Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China

G Model PRECAM-3496; No. of Pages 20

ARTICLE IN PRESS Precambrian Research xxx (2012) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Precambrian Research journal homepage: www.elsevier.com/locate/precamres

Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China Da Li a,b,∗ , Hong-Fei Ling a,∗ , Graham A. Shields-Zhou b,∗ , Xi Chen a , Lorenzo Cremonese b , Lawrence Och b , Matthew Thirlwall c , Christina J. Manning c a

State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China Deptartment of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK c Deptartment of Earth Sciences, Royal Holloway, University of London, Egham Hill, Egham, Surrey TW20 0EX, UK b

a r t i c l e

i n f o

Article history: Received 18 March 2011 Received in revised form 23 December 2011 Accepted 6 January 2012 Available online xxx Keywords: Carbon isotopes Strontium isotopes Ediacaran–Cambrian Xiaotan section Yangtze platform

a b s t r a c t This study reports a high-resolution carbon and strontium isotope profile for the Ediacaran–Cambrian Xiaotan section, situated in northeastern Yunnan, South China. Xiaotan section represents a more distal setting than the more condensed Meishucun section in eastern Yunnan, and is, biostratigraphically and chemostratigraphically, the best constrained section on the Yangtze platform covering the Ediacaran–Cambrian interval. The carbonate carbon isotopic ratios of the late Ediacaran, upper Dengying Formation to the early Cambrian, Zhujiaqing Formation, exhibit a large negative excursion (N1, −12.2‰) in the Daibu Member, just below the first appearance of small shelly fossils (SSF) at the base of the Zhongyicun Member, and a sustained pre-‘Tommotian’ positive excursion (P4, ı13 C up to +7.3‰) which coincides with the occurrence of the Heraultipegma yunnanensis (=Watsonella crosbyi) Assemblage SSF in the Dahai Member. Least altered strontium isotope ratios, based on sequential acid leaching of limestones as well as dolostones and phosphorites following rigid petrographic and geochemical selection, reveal a systematic decreasing trend from 0.7085–6 in the latest Ediacaran (Daibu Member) to about 0.7082–3 by the FAD of Watsonella crosbyi in China (Dahai Member). The carbon and strontium isotope features of the Xiaotan section can be correlated with those from sections in Morocco, Mongolia and Siberia, and confirm a decrease in seawater 87 Sr/86 Sr during Cambrian Stage 1 accompanied by C-isotope oscillations, which together may assist global stratigraphic correlation of the early Cambrian. As previously intimated by C-isotopes and biostratigraphy, this new Sr-isotope evidence confirms a depositional gap during the pre-Tommotian in the Aldan River area of SE Siberia. The decreasing trend of 87 Sr/86 Sr in the Cambrian Stage 1 marks a temporary (∼16 Myrs) reversal of the overall increasing trend during the Ediacaran and Cambrian periods, and potentially records a short-term decrease in continental weathering within a prolonged interval of generally increasing weathering rates. Other factors, such as increased submarine hydrothermal alteration, ocean spreading rates and/or chemical weathering of volcanic provinces and young carbonate dissolution might also have affected this decrease in seawater 87 Sr/86 Sr. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Earth’s biosphere witnessed a revolutionary change from soft bodied animals to skeletal animals across the Precambrian–Cambrian (PC–C) boundary (synonymous with Ediacaran–Cambrian boundary). Despite this, the Global Stratotype Section and Point of the PC–C boundary was established in 1992 at Fortune Head, SE Newfoundland, Canada on the basis of trace

∗ Corresponding authors. E-mail addresses: [email protected] (D. Li), hfl[email protected] (H.-F. Ling), [email protected] (G.A. Shields-Zhou).

fossils in siliciclastic settings (Brasier et al., 1994a; Landing, 1994) which are not abundant in carbonate platform areas such as South China. Since 1992, research on two previous PC–C stratotype candidates (Siberian and Yangtze platforms) has continued nevertheless with both chemostratigraphic (carbon and strontium isotopes) and biostratigraphic (mainly small shelly fossils, SSF) studies. Several SSF zones have been established for PC–C boundary sections and can in some cases be useful for global correlation. Among these SSFs, the first appearance datum (FAD) of Watsonella species has been proposed increasingly as a correlation tool, and more recently as a potential diagnostic marker fossil for the stage subdivision of Cambrian Series 1 (Terreneuvian) (Landing, 1989; Steiner et al., 2007). Carbon isotope stratigraphy has also proved

0301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2012.01.002

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20 2

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

Fig. 1. Location map of Xiaotan, Laolin and Meishucun sections.

useful for global stratigraphic correlation of the PC–C transition. For example, more and more evidence points to a negative Cisotope anomaly around the FAD of small shelly fossils, and one or more positive pre-‘Tommotian’ C-isotope excursions (Landing et al., 2007; Li et al., 2009; Zhu et al., 2005). However, stratigraphic subdivision of the lower Cambrian is still being debated due to the lack of a global stratigraphic framework based on fossils for this time. Strontium isotopes retain long-term global seawater signals, and can be used together with carbon isotopes to test stratigraphic correlation hypotheses based on fossils (Melezhik et al., 2001). 87 Sr/86 Sr ratios determined on authigenic sediments can therefore provide constraints on stratigraphic subdivision and palaeoenvironmental changes such as the weathering history of the Earth (e.g. Halverson et al., 2007; Maloof et al., 2010a). Ediacaran (Precambrian)–Cambrian rocks outcrop widely in South China; the Three Gorges area, Hubei and E Yunnan are two of the best areas for Ediacaran–Cambrian (Ed–C) boundary studies. For example, one of the previous PC–C boundary GSSP candidates was the Meishucun section in E Yunnan (Fig. 1). However, the FAD there of SSF has been much debated (Luo et al., 1982; Qian and Bengtson, 1989; Qian et al., 1996), while the negative Ed–C boundary C-isotope excursion has not been identified clearly (Brasier et al., 1990). The Laolin section, 200 km north of the Meishucun section, in Huize county of Yunnan (Fig. 1), was suggested to be a parastratotype section (Qian et al., 2002), and C-isotope stratigraphy was since carried out (Li et al., 2009; Shen and Schidlowski, 2000). 200 km further north of the Laolin section, at Xiaotan (Fig. 1), a relatively thick succession, containing carbonate rocks, phosphorites, shales and sandstones was found to be continuously exposed along the Jinsha River, and is now recognized to have considerable potential for stratigraphic studies (Li and Xiao, 2004; Zhou et al., 1997). This paper presents a detailed carbon and strontium isotope stratigraphy for the Xiaotan section in SW China, and discusses

the evolution of the contemporaneous ocean, with respect to changes in biological activity, surface environment and dynamics of the solid earth. This is the first detailed study of composite Cisotope, Sr-isotope and bio-stratigraphy of the early Cambrian on the Yangtze platform, S China. 2. Stratigraphic setting The Xiaotan section is located on the southern bank of the Jinsha River, Yongshan County in NE Yunnan Province (Fig. 1). The Jinsha River marks the boundary between the Yunnan and Sichuan provinces here. The rocks of this section are very well exposed at the bottom of a deep valley formed by the perennial erosion of the rapid waters (Fig. 2). The investigated Xiaotan section comprises, from oldest to youngest, the upper Donglongtan Member, Jiucheng Member and Baiyanshao Member of the Dengying Formation, the Daibu Member, Zhongyicun Member and Dahai Member of the Zhujiaqing Formation, the Shiyantou Formation and Yu’anshan Formation (Fig. 3a). The Donglongtan Member contains dolostones, and is overlain by shales of the Jiucheng Member which have been eroded away by the river at Xiaotan. The Baiyanshao Member comprises grey to dark grey, thickly bedded to massive dolostone (Fig. 4g). The Daibu Member comprises interbedded thin-intermediate, dark, dolomitic cherts and yellowish siliceous dolostone (Fig. 4e), which represents a transgressive systems tract (TST). The Daibu Member is a widespread lithostratigraphic unit in NE Yunnan but is absent in the Meishucun area (Qian et al., 1996; Zhu et al., 2001). The lower part of the Zhongyicun Member comprises grey, thick beds of laminated phosphorite containing the Anabarites trisulcatus-Protohertzina anabarica Assemblage (Zone I) and Siphogonuchites triangularis-Paragloborilus subglobosus Assemblage (Zone II) SSFs; these two SSF assemblages have not been clearly distinguished at Xiaotan (Li and Xiao, 2004). The Zhongyicun Member

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20

ARTICLE IN PRESS 3

Fig. 2. Photomosaic of the Xiaotan section exposed along the Jinsha River.

D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20 4

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

Fig. 3. Stratigraphic columns of the (a) Xiaotan, (b) Laolin and (c) Meishucun sections in E Yunnan, SW China. Small shelly fossils (SSF) zones: A.-P. Zone = Anabarites trisulcatus-Protohertzina anabarica Assemblage Zone; S.-P. Zone = Siphogonuchites triangularis-Paragloborilus subglobosus Assemblage Zone; H. Zone = Heraultipegma yunnanensis [=Watsonella crosbyi (Grabau, 1900); Landing, 1989] Assemblage Zone. HST = highstand system tracts; SB3 = third-order sequence boundary; TST = transgressive system tracts; MFS = maximum flooding surfaces; cs = condensed sections. BYS = Baiyanshao Member; DB = Daibu Member; ZYC = Zhongyicun Member; DH = Dahai Member; SYT = Shiyantou Formation. Note only stratigraphic heights of the Zhujiaqing Formation at both Xiaotan and Laolin are shown in scale, while height scale of the Meishucun section has been enlarged, and that of the Dengying Formation and Shiyantou Formation at Xiaotan has been shortened.

is a phosphatic unit (Fig. 4c) which contacts the Daibu Member below and the Dahai Member above both with transitional boundaries. The Dahai Member comprises pale, thickly bedded limestone (Fig. 4a) and contains Heraultipegma yunnanensis [=Watsonella crosbyi (Grabau, 1900)] Assemblage (Zone III) SSFs. The Shiyantou Formation comprises grey to dark grey, thickly to thinly bedded quartz siltstone in its lower part, which represents a condensed

section (cs) (Zhu et al., 2001), and contains a SinosachitesTannuolina SSF Assemblage (Zone IV) in its upper part (Li and Xiao, 2004) which extends to the basal Yu’anshan Formation. Trilobites first occur near the base of the Yu’anshan Formation. Sedimentary and litho-facies evolution at Xiaotan section indicates that the late Ediacaran and early Cambrian successions in NE Yunnan were deposited in an offshore intra-basin within the

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

5

Fig. 4. Outcrop pictures and microscope photos (plane-polarized light) of thin-sections from Xiaotan section. (a) and (b) Limestone in Dahai Member; (c) and (d) phosphorite in Zhongyicun Member; (e) and (f) interbedded dolostone and chert in Daibu Member; (g) and (h) dolostone in Baiyanshao Member.

Yangtze Platform. Phosphogenesis in the Zhongyicun Member took place at the transition from a transgressive systems tract to a condensed interval, which suggests that phosphorites sedimentation rates were slow. The gradual transition from thinly bedded Zhongyicun phosphorite layers to thickly bedded Dahai limestones represents a rise in sea-level and consequent increase of sedimentary rate. The Xiaotan section exhibits a similar chemostratigraphic and biostratigraphic framework to the Laolin section (Fig. 3a and b), although the former section is thicker with better field exposure. The previous PC–C GSSP candidate Meishucun, located west of the

Dianchi Fault (Fig. 1), is typical of the more condensed successions in E Yunnan (Fig. 3c). Numerous ages of a bentonite tuff bed (Bed 5) in the upper Zhongyicun Member at Meishucun section have been published, and determined as 535.2 ± 1.7 Ma (Zhu et al., 2009), or 536.5 ± 2.5 Ma by cathodoluminescence (CL) imaging and in situ U–Pb dating with LA-ICP-MS and nano-SIMS (Sawaki et al., 2008), or 539.4 ± 2.9 Ma by SHRIMP dating (Compston et al., 2008), or ca. 533 Ma estimated by ID-TIMS analyses (Brooks et al., 2006), or 538.2 ± 1.5 Ma by calculated U/Pb dating of zircons on SHRIMP (Jenkins et al., 2002) (Fig. 3c). This age can be correlated to the middle of the Zhongyicun Member at the Xiaotan section, in between

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

6

the “lower phosphorite” and “upper phosphorite” layers. A further U/Pb SHRIMP age of 526.52 ± 1.1 Ma was reported by Compston et al. (2008) from the basal Shiyantou Formation at Meishucun section.

