Evolution of Carbon Isotope Composition in Potential Global Stratotype Section and Point at Luoyixi, South China, for the Base of the Unnamed Global Seventh Stage of Cambrian System

Journal of China University of Geosciences, Vol. 19, No. 1, p.9–22, February 2008 Printed in China

ISSN 1002-0705

Evolution of Carbon Isotope Composition in Potential Global Stratotype Section and Point at Luoyixi, South China, for the Base of the Unnamed Global Seventh Stage of Cambrian System Zuo Jingxun* (Ꮊ᱃࢟) Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China; Henan Institute of Geological Survey, Zhengzhou 450001, China Peng Shanchi (ᕁ୘∴), Zhu Xuejian (ᴅᄺࠥ), Qi Yuping (⼕⥝ᑇ), Lin Huanling (ᵫ⛩Ҹ), Yang Xianfeng (ᴼᰒዄ) State Key Laboratory of Palaeontology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China ABSTRACT: This work deals with the evolution of carbon isotope composition in the Luoyixi (㔫ձ⑾) Section, a candidate of the Global Standard Stratotype-section and Point (GSSP), defining the base of the as-yet-undefined seventh stage of Cambrian System at the first appearance of the cosmopolitan agnostoid Lejopyge laevigata. This level is favored in a vote of International Subcommission on Cambrian Stratigraphy (ISCS) as the biohorizon for defining the base of a global stage. Two hundred and sixty-four samples for carbon and oxygen isotope analysis have been collected from the carbonate successions at an interval of 0.25 to 0.5 m in this section. Results of the carbon isotope data exhibit a remarkable disciplinarian trend. The pattern of the carbon isotope evolution is gently undulant with a relatively long period during the underlying Drumian Stage, and then the values of į13C fluctuate sharply with a short period in provisional seventh stage. The onset of sharp fluctuation in the į13C values begins at the proposed GSSP level, defining the base of the global seventh stage, where į13C values change from a gentle trend to a sharp trend. Distinct covariant-relationships among į13C, į18O, and sea level fluctuations suggest that a warming change in paleoclimate took place during the early global seventh stage, which led to a positive shift in į13C values. KEY WORDS: carbon isotope composition, global seventh stage of Cambrian System, Luoyixi Section, northwestern Hunan (␪फ). 

This article is supported by the National Natural Science Foundation of China (Nos. 40672023, 40332018), the Chinese Academy of Sciences (KZCX2-YW-122) and the Major Basic Research Project of MST (2006CB806400). *Corresponding author: [email protected] Manuscript received August 30, 2007. Manuscript accepted November 10, 2007.

INTRODUCTION The trend of carbon isotope composition from carbonate successions can be used as an important proxy for stratigraphic division and correlation, definition of key stratigraphical boundaries, reconstruction of paleogeography and paleoenvironment, implication of mass extinction, and recovery of the ecosystem (Yang et al., 2005; Zuo, 2003; Hesselbo et al., 2002; Saltzman, 2002; Saltzman

10

Zuo Jingxun, Peng Shanchi, Zhu Xuejian, Qi Yuping, Lin Huanling and Yang Xianfeng

et al., 1998; Baud et al., 1996, 1989). Moreover, carbon isotopic stratigraphy has been playing an important role in the establishment of the Global Standard Stratotype-section and Points. Cambrian strata are extensively developed and exposed in South China. Discoveries of the Niutitang, Chengjiang, and Kaili faunas (Hou et al., 1999; Zhao et al., 1999a, b, 1994; Chen et al., 1996), and the erection of the first intra-Cambrian GSSP for the Furongian Series and Paibian Stage at Paibi, Huayuan, northwestern Hunan, South China, are the two most important achievements that attract worldwide attention in paleontological and stratigraphical studies (Peng et al., 2004a, b). Recently, a stratigraphic framework on the subdivision of the Cambrian System with four series and ten stages and a number of key levels that mark the bases of stages has been established by the International Subcommission on Cambrian Stratigraphy (ISCS) (Babcock et al., 2005; Peng and Babcock, 2005). On the basis of the new framework, a number of new GSSPs that define the yet unnamed global Cambrian stages will be erected within next years. With candidate stratotype sections in northwestern Hunan, western Zhejiang, and eastern Guizhou, South China holds a great potential of erecting more GSSPs, especially those for the bases of the fifth, seventh, ninth, and tenth stages (Peng, 2006). As one of the requirements for the erection of GSSPs, the carbon isotope record has been well studied for the Precambrian–Cambrian transition (Shen and Schidlowski, 2000; Walter et al., 2000; Kaufman et al., 1996; Magaritz et al., 1986), the traditional Lower–Middle Cambrian transition (Guo et al., 2005), and the base of the Furongian Series (Saltzman et al., 2000). However, as a significant region for the global Cambrian chronostratigraphical studies, South China received no detailed carbon isotope studies for the boundary intervals of other potential global stages within the Cambrian System. In recent years, the authors have devoted themselves to study the potential Cambrian GSSPs in South China. As one of the efforts, this article deals with the carbon isotope stratigraphy of the Luoyixi Section, a GSSP candidate for defining the base of the global seventh stage of the Cambrian System (Peng et al., 2006).

