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 Cambrian 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 Geology 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 trilobites 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 Hunan, 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 Furongian Series. Three distinctive negative excursions CNEwc-1, 2, 3 were separately tested in the Lower Terreneuvian 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 Guzhangian Stage (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, Jiangshanian 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 trilobite 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 Ordovician 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 carboniferous 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)