Sr isotope excursion across the Precambrian–Cambrian boundary in the Three Gorges area, South China

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Sr isotope excursion across the Precambrian–Cambrian boundary in the Three Gorges area, South China Yusuke Sawaki⁎, Takeshi Ohno, Yusuke Fukushi, Tsuyoshi Komiya, Tomoko Ishikawa, Takafumi Hirata, Shigenori Maruyama Department of Earth and Planetary Sciences, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan Received 6 July 2007; received in revised form 8 November 2007; accepted 20 November 2007 Available online 3 January 2008

Abstract The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life. However, the scarcity of wellpreserved outcrops across the boundary leaves an obstacle in decoding surface environmental changes and patterns of biological evolution. In south China, strata through the PC/C boundary are almost continuously exposed and contain many fossils, suitable for study of environmental and biological change across the PC/C boundary. We undertook deep drilling at four sites in the Three Gorges area to obtain continuous and fresh samples without surface alteration and oxidation. 87Sr/86Sr ratios of the fresh carbonate rocks, selected based on microscopic observation