GEOLOGICAL SETTINGS Lithofacies of the Cambrian System in South China were affected by the synchronal rift, which developed along the southeastern margin of the Yangtze platform (Chen, 1991). This area lies on the northwestern side of the rift, which is a broad and extensive Yangtze platform. Areas that lie on the other side of the rift are the Jiangnan slope belt in roughly northeast-trending and the Jiangnan basin. Turbiditic sediments are extensively developed on the Jiangnan slope belt (Qin and Zhao, 1993; Liu et al., 1990; Gao and Duan, 1985). Several key sections such as the Paibi, Luoyixi and Wangcun sections in northwestern Hunan are located on the Jiangnan slope belt (Fig. 1). Sedimental characteristics of the Cambrian System of the Wangcun and Luoyixi sections on both sides of the Youshui River, Yongshun County, have been discussed by previous investigators (Zuo et al., 2006a; Fu et al., 1999). Two lithostratigraphical units, the Aoxi and Huaqiao formations in the Wangcun Section have been revised, with new concepts, by Peng and Robison (2000), and a Middle–Late Cambrian (Wulingian-Furongian epochs) biostratigraphic succession has been documented on the basis of the agnostid trilobites in the same literature. Pre-Furongian carbon isotope stratigraphy has been discussed previously by Zhu et al. (2004). The Luoyixi Section dealt with in this article is considered as one of the candidate stratotypes for the base of the as-yet-unnamed global seventh stage that is the uppermost stage of the equally as-yet-unnamed global third series of the Cambrian System. It is located on the south bank of the Youshui River, opposite to the Wangcun Section, and was previously nominated as the South Wangcun Section. Upsection, the Cambrian in the Luoyixi Section is divided into five formations: the Muchang, Tsinghsutung, Aoxi, Huaqiao, and Zhuitun formations, with a total thickness up to 1 800 m. The Cambrian System overlies conformably on the Neoproterozoic Liuchapo Formation and is overlain by the Early Ordovician Nanjinguan Formation. Lithostratigraphically, the Muchang Formation, the lowermost unit within the Cambrian System in western and northwestern Hunan, is primarily composed of dark carbonaceous shales with minor

Evolution of Carbon Isotope Composition in Potential Global Stratotype Section at Luoyixi, South China North China platform

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Yichang

Chengdu

Xichang

Hefei

Xiangfan

Dazhou Kangding

11

Meizhou

Guangzhou

Nanning 0

250

500 km

N

Luoyixi Section

Qinzhou Zhanjiang

Figure 1. Map showing location of the Luoyixi Section, northwestern Hunan, South China. limestone and sandstone interlayers in the basal and middle parts. The Tsinghsutung Formation is composed of dolostone. The Aoxi Formation is made up of gray thick-bedded dolostone with minor shales in the lower part and calcareous mudstone in the upper part. The Huaqiao Formation consists of gray argillaceous limestone containing lenticular limestone, limestone-ribbon or limestone breccias. The Zhuitun Formation, the uppermost lithostratigraphical unit of Cambrian, consists only of light gray massive dolostone. The į13C curves in this study have been correlated by using the standard trilobite zones. The Cambrian successions of the Jiangnan slope belt are richly fossiliferous. Based on the first appearances of Tricophycus pedum and trilobite species that are significant in global or intercontinental stratigraphical correlation, a four-fold Cambrian chronostratigraphical framework including the Diandongian, Qiandongian, Wulingian and the Furongian series within the Cambrian System has been established in South China (Peng et al., 2004c, 2001; Peng, 2003). The Wulingian Series is further subdivided into three regional stages, namely as the Taijiangian, Wangcunian, and Youshuian stages in ascending order (Fig. 2). Five trilobite zones are recognized in the Wangcunian Stage, including Ptychagnostus punctuosus, Goniagnostus nathorsti, Lejopyge armata, Lejopyge laevigata and Proagnostus

bulbus zones. The Wulingian Series is a regional equivalent of the as-yet-unnamed global third series proposed recently by the ISCS. The studied interval on the carbon isotope stratigraphy in this article includes the upper part of the Wangcunian Stage, which includes the Goniagnostus nathorsti Zone through the Proagnostus bulbus Zone (Peng, 2003). This interval bears the lowest occurrence of Lejopyge laevigata, which is considered to be the key level to define the base of a global stage of the Cambrian System, i.e. the seventh stage of the Cambrian or the uppermost stage of the global third series. Therefore, the study on the carbon and oxygen isotopic record of the Lejopyge laevigata-bearing interval in the Luoyixi Section has a significant impact on the definition of the boundary. METHODS Two hundred and sixty-four samples of carbonates, for carbon and oxygen isotope analysis, have been collected from the lower part of the Huaqiao Formation, at an interval of 0.25–0.5 m. In order to generate relatively high-resolution, continuous stable isotope curves, bulk carbonate samples are carefully collected to avoid later alteration and calcite veins at a later stage. Samples were preferentially drilled from fresh rock surfaces with micro-driller and about 0.2 mg of carbonate powder was used for carbon isotope analysis. Powders to be

Zuo Jingxun, Peng Shanchi, Zhu Xuejian, Qi Yuping, Lin Huanling and Yang Xianfeng

12

System Litho. unit

Global and South China series

South China stages

FAD of Lotagnostus americanus

Huaqiao Fm.

Furongian

Cambrian

GSSP horizons or provisional stratigraphic tie point proposed by ISCS

Stage 10

Zhuitun Fm.

Stage 9

FAD of Agnostotes orientalis

Paibian FAD of Glyptagnostus reticulatus Youshuian Series 3 (Wulingian, South China)

Aoxi Fm.

Wangcunian

Stage 7 Interval for carbon isotope study dealt with this paper

FAD of Lejopyge laevigata Drumian FAD of Ptychagnostus atavus

Taijiangian

Stage 5 FAD of Oryctocephalus indicus

Series 2 Tsinghsutung Fm.

Global stage

(Qiandongian, South China)

Stage 4 FAD of Arthricocephalus chauveaui Stage 3

Muchang Fm.