3. Analytical methods More than 200 samples were collected continuously from the ∼300 m-long Xiaotan section. Sample lithologies included dolostone of the Donglongtan Member and the Baiyanshao Member, dolomitic chert, cherty dolostones of the Daibu Member, phosphatic dolostone, dolomitic phosphorite of the Zhongyicun Member, limestone of the Dahai Member and calcareous shale of the basal Shiyantou Formation (Table 1). Thin-sections of the rock samples were scrutinized under a polarizing microscope in order to identify the best-preserved rock components destined for element and isotope analyses. The powders were then drilled on the counterpart of the rock slabs while avoiding visible alteration, such as calcitic micro-veins and interstitial cements. One aliquot of the sample powders was reacted with orthophosphoric acid at 70 ◦ C for 1 h to extract CO2 following the principles first determined by McCrea (1950) and Craig (1953). C- and O-isotope compositions were determined using an online analysis system, Finnigan Gasbench II + Delta Plus XP at the State Key Laboratory for Mineral Deposits Research, Nanjing University. Chinese GBW00405 carbonate standards (TTB-1 and TTB-2) were placed after every ten samples in order to check for memory effects and isotopic calibration. The external deviations of C- and O-isotope values are both within 0.1‰, and the data are presented using ı-notation in parts per thousand relative to the international V-PDB standard (Table 1). Another aliquot of the sample powders was leached by excess 10% hydrochloric acid at room temperate for at least 12 h. The concentrations of some diagnostic major and trace elements (Ca, Mg, Fe, Mn and Sr) were determined using ICP-AES at the Wolfson Laboratory of University College London. The precisions are generally better than 5% for the analysed elements based on long-term reproducibility of the lab measurement. A portion of the sample sets (more than 40) was then selected for Sr isotope stratigraphy (SIS) study following strict petrographic and geochemical selection criteria. Samples with high Sr and CaO + MgO concentrations, low Mn/Sr ratios and plausible O-isotopes were selected, and only the primary rock component (pure, fine-grained calcite, dolomite or apatite) were selected and micro-drilled or milled for SIS studies. For limestone and dolostone samples, the resultant fine powders were pre-leached by 0.2 M acetic acid by calculated volume in order to dissolve around one-third of carbonates (by applying the CaO + MgO data), aiming to remove silicate-bound contaminants and altered carbonate components, leaving the targeted carbonate components to be partially dissolved by weak acetic acid for SIS study, here again around one-third of carbonates were dissolved by calculated amounts of acid. For phosphorite samples, after all the carbonates were leached out, the targeted phosphates were dissolved by 0.05 M nitric acid, and here again incomplete dissolution was employed. Sr was eluted from solutions by ion exchange chromatography through columns; Sr isotope values were determined using TIMS at Royal Holloway, University of London and Nanjing University. Repeated measurement of the Sr standard SRM 987 yielded 87 Sr/86 Sr = 0.710250 (±13) (n = 8) at Royal Holloway, and 0.710252 (±16) (n = 40) at Nanjing, respectively. The resulting 87 Sr/86 Sr data and their corresponding 2 standard error of mean are listed in Table 1. A more detailed procedure is outlined in a separate publication (Li et al., 2011), which presents a simple protocol for future SIS studies of bulk marine carbonate and phosphorite rocks.

4. Results 4.1. Petrographic observations Microscopic observations are crucial for identifying the sample lithology and for choosing the appropriate rock component for Sr isotope analysis. Abbreviate descriptions of all these samples (more than 200) are listed in Table 1. The typical petrographic photos of each member across the Ed–C boundary can be seen in Fig. 4 and described as follows. The Dahai Member contains microsparite, sparite, and partially recrystallized pelsparite. The typical wellpreserved microsparite samples (Fig. 4b) contained fine-grained calcite with minimal siliciclastic content. The Zhongyicun Member is a phosphorite-bearing unit. The peloidal phosphate (Fig. 4d) can be easily recognized in thin-sections with diameter of tens to hundreds of micrometers. The Daibu Member is characteristic of interbedded cherts and dolostones (Fig. 4e) in the outcrop. The dolostones in this member are more coarsely grained with diameters mostly higher than 100 ␮m (Fig. 4f). The Baiyanshao Member contains dolosparite, dolomicrosparite, and partially recrystallized dolomicrosparite (see Fig. 4h for example). 4.2. Diagenetic evaluation The C, O and Sr isotope compositions and Mn/Sr, Mg/Ca, Rb/Sr ratios are listed in Table 1, and shown in Figs. 5 and 6. Before discussing chemostratigraphic correlation and environmental implications of these data, diagenetic effects need first to be evaluated. Progressive diagenetic alteration of carbonates generally raises Mn contents and at the same time lowers Sr contents, as well as ı13 C and ı18 O values of carbonate rocks (Bartley et al., 1998; Derry et al., 1994; Veizer et al., 1999). Thus, Mn/Sr ratios and correlations between ı13 C, ı18 O values and Mn/Sr ratios can be used sometimes to discriminate diagenetically altered from unaltered samples. Almost all samples from Xiaotan section have Mn/Sr ratios lower than 10, while cross-plots of Mn/Sr versus ı18 O and Mn/Sr versus ı13 C show no clear correlations (Figs. 5c and d). The exceptions are some samples from the Baiyanshao and Daibu Members which exhibit Mn/Sr ratios higher than 10 (Figs. 5c and d). However, these samples with Mn/Sr ratios between 10 and 20 tend to have the most positive ı13 C and ı18 O values among all measured samples; for samples with a high and large range of Mn/Sr (20–100), their ı13 C and ı18 O values are generally intermediate in the whole range of all samples (Figs. 5c and d). Taken individually, no member of the Xiaotan section shows any such negative correlation either, which may suggest that in the case of Xiaotan, the Mn/Sr ratio may not be an adequate tracer of diagenetic alteration, cf. Laolin section in NE Yunnan area (Li et al., 2009). Also, Derry (2010) indicated that the low Mn/Sr in ancient carbonates do not necessarily imply preservation of primary values, and admonished against the use of Mn/Sr ratios as criteria. ı18 O < −10‰, and positive correlation between ı13 C and ı18 O are considered to be diagnostic of diagenetic alteration as well. Almost half of our ı18 O values are below −10‰. However, crossplots of ı13 C versus ı18 O show no positive correlation, which excludes the possibility of post-depositional fluid alteration. The exceptions are nine samples with extremely low ı18 O values (<−16‰), which plot toward the lower left part of the trend arrow in Fig. 5a. Five dolomitic phosphorite samples of Zhongyicun Member (five red open points in Fig. 5a) lie also within the diagenesis trend toward the lower-left part of the ı13 C–ı18 O diagram, so they are also susceptible to diagenesis. And these five ı13 C data points have low ı18 O values (lower than −12‰) compared with a general trend in the Zhongyicun Member (Fig. 7). All these 13 samples are

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

Lithology

Height (m)

ı13 C (PDB, ‰)

ı18 O (PDB, ‰)

XT-6 XT-5 XT-4 XT-3 XT-2 XT-1 XT-101 XT-102 XT-103 XT-104 XT-105 XT-106 XT-107 XT-108 XT-109 XT-110 XT-111 XT-112 XT-113 XT-114 XT-115 XT-116 XT-117 XT-118 XT-119 XT-120 XT-121 XT-122 XT-123 XT-124 XT-125 XT-126 XT-127 XT-128 XT-129 XT-130 XT-131 XT-132 XT-133 XT-134 XT-135 XT-136 XT-137 XT-138 XT-139 XT-140 XT-141 XT-142 XT-143 XT-144 XT-145 XT-146 XT-147 XT-148

Shiyantou Fm. Shiyantou Fm. Shiyantou Fm. Shiyantou Fm. Shiyantou Fm. Shiyantou Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm.

Calcareous siltstone Calcareous siltstone Calcareous siltstone Calcareous siltstone Calcareous siltstone Calcareous siltstone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone

284.1 282.6 281.3 280.3 279.3 279.0 278.9 278.3 277.9 276.9 276.1 275.5 274.5 273.8 273.0 272.5 270.5 269.9 268.7 267.3 265.3 263.3 261.9 260.7 259.4 258.2 257.2 255.2 253.9 252.9 251.7 250.5 248.6 246.7 245.9 244.3 243.3 241.8 240.7 239.5 237.5 235.9 234.9 233.3 231.7 230.8 229.4 228.3 226.6 224.6 222.6 221.1 219.6 218.1

−5.77 −13.06 −11.11 −12.29 −0.42 0.64 −4.11 −0.25 −0.94 −0.19 −0.10 −0.22 −0.24 1.04 3.71 4.16 5.52 5.74 6.19 6.47 5.68 5.92 7.22 6.13 6.96 7.06 6.69 5.99 6.26 6.46 6.66 6.62 6.68 6.86 7.10 6.71 6.94 7.32 6.47 7.11 7.15 7.18 6.92 7.11 6.39 5.96 6.27 6.25 6.72 6.34 6.53 5.87 6.29 5.83

−15.71 −18.30 −17.09 −18.31 −14.02 −14.71 −14.96 −14.83 −14.49 −14.91 −15.11 −15.11 −15.10 −15.20 −10.88 −11.89 −11.18 −12.58 −12.32 −12.30 −12.46 −12.47 −11.46 −11.46 −11.30 −10.99 −10.99 −11.64 −11.80 −11.91 −12.08 −11.82 −11.92 −10.90 −10.78 −11.28 −11.12 −11.24 −12.00 −12.07 −12.17 −12.46 −12.66 −10.90 −12.17 −12.07 −12.16 −12.33 −12.25 −11.99 −11.36 −12.06 −12.09 −11.84

87

Sr/86 Sr

Error (2)

0.710796

0.000010

0.711959

0.000005

0.711718 0.711320

0.000009 0.000009

0.708303

0.000004

0.708382

0.000004

0.708311

0.000005

0.708261

0.000005

0.708344

0.000006

0.708282

0.000028

0.708288

0.000007

0.708270 0.708577

0.000011 0.000006

0.708314

0.000008

0.708269

0.000006

0.708284

0.000011

CaO (%)

Sr (ppm)

Mn (ppm)

Rb (ppm)

Mn/Sr (w/w)

n.a. n.a. n.a. n.a. n.a. n.a. 45.7 36.0 38.7 40.1 43.0 45.7 39.8 34.0 52.1 49.9 51.7 52.1 50.3 47.6 47.5 50.5 51.8 47.3 47.9 50.3 49.0 50.4 53.1 50.3 49.2 49.4 48.9 49.4 50.4 48.7 49.6 49.6 49.6 47.3 48.4 48.7 49.3 49.2 51.0 47.9 48.9 49.5 49.5 48.8 50.5 45.5 47.2 50.2

n.a. n.a. n.a. n.a. n.a. n.a. 757.0 1716.7 1497.6 1827.7 1848.8 1806.6 2041.4 1530.7 660.5 950.7 2008.5 3391.8 4665.0 4489.0 2185.3 4014.1 4632.4 6916.6 5021.6 4782.6 5289.2 2654.3 3186.8 4921.8 4457.3 4436.2 4216.9 2458.3 4103.2 4033.7 2742.4 4656.7 2376.6 2804.5 3954.8 3423.1 4089.9 3998.4 4998.8 1772.8 1675.5 2211.3 3191.6 2457.7 3661.8 3094.0 3309.9 2517.5

n.a. n.a. n.a. n.a. n.a. n.a. 360.2 530.4 402.4 336.6 280.5 240.8 282.0 214.5 433.0 294.5 210.6 248.5 313.9 323.9 342.6 203.5 94.3 83.2 136.0 149.5 152.1 89.6 105.6 71.2 112.8 125.8 192.6 194.9 101.7 116.4 123.3 89.9 143.9 100.6 86.5 77.0 196.2 173.8 61.2 216.5 170.3 175.5 97.8 113.8 111.9 127.7 270.7 111.9

n.a. n.a. n.a. n.a. n.a. n.a. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 1.77 b.d.l. b.d.l. 1.19 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 4.43 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 102.47 b.d.l. b.d.l. b.d.l. 0.71 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 9.53 b.d.l.

0.48 0.31 0.27 0.18 0.15 0.13 0.14 0.14 0.66 0.31 0.10 0.07 0.07 0.07 0.16 0.05 0.02 0.01 0.03 0.03 0.03 0.03 0.03 0.01 0.03 0.03 0.05 0.08 0.02 0.03 0.04 0.02 0.06 0.04 0.02 0.02 0.05 0.04 0.01 0.12 0.10 0.08 0.03 0.05 0.03 0.04 0.08 0.04

Mg/Ca (w/w)

0.01 0.04 0.02 0.02 0.02 0.01 0.02 0.02 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.00

Rb/Sr (w/w)

0.0027

0.0004

0.0008

D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

Stratigraphic unit

0.0220

0.0002

0.0029

ARTICLE IN PRESS

Sample

G Model

PRECAM-3496; No. of Pages 20

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

Table 1 Analytical results of carbonate carbon and oxygen isotope compositions, carbonate or phosphate strontium isotope ratios, and chemical concentrations or ratios of hydrochloric acid leachable parts of samples from the Xiaotan section, NE Yunnan, S China.

7

Height (m)

ı13 C (PDB, ‰)

ı18 O (PDB, ‰)

XT-149 XT-150 XT-151 XT-152 XT-153 XT-154 XT-155 XT-156 XT-157 XT-158 XT-159 XT-160 XT-161 XT-162 XT-165 XT-166 XT-168 XT-169 XT-170 XT-174 XT-176 XT-178 XT-180 XT-182 XT-183 XT-184 XT-185 XT-186 XT-187 XT-188 XT-189 XT-190 XT-191 XT-192 XT-193 XT-194 XT-195 XT-196 XT-197 XT-198 XT-201 XT-202 XT-203 XT-204 XT-206 XT-208 XT-209 XT-211 XT-212 XT-214 XT-215 XT-216 XT-222 XT-224 XT-227

Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Dahai Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm.

Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Phosphatic limestone Phosphatic limestone Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Phosphatic limestone Dolomitic phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Calcareous phosphorite Limestone Limestone Limestone Phosphatic limestone Phosphatic limestone Limestone Phosphatic limestone Dolomitic phosphorite Phosphatic dolostone Phosphatic dolostone Dolomitic phosphorite Phosphatic dolostone Dolomitic phosphorite Dolomitic phosphorite Phosphatic dolostone Dolomitic phosphorite Dolomitic phosphorite Dolomitic phosphorite Dolomitic phosphorite Phosphatic limestone Dolomitic phosphorite Dolomitic phosphorite Dolomitic phosphorite

217.1 215.6 214.5 214.1 213.1 212.3 211.3 210.7 210.7 209.2 208.6 207.3 206.3 206.0 204.0 203.0 201.1 200.4 200.0 196.2 195.4 193.5 190.5 188.2 187.2 186.2 185.2 183.9 182.9 181.9 180.9 179.9 178.9 178.4 177.4 176.4 175.4 174.4 173.4 172.0 168.7 168.5 168.3 168.1 166.5 165.9 165.4 163.6 162.5 161.2 160.2 159.4 154.5 152.5 149.6

4.63 4.73 2.15 2.32 1.05 0.87 1.37 0.92 1.05 −1.36 −0.58 −0.87 0.02 −6.04 −6.44 −8.74 −2.89 −3.64 −5.21 −1.89 −3.55 −2.97 −3.18 −4.86 −10.30 −8.81 −7.89 −5.52 −10.27 −7.37 −2.03 −5.29 −8.39 −1.40 −5.06 −4.39 −2.64 −4.39 −4.23 −3.58 −4.38 −5.26 −8.15 −4.32 −10.43 −7.73 −5.87 b.d.l. −4.84 −4.41 −3.30 −10.37 −4.73 −3.41 −9.88

−11.36 −11.15 −11.66 −11.84 −11.82 −11.78 −11.51 −11.52 −10.82 −10.70 −10.79 −10.59 −10.53 −13.50 −13.10 −14.70 −11.11 −10.33 −10.10 −10.02 −11.19 −9.95 −9.88 −10.70 −14.35 −12.60 −11.94 −10.52 −13.80 −11.09 −9.04 −9.02 −12.49 −9.76 −9.03 −10.48 −9.84 −9.98 −9.58 −7.92 −10.01 −9.04 −12.29 −8.50 −12.71 −11.83 −6.52 b.d.l. −10.23 −10.37 −9.68 −12.71 −9.90 −9.38 −14.37

87

Sr/86 Sr

0.708486

0.708628

Error (2)

0.000007

0.000007

0.708507 0.708767

0.000010 0.000005

0.708412

0.000011

0.708588

0.000010

0.708749

0.000009

0.708726

0.000010

0.708623

0.000006

0.708823

0.000010

0.708922

0.000010

0.708607 0.708876

0.000009 0.000004

0.708768

0.000009

CaO (%)

Sr (ppm)

Mn (ppm)

Rb (ppm)

49.2 47.2 40.8 54.4 47.8 47.8 46.5 45.3 45.8 42.4 11.9 40.4 33.9 39.9 16.7 1.4 40.6 16.9 5.3 4.9 9.1 50.2 46.5 4.1 4.2 7.6 30.3 23.9 6.1 21.3 32.8 22.2 24.4 36.9 21.2 14.3 9.0 48.5 18.6 4.8 25.1 27.3 3.2 21.2 b.d.l. 1.2 37.5 0.0 5.9 1.9 0.5 32.8 5.6 4.3 b.d.l.

2557.8 3086.0 2726.8 1712.6 1888.2 1716.0 2383.2 2584.8 1936.8 2233.5 2694.8 1727.8 2293.4 6568.9 5119.8 5171.7 4858.5 2458.8 2497.0 1611.8 2344.6 2056.1 1928.1 1787.8 2171.1 2206.5 3567.7 3614.5 3693.6 3422.2 1872.5 4451.3 3444.9 1629.1 3276.5 3357.8 8635.5 2613.5 4117.5 1427.0 1920.2 1610.7 852.0 1528.9 b.d.l. 4658.9 2570.9 747.0 2435.8 1537.1 1754.8 3743.8 1598.1 1082.3 b.d.l.

239.4 310.0 1859.5 1634.2 1219.5 1081.8 1294.1 1033.6 1143.8 467.3 2352.5 591.7 1059.3 34.0 81.0 3551.9 1969.0 2154.4 2487.8 703.8 989.2 458.3 307.9 1545.8 520.5 692.9 81.2 398.1 114.4 269.9 764.1 268.9 114.3 772.7 186.5 248.3 2886.0 695.6 604.4 9428.0 663.3 1140.5 2804.6 5105.6 b.d.l. 3019.9 695.8 401.7 363.4 9280.2 3152.2 26.3 994.9 767.5 b.d.l.

b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 8.62 b.d.l. b.d.l. 110.32 b.d.l. b.d.l. 12.84 b.d.l. 780.73 b.d.l. 61.36 b.d.l. 117.17 119.44 b.d.l. b.d.l. b.d.l. 34.19 b.d.l. 2.67 b.d.l. b.d.l. b.d.l. b.d.l. 8.54 36.06 b.d.l. b.d.l. 65.41 0.43 b.d.l. 69.06 419.72 252.70 137.46 471.35 158.54 b.d.l. 3442.16 1.73 16.51 128.71 171.32 1898.95 b.d.l. 616.27 254.48 b.d.l.

Mn/Sr (w/w) 0.09 0.10 0.68 0.95 0.65 0.63 0.54 0.40 0.59 0.21 0.87 0.34 0.46 0.01 0.02 0.69 0.41 0.88 1.00 0.44 0.42 0.22 0.16 0.86 0.24 0.31 0.02 0.11 0.03 0.08 0.41 0.06 0.03 0.47 0.06 0.07 0.33 0.27 0.15 6.61 0.35 0.71 3.29 3.34 0.65 0.27 0.54 0.15 6.04 1.80 0.01 0.62 0.71

Mg/Ca (w/w) 0.00 0.01 0.02 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.14 0.04 0.04 0.00 0.01 0.65 0.01 0.12 0.14 0.05 0.08 0.01 0.00 0.04 0.04 0.01 0.01 0.00 0.01 0.02 0.06 0.02 0.01 0.01 0.03 0.03 0.16 0.01 0.05 0.41 0.11 0.20 0.12 0.34

0.25 0.13 0.28 0.68 0.00 0.35 0.39

Rb/Sr (w/w)

G Model

Lithology

0.0033

0.0409

0.0020 0.1510 0.0250 0.0727 0.0509

0.0157 0.0007

0.0019 0.0105

0.0195 0.0000 0.0168 0.2941 0.1316 0.0853 0.5532 0.1037 0.7388 0.0007 0.0221 0.0528 0.1115 1.0821 0.3856 0.2351

D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

Stratigraphic unit

ARTICLE IN PRESS

Sample

PRECAM-3496; No. of Pages 20

8

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

Table 1 (Continued )

ı13 C (PDB, ‰)

ı18 O (PDB, ‰)

87

XT-230 XT-231 XT-232 XT-234 XT-238 XT-240 XT-241 XT-242 XT-243 XT-244 XT-245 XT-246 XT-247 XT-248 XT-249 XT-250 XT-251 XT-252 XT-253 XT-254 XT-255 XT-256 XT-257 XT-258 XT-259 XT-260 XT-263 XT-264 XT-265 XT-266 XT-267 XT-268 XT-269 XT-270 XT-271 XT-272 XT-273 XT-274 XT-275 XT-276 XT-277 XT-278 XT-279 XT-280 XT-281 XT-282 XT-283 XT-284 XT-285 XT-286 XT-287 XT-288 XT-289 XT-290 XT-291 XT-292

Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Zhongyicun Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Daibu Mb., Zhujiaqing Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm.

Phosphatic dolostone Phosphatic dolostone Dolomitic phosphorite Phosphatic dolostone Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Dolomitic chert Siliceous dolostone Dolomitic chert Dolomitic chert Siliceous dolostone Siliceous dolostone Dolomitic chert Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone

137.7 137.0 136.2 134.7 106.9 104.4 103.6 102.7 102.0 100.0 98.9 97.9 96.9 95.4 93.9 92.9 91.0 89.7 87.7 86.8 85.8 85.2 82.6 81.6 80.6 78.6 75.7 75.6 74.9 74.3 73.0 72.1 71.3 70.0 69.6 68.9 68.2 67.6 67.0 66.1 64.2 63.2 61.4 60.4 58.4 56.4 54.4 51.4 48.4 46.4 43.9 42.4 41.4 39.6 38.2 36.9

−5.23 −4.22 −4.66 −5.27 −5.51 −4.24 −4.86 −5.18 −5.83 −4.33 −13.40 −7.27 −6.29 −7.22 −7.16 −7.51 −13.90 −8.12 −13.51 −14.21 −8.67 −9.11 −8.50 −7.96 −12.18 −7.84 −5.81 −2.62 −2.29 −1.55 −0.86 −0.80 −0.77 0.19 0.41 0.88 1.10 0.55 0.62 0.65 1.21 0.68 0.90 1.23 1.45 0.60 0.55 1.36 1.35 1.38 1.22 0.84 1.23 1.48 0.86 1.47

−8.38 −9.01 −11.39 −8.24 −9.07 −11.10 −8.40 −8.44 −8.01 −9.59 −16.97 −7.62 −8.65 −7.66 −8.37 −7.92 −16.85 −9.05 −17.97 −18.34 −11.63 −7.47 −10.44 −9.83 −7.09 −6.96 −10.07 −7.89 −8.07 −8.06 −8.13 −8.16 −7.05 −7.75 −7.55 −6.21 −7.55 −8.33 −9.08 −8.37 −3.50 −6.57 −9.85 −6.59 −3.90 −3.15 −6.39 −6.46 −6.49 −5.39 −7.18 −8.01 −6.84 −5.62 −6.83 −3.90

Sr/86 Sr

Error (2)

CaO (%)

Sr (ppm)

0.708654 0.708718

0.000009 0.000003

0.708531

0.000009

27.8 17.1 1.1 26.3 3.4 1.5 1.0 3.1 2.2 3.7 b.d.l. 2.8 0.8 9.8 1.5 0.9 b.d.l. 0.7 0.0 b.d.l. 0.5 28.6 0.2 0.5 28.3 28.0 1.4 22.7 22.3 27.3 23.5 24.7 26.8 25.7 23.3 26.2 25.0 24.1 26.6 21.2 17.4 14.3 26.0 23.9 18.8 14.4 13.6 24.0 21.7 23.4 25.7 26.4 26.2 21.1 26.4 18.9

1586.9 1199.1 1534.0 1222.9 458.9 91.5 b.d.l. 196.2 114.2 42.1 b.d.l. 401.1 b.d.l. 185.3 b.d.l. 1274.9 b.d.l. 214.3 283.2 b.d.l. 273.6 58.8 1169.8 262.7 136.8 112.5 586.5 91.1 90.8 79.5 83.0 77.9 83.2 83.7 107.2 110.2 109.8 93.1 86.4 113.1 147.6 99.1 103.7 117.9 149.8 189.8 145.2 95.0 109.7 114.6 86.3 72.0 103.2 131.4 83.3 146.0

0.711804

0.708550

0.712326

0.710219

0.711988

0.000032

0.000009

0.000005

0.000013

0.000005

Mn (ppm) 1904.1 2752.0 7095.3 1919.5 1597.7 7158.7 b.d.l. 5008.1 5175.3 2574.0 b.d.l. 5075.6 b.d.l. 3549.4 b.d.l. 36,909.3 b.d.l. 5201.2 557.7 b.d.l. 15,552.7 315.0 69,904.9 23,888.2 390.1 726.2 12,313.8 1294.4 1101.2 916.1 1146.9 1008.1 1136.1 1127.4 1428.7 1262.3 1192.1 1240.0 1197.0 1526.7 1948.1 3469.1 1308.4 1059.9 1722.6 1394.6 932.8 864.9 980.5 1002.7 923.5 944.6 974.5 1206.7 1152.0 1477.0

Rb (ppm)

Mn/Sr (w/w)

Mg/Ca (w/w)

Rb/Sr (w/w)

22.21 15.27 573.80 b.d.l. b.d.l. 608.20 b.d.l. 407.89 407.02 248.97 b.d.l. 858.34 b.d.l. 46.73 b.d.l. 485.31 b.d.l. 1768.24 13.01 b.d.l. 746.25 47.58 b.d.l. 1100.65 b.d.l. b.d.l. 212.30 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 37.99 20.48 28.85 1.13 26.55 b.d.l. b.d.l. b.d.l. 6.63 10.93 120.97 b.d.l. 50.29 123.76 61.66 45.24 50.79 15.89 3.75 b.d.l. b.d.l. 18.78 26.62 92.93

1.20 2.29 4.63 1.57 3.48 78.25

0.21 0.41 0.31 0.34 0.29 0.52 0.72 0.57 0.55 0.55

0.0140 0.0127 0.3741

25.52 45.34 61.17 12.65

6.6470 2.0790 3.5641 5.9138 2.1400

28.95

0.48 0.76 0.46 0.70 0.29

24.27 1.97

0.53 0.42

8.2512 0.0459

56.84 5.36 59.76 90.93 2.85 6.45 21.00 14.20 12.13 11.52 13.82 12.94 13.66 13.47 13.33 11.45 10.86 13.32 13.85 13.50 13.20 35.02 12.61 8.99 11.50 7.35 6.42 9.10 8.94 8.75 10.70 13.11 9.44 9.18 13.82 10.12

0.54 0.26 0.98 0.52 0.26 0.33 0.23 0.37 0.37 0.27 0.31 0.35 0.34 0.34 0.36 0.41 0.27 0.35 0.33 0.35 0.32 0.35 0.39 0.27 0.37 0.39 0.37 0.35 0.36 0.24 0.29 0.26 0.35 0.27 0.32 0.30

2.7275 0.8092

19.15

0.2522 0.3807

4.1898

0.3620

0.4566 0.2447 0.2691 0.0103 0.2418

0.0449 0.1103 1.1665 0.3357 0.6521 0.4247 0.4762 0.4630 0.1387 0.0435

0.1429 0.3196 0.6365

G Model

Height (m)

D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

Lithology

ARTICLE IN PRESS

Stratigraphic unit

PRECAM-3496; No. of Pages 20

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

Sample

9

G Model

PRECAM-3496; No. of Pages 20

10

Stratigraphic unit

Lithology

Height (m)

XT-293 XT-294 XT-296 XT-297 XT-298 XT-299 XT-300 XT-301 XT-302 XT-303 XT-304 XT-305 XT-306 XT-307 XT-308 XT-309 XT-310 XT-311 XT-312 XT-313 XT-314 XT−315 XT-316 XT-317 XT-318 XT−319 XT-320 XT-321 XT-322

Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Baiyanshao Mb., Dengying Fm. Donglongtan Mb., Dengying Fm. Donglongtan Mb., Dengying Fm. Donglongtan Mb., Dengying Fm. Donglongtan Mb., Dengying Fm. Donglongtan Mb., Dengying Fm. Donglongtan Mb., Dengying Fm. Donglongtan Mb., Dengying Fm. Donglongtan Mb., Dengying Fm. Donglongtan Mb., Dengying Fm. Donglongtan Mb., Dengying Fm.

Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone Dolostone

34.7 33.1 29.3 27.3 25.3 23.3 21.3 19.5 18.2 17.2 15.0 12.9 11.0 9.2 7.9 6.7 4.6 2.5 0.0 −20.0 −22.0 −24.0 −26.0 −28.0 −30.0 −32.0 −34.0 −36.0 −37.0

ı13 C (PDB, ‰)

ı18 O (PDB, ‰)

1.00 0.78 1.77 2.26 2.27 2.32 1.43 2.64 3.02 3.03 2.50 2.44 2.64 0.06 2.17 2.55 2.33 1.83 2.98 1.30 1.30 1.25 0.82 1.15 0.96 0.11 0.71 0.90 0.79

−7.20 −6.81 −6.91 −5.73 −6.55 −6.23 −6.31 −5.15 −5.92 −4.43 −3.65 −3.96 −6.20 −5.66 −5.79 −6.82 −4.04 −5.95 −3.52 −5.64 −6.83 −4.39 −7.43 −5.66 −7.06 −6.27 −6.93 −7.08 −7.38

b.d.l. – below detectable limitl; n.a. – not analysed. Italic values indicate diagenetically altered data which are not included in the stratigraphic curve.

87

Sr/86 Sr

0.710007

Error (2)

0.000005

0.710222

0.000011

0.710339

0.000006

0.710729

0.000033

CaO (%)

Sr (ppm)

Mn (ppm)

Rb (ppm)

Mn/Sr (w/w)

Mg/Ca (w/w)

Rb/Sr (w/w)

24.9 11.4 26.1 21.1 25.7 26.4 25.9 23.8 26.4 25.3 22.9 20.4 27.4 25.2 29.9 29.5 23.1 25.5 29.8 29.1 26.1 29.0 26.9 28.9 28.5 28.8 27.6 33.3 13.2

119.1 263.9 77.7 95.2 66.9 76.3 85.6 98.0 52.7 80.0 99.4 112.9 143.6 97.8 78.1 71.0 90.3 99.0 82.3 76.4 62.4 76.3 74.3 80.2 95.6 102.6 75.1 58.2 b.d.l.

1190.6 2474.4 716.6 760.9 793.4 363.6 528.9 1166.3 456.5 799.5 771.4 810.7 673.5 3758.8 785.2 1125.5 562.0 661.3 281.0 263.8 488.0 230.8 253.4 318.8 242.7 279.0 380.7 371.3 b.d.l.

24.26 82.36 b.d.l. b.d.l. 1.29 2.64 b.d.l. 13.42 39.74 19.53 b.d.l. 31.86 6.30 b.d.l. b.d.l. 78.36 5.88 b.d.l. 60.56 b.d.l. 6.86 b.d.l. 14.85 b.d.l. b.d.l. 1.15 b.d.l. 47.87 105.11

10.00 9.37 9.22 7.99 11.86 4.76 6.18 11.90 8.66 9.99 7.76 7.18 4.69 38.42 10.05 15.86 6.22 6.68 3.42 3.45 7.82 3.03 3.41 3.98 2.54 2.72 5.07 6.38

0.29 0.44 0.27 0.27 0.31 0.29 0.26 0.34 0.33 0.35 0.36 0.30 0.30 0.32 0.29 0.41 0.33 0.33 0.39 0.24 0.29 0.38 0.27 0.26 0.26 0.19 0.29 0.38 0.46

0.2037 0.3121

0.0193 0.0346 0.1369 0.7541 0.2441 0.2822 0.0439

1.1037 0.0651 0.7358 0.1099 0.1999

0.0112 0.8225

D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

Sample

ARTICLE IN PRESS

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

Table 1 (Continued )

G Model PRECAM-3496; No. of Pages 20

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

11

Fig. 5. Cross-plots of ı18 O–ı13 C (a), ı18 O–Mg/Ca (b), ı18 O–Mn/Sr (c) and ı13 C–Mn/Sr (d) for Xiaotan section in NE Yunnan, SW China. Open data points indicate that the samples experienced late diagenesis or fluid alteration, and are not included in the stratigraphic curve. Data points in the circled area in Fig. 5(a) were not subjected to diagenetic alteration as detailed in the text.

considered by us to have been modified by diagenesis (for example, fluid-rock interaction), although none of them have Mn/Sr ratios higher than 10, and are thus not included in the ı13 C and ı18 O evolutionary curve (Fig. 7). Dolomitization is believed to affect the carbonate ı18 O value (Vasconcelos et al., 2005). Although cross-plots between ı18 O and Mg/Ca ratios show no correlation in our study (Fig. 5b), samples with high Mg/Ca ratios (0.4–0.6) tend to have higher ı18 O values than those with low Mg/Ca ratios (0–0.2), which indicates that the former (dolostones) were not significantly affected by meteoric or high-temperature alteration as isotopic differences are likely to be the result of equilibrium isotopic fractionation (Vasconcelos et al., 2005). Therefore, ı18 O screening proxies are more ambiguous in case of dolomitization, which is a process generally elevates ı18 O values. Our strontium isotope data were determined on calcite, dolomite and phosphate rock components selected on rigid petrographic and geochemical criteria. Although post-depositional alteration through interaction with detrital components generally increases 87 Sr/86 Sr ratios, contamination of clay minerals and tiny silicate minerals originated from basaltic rocks could decrease 87 Sr/86 Sr values. We have checked the thin-sections to avoid the unnecessary detrital components, and have performed acetic acid pre-leach before incomplete dissolution aiming to further exclude to possibility of detrital contaminations.

In order to exclude obviously diagenetically altered 87 Sr/86 Sr values, various parameters were used to calibrate the Sr-isotope results (Figs. 6a–f). No obvious correlation between 87 Sr/86 Sr and these parameters occurred. Limestones generally have higher Sr concentrations than dolostones; samples with relatively low Sr concentrations are therefore more likely to have been subjected to diagenetic alteration, and are thus are crossed out in the figure (Fig. 6a). Sample purity (high CaO + MgO concentrations) is important to screen up the Sr isotope data, and the purist carbonates tend to have the lowest 87 Sr/86 Sr values (Fig. 6b). Samples with low ı18 O are considered to be affected by post-depositional diagenetic fluid. However, the general assumption that lower ı18 O values reflect a greater degree of alteration becomes less useful in the case of dolomitization which can lead to higher ı18 O due to equilibrium isotopic fractionation (Li et al., 2011). Based on extremely low ı18 O values (<−14‰), we exclude four limestone samples and one dolostone in Fig. 6c. The Mn/Sr screening cut-off commonly used for carbonates ranges from 1 to 3; however, limestones should have a stricter Mn/Sr cut-off than dolostones, to as low as 0.1 (e.g. Li et al., 2011). Based on Mn/Sr ratios, we cross out obviously altered data in Fig. 6d. Samples with Rb/Sr higher than 0.01 have 87 Sr/86 Sr ratios that obviously deviate from contemporaneous seawater; and this deviation might be attributed to the decay of 87 Rb. Furthermore, as proposed by Elderfield (1986), the Rb/Sr ratio in carefully cleaned carbonates is generally of the order of 0.001 (as

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20 12

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

Fig. 6. Cross-plots of 87 Sr/86 Sr versus 1/Sr concentrations (a), percentage of CaCO3 ± MgCO3 (b), ı18 O (c), Mn/Sr (d), Rb/Sr (e) and Mg/Ca (f) for Xiaotan section in NE Yunnan, SW China. Arrows point toward the region of least diagenetic alteration. Obviously diagenetically altered 87 Sr/86 Sr data points are corssed out using various parameters in Fig. 6a–f. Open data points indicate diagenetically altered samples which are not included in the stratigraphic curve. Sr isotope values were determined on petrographically scrutinized rock components, as indicated by L – Limestone, P – Phosphorite, D – Dolostone.

this is mostly in our case), in which case no correction of original 87 Sr/86 Sr values for Rb decay is necessary. Mg/Ca ratios are clearly of no diagenetic significance for entirely dolomitised samples (Fig. 6f). 4.3. C- and Sr-isotope curve Fig. 7 shows a C and Sr isotope evolutionary curve for the Xiaotan section across the Ediacaran–Cambrian transition. The ı13 C values of the uppermost Donglongtan Member and Baiyanshao Member remain stable between 0 and +3‰, and decrease sharply to −12.2‰ in the lower Daibu Member. In the Daibu Member, the ı13 C values gradually rise to −4.2‰, after which a 26-m section lacks data due to talus cover. The ı13 C curve shows three positive and negative oscillations between −8.8‰ and −1.4‰ in the Zhongyicun Mb, and subsequently rises to ∼+6‰ at the base of the overlying Dahai Member. The Dahai limestones record a plateau of stable and high ı13 C values around +6 to +7‰ with the highest value of 7.3‰. Thereafter, ı13 C values plummet remarkably to a nadir of −5.8‰ in the basal Shiyantou Formation. Consequently, this profile shows clearly two large negative ı13 C excursions (N1 and N2) and one large positive ı13 C excursion (P4), thus providing a valuable tool for intrabasinal and interbasinal correlations during the Ediacaran–Cambrian transition. ı18 O varies in the Baiyanshao and Daibu Member, with values between around −4‰ and −8‰, and then declines to −11.6‰ in the lower part of the Daibu Member. Average values decrease consistently from −10‰ in the Daibu Member, through the Zhongyicun Member to −12‰ in the Dahai Member. The Dahai–Shiyantou boundary witnesses another quick drop in ı18 O to ∼−15‰. Consequently, oxygen isotope values are lowest (−15.7‰) at the base of Shiyantou Formation. The high resolution (∼1 m) C-, and O-isotope curve shown here for the Xiaotan section agrees with the sparse one

previously published by Zhou et al. (1997) in a domestic journal in Chinese. Organic carbon isotope data yielded stratigraphic trends (Cremonese et al., this issue). The covariation of ı13 Corg with ı13 Ccarb , shown in Fig. 7 implies that carbonate and organic carbon cycled through the ocean as in the Phanerozoic, implying that the trends in ı13 Corg can be used for stratigraphic correlation, thus extending the potential to correlate Xiaotan section with other sections globally into the Atdabanian Stage. In general, diagenesis tends to increase carbonate 87 Sr/86 Sr relative to pristine values due to the release of radiogenic Sr from silicate minerals. Thus, it is most frequently appropriate to draw a best-estimate curve using the lower limits defined by the data (Jones et al., 1994), provided no contaminations of clay minerals originated from basaltic (non-radiogenic) rocks. Seawater 87 Sr/86 Sr evolution across the Ediacaran–Cambrian transition can be estimated along the lowermost data points in each stratigraphic level (Fig. 7). A gradual drop of best-preserved Sr isotope values from ∼0.7085–6 to ∼0.7082–3 occurred from the Daibu Member to the Dahai Member. Viewed against the long-term increase in strontium isotope ratios from the late Neoproterozoic to the late Cambrian, this decrease would seem merely to represent a ‘short-term’ reversal. 5. Discussions 5.1. Intrabasinal chemostratigraphic correlations The carbon isotope curve for Xiaotan section, Laolin section ∼200 km further south (Li et al., 2009) and Meishucun section ∼400 km further south (Brasier et al., 1990) can easily be correlated (Figs. 7b–d). The most expanded SSF-constrained C-isotope profile in Yunnan can be seen at Xiaotan or Laolin section, both of

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

13

Fig. 7. Composite profiles of ı13 Ccarb , ı18 Ocarb , Mn/Sr and 87 Sr/86 Sr (this study), together with ı13 Corg of organic matter, ı13 C data (Cremonese et al., this issue) for Xiaotan section in Yunnan, SW China. Small shelly fossils (SSF) zones: A.-P. Zone = Anabarites trisulcatus-Protohertzina anabarica Assemblage Zone; S.-P. Zone = Siphogonuchites triangularis-Paragloborilus subglobosus Assemblage Zone; H. Zone = Heraultipegma yunnanensis Assemblage Zone. HST = highstand system tracts; SB3 = third-order sequence boundary; TST = transgressive system tracts; MFS = maximum flooding surfaces; cs = condensed sections. Open data points indicate that samples experienced later diagenesis or fluid alteration, and are not included in the stratigraphic curve.

