with a significant increase in 87/ Sr (Mon- Jiangshanian Stage. tañez et al., 2000) coincides with the mass extinction of Ar- chaeocyathids and Redlichiid and Olenellid trilobites (Babcock et 5 DISCUSSION al., 2015; Zhu et al., 2006). The coincidence of these events sug-

Carbon-isotope excursions across the Pre-Cambrian/ gests that CNEwc-2 would be caused by a large scale Cambrian boundary, the /Series 3 boundary, sea-level-fall event resulting in an enhancement in continental and in the lower part of the Furongian Series are well correlated weathering. In the late Epoch 2, the worldwide sea-level-fall worldwide. These excursions represent several large-scale per- event (Mei et al., 2007; Álvaro and Vennin, 1998) led to the turbations in the global carbon cycles, and such perturbations deterioration of paleoceanographic environments, such as the are associated with changes in primary biogenic productivities, exposure of continental shelf and dwindling of the marine bio- paleoclimate, and sea-level fluctuations. Zhu et al. (2006) pro- sphere, which may have aggravated the mass extinctions. posed an integrated Cambrian carbon-isotope stratigraphic The positive carbon-isotope excursion CPEwc-2 covers the framework, and related carbon-isotope excursions to mass ex- entire Paibian Stage in the Wangcun Section and corresponds to 13 tinctions occurred during the Cambrian Period. We now tenta- the positive excursion CPEdb-2 in the Duibian A Section; δ C tively discuss the Cambrian carbon-isotope excursions identi- in both sections speedily increases from approximately 1.0‰ to fied in the deepwater facies of the Duibian A Section of western approximately 4.0‰. This positive excursion with an increas- Zhejiang, and in the slope-belt facies of the Wangcun Section ing trend is similar to the SPICE in the lower part of the tradi- of western Hunan, South China, and interpret these excursions tional Upper Cambrian in North America, Siberia, and Austra- associated with the mass extinctions, sea-level changes, and lia (Glumac and Mutti, 2007; Saltzman et al., 2004, 2000, 1998; paleoclimate changes (Fig. 5). Glumac, 2001; Glumac and Walker, 1998). The SPICE com- The carbon-isotope profile shows a large negative excur- mences on the mass extinction of the Marjumiid biomore and sion (CNEwc-1) with a minimum value of -6.0‰ in the Terre- ends above the mass extinction of the Pterocephaliid biomore neuvian Series of the Wangcun Section, and this negative ex- in the Laurentia (Babcock et al., 2015; Zhu et al., 2006). The cursion can be compared with CNEdb-1 in the deepwater basin SPICE in China contemporaneously occurs in the sea-level facies of the Duibian A Section of western Zhejiang. This nega- highstand successions, which consist of laminated argillaceous tive excursion can also be traced in Siberia and Iran (Brasier limestone interbedded with dark calcareous shale, microbialites and Sukhov, 1998; Derry et al., 1994; Brasier et al., 1990). The and stromatolites (Zhang et al., 2015).

Carbon-Isotope Excursions Recorded in the Cambrian System, South China 489

The CPEwc-3 positive excursion (Fig. 5), a new positive are interpreted to be associated with high primary oceanic pro- excursion above the SPICE, has been discovered in both the ductivities, and negative carbon-isotope excursions are inter- slope-belt facies of the Wangcun Section and in the deepwater preted to be associated with mass extinctions and weathering of 13 facies of the Duibian A Section (CPEdb-2). δ C of this positive organic matter on the upper part of the continental shelf during excursion developed on high δ13C background reaches to 2.0‰ periods of Cambrian sea-level fall. from 0 in the Duibian A Section and reaches to 3.0‰ from (4) Additionally, the newly discovered positive carbon- 1.0‰ in the Wangcun Section respectively. The positive excur- isotope excursion in strata above the SPICE, together with the sion CPEwc-3 reflects the abnormal carbon cycles associated three negative carbon-isotope spike excursions in the upper- with a rising sea-level event in South China during the Late most Cambrian, reflect that carbon cycles changed very fre- Cambrian that developed on the basis of high primary biogenic quently during the Late Cambrian Period. They are separately productivities. associated with sea-level rising and falling events that occurred

The CNEwc-3 is a compound negative excursion, including in the Late Cambrian. three adjacent spike negative shifts, apparently occurs in the uppermost Cambrian in the slope facies of the Wangcun Sec- ACKNOWLEDGMENTS tion of western Hunan, whereas the shift is displaced by three This work was supported by the National Natural Science adjacent small scale shifts toward low values on high δ13C Foundation of China (Nos. 41672028, 41672002, 41330101, background values in the deepwater basin facies of the Duibian 41221001). The authors would like to thank Prof. Laishi Zhao A Section of western Zhejiang (Fig. 5). Similarly, three adja- from the State Key Lab of Geological Process and Mineral cent negative spike shifts in the uppermost Cambrian were also Resources of China University of Geosciences, Wuhan, Hubei detected in the Yankong Section of Guizhou Province, South Province, China. Professor Xinggong Kong from the Labora- China (Zuo et al., 2008a). Of the three negative shifts, the mid- tory of Stable Isotope Analysis of the Nanjing Normal Univer- dle one corresponds to the HERB event, which was succes- sity is acknowledged for the help in processing and analyzing sively tested in western Newfoundland, Canada, the eastern the samples. The final publication is available at Springer via Precordillera, Argentina, and in the Kulyumbe River Section in https://doi.org/10.1007/s12583-017-0963-x. northwestern Siberia (Miller et al., 2015, 2011; Kouchinsky et al., 2008; Sial et al., 2008). This compound excursion coincides REFERENCE CITED with horizons of mass extinction of the Ptychaspid biomere, Álvaro, J. J., Vennin, E., 1998. Stratigraphic Signature of a Terminal Early and high 87Sr/86Sr ratios for the uppermost Cambrian (Ebneth et Cambrian Regressive Event in the Iberian Peninsula. 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