FAD of Trilobite Terreneuvian (Diandongian, South China)

Stage 2 FAD of SSF or Archaeocyaths Fortunian

EdiacaLiuchapo Fm. ran

FAD of Trichophycus pedum

Figure 2. Stratigraphical intervals for carbon isotopic study in the Luoyixi Section, northwestern Hunan, South China. the Luoyixi Section (Fig. 3). Additionally, values of į18O also are used as an important marker to judge carbonate diagenesis. For instance, values of į18O that are lighter than -10‰ or -11.0‰ (Kaufman et al., 1993; Derry et al., 1992) suggest that the carbonates could be experiencing stronger diagenesis. į18O values of almost all samples collected from the Wulingian Series in the Luoyixi Section range from -6.63‰ to 3.0 2.0

1.0 -12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

18

¥ O (ă) (PDB) -1.0

¥13C (ă) ( PDB )

analyzed were roasted for 1 h to remove volatile contaminants and then reacted with anhydrous phosphoric acid in vacuum bottles at 25 ć. All samples were analyzed at the Laboratory of Stable Isotopes, Faculty of Geographical Sciences, Nanjing Normal University, Jiangsu Province, where carbonate powders were conducted in an online carbonate preparation line with a Finnigan-MAT 253 mass spectrometer. Data are reported in per mill (‰), relative to the PDB standard. An international standard was first calibrated by using NBS-19 (į13C = 1.95‰, į18O = -2.2‰; PDB standard). Precision is monitored through daily analyses of carbonate standards and is better than 0.1‰ for both carbon and oxygen isotope compositions. Fractionation between carbon and oxygen isotopic compositions in marine carbonate and in pore-water took place during diagenesis. Later alteration can produce much lighter į18O, on account of exchanges with meteoric waters. Based on studies a positive relationship between į13C and į18O occurs in altered carbonate rocks (Wang and Feng, 2002; Qing and Veizer, 1994). The relative-coefficient of į13C and į18O is 0.056 in an interval of the Wulingian Series in

-2.0

-3.0 13

Figure 3. Relationship between į C and į18O values from the Luoyixi Section, northwestern Hunan, South China.

Evolution of Carbon Isotope Composition in Potential Global Stratotype Section at Luoyixi, South China

-9.99‰, indicating a weaker diagenesis of the carbonates, although five samples, S-340, S-364, S-510, S-512 and S-526 have į18O values shifting from -10.0‰ to -10.49‰. Therefore, oxygen isotopic composition in bulk carbonate indicates that the carbonates keep primary record of marine sediments. DISCUSSION Evolution of Carbon Isotope Composition į13C values across the transition from the upper Drumian Stage to the lower global seventh stage are present in the stratigraphic profile in Fig. 4 (also see Table 1). Samples collected from the lower part of the Huaqiao Formation have a maximum į13C value of 1.99‰, a minimum į13C value of -2.47‰ and a mean value of -0.09‰. Most of the į13C values fall into the range of -0.98‰ to 1.22‰. Disciplinarian changes in į13C values can be seen on the curve generated from the measured section. Based on the periodic occurrence of low į13C values, the trend of carbon isotope evolution through the lower part of the Huaqiao Formation has been divided into six cycles (C1–C6). Characteristics of carbon isotopic cycles are as follows. The upper part of the carbon isotopic cycle C1 corresponds to the middle part of the Goniagnostus nathorsti Zone in the upper Drumian Stage (Babcock et al., 2004) and spans a lithostratigraphical interval of 17 m thick (Fig. 4). The carbon isotopic curve presents a wide shape with a limited range of fluctuations, in which the į13C values oscillate between -0.68‰ and 0.53‰ with a mean value of -0.17‰. į13C values drop gradually from positive to negative values in the upper part of the cycle. spans a Carbon isotopic cycle C2 lithostratigraphical interval of 25 m thick and corresponds to a biostraigraphical interval occupied by the uppermost part of the Goniagnostus nathorsti Zone and the Lejopyge armata Zone as a whole, which is the uppermost bio-zone of the Drumian Stage. The carbon isotopic curve also shows a wide shape with a small range of fluctuation, in which į13C values rise gradually from -0.85 ‰ to 0.15‰ near the base of the Lejopyge armata Zone before a gentle fall appears in the upper part of the bio-zone with a minimum value of -0.85‰ at the base of the Lejopyge laevigata

13

Zone, where the proposed GSSP position for global seventh stage (Peng and Babcock, 2005) lies. Compared to the lower part of the cycle C2, the value of į13C in the upper part of the cycle drops more quickly. Carbon isotopic cycle C3 occurs in the lower part of the Lejopyge laevigata Zone and spans a lithostratigraphical interval of 24 m thick with a mean į13C value of -0.10‰. In the lower part of the cycle, į13C keeps undulated increase in values from -0.84‰ to 0.75‰, whereas it drops quickly in the upper part of the cycle with values ranging from 0.75‰ to -0.83‰. The middle part of the cycle shows a distinctively positive excursion. Carbon isotopic cycle C4 occurs in the middle part of the Lejopyge laevigata Zone, occupying a 17 m thick lithostratigraphic interval. The į13C value oscillates notably within the cycle, ranging from -1.46‰ to 1.95‰ with a mean value of -0.02‰. An undulated increase occurs in the lower part of cycle C4 and is followed by a drop in the upper part of the cycle and thus displays a relatively positive excursion in the middle part of the cycle. The į13C value drops to -1.46‰ in the middle part of the Lejopyge laevigata Zone. Compared to the highest value in the middle part of the cycle, the oscillation extent of į13C in cycle C4 reaches up to 3.41‰. Carbon isotopic cycle C5 has been detected within the upper part of the Lejopyge laevigata Zone, spanning a lithostratigraphical interval of 22.5 m thick. In the lower part of this cycle the į13C value oscillates sharply, but in the upper part of the cycle the į13C value fluctuates in a range of -0.98‰ to 1.99‰, with the exception of the minimum value of -2.44‰. The mean value of į13C in cycle C5 is 0.25‰. In general, the į13C values in the cycle show an increasing trend in the lower part, and a positive excursion trend with the maximum oscillated extent of 3.45‰ in the middle part of the cycle. Carbon isotopic cycle C6 is 20.5 m in thickness, which corresponds to the lower part of the Proagnostus bulbus Zone. The į 13 C value varies within a range of -0.67‰ to 1.14‰ with the mean value of -0.11‰. The evolution pattern of the carbon isotope shows a sharp increase in the į13C values in

14

Zuo Jingxun, Peng Shanchi, Zhu Xuejian, Qi Yuping, Lin Huanling and Yang Xianfeng

Figure 4. Evolution of carbon and oxygen isotopes in the Wulingian Series in the Luoyixi Section, northwestern Hunan. 1. Massive argillaceous limestone; 2. ribbon argillaceous limestone; 3. limestone contains argillaceous ribbons; 4. limestone; 5. massive argillaceous limestone with lenticular limestone; 6. lenticular or breccia limestone; Sp-01. number of parasequence; C1. number of carbon isotopic cycle.