Fig. 8. C-isotope stratigraphic correlation scheme between sections on Yangtze platform. Note the scale for the much thinner Meishucun section is not equal to that for the others, and the thicknesses are not proportional to time. The first appearance data (FAD) of some diagnostic fossils are marked the stratigraphic level.

which are located east of the Dianchi Fault, and represent a more offshore platformal marine environment than those sections west of the fault (Meishucun section included) (Qian et al., 1996). Both C-isotopes and SSF zones indicate that there are depositional gaps below the base of Zhongyicun Member, and between the Shiyantou Formation and the Dahai Member at Meishucun section (Li et al., 2009). Thus, at Meishucun, the large negative ı13 C excursion near

the Ed–C boundary (<−4‰) and the large positive excursion in the Dahai Member (>4‰) are missing (Fig. 8d). Ed–C boundary strata between Yunnan sections and the Anjiahe section in the Three Gorges area, S China (Ishikawa et al., 2008), ∼1000 km further east, can be readily correlated using two prominent C-isotope features (N1 with N1, P4 with P2, respectively) (Figs. 7a and b). However, the negative ı13 C anomaly (N1)

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20

ARTICLE IN PRESS

14

D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

occurred together with the first appearance of SSFs in the Three Gorges area (Ishikawa et al., 2008), while the N1 anomaly occurred beneath SSF Zone I at Xiaotan section. The positive anomaly P2 lies below the FAD of Aldanella in the Three Gorges area, while at Xiaotan P4 lies within SSF Zone III (Watsonella crosbyi). These differences may be caused by the variable relative completeness of fossil records or post-depositional preservation. On the other hand, the Ed–C boundary strata in the Three Gorges area are more condensed than those of Yunnan, while the biostratigraphy (SSF zones) of the former is not as well constrained as in Yunnan (Fig. 8a). For example, Watsonella has not been found in the Three Gorges area, while the Heraultipegma yunnanensis (=Watsonella crosbyi) SSF Zone has been established as SSF Zone III in Yunnan. Consequently, the best Ed–C boundary section of the Yangtze platform is located in NE Yunnan. Assuming that this chemostratigraphic correlation scheme is valid, the P4 (+7.3) at Xiaotan is still much higher than L4 (+3.5) at Laolin and P2 (+4.2) in the Three Gorges area. These discrepancies might be caused by different depositional depths, as argued by the large surface-to-deep ocean ı13 C gradient observed across the Ediacaran Yangtze Platform (Jiang et al., 2007). 5.2. Global chemostratigraphic correlations Fig. 9 shows our attempt to correlate the combined carbon and strontium isotope curve of Xiaotan section to others from Morocco (Maloof et al., 2010a, 2005), Mongolia (Brasier et al., 1996), NW Siberia (Kaufman et al., 1996) and SE Siberia (Brasier et al., 1994b; Derry et al., 1994; Nicholas, 1996). The upper and lower limits of the correlated stratigraphy (yellow area in Fig. 9) tentatively correspond to the Ediacaran/Cambrian boundary and NemakitDaldynian/Tommotian boundary, respectively. Except for Morocco where there are no biostratigraphic constraints, these chemostratigraphic correlations are well calibrated by biostratigraphic markers and thus are globally representative, and will be discussed below. 5.2.1. Ediacaran–Cambrian boundary ı13 C correlation The Ediacaran–Cambrian boundary is conterminant with the base of the first Stage/Age of the Cambrian System/Period, which is called the Fortunian (Landing et al., 2007). This boundary is a ratified Global Stratotype Section and Point (GGSP) established at Fortune Head in SE Newfoundland at the FAD of the trace fossil Treptichnus pedum (Brasier et al., 1994a; Landing, 1994; Narbonne et al., 1987). Meanwhile, the PC–C boundary GSSP candidates based on SSF zones in China (Meishucun) and Siberia (Dvortsy) were rejected in favor of the T. pedum Ichnozone. Early chemostratigraphic studies showed some negative ı13 C values around the Precambrian–Cambrian transition in South China (Hsu et al., 1985), Morocco (Tucker, 1986), and Siberia (Magaritz et al., 1986). Since then, numerous carbon isotope studies around the PC–C transition have been carried out worldwide over the past two decades, with distinct negative ı13 C excursions (<−3.5‰) found to be associated with the Ed–C transition from the Aldan-Lena area in SE Siberia (Brasier et al., 1994b; Kouchinsky et al., 2005; Magaritz et al., 1991; Pelechaty, 1998), Anabar uplift in NW Siberia (Kaufman et al., 1996; Knoll et al., 1995b; Kouchinsky et al., 2001; Pokrovsky and Missarzhevsky, 1993), NW margin of the Siberian platform (Bartley et al., 1998; Kouchinsky et al., 2007), Olenek in N Siberia (Khomentovsky and Karlova, 1993; Knoll et al., 1995a; Pelechaty et al., 1996a,b), NW Canada (Narbonne et al., 1994), Mongolia (Brasier et al., 1996), Morocco (Magaritz et al., 1991; Maloof et al., 2005, 2010b), Oman (Amthor et al., 2003), Iran (Brasier et al., 1990; Kimura et al., 1997), Namibia (Grotzinger et al., 1995), and S China (Brasier et al., 1990; Ishikawa et al., 2008; Li et al., 2009).

In mixed carbonate–siliciclastic systems, such as in Siberia, Mongolia and S China, the Ed–C boundary has been more commonly defined by the SSF zones than by official T. pedum Ichnozone, while the associated deep negative ı13 C excursions (more than 3.5‰ and back again) has also been suggested as a surrogate for defining the Ed–C boundary. In case of no preservation of either SSFs or T. pedum, such as in Oman and Morocco, only the marker negative ı13 C excursion can be used for identifying the Ed–C boundary. Therefore, it has been suggested that the Ed–C boundary can be better correlated using these negative ı13 C excursions than with biostratigraphy, referred to as the Basal Cambrian C-isotope Excursion (BACE) (Landing et al., 2007; Zhu et al., 2005). However, synchroneity of variable signal is always a highly debated issue. Since many ı13 C correlations ignore fossil tie points, synchroneity of ı13 C trends needs to be assumed rather than synchroneity of first appearances of animal taxa (Maloof et al., 2010a). This assumption is favorable due to the following reasons. On one hand, the carbon cycle in the ocean reaches global homogeneity on time scales larger than 105 y; thus the well-preserved ı13 C signal from different carbonates platforms can be comparable. Although isotope records may be truncated by stratigraphic hiatuses, and the magnitude of the excursions can be varied due to the depth control or the post-depositional changes, the morphology of ı13 C curves remains intact and can be compared from section to section. On the other hand, lower Cambrian fossils can be strongly faciescontrolled, and are relatively easily affected by poor preservation or hiatuses in carbonate platform areas. As a consequence, some potentially diagnostic fossils may not necessarily have the same FAD globally. In Fig. 9, the BACE-type negative ı13 C excursion ‘N1’ from Xiaotan section is correlated with excursion ‘W’ from SE Siberia (Fig. 9e), excursion ‘N’ from NW Siberia (Fig. 9d), and excursion ‘W’ from SW Mongolia (Fig. 9c). The BACE negative ı13 C excursions from Yunnan, S China, SE & NW Siberia are all found immediately below the FAD of SSFs at those localities. This is the most common case worldwide when the C-isotope and FAD of SSF around the Ed–C boundary were compared, except in Mongolia and Olenek uplift where the BACE minima were found to be above the Anabarites trisculcatus FAD (Fig. 9c). At the Anti-Atlas Mountain sections of Morocco, there are no useful biostratigraphic constraints, and so only the BACE itself can be used as a surrogate for the Ed–C boundary (Fig. 9b) by correlation with the negative ı13 C excursion N1 from Xiaotan section. This correlation scheme further supports the preference for chemostratigraphy over biostratigraphy, and may indicate a global biogeochemical event during the Ediacaran–Cambrian transition. The <−7‰ ı13 C excursion (N1) from Xiaotan section requires us to consider input from an isotopically light carbon reservoir, e.g. methane or a DOC pool (but this needs large quantity of DOC considering the short duration of the N1 anomaly). 5.2.2. The Nemakit-Daldynian/Tommotian boundary ı13 C correlation The Tommotian lower boundary is defined at the base of the Nochoroicyathus sunnaginicus zone at Dvortsy in SE Siberia (Rozanov et al., 1969), and was subsequently considered as the basal division of the Cambrian System at that time (Cowie and Rozanov, 1983; Magaritz et al., 1986; Rozanov, 1984). The NemakitDaldynian stage was established by Khomentovsky in 1976, and a synonymy “Manykay” was used by Missarzhevsky in 1983, which both refer to the chronological stage immediately before the Tommotian. Early chemostratigraphic studies showed that carbonate ı13 C values rise from −4‰ to +3.4‰ (peak I in Fig. 9e) toward the base of the Tommotian (Brasier et al., 1994b; Magaritz et al., 1986). Strata of the upper Yudoma Formation, containing positive ı13 C excursions Z and I (Fig. 9e), coincide with the Siberian SSF

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model

PRECAM-3496; No. of Pages 20 D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

ARTICLE IN PRESS

15

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

Fig. 9. Global C-, Sr-isotope stratigraphic correlation scheme in the early Cambrian. Filled data points of strontium isotope ratios indicate least altered samples, while hollow ones represent probably altered samples. Note the scale bars of depth are consistent for (a) Xiaotan, S China, (d) Kotuikan, NW Siberia, and (e) Aldan and Lena, SW Siberia. The depositional depths of early Cambrian strata from (b) Anti-Atlas, Morocco, and (c) SW Mongolia are much thicker than the others. Labels 1p–7p in (b) Morocco were named in Sukharikha in Siberia (Kouchinsky et al., 2007), then were compared to Morocco by Maloof et al. (2010a).

G Model PRECAM-3496; No. of Pages 20 16

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

zones “Anabarites trisulcatus” and “Purella antiqua”, two of which were used to define the range of the biostratigraphic NemakitDaldynian stage (Brasier et al., 1994b; Khomentovsky and Karlova, 1993). A karstic unconformity was, however, suspected to have concentrated fauna of diverse ages into transgressive basal Tommotian Pestrotsvet Formation beds (Knoll et al., 1995b) and even grainstone-filled cavities below the unconformity within the UstYudoma Formation (Khomentovsky and Karlova, 1993). A larger positive ı13 C excursion (5.4‰) in an interval during which N. sunnaginicus Zone species more gradually emerged than at Aldan river, was found by Pokrovsky and Missarzhevsky (1993) and named I (Fig. 9d) by Kaufman et al. (1996) and Knoll et al. (1995b), occurring in shallow-marine limestones of the Medvezhya Formation in the western Anabar uplift, NW Siberia. This excursion (between peak I and II in SE Siberia) could not be correlated into the Aldan and Lena River area (Fig. 9d and e), verifying the depositional gap between the Yudoma Formation and the overlying Pestrotsvet Formation. More recently, oscillatory positive excursions after I and within the N. sunnaginicus zone have been found in a more distal depositional environment in Siberia, such as Ia , Ib (both ∼3.5‰) at the Bol’shaya Kuonamka section of the eastern Anabar uplift (Kouchinsky et al., 2001), and In (3–4.5‰) at the Selinde section in SE Siberia (Kouchinsky et al., 2005), and 6p (6.4%) and 7p (∼1.5‰) at Sukharikha River on the western margins of Siberia (Kouchinsky et al., 2007). As the abrupt appearance of diverse N. sunnaginicus zone species in the stratotype Aldan river area is a consequence of a pre-Tommotian hiatus, the Nemakit-Daldynian boundary can be placed at the FAD of the last N. sunnaginicus zone taxa, which is a level above In -type positive excursions (Kaufman et al., 1996; Knoll et al., 1995b), leading to a greatly expanded Nemakit-Daldynian stage. In SW Mongolia, the positive C-isotope excursions F (Fig. 9c, 5.1‰) below the Tommotian-type Tiksitheca licis zone, have been correlated with I in NW Siberia likewise (Brasier et al., 1996). In the Anti-Atlas mountains of Morocco, the last positive ı13 C anomaly 6p (>6‰) is followed by a shift to <−4‰ and ends the abrupt ı13 C oscillations of the earliest Cambrian, which then lead to the smaller ı13 C oscillations which characterize the succeeding ‘Tommotian’ and ‘Atdabanian’ stages (Maloof et al., 2005, 2010b). The Moroccan ı13 C oscillations were found to match 1p to 7p of the ‘Nemakit-Daldynian’ stage of the Sukharikha section peak-for-peak (Maloof et al., 2010a). If this correlation is accepted, the U/Pb age (525.34 ± 0.09 Ma, Maloof et al., 2010b) associated with the Moroccan ı13 C peak 6p (Fig. 9b, the last large peak before the Tommotian) can be correlated with the In -type positive excursions in Siberia. However, there are no robust fossil constraints in Morocco to confirm this. On the Yangtze Platform of South China, a trend of generally increasing ı13 C values and several positive excursions have also been recognized, such as a ∼1‰ positive excursion at the Meishucun section (Brasier et al., 1990) and a stratigraphically higher positive excursion L4 (3.5‰) at the Laolin section (Li et al., 2009) and P4 plateau at the Xiaotan section (as high as 7.3‰) (this study). These latter two excursions can be referred to by the name “ZHUCE”: the last large ı13 C peak in the Zhujiaqing Formation (Zhu et al., 2005, 2007). ZHUCE has been correlated with peak I of the upper Yudoma Fm at Dvortsy, SE Siberia, but has been correlated alternatively with Tommotian peaks II and III of Siberia (Parkhaev and Demidenko, 2010). ZHUCE is considered to be a late pre-Tommotian feature as it shares some common fossils with the Siberian N. sunnaginicus zone in the sub-Tommotian unconformity and is associated with the Heraultipegma yunnanensis (=Watsonella crosbyi) Zone in South China (Zhu et al., 2001). Thus the peak P4 (ZHUCE-type) at Xiaotan can be correlated with the In -type positive excursions (the uppermost excursion in this series of high positive peaks) in Siberia, the peak 6p in Morocco, and the peak F in