Evolution of Carbon Isotope Composition in Potential Global Stratotype Section at Luoyixi, South China Table 1 Sample

15

Data of carbon and oxygen isotopes, Luoyixi Section, northwestern Hunan (PDB)

Lithology

Depth (m) į13C (‰)

į18O (‰) Sample Lithology

Depth (m)

į13C (‰)

į18O (‰)

S-528

Argillaceous limestone

211.4

-0.22

-9.30

S-396

Micritic limestone

178.4

0.11

-9.38

S-526

Argillaceous limestone

210.9

-0.20

-10.91

S-394

Micritic limestone

177.9

0.15

-8.62

S-524

Argillaceous limestone

210.4

-0.14

-9.20

S-392

Micritic limestone

177.4

-0.04

-9.26

S-522

Argillaceous limestone

209.9

-0.29

-8.67

S-390

Micritic limestone

176.9

-0.21

-8.76

S-520

Micritic limestone

209.4

-0.58

-9.14

S-388

Micritic limestone

176.4

0.56

-9.04

S-518

Argillaceous limestone

208.9

-0.22

-9.99

S-386

Micritic limestone

175.9

0.61

-9.05

S-516

Argillaceous limestone

208.4

-0.37

-9.95

S-384

Micritic limestone

175.4

0.77

-9.35

S-514

Micritic limestone

207.9

-0.08

-9.08

S-382

Micritic limestone

174.9

1.99

-9.36

S-512

Argillaceous limestone

207.4

-0.14

-10.15

S-380

Micritic limestone

174.4

0.79

-9.01

S-510

Micritic limestone

206.9

-0.15

-10.11

S-378

Micritic limestone

173.9

-0.27

-9.36

S-508

Argillaceous limestone

206.4

-0.22

-9.03

S-376

Micritic limestone

173.4

-0.19

-9.49

S-506

Argillaceous limestone

205.9

-0.39

-8.83

S-374

Micritic limestone

172.9

0.89

-9.41

S-504

Argillaceous limestone

205.4

-0.38

-9.81

S-372

Micritic limestone

172.4

0.87

-9.49

S-502

Argillaceous limestone

204.9

-0.50

-8.85

S-370

Micritic limestone

171.9

0.80

-9.53

S-500

Micritic limestone

204.4

-0.50

-8.94

S-368

Micritic limestone

171.4

1.19

-8.81

S-498

Argillaceous limestone

203.9

-0.40

-8.19

S-366

Micritic limestone

170.9

0.27

-8.45

S-496

Argillaceous limestone

203.4

-0.40

-8.77

S-364

Micritic limestone

170.4

-0.19

-10.42

S-494

Argillaceous limestone

202.9

-0.27

-8.05

S-362

Micritic limestone

169.9

0.99

-9.16

S-492

Argillaceous limestone

202.4

-0.35

-8.66

S-360

Argillaceous limestone

169.4

0.09

-9.35

S-490

Argillaceous limestone

201.9

-0.67

-8.46

S-358

Argillaceous limestone

168.9

0.03

-9.34

S-488

Argillaceous limestone

201.4

-0.13

-9.24

S-356

Micritic limestone

168.4

0.08

-9.21

S-486

Argillaceous limestone

200.9

-0.42

-9.18

S-354

Micritic limestone

167.9

-2.47

-8.96

S-484

Argillaceous limestone

200.4

-0.35

-8.70

S-352

Micritic limestone

167.4

0.15

-9.18

S-482

Argillaceous limestone

199.9

-0.22

-9.14

S-350

Micritic limestone

166.9

-0.29

-9.46

S-480

Argillaceous limestone

199.4

-0.17

-9.37

S-348

Argillaceous limestone

166.4

-0.22

-8.42

S-478

Argillaceous limestone

198.9

0.04

-9.59

S-346

Argillaceous limestone

165.9

0.86

-9.33

S-476

Argillaceous limestone

198.4

0.20

-9.46

S-344

Micritic limestone

165.4

-0.30

-9.44

S-474

Argillaceous limestone

197.9

0.38

-8.09

S-342

Micritic limestone

164.9

-0.98

-8.54

S-472

Micritic limestone

197.4

0.41

-8.04

S-340

Micritic limestone

164.4

0.09

-10.03

S-470

Micritic limestone

196.9

0.79

-9.18

S-338

Argillaceous limestone

163.9

-0.39

-9.06

S-468

Argillaceous limestone

196.4

-0.38

-9.18

S-337

Argillaceous limestone

163.7

0.44

-9.29

S-466

Argillaceous limestone

195.9

0.56

-8.85

S-336

Argillaceous limestone

163.4

0.41

-9.72

S-464

Argillaceous limestone

195.4

1.14

-9.43

S-334

Argillaceous limestone

162.9

-1.46

-9.41

S-462

Argillaceous limestone

194.9

0.96

-9.49

S-332

Argillaceous limestone

162.4

-0.28

-9.40

S-460

Argillaceous limestone

194.4

0.31

-9.32

S-330

Argillaceous limestone

161.9

-0.09

-8.36

S-458

Micritic limestone

193.9

-0.11

-9.63

S-328

Argillaceous limestone

161.4

-0.68

-9.16

S-456

Argillaceous limestone

193.4

0.00

-8.93

S-326

Argillaceous limestone

160.9

-0.47

-9.28

S-454

Argillaceous limestone

192.9

-0.26

-9.01

S-324

Argillaceous limestone

160.4

0.97

-9.25

S-452

Argillaceous limestone

192.4

-0.29

-8.02

S-322

Argillaceous limestone

159.9

0.12

-9.44

S-450

Argillaceous limestone

191.9

0.00

-9.34

S-320

Argillaceous limestone

159.4

-0.24

-9.21

S-448

Argillaceous limestone

191.4

-0.27

-9.32

S-318

Argillaceous limestone

158.9

0.65

-9.37

S-446

Argillaceous limestone

190.9

-0.22

-8.52

S-316

Micritic limestone

158.4

0.25

-9.39

S-444

Argillaceous limestone

190.4

0.03

-8.87

S-314

Micritic limestone

157.9

-0.58

-9.72

S-442

Ribbon limestone

189.9

0.07

-9.13

S-312

Argillaceous limestone

157.4

-0.40

-9.44

S-440

Ribbon limestone

189.4

0.15

-9.30

S-310

Argillaceous limestone

156.9

-0.43

-9.60

Zuo Jingxun, Peng Shanchi, Zhu Xuejian, Qi Yuping, Lin Huanling and Yang Xianfeng

16 Continued Sample

Lithology

Depth (m)

į13C (‰)

į18O (‰) Sample Lithology

Depth (m)

į13C (‰)

į18O (‰)

S-438

Ribbon limestone

188.9

0.15

-9.45

S-308

Argillaceous limestone

156.4

0.13

-9.58

S-436

Ribbon limestone

188.4

-0.27

-8.53

S-307

Argillaceous limestone

155.9

1.95

-9.48

S-434

Ribbon limestone

187.9

-0.14

-8.58

S-304

Argillaceous limestone

155.4

0.98

-8.72

S-432

Ribbon limestone

187.4

-0.30

-8.85

S-302

Argillaceous limestone

154.9

1.00

-9.48

S-430

Ribbon limestone

186.9

-0.02

-9.26

S-300

Argillaceous limestone

154.4

-0.05

-9.94

S-428

Ribbon limestone

186.4

-0.08

-9.21

S-298

Argillaceous limestone

153.9

0.05

-9.36

S-426

Ribbon limestone

185.9

0.31

-9.31

S-296

Argillaceous limestone

153.4

-0.33

-8.25

S-424

Ribbon limestone

185.4

0.10

-9.27

S-294

Argillaceous limestone

152.9

-0.03

-9.39

S-422

Ribbon limestone

184.9

-0.17

-9.38

S-292

Argillaceous limestone

152.4

-0.50

-8.18

S-420

Ribbon limestone

184.4

0.28

-9.36

S-290

Argillaceous limestone

151.9

-0.50

-9.08

S-418

Micritic limestone

183.9

-0.18

-8.80

S-288

Argillaceous limestone

151.4

-0.36

-9.29

S-416

Micritic limestone

183.4

0.41

-9.45

S-286

Argillaceous limestone

150.9

-0.11

-9.25

S-414

Micritic limestone

182.9

1.11

-9.40

S-284

Argillaceous limestone

150.4

1.22

-9.47

S-412

Micritic limestone

182.4

0.32

-9.46

S-282

Argillaceous limestone

149.9

0.14

-9.23

S-410

Micritic limestone

181.9

-0.22

-9.41

S-280

Argillaceous limestone

149.4

-0.23

-9.41

S-408

Micritic limestone

181.4

0.44

-9.33

S-278

Argillaceous limestone

148.9

-0.60

-9.33

S-406

Micritic limestone

180.9

0.86

-9.45

S-276

Argillaceous limestone

148.4

0.31

-9.54

S-404

Micritic limestone

180.4

1.08

-9.37

S-274

Argillaceous limestone

147.9

-0.47

-9.46

S-402

Micritic limestone

179.9

-0.48

-9.31

S-272

Argillaceous limestone

147.4

-0.04

-9.62

S-400

Micritic limestone

179.4

0.95

-9.18

S-270

Argillaceous limestone

146.9

-0.17

-9.26

S-398

Micritic limestone

178.9

0.13

-9.41

S-268

Argillaceous limestone

146.4

-0.29

-9.47

S-266

Argillaceous limestone

145.9

-0.14

-9.39

S-132

Argillaceous limestone

112.4

-0.25

-8.99

S-264

Argillaceous limestone

145.4

-0.27

-9.69

S-130

Argillaceous limestone

111.9

0.15

-9.58

S-262

Argillaceous limestone

144.9

-0.75

-9.49

S-128

Argillaceous limestone

111.4

-0.09

-9.37

S-260

Argillaceous limestone

144.4

-0.83

-8.62

S-126

Argillaceous limestone

110.9

-0.07

-9.37

S-258

Argillaceous limestone

143.9

-0.47

-9.20

S-124

Micritic limestone

110.4

-0.16

-9.58

S-256

Micritic limestone

143.4

-0.43

-8.52

S-122

Micritic limestone

109.9

-0.14

-9.12

S-254

Argillaceous limestone

142.9

-0.10

-9.53

S-120

Argillaceous limestone

109.4

-0.29

-8.42

S-252

Argillaceous limestone

142.4

0.10

-8.10

S-118

Micritic limestone

108.9

-0.32

-9.39

S-250

Argillaceous limestone

141.9

0.35

-9.24

S-114

Micritic limestone

107.9

0.05

-9.59

S-248

Micritic limestone

141.4

0.39

-9.27

S-112

Argillaceous limestone

107.4

-0.60

-7.97

S-246

Micritic limestone

140.9

0.03

-8.90

S-110

Argillaceous limestone

106.9

-0.13

-9.17

S-244

Micritic limestone

140.4

-0.23

-8.43

S-108

Argillaceous limestone

106.4

-0.20

-9.65

S-242

Micritic limestone

139.9

-0.11

-8.79

S-106

Argillaceous limestone

105.9

-0.59

-8.61

S-240

Argillaceous limestone

139.4

-0.19

-8.87

S-104

Micritic limestone

105.4

0.04

-9.53

S-238

Argillaceous limestone

138.9

0.13

-9.49

S-102

Argillaceous limestone

104.9

-0.28

-8.36

S-236

Argillaceous limestone

138.4

-0.14

-8.24

S-100

Argillaceous limestone

104.4

-0.59

-8.02

S-234

Argillaceous limestone

137.9

-0.09

-8.47

S-098

Argillaceous limestone

103.9

-0.36

-9.52

S-232

Argillaceous limestone

137.4

0.75

-9.39

S-096

Argillaceous limestone

103.4

-0.31

-8.98

S-230

Argillaceous limestone

136.9

-0.24

-8.98

S-094

Argillaceous limestone

102.9

-0.35

-9.62

S-228

Micritic limestone

136.4

0.51

-9.55

S-092

Argillaceous limestone

102.4

-0.52

-7.79

S-226

Argillaceous limestone

135.9

0.03

-8.46

S-090

Argillaceous limestone

101.9

-0.61

-9.12

S-224

Micritic limestone

135.4

-0.05

-7.90

S-088

Argillaceous limestone

101.4

-0.22

-8.54

S-222

Argillaceous limestone

134.9

0.22

-8.62

S-086

Argillaceous limestone

100.9

-0.42

-7.81

S-220

Argillaceous limestone

134.4

0.69

-9.40

S-084

Micritic limestone

100.4

-0.35

-7.75

Evolution of Carbon Isotope Composition in Potential Global Stratotype Section at Luoyixi, South China

17

Continued Sample

Lithology

Depth (m) į13C (‰)

į18O (‰) Sample Lithology

Depth (m)

į13C (‰)

į18O (‰)