Mongolia both by biostratigraphy and C-isotope curves (Figs. 8a–d). The lack of a ZHUCE positive excursion at Meishucun section may be explained by a hiatus there as the Dahai Member lacks an upper limestone unit in the Meishucun area (west of the Dianchi Fault, Yunnan). It should be noted that the morphology of P4 positive excursion in Xiaotan section occurs as a plateau, differs from other In -type positive excursions (6p in Morocco for example). Considering the high sedimentation rate of the Dahai Member (thick-bedded to massive platformal limestones), we can envisage the long-lasting feature of the P4 excursion. Meanwhile, in NE Yunnan (Xiaotan and Laolin sections), the lack of cyclical positive excursions (1p–7p in Sukharikha, Siberia for example) may be attributed to the local restriction with the global ocean. Local sea-level change may affect the distribution of mineralogies and the depositional and diagenetic environments that control ı13 C on shallow-water carbonate platforms, thus causing deviation of the ı13 C curve from the global mean (positive ı13 C excursions except P4 below zero at Xiaotan). As there is a third-order sequence boundary in the lower part of the Dahai Member (i.e. the lower part of the P4 excursion) at Xiaotan, the P4 plateau might correspond to the 5p + 6p peaks in Siberia and Morocco. This possibility is supported further by detailed comparison of these areas’ 87 Sr/86 Sr data, as a similar decreasing trend from 0.7085 to 0.7081–2 is found at both Xiaotan and Morocco. 5.2.3. Nemakit-Daldynian Sr isotope stratigraphy Carbon isotope chemostratigraphy reflects global changes in non-restricted basins of the surface ocean, and thus can be a reliable tool for stratigraphic correlation. However, arguments have been raised to suggest that highly 13 C-depleted C-isotope ratios in Neoproterozoic marine carbonates represent groundwater influx of photosynthetic carbon from terrestrial phytomass rather than reflecting global marine carbon cycle perturbations (Bristow and Kennedy, 2008; Knauth and Kennedy, 2009; Zhao and Zheng, 2010). Trends toward enrichment in 13 C seen in many Neoproterozoic carbonates are sometimes considered to be caused by elevated bioproductivity and/or enhanced evaporation in shallow marine, near-coastal, temporarily restricted depositional environments and formed by microbial mediation rather than direct precipitation from ambient seawater (Frimmel, 2010). Such concerns are mitigated to some extent by the demonstrated ı13 Ccarb –ı13 Corg covariation in our data, however, the problem of reliability of C-isotope stratigraphic correlation when C-isotopes are used alone in Neoproterozoic strata (e.g. Kennedy et al., 1998) can also be resolved by Strontium Isotope Stratigraphy (SIS). Due to the longer oceanic residence time of Sr relative to C, the isotopic composition of the former element better records long-term global oceanic composition changes and thus can be a reliable tool for chemostratigraphic correlation and indirect age determinations (Melezhik et al., 2001), provided that well preserved marine authigenic components can be identified, and appropriate dissolution methods used (Li et al., 2011). Seawater 87 Sr/86 Sr rose from <0.7055 to >0.7080 during the Neoproterozoic (1000–542 Ma) (Halverson et al., 2007; Shields, 2007), and finally reached its maximum value of about 0.7091 during the late Cambrian at ∼500 Ma (McArthur et al., 2001; Veizer et al., 1999). Against a backdrop of generally increasing 87 Sr/86 Sr, a temporary reversal has been reported worldwide in early Cambrian strata. In Morocco, the lowest 87 Sr/86 Sr values of carbonates from each stratigraphic level in the Tifnout Member decline monotonously from ∼0.7086 to ∼0.7082 in the ‘NemakitDaldynian’ stage there, and begin to rise in the ‘Tommotian’ stage eventually reaching values as high as ∼0.7090 (Fig. 9b) (Maloof et al., 2010a). In Mongolia, least-altered 87 Sr/86 Sr values decline from ∼0.7085 to ∼0.7082 in the ‘Nemakit-Daldynian’ stage (Fig. 9c)

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

(Brasier et al., 1996). Sr-isotope data show an even lower value of ∼0.7080 in the ‘Tommotian’ stage in Mongolia (Fig. 9c), which has been correlated with similar low values from in the limestones of the basal Tommotian of SW Siberia (Fig. 9e) (Brasier et al., 1994c; Derry et al., 1994; Nicholas, 1996). However, at Aldan and Lena River area, SE Siberia, the least-altered 87 Sr/86 Sr ratios fall abruptly from around 0.7085 down to 0.7081 during the transitional period from ‘Nemakit-Daldynian’ to ‘Tommotian’ (Fig. 9e), which suggests a depositional gap during pre-‘Tommotian’ deposition in the AldanLena area, as further evidenced by C-isotope and SSFs occurrences (Knoll et al., 1995b). At Kotuikan River, NW Siberia, the 87 Sr/86 Sr curve did not show a gradually decreasing trend of 0.0004–6 but decreased slightly by 0.0001–2 in the ‘Nemakit-Daldynian’ stage (Fig. 9d) (Kaufman et al., 1996). In the Three Gorges area of South China, carbonate 87 Sr/86 Sr values maintain values around 0.7084–5 in the latest Ediacaran (Fig. 9a) (Sawaki et al., 2010), after which as shown from our Xiaotan section data, least altered 87 Sr/86 Sr values declined gradually from ∼0.7085–6 to ∼0.7082 in the ‘NemakitDaldynian’ stage (Fig. 9a). These similarities of absolute values and evolutionary trends in 87 Sr/86 Sr through early Cambrian strata of China, Morocco, Mongolia and Siberia corroborate global C-isotope stratigraphic correlation schemes, indicating that ı13 C ratios were inherited from ambient seawater during sedimentary deposition. Furthermore, least-altered ratios of 0.7085–6 falling to ∼0.7082, together with carbon isotope oscillation from N1 to P4 verify the stratigraphic correlation using SSF zones, and suggest that the Zhongyicun and Dahai Member at Xiaotan correspond to a Pre-‘Tommotian’ stage, represented by a sedimentary gap at the Dvortsy section in eastern Siberia. 5.2.4. Implications for the Cambrian Stage 1/2 boundary The Fortunian Stage at the base of the Cambrian can be correlated worldwide using ichnozonation (Landing et al., 2007). In practice though, generally due to the lack of shelly fossils and isotopic potential in Newfoundland, basal Cambrian strata are more commonly correlated using a combination of trace fossils, small shelly fossils (SSF) and the BACE C-isotope excursion. The definition of the top of this stage, and thus base of the overlying Cambrian Stage 2, remains undefined. Suitable index fossils (such as acritarchs, archaeocyathids or SSFs) or auxiliary tools (such as specific carbon isotope excursions) have been suggested to define and correlate the base of Stage 2 (Landing, 1998; Rozanov et al., 2008). Xiaotan section is well constrained by SSFs and now by C-, Sr-isotope stratigraphy, providing potential for global stratigraphic correlation of this part of the early Cambrian. The ZHUCE-type ı13 C excursion P4 is coincident worldwide with lowermost 87 Sr/86 Sr values of ∼0.7082, and with the occurrence of Heraultipegma yunnanensis (=Watsonella crosbyi) SSF Assemblage Zone in Siberia, Mongolia and China. The stratigraphic level immediately above P4 and the W. crosbyi zone at the Dahai/Shiyantou boundary can be assigned an age of ∼525 Ma by correlation with Morocco, and such correlation is roughly consistent with the age (526.5 Ma) of basal Shiyantou Formation at Meishucun section (Compston et al., 2008). This correlation also implies that the beginning of this ı13 C peak and the W. crosbyi FAD is older than 525 Ma in South China. 5.3. Indirect dating of Xiaotan section The C-, Sr-isotopic and biostratigraphically well-constrained sedimentary units of the Xiaotan section given here can be readily correlated with other contemporaneous successions worldwide (Fig. 9). Age constraints from China and elsewhere can thus be used to provide approximate ages for early Cambrian strata at Xiaotan, and correlative strata worldwide.

17

The latest and most precise age of 535.2 ± 1.7 Ma (Zhu et al., 2009) was yielded in the bentonite tuff bed in the upper Zhongyicun Member, and a U/Pb SHRIMP age of 526.52 ± 1.1 Ma was reported by Compston et al. (2008) from the basal Shiyantou Formation at Meishucun section. A new precise SHRIMP U–Pb zircon age of 532.3 ± 0.7 Ma has been reported from a volcanic ash bed in the lowermost black shale sequence of the Niutitang Formation in Guizhou, China (Jiang et al., 2009), which is at a stratigraphic level immediately above the Ed–C boundary, but is otherwise poorly constrained stratigraphically due to the condensed nature of that section. The dated horizon thus lies stratigraphically close to the postı13 C peak zero crossing close to the Dahai/Shiyantou boundary at Xiaotan, which has been dated in Morocco. In Morocco, a weighted mean 206 Pb/238 U age of 524.84 ± 0.09 Ma was reported from a tuff bed at the zero crossing of the decreasing ı13 C curve sector after the last positive anomaly 6p, and in addition, a refined 206 Pb/238 U age of 525.34 ± 0.09 Ma in an tuff within peak 6p (Maloof et al., 2010b). Using the more precise, but mutually consistent age constraints from Morocco implies that the ages of the Dahai ı13 C peak and Dahai/Shiyantou boundary are approximately 525.3 Ma and 524.8 Ma, respectively. The PC–C boundary was defined by a short-lived negative carbon isotope anomaly and extinction of Ediacaran-type skeletal fossil (Cloudina and Namacalathus) in S Oman (Amthor et al., 2003). Precise dating of zircons from volcanic ash beds in Oman yielded weighted mean 206 Pb/238 U dates of 542.3 ± 0.2 Ma and 541.0 ± 0.2 Ma, below and during the Ed–C boundary ı13 C negative anomaly (−5‰), respectively, assigning a most commonly used age of ca. 542 Ma to the basal Cambrian System (Amthor et al., 2003; Bowring et al., 2007). Taking these age constraints into account, the strata at Xiaotan containing the N1 excursion (BACE) in the lower Daibu Member and the zero crossing immediately above the P4 excursion (ZHUCE) at the Dahai/Shiyantou boundary could be assigned ages of ∼541 Ma and ∼525 Ma, respectively (Fig. 9). Therefore, the ‘Nemakit-Daldynian’ stage constrained by biostratigraphy, and containing China SSF Zones I to III in the Zhongyicun and Dahai members, seems likely to have lasted for 16 Myrs. 5.4. Palaeoenvironmental implications 5.4.1. C versus Sr isotopes The composite detailed C-, O- and Sr-isotope chemostratigraphy of the Xiaotan section constrains the chemical evolution of ambient seawater and dynamics of the solid earth during the early Cambrian (Fig. 10a). This interval was associated with a generally rising trend in ı13 C interspersed with rapid reorganizations of the carbon cycle, superimposed on a smooth decrease in 87 Sr/86 Sr values. The N1 phase (Fig. 10a) is characterized by ı13 C ratios lower than −5‰ (as low as −12‰) and 87 Sr/86 Sr values at 0.7085–6. By contrast, the P4 phase (Fig. 10a) is characterized by a plateau of ı13 C ratios around 6‰ and relatively lower least-altered 87 Sr/86 Sr values of 0.7082–3. 5.4.2. Carbon isotope Interpretations Carbonate carbon isotopes mimic the ı13 C ratio of dissolved inorganic carbon (DIC) in the global ocean, which is normally higher than that of dissolved organic carbon (DOC) due to biogenic carbon fractionation and organic burial (Hayes et al., 1999). Carbonate ı13 C may be subjected to a number of different perturbations, such as weathering, volcanism, riverine and hydrothermal input, and ocean ventilation (Kump and Arthur, 1999). The Ed–C boundary-associated ı13 C anomalies may indicate a global event during this interval. The extreme nature of the negative carbonate ı13 C anomaly at Xiaotan (−12‰), compared to its equivalents in Siberia, Mongolia and Morocco, is hard to explain and could be