S-218

Micritic limestone

133.9

0.02

-9.49

S-082

Argillaceous limestone

99.9

0.03

-9.25

S-216

Micritic limestone

133.4

0.30

-9.28

S-080

Micritic limestone

99.4

-0.05

-9.43

S-214

Argillaceous limestone

132.9

0.13

-9.15

S-078

Argillaceous limestone

98.9

-0.67

-8.68

S-212

Argillaceous limestone

132.4

0.18

-9.43

S-076

Argillaceous limestone

98.4

0.06

-9.78

S-210

Argillaceous limestone

131.9

-0.26

-9.35

S-074

Argillaceous limestone

97.9

-0.66

-8.32

S-208

Argillaceous limestone

131.4

-0.04

-9.24

S-072

Argillaceous limestone

97.4

-0.06

-9.50

S-206

Argillaceous limestone

130.9

0.01

-9.38

S-070

Argillaceous limestone

96.9

-0.73

-7.60

S-204

Argillaceous limestone

130.4

-0.34

-8.61

S-068

Argillaceous limestone

96.4

-0.64

-7.75

S-202

Argillaceous limestone

129.9

-0.56

-9.37

S-066

Argillaceous limestone

95.9

-0.64

-8.83

S-200

Argillaceous limestone

129.4

-0.48

-9.40

S-064

Micritic limestone

95.4

-0.31

-7.93

S-198

Argillaceous limestone

128.9

0.17

-9.38

S-062

Argillaceous limestone

94.9

-0.15

-7.57

S-196

Argillaceous limestone

128.4

-0.55

-8.84

S-060

Argillaceous limestone

94.4

-0.18

-9.35

S-194

Argillaceous limestone

127.9

-0.63

-9.37

S-058

Micritic limestone

93.9

-0.52

-7.55

S-192

Argillaceous limestone

127.4

-0.84

-8.18

S-056

Argillaceous limestone

93.4

-0.68

-8.00

S-190

Argillaceous limestone

126.9

-0.76

-8.91

S-054

Argillaceous limestone

92.9

-0.59

-8.28

S-188

Argillaceous limestone

126.4

-0.33

-9.30

S-052

Argillaceous limestone

92.4

-0.13

-9.30

S-186

Argillaceous limestone

125.9

-0.23

-9.61

S-050

Argillaceous limestone

91.9

-0.12

-8.96

S-184

Argillaceous limestone

125.4

-0.47

-9.41

S-048

Argillaceous limestone

91.4

-0.34

-7.67

S-181

Argillaceous limestone

124.9

-0.56

-9.86

S-046

Argillaceous limestone

90.9

-0.02

-9.47

S-180

Argillaceous limestone

124.4

-0.43

-8.73

S-044

Argillaceous limestone

90.4

-0.61

-8.91

S-178

Argillaceous limestone

123.9

-0.73

-8.69

S-042

Argillaceous limestone

89.9

-0.51

-9.26

S-176

Micritic limestone

123.4

-0.57

-8.07

S-040

Argillaceous limestone

89.4

0.10

-9.08

S-174

Argillaceous limestone

122.9

-0.19

-9.10

S-038

Argillaceous limestone

88.9

-0.25

-8.04

S-172

Argillaceous limestone

122.4

-0.39

-8.91

S-036

Micritic limestone

88.4

-0.07

-6.63

S-170

Argillaceous limestone

121.9

-0.85

-9.56

S-034

Argillaceous limestone

87.9

-0.15

-6.91

S-168

Argillaceous limestone

121.4

-0.58

-9.23

S-032

Argillaceous limestone

87.4

-0.22

-6.91

S-164

Micritic limestone

120.4

-0.55

-9.26

S-030

Micritic limestone

86.9

0.13

-9.69

S-162

Argillaceous limestone

119.9

-0.62

-8.98

S-028

Argillaceous limestone

86.4

-0.23

-9.38

S-160

Micritic limestone

119.4

-0.71

-9.49

S-026

Argillaceous limestone

85.9

-0.24

-7.54

S-158

Argillaceous limestone

118.9

-0.39

-8.93

S-024

Argillaceous limestone

85.4

-0.27

-7.12

S-156

Micritic limestone

118.4

-0.12

-9.74

S-022

Argillaceous limestone

84.9

0.24

-9.69

S-154

Argillaceous limestone

117.9

-0.76

-8.32

S-020

Micritic limestone

84.4

-0.06

-6.81

S-152

Argillaceous limestone

117.4

-0.43

-8.64

S-018

Argillaceous limestone

83.9

-0.46

-7.58

S-150

Argillaceous limestone

116.9

0.01

-9.12

S-016

Argillaceous limestone

83.4

0.18

-9.41

S-148

Argillaceous limestone

116.4

-0.09

-8.00

S-014

Micritic limestone

82.9

-0.13

-7.17

S-146

Argillaceous limestone

115.9

-0.10

-7.81

S-012

Argillaceous limestone

82.4

-0.19

-6.82

S-144

Argillaceous limestone

115.4

-0.22

-7.45

S-010

Argillaceous limestone

81.9

0.14

-9.35

S-142

Argillaceous limestone

114.9

-0.37

-9.45

S-008

Argillaceous limestone

81.4

0.05

-7.60

S-140

Argillaceous limestone

114.4

0.03

-9.32

S-006

Argillaceous limestone

80.9

-0.03

-7.33

S-138

Micritic limestone

113.9

0.03

-9.37

S-004

Argillaceous limestone

80.4

0.01

-7.02

S-136

Argillaceous limestone

113.4

0.01

-9.36

S-002

Argillaceous limestone

79.9

0.53

-9.55

S-134

Argillaceous limestone

112.9

-0.14

-8.98

S-000

Argillaceous limestone

79.4

0.31

-9.48

18

Zuo Jingxun, Peng Shanchi, Zhu Xuejian, Qi Yuping, Lin Huanling and Yang Xianfeng

the lower part of the cycle, a relatively positive excursion with large oscillated extent of 1.7‰ in the middle part, and a sharp drop in the upper part of the cycle. As discussed in details earlier, the evolution pattern of the carbon isotope in the undefined global third series (or Wulingian Series as used in South China) of the Cambrian System in the Luoyixi Section presents three features: (1) pattern of carbon isotope evolution, characterized by small range oscillation, is more opened in the late Drumian Stage than in the undefined global seventh stage; (2) į13C values periodically change in the upper and the lower parts of the Lejopyge laevigata Zone, but sharply oscillate with a trend to higher values in the middle part of the biozone, suggesting an isotopic abnormality occurs in the lower part of the undefined global seventh stage; (3) the first appearance datum (FAD) of the cosmopolitan agnostoid Lejopyge laevigata, which indicates the base of the global seventh stage, occurs at 121.3 m above the top of Huaqiao Formation, where the trend of į13C changes from a relatively stable pattern to a sharply fluctuated pattern. Factors Controlling Carbon Isotope Evolution Many models, such as, the sedimentary basins, paleogeographical environment, redox process, sea-level fluctuations, mass extinction and recovery, evaporation process, volcanic activities, methane release, changes of the global climate, and diagenesis, are applied to interpret carbon isotope excursions (Zuo et al., 2006b; Jacobsen, 2001; Hesselbo et al., 2000; Hoffman et al., 1998; Dickens et al., 1995; Holser et al., 1989). Two mass trilobite extinctions were recognized in Cambrian, based on the studies of biostratigraphy. One trilobite extinction, the redlichiid-olenellid extinction, occurred in the latest traditionally Early Cambrian, possibly in two pulses (Erwin, 2001) and the other trilobite extinction occurred at the GSSP of the Furongian Series (Peng and Robison, 2000; Öpik, 1966; Palmer, 1965). The evolutional trend of the carbon isotope across the transition from the uppermost part of the global second series to the lowermost part of the global third series exhibits a large scale negative excursion on the Jiangnan slope

belt, South China (Guo et al., 2005; Zhu et al., 2004), which is termed as ROECE (Zhu et al., 2006). However, the carbon isotope composition from the carbonate strata of the lower part of the Furongian Series in South China suggests that a remarkable positive carbon isotope excursion (SPICE) begins near the base of the Glyptagnotus reticulatus Zone, which is coeval with the damesellid trilobite extinction. The SPICE excursion can also be recognized in the Kyrshabakty Section, southern Kazakhstan; the Core Section, northwestern Queensland, Australia; the Shingle Pass Section, the Great Basin, USA and in the Andearum-3 Core Drilling, Scania, Sweden (Ahlberg et al., 2006; Saltzman et al., 2000). Surprisingly, the first trilobite extinction, the redlichiid-olenellid extinction coincides with the distinct negative carbon isotope excursion beginning near the top of the global series 2, but the second trilobite extinction, damesellid trilobite extinction coincides with the onset of large scale positive SPICE carbon isotope excursion in the basal part of the Furongian Series (a reduced Upper Cambrian as used traditionally). The causes of the two different carbon isotope excursions seem to be associated with sea level fluctuations. The first-order sequence surface was identified on the top of the traditional Lower Cambrian Series in Sichuan, Guizhou and Hunan, South China, terrigenous clasts interlayers occur above the sequence stratigraphic surface (Yang and Xu, 1997), suggesting that a rapid large-scale sea-level falling had began in the late period of the lower half of the Cambrian, when the marine ecosystematic environment devastated gradually, the shelf of the continent was exposed after sea water regression, the organic matters oxygenated, the methane buried in the sediments was released, and the C12-rich was moved to the sea water or went into the atmosphere, therefore, the carbon isotope ratio values from the marine carbonates showed distinct negative excursion during the interval from the Cambrian Epoch 2 to the early of Cambrian Epoch 3. Another sea-level falling was identified, which began with the onset of the Furongian Series in the western United States, where one of second-order sequence boundaries occurred near the terminal of the SPICE excursion (Glumac and Mutti, 2007; Saltzman et al., 2004). Trilobite

Evolution of Carbon Isotope Composition in Potential Global Stratotype Section at Luoyixi, South China

extinction associated with this sea-level falling reached up to eighty species (Öpik, 1966). The onset of the SPICE positive carbon isotope excursion was consistent with the horizon of mass extinction. A large amount of marine organic matter was quickly buried by sediments during gentle sea level falling in the early Furongian Epoch. Up to now, no other mass trilobite extinction event has been recognized in the undefined global third series of the Cambrian System, therefore, shifts of carbon and oxygen isotope within the transition from the Drumian Stage to the global seventh stage is likely related to sea level fluctuations. Furthermore, several small-scale sea level fluctuations have been recognized in the measured Luoyixi Section, northwestern Hunan, South China, based on the parasequences of carbonates identified in field investigations. Parasequences of carbonate successions are well developed in the Luoyixi Section. Sixteen parasequences have been recognized in the low part of the Huaqiao Formation (Zuo et al., 2006a). In general, the upper part of each parasequence composed of deep-water carbonates of gray ribbon limestone, thin-bedded argillaceous limestone