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20 18

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

Fig. 10. Palaeoenvironmental implications of the early Cambrian Sr- and C-isotope record at Xiaotan section.

put down to diagenetic alteration. Notwithstanding this, the existence of negative anomalies lower than −5‰ explained by mantle input alone (−5‰) still requires an additional, primary explanation. Rothman et al. (2003) proposed a non-steady state model involving decoupled reduced and oxidized carbon reservoirs to explain the large carbon isotope fluctuations. This mechanism was then advanced primarily to explain the Shuram-Wonoka anomaly in the Ediacaran ocean (Fike et al., 2006; Jiang et al., 2007; McFadden et al., 2008). On the time scale we envisage (extrapolating from Oman and Morocco), ı13 C decreased over approximately 1–2 Myrs during the N1 phase. A decrease from +2‰ to −12‰ would require a pool of DOC and oxidants (O2 or sulfate), which may be implausibly large (Maloof et al., 2010a). Therefore, other sources 13 C-deplet ed–carbon need to be considered, such as methanogenesis. In the P4 positive ı13 C plateau, the ı13 Ccarb rose in parallel with ı13 Corg , which indicates that DOC reservoir was smaller than the DIC reservoir as is the case today. 5.4.3. Strontium isotope interpretations The strontium isotope composition of seawater is influenced on geological time scales by changes in the rates of continental silicate weathering relative to ocean crust alteration, as well as carbonate dissolution (Shields, 2007). The oceanic contribution of relatively 87 Sr-depleted materials is related to the mid-ocean ridge spreading rate. Continental silicate weathering generally brings 87 Sr-enriched materials to the ocean (e.g. Derry and France Lanord, 1996; Edmond, 1992; Richter et al., 1992), however, the relatively rapid weathering of young continental or submarine carbonate functions as a buffer to seawater Sr isotope compositions. The decline in least-altered 87 Sr/86 Sr values from phase N1 to phase P4 was therefore possibly caused by one or all of the following changes as illustrated in Fig. 10b: (1) an increase in the ocean spreading rates and thus increase in the flux of MORB (∼0.703) under submarine weathering; (2) young carbonate weathering, for example, of carbonate formed earlier in Earth history such as the underlying Doushantuo Formation (∼0.708) (Sawaki et al., 2010) possibly dissolved during transgression in early Cambrian; (3) continental

or submarine weathering of large juvenile igneous provinces; (4) a decrease of the 87 Sr/86 Sr ratio and/or the flux of continental silicates undergoing weathering. Sr isotopes can also be interpreted in the light of solid earth dynamics. For example, the break-up of the Rodinian supercontinent likely accelerated continental runoff and erosion, thus leading to Neoproterozoic glaciation (Donnadieu et al., 2004; Halverson et al., 2007). High continental weathering of the continent interior surfaces due to the supercontinent break-up could have increased global seawater 87 Sr/86 Sr toward the end of the Ediacaran. In the early Cambrian, the beginning of Gondwana-Pangaea assembly may have isolated continental interiors from moisture sources which could have led to a reduction in the continental weathering flux of Sr to the oceans (Halverson et al., 2007). The opening of the Iapetus Ocean in the early Cambrian may have increased mid-ocean-ridge length and contributed additional hydrothermal 87 Sr/86 Sr source to the ocean (Maloof et al., 2010a). Thus, a multicause mechanism involving increasing submarine hydrothermal alteration, higher ocean spreading rates, and decreased continent silicate weathering rates could have lowered seawater 87 Sr/86 Sr to 0.7082–3 in the latest Fortunian Stage of the early Cambrian. 6. Conclusions (1) Confirming previous work (Zhou et al., 1997), the C isotope curve at Xiaotan section depicts a large negative excursion N1 (BACE) (−12.2‰) in the Daibu Member just below the Ediacaran–Cambrian boundary and below the first SSF FAD in the basal Zhongyicun Member, and a large positive excursion P4 (ZHUCE) (7.3‰) in the overlying Dahai Member. This type of C isotope curve is characteristic of the early Cambrian of the Yangtze platform and can be correlated globally. (2) The Sr isotope values show a decreasing trend from 0.7085–6 to 0.7082–3 across the Ediacaran–Cambrian transition in South China which can also be correlated globally. (3) The preference for chemostratigraphy over biostratigraphy is favored in stratigraphic correlation and subdivision of the

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

early Cambrian. Strontium isotope stratigraphy provides a more definitive and precise tool for stratigraphic correlations and indirect age determinations compared to the carbon isotope approach. Using recent geochronological constraints and our global correlation based on C-, Sr-, and biostratigraphy, the Nemakit-Daldynian stage in Siberia, which may approximate the Cambrian Stage 1 (Fortunian), appears to have lasted approximately 16 Myrs. (4) A short-term lessening of continental weathering punctuating an overall increase in terrestrial weathering rates may have been responsible for the temporary reversal in the increasing seawater 87 Sr/86 Sr trend during the Cambrian Period. Other factors, such as increasing hydrothermal events and oceanic crust alteration, young carbonate dissolution, and chemical weathering of juvenile volcanic provinces, might also have decreased the seawater Sr isotopes at this time. Acknowledgements We thank Prof. Maoyan Zhu and Prof. Junming Zhang for their guide in the fieldwork. DL is grateful to Tim Atkinson and Tony Osborn at UCL, and Wei Pu at Nanjing University for assistance with chemical analysis. Authors acknowledge National Science Foundation of China (Grants. 40872025 and 41102018), DFG Forschergruppe 736 “The Precambrian-Cambrian ecosphere revolution”, and China Scholarship Council for overseas study. References Amthor, J.E., Grotzinger, J.P., Schroder, S., Bowring, S.A., Ramezani, J., Martin, M.W., Matter, A., 2003. Extinction of Cloudina and Namacalathus at the Precambrian–Cambrian boundary in Oman. Geology 31, 431–434. Bartley, J.K., Pope, M., Knoll, A.H., Semikhatov, M.A., Petrov, P.Y., 1998. A Vendian–Cambrian boundary succession from the northwestern margin of the Siberian Platform; stratigraphy, palaeontology, chemostratigraphy and correlation. Geol. Mag. 135, 473–494. Bowring, S.A., Grotzinger, J.P., Condon, D.J., Ramezani, J., Newall, M.J., Allen, P.A., 2007. Geochronologic constraints on the chronostratigraphic framework of the Neoproterozoic Huqf Supergroup, Sultanate of Oman. Am. J. Sci. 307, 1097–1145. Brasier, M., Cowie, J., Taylor, M., 1994a. Decision on the Precambrian–Cambrian boundary stratotype. Episodes 17, 3–8. Brasier, M.D., Corfield, R.M., Derry, L.A., Rozanov, A.Y., Zhuravlev, A.Y., 1994b. Multiple Delta-13 C excursions spanning the Cambrian explosion to the Botomian Crisis in Siberia. Geology 22, 455–458. Brasier, M.D., Magaritz, M., Corfield, R., Luo, H., Wu, X., Ouyang, L., Jiang, Z., Hamdi, B., He, T., Fraser, A.G., 1990. The carbon- and oxygen-isotope record of the Precambrian–Cambrian boundary interval in China and Iran and their correlation. Geol. Mag. 127, 319–332. Brasier, M.D., Rozanov, A.Y., Zhuravlev, A.Y., Corfield, R.M., Derry, L.A., 1994c. A carbon-isotope reference scale for the lower Cambrian succession in Siberia—report of Igcp Project-303. Geol. Mag. 131, 767–783. Brasier, M.D., Shields, G., Kuleshov, V.N., Zhegallo, E.A., 1996. Integrated chemo- and biostratigraphic calibration of early animal evolution; neoproterozoic—early Cambrian of Southwest Mongolia. Geol. Mag. 133, 445–485. Bristow, T.F., Kennedy, M.J., 2008. Carbon isotope excursions and the oxidant budget of the Ediacaran atmosphere and ocean. Geology 36, 863–866. Brooks, B., Crowley, J., Bowring, S., Cervato, C., Jin, Y., 2006. A new U/Pb date for the basal Meishucun section and implications for the age of the Cambrian explosion. American Geophysical Union, Fall Meeting 2006. Abstract, p. 0568. Compston, W., Zhang, Z., Cooper, J.A., Ma, G., Jenkins, R.J.F., 2008. Further SHRIMP geochronology on the early Cambrian of South China. Am. J. Sci. 308, 399–420. Cowie, J.W., Rozanov, A.Y., 1983. Precambrian Cambrian boundary candidate, Aldan River, Yakutia, USSR. Geol. Mag. 120, 129–139. Craig, H., 1953. The geochemistry of the stable carbon isotopes. Geochim. Cosmochim. Acta 3, 53–92. Cremonese, L., Struck, U., Shields-Zhou, G., Och, L., this issue. Nitrogen isotope evolution across the Precambrian–Cambrian transition of South China. Precambrian Res. Derry, L.A., 2010. A burial diagenesis origin for the Ediacaran Shuram-Wonoka carbon isotope anomaly. Earth Planet. Sci. Lett. 294, 152–162. Derry, L.A., Brasier, M.D., Corfield, R.M., Rozanov, A.Y., Zhuravlev, A.Y., 1994. Srisotope and C-isotope in lower Cambrian carbonates from the Siberian Craton—a paleoenvironmental record during the Cambrian explosion. Earth Planet. Sci. Lett. 128, 671–681. Derry, L.A., France Lanord, C., 1996. Neogene Himalayan weathering history and river Sr-87/Sr-86 impact on the marine Sr record. Earth Planet. Sci. Lett. 142, 59–74.

19

Donnadieu, Y., Godderis, Y., Ramstein, G., Nedelec, A., Meert, J., 2004. A ‘snowball Earth’ climate triggered by continental break-up through changes in runoff. Nature 428, 303–306. Edmond, J.M., 1992. Himalayan tectonics, weathering processes, and the strontium isotope record in marine limestones. Science 258, 1594–1597. Elderfield, H., 1986. Strontium isotope stratigraphy. Palaeogeogr. Palaeoclimatol. Palaeoecol. 57, 71–90. Fike, D.A., Grotzinger, J.P., Pratt, L.M., Summons, R.E., 2006. Oxidation of the Ediacaran Ocean. Nature 444, 744–747. Frimmel, H.E., 2010. On the reliability of stable carbon isotopes for Neoproterozoic chemostratigraphic correlation. Precambrian Res. 182, 239–253. Grotzinger, J.P., Bowring, S.A., Saylor, B.Z., Kaufman, A.J., 1995. Biostratigraphic and geochronologic constraints on early animal evolution. Science 270, 598–604. Halverson, G.P., Dudás, F.Ö., Maloof, A.C., Bowring, S.A., 2007. Evolution of the 87Sr/86Sr composition of Neoproterozoic seawater. Palaeogeogr. Palaeoclimatol. Palaeoecol. 256, 103–129. Hayes, J.M., Strauss, H., Kaufman, A.J., 1999. The abundance of C-13 in marine organic matter and isotopic fractionation in the global biogeochemical cycle of carbon during the past 800 Ma. Chem. Geol. 161, 103–125. Hsu, K.J., Oberhansli, H., Gao, J.Y., Sun, S., Chen, H.H., Krahenbuhl, U., 1985. Strangelove ocean before the Cambrian explosion. Nature 316, 809–811. Ishikawa, T., Ueno, Y., Komiya, T., Sawaki, Y., Han, J., Shu, D., Li, Y., Maruyama, S., Yoshida, N., 2008. Carbon isotope chemostratigraphy of a Precambrian/Cambrian boundary section in the Three Gorge area, South China: prominent global-scale isotope excursions just before the Cambrian explosion. Gondwana Res. 14, 193–208. Jenkins, R.J.F., Cooper, J.A., Compston, W., 2002. Age and biostratigraphy of Early Cambrian tuffs from SE Australia and southern China. J. Geol. Soc. Lond. 159, 645–658. Jiang, G.Q., Kaufman, A.J., Christie-Blick, N., Zhang, S.H., Wu, H.C., 2007. Carbon isotope variability across the Ediacaran Yangtze platform in South China: Implications for a large surface-to-deep ocean delta C-13 gradient. Earth Planet. Sci. Lett. 261, 303–320. Jiang, S.-Y., Pi, D.-H., Heubeck, C., Frimmel, H., Liu, Y.-P., Deng, H.-L., Ling, H.-F., Yang, J.-H., 2009. Early Cambrian ocean anoxia in South China. Nature 459, E5–E6. Jones, C.E., Jenkyns, H.C., Coe, A.L., Hesselbo, S.P., 1994. Strontium isotopic variations in Jurassic and cretaceous seawater. Geochim. Cosmochim. Acta 58, 3061–3074. Kaufman, A.J., Knoll, A.H., Semikhatov, M.A., Grotzinger, J.P., Jacobsen, S.B., Adams, W., 1996. Integrated chronostratigraphy of Proterozoic–Cambrian boundary beds in the western Anabar region, northern Siberia. Geol. Mag. 133, 509–533. Kennedy, M.J., Runnegar, B., Prave, A.R., Hoffmann, K.H., Arthur, M.A., 1998. Two or four neoproterozoic glaciations? Geology 26, 1059–1063. Khomentovsky, V.V., Karlova, G.A., 1993. Biostratigraphy of the vendian Cambrian beds and the lower Cambrian boundary in Siberia. Geol. Mag. 130, 29–45. Kimura, H., Matsumoto, R., Kakuwa, Y., Hamdi, B., Zibaseresht, H., 1997. The Vendian–Cambrian delta C-13 record, North Iran: evidence for overturning of the ocean before the Cambrian explosion. Earth Planet. Sci. Lett. 147, E1–E7. Knauth, L.P., Kennedy, M.J., 2009. The late Precambrian greening of the Earth. Nature 460, 728–732. Knoll, A.H., Grotzinger, J.P., Kaufman, A.J., Kolosov, P., 1995a. Integrated approaches to terminal Proterozoic stratigraphy: an example from the Olenek Uplift, northeastern Siberia. Precambrian Res. 73, 251–270. Knoll, A.H., Kaufman, A.J., Semikhatov, M.A., Grotzinger, J.P., Adams, W., 1995b. Sizing up the sub-Tommotian unconformity in Siberia. Geology 23, 1139–1143. Kouchinsky, A., Bengtson, S., Missarzhevsky, V.V., Pelechaty, S., Torssander, P., Val’kov, A.K., 2001. Carbon isotope stratigraphy and the problem of a preTommotian stage in Siberia. Geol. Mag. 138, 387–396. Kouchinsky, A., Bengtson, S., Pavlov, V., Runnegar, B., Torssander, P., Young, E., Ziegler, K., 2007. Carbon isotope stratigraphy of the Precambrian–Cambrian Sukharikha River section, northwestern Siberian platform. Geol. Mag. 144, 609–618. Kouchinsky, A., Bengtson, S., Pavlov, V., Runnegar, B., Val’kov, A., Young, E., 2005. Pre-Tommotian age of the lower Pestrotsvet formation in the Selinde section on the Siberian platform: carbon isotopic evidence. Geol. Mag. 142, 319–325. Kump, L.R., Arthur, M.A., 1999. Interpreting carbon-isotope excursions: carbonates and organic matter. Chem. Geol. 161, 181–198. Landing, E., 1989. Paleoecology and distribution of the early Cambrian Rostroconch Watsonella-Crosbyi Grabau. J. Paleontol. 63, 566–573. Landing, E., 1994. Precambrian–Cambrian boundary global stratotype ratified and a new perspective of Cambrian time. Geology 22, 179–182. Landing, E., 1998. Cambrian subdivisions and correlations: Introduction. Can. J. Earth Sci. 35, 321–322. Landing, E., Peng, S., Babcock, L.E., Geyer, G., Moczydlowska-Vidal, M., 2007. Global standard names for the lowermost Cambrian series and stage. Episodes 30, 287–289. Li, D., Ling, H.-F., Jiang, S.-Y., Pan, J.-Y., Chen, Y.-Q., Cai, Y.-F., Feng, H.-Z., 2009. New carbon isotope stratigraphy of the Ediacaran–Cambrian boundary interval from SW China: implications for global correlation. Geol. Mag. 146, 465–484. Li, D., Shields-Zhou, G., Ling, H.-F., Thirlwall, M., 2011. Dissolution methods for strontium isotope stratigraphy: guidelines for the use of bulk marine carbonate and phosphorite rocks. Chem. Geol. 290, 133–144. Li, G.X., Xiao, S.H., 2004. Tannuolina and Micrina (Tannuolinidae) from the lower Cambrian of eastern Yunnan, South China, and their scleritome reconstruction. J. Paleontol. 78, 900–913. Luo, H., Jiang, Z., Wu, X., Song, X., Ouyang, L., 1982. The Sinian–Cambrian Boundary in Eastern Yunnan: Yunnan China. Yunnan People’s Publishing House, Kuming.