19

interbedded with micritic limestone indicates the carbonate successions with relatively heavier į13C values was deposited during sea-level rising stages. Shallow-water facies association composed of dark-gray medium-thick-bedded argillaceous limestone containing richly lenticular limestone-bodies with relatively lighter į13C values is abundant in the lower part of each parasequences. The micritic lenticular limestone-bodies margined clearly with the ambient rocks indicate that the argillaceous carbonate was deposited during sea-level falling stage. Similarly, the evolutional periods of oxygen isotope are consistent with parasequences composed of carbonate successions. The upper part of each parasequence made up of deep-water carbonate has lighter į 18 O values than the lower part of each parasequence composed of shallow-water carbonate (Fig. 5). The positive carbon isotope shift detected in the early part of global seventh stage corresponds to the high sea-level facies associations with the low į 18 O values, suggesting that a good covariant-relationship existed among į13C, į18O and sea-level fluctuations (Fig. 4). This indicates that a

Figure 5. Relationship between carbon and oxygen isotope values with sea level fluctuations. 1. Massive argillaceous limestone; 2. ribbon argillaceous limestone; 3. limestone; 4. lenticular limestone.

Zuo Jingxun, Peng Shanchi, Zhu Xuejian, Qi Yuping, Lin Huanling and Yang Xianfeng

20

warming change in paleoclimate took place during the early global seventh stage, when a large amount of fresh water flowed into the ocean, and made the sea area was largely extended. This causes the increase of primary biogenic productivities, the high absorption of carbon-12 into organic matters. As a result, and the carbonates deposited during this time obtained high į13C values. During the early global seventh stage, the highest sea-level stage, the carbonate successions deposited on the Jiangnan slope belt were frequently affected by the ocean currents; therefore, the trend of į13C evolution hovered sharply.

Axhaimer, N., ed., The Lower and Middle Cambrian of Sweden: Trilobites, Biostratigraphy and Intercontinental Correlation. Litholund Theses 10, Doctorial Thesis, Department of Geology, Lund University, Lund. VII-1–13 Babcock, L. E., Peng, S. C., Geyer, G., et al., 2005. Changing Perspectives

on

Cambrian

Chronostratigraphy

and

Progress toward Subdivision of the Cambrian System. Geosciences Journal, 9: 101–106 Babcock, L. E., Rees, M. N., Robison, R. A., et al., 2004. Potential Global Standard Stratotype-Section and Point (GSSP) for a Cambrian Stage Boundary Defined by the First Appearance of the Trilobite Ptychagnostus atavus, Drum Mountains, Utah, USA. Geobios, 37: 149–158

CONCLUSIONS (1) Carbon isotope ratios fluctuate gently in longer periods in the late global Drumian Stage, but fluctuate sharply in shorter periods in the early global seventh stage, implicating an abnormality of global carbon isotope cycle took place during this time. (2) The onset of sharp fluctuation in į13C values begins at the proposed GSSP level defining the base of the global seventh stage, where į13C values change from a gentle trend to a sharp trend in the Luoyixi Section. (3) Distinct covariant-relationship among į13C, į18O and sea level fluctuations indicates that a warming change in paleoclimate took place during the early global seventh stage, which led to a positive shift in į13C values.

Baud, A., Atudorei, V., Sharp, Z., 1996. Late Permian and Early Triassic Evolution of the Northern Indian Margin: Carbon Isotope and Sequence Stratigraphy. Geodinamica Acta (Paris), 9(2–3): 57–77 Baud, A., Magaritz, M., Holser, W. T., 1989. Permian-Triassic of the Tethys: Carbon Isotope Studies. Geolcgische Rundschau, 78: 649–677 Chen, J. Y., Zhou, G. Q., Zhu, M. Y., et al., 1996. The Chengjiang Biota: A Unique Window of the Cambrian Explosion.

National

Museum

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Science,

Taichung. 222 (in Chinese with English Abstract) Chen, Z. M., 1991. A Discussion on Geologic Background for Carbonate Gravity Flows in Early Palaeozoic Carbonate Rocks on Yangtze Platform. Scientia Geologica Sinica, 4: 337–345 (in Chinese with English Abstract) Derry, L. A., Kaufman, A. J., Jacobsen, S. B., 1992. Sedimentary Cycling and Environmental Change in the

ACKNOWLEDGMENTS The authors would like to thank Dr. Zhang Gangya from the Nanjing Institute of Soil Science, Chinese Academy of Sciences, and Dr. Kong Xinggong from the Laboratory of Stable Isotope Analysis, the Nanjing Normal University for their helps in analyzing the samples. This work is supported by the National Natural Science Foundation of China (Nos. 40672023, 40332018), the Chinese Academy of Sciences (KZCX2-YW-122), and the Major Basic Research Project of MST (2006CB806400).

Late Proterozoic: Evidence from Stable and Radiogenic Isotopes.

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1317–1329 Dickens, G. R., O’Neil, J. R., Rea, D. K., et al., 1995. Dissociation of Oceanic Methane Hydrate as a Cause of the Carbon Isotope Excursion at the End of the Paleocene. Paleoceanography, 10: 965–972 Erwin, D., 2001. Lessons from the Past: Biotic Recoveries from Mass Extinctions. PNAS, 98(10): 5399–5403 Fu, Q. L., Zhou, Z. C., Peng, S. C., et al., 1999. Sedimentology of Candidate Sections for the Middle-Upper Cambrian Boundary Stratotype in Western Hunan, China. Scientia

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