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002

G Model PRECAM-3496; No. of Pages 20 20

ARTICLE IN PRESS D. Li et al. / Precambrian Research xxx (2012) xxx–xxx

Magaritz, M., Holser, W.T., Kirschvink, J.L., 1986. Carbon-isotope events across the Precambrian Cambrian boundary on the Siberian platform. Nature 320, 258–259. Magaritz, M., Kirschvink, J.L., Latham, A.J., Zhuravlev, A.Y., Rozanov, A.Y., 1991. Precambrian Cambrian boundary-problem—carbon isotope correlations for Vendian and Tommotian time between Siberia and Morocco. Geology 19, 847–850. Maloof, A.C., Porter, S.M., Moore, J.L., Dudas, F.O., Bowring, S.A., Higgins, J.A., Fike, D.A., Eddy, M.P., 2010a. The earliest Cambrian record of animals and ocean geochemical change. Geol. Soc. Am. Bull. 122, 1731–1774. Maloof, A.C., Ramezani, J., Bowring, S.A., Fike, D.A., Porter, S.M., Mazouad, M., 2010b. Constraints on early Cambrian carbon cycling from the duration of the Nemakit-Daldynian–Tommotian boundary delta C-13 shift, Morocco. Geology 38, 623–626. Maloof, A.C., Schrag, D.P., Crowley, J.L., Bowring, S.A., 2005. An expanded record of Early Cambrian carbon cycling from the Anti-Atlas margin, Morocco. Can. J. Earth Sci. 42, 2195–2216. McArthur, J.M., Howarth, R.J., Bailey, T.R., 2001. Strontium isotope stratigraphy: LOWESS version 3: Best fit to the marine Sr-isotope curve for 0–509 Ma and accompanying look-up table for deriving numerical age. J. Geol. 109, 155–170. McCrea, J.M., 1950. Isotopic chemistry of carbonates and a paleo-temperature scale. J. Chem. Phys. 18, 849–857. McFadden, K.A., Huang, J., Chu, X.L., Jiang, G.Q., Kaufman, A.J., Zhou, C.M., Yuan, X.L., Xiao, S.H., 2008. Pulsed oxidation and bioloical evolution in the Ediacaran Doushantuo formation. Proc. Natl. Acad. Sci. U.S.A. 105, 3197–3202. Melezhik, V.A., Gorokhov, I.M., Kuznetsov, A.B., Fallick, A.E., 2001. Chemostratigraphy of neoproterozoic carbonates: implications for ‘blind dating’. Terra Nova 13, 1–11. Narbonne, G.M., Kaufman, A.J., Knoll, A.H., 1994. Integrated chemostratigraphy and biostratigraphy of the Windermere Supergroup, northwestern Canada: implications for Neoproterozoic correlations and the early evolution of animals. Geol. Soc. Am. Bull. 106, 1281–1292. Narbonne, G.M., Myrow, P.M., Landing, E., Anderson, M.M., 1987. A candidate stratotype for the Precambrian–Cambrian boundary, Fortune Head Burin Peninsula, southeastern Newfoundland. Can. J. Earth Sci. 24, 1277–1293. Nicholas, C.J., 1996. The Sr isotopic evolution of the oceans during the ‘Cambrian explosion’. J. Geol. Soc. Lond. 153, 243–254. Parkhaev, P., Demidenko, Y., 2010. Zooproblematica and mollusca from the Lower Cambrian Meishucun section (Yunnan, China) and taxonomy and systematics of the Cambrian small shelly fossils of China. Paleontol. J. 44, 883–1161. Pelechaty, S.M., 1998. Integrated chronostratigraphy of the Vendian System of Siberia: implications for a global stratigraphy. J. Geol. Soc. Lond. 155, 957–973. Pelechaty, S.M., Grotzinger, J.P., Kashirtsev, V.A., Zhernovsky, V.P., 1996a. Chemostratigraphic and sequence stratigraphic constraints on Vendian–Cambrian basin dynamics, northeast Siberian craton. J. Geol. 104, 543–563. Pelechaty, S.M., Kaufman, A.J., Grotzinger, J.P., 1996b. Evaluation of delta C-13 chemostratigraphy for intrabasinal correlation Vendian strata of northeast Siberia. Geol. Soc. Am. Bull. 108, 992–1003. Pokrovsky, B.G., Missarzhevsky, V.V., 1993. Isotopic correlation of Precambrian and Cambrian of the Siberian platform. Doklady Akad. Nauk 329, 768–771 (in Russian). Qian, Y., Bengtson, S., 1989. Paleontology and biostratigraphy of the Early Cambrian Meishucunian Stage in Yunnan Province, South China. Fossils Strata 24, 156. Qian, Y., He, T., Jiang, Z., 1996. New investigation of Precambrian–Cambrian boundary sections in eastern Yunnan. Acta Micropalaeontol. Sin. 13, 225–240 (in Chinese with English abstract). Qian, Y., Zhu, M., Li, G., Jiang, Z., Iten, H.V., 2002. A supplemental Precambrian–Cambrian boundary global stratotype section in SW China. Acta Palaeontol. Sin. 41, 19–26.

Richter, F.M., Rowley, D.B., DePaolo, D.J., 1992. Sr isotope evolution of seawater: the role of tectonics. Earth Planet. Sci. Lett. 109, 11–23. Rothman, D.H., Hayes, J.M., Summons, R.E., 2003. Dynamics of the Neoproterozoic carbon cycle. Proc. Natl. Acad. Sci. U.S.A. 100, 8124–8129. Rozanov, A.Y., 1984. The Precambrian Cambrian boundary in Siberia. Episodes 7, 20–24. Rozanov, A.Y., Khomentovsky, V.V., Shabanov, Y.Y., Karlova, G.A., Varlamov, A.I., Luchinina, V.A., Pegel, T.V., Demidenko, Y.E., Parkhaev, P.Y., Korovnikov, I.V., Skorlotova, N.A., 2008. To the problem of stage subdivision of the Lower Cambrian. Stratigr. Geol. Correl. 16, 1–19. Rozanov, A.Y., Missarzhevsky, V., Volkova, N., Voronova, L., Krylov, I., Keller, B., Korolyuk, I., Lendzion, K., Michniak, R., Pychova, N., 1969. The Tommotian stage and the Cambrian lower boundary problem. Trudy Geol. Inst. Akad. Nauk SSSR 206, 379 (in Russian). Sawaki, Y., Nishizawa, M., Suo, T., Komiya, T., Hirata, T., Takahata, N., Sano, Y., Han, J., Kon, Y., Maruyama, S., 2008. Internal structures and U–Pb ages of zircons from a tuff layer in the Meishucunian formation, Yunnan Province, South China. Gondwana Res. 14, 148–158. Sawaki, Y., Ohno, T., Tahata, M., Komiya, T., Hirata, T., Maruyama, S., Windley, B.F., Han, J., Shu, D., Li, Y., 2010. The Ediacaran radiogenic Sr isotope excursion in the Doushantuo formation in the Three Gorges area South China. Precambrian Res. 176, 46–64. Shen, Y., Schidlowski, M., 2000. New C isotope stratigraphy from southwest China: implications for the placement of the Precambrian–Cambrian boundary on the Yangtze Platform and global correlations. Geology 28, 623–626. Shields, G.A., 2007. A normalised seawater strontium isotope curve: possible implications for Neoproterozoic-Cambrian weathering rates and the further oxygenation of the Earth. eEarth 2, 35–42. Steiner, M., Li, G.X., Qian, Y., Zhu, M.Y., Erdtmann, B.D., 2007. Neoproterozoic to early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China). Palaeogeogr. Palaeoclimatol. 254, 67–99. Tucker, M.E., 1986. Carbon isotope excursions in Precambrian/Cambrian boundary beds, Morocco. Nature 319, 48–50. Vasconcelos, C., McKenzie, J.A., Warthmann, R., Bernasconi, S.M., 2005. Calibration of the delta O-18 paleothermometer for dolomite precipitated in microbial cultures and natural environments. Geology 33, 317–320. Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, F., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O.G., Strauss, H., 1999. Sr-87/Sr-86, delta C-13 and delta O-18 evolution of Phanerozoic seawater. Chem. Geol. 161, 59–88. Zhao, Y.-Y., Zheng, Y.-F., 2010. Stable isotope evidence for involvement of deglacial meltwater in Ediacaran carbonates in South China. Chem. Geol. 271, 86–100. Zhou, C., Zhang, J., Li, G., Yu, Z., 1997. Carbon and oxygen isotopic record of the early Cambrian from the Xiaotan section, Yunnan, South China. Sci. Geol. Sin., 201–211 (in Chinese with English abstract). Zhu, M.-Y., Babcock, L.E., Peng, S.-C., 2005. Advances in Cambrian stratigraphy and paleontology: integrating correlation techniques, paleobiology, taphonomy and paleoenvironmental reconstruction. Palaeoworld 15, 217–222. Zhu, M., Li, G., Zhang, J., Steiner, M., Qian, Y., Jiang, Z., 2001. Early Cambrian stratigraphy of east Yunnan, southwestern China: a synthesis. Acta Palaeontol. Sin. 40, 4–39. Zhu, M., Strauss, H., Shields, G.A., 2007. From snowball earth to the Cambrian bioradiation: calibration of Ediacaran–Cambrian earth history in South China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 254, 1–6. Zhu, R., Li, X., Hou, X., Pan, Y., Wang, F., Deng, C., He, H., 2009. SIMS U–Pb zircon age of a tuff layer in the Meishucun section Yunnan, southwest China: constraint on the age of the Precambrian–Cambrian boundary. Sci China Ser. D: Earth Sci. 52, 1385–1392.

Please cite this article in press as: Li, D., et al., Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China. Precambrian Res. (2012), doi:10.1016/j.precamres.2012.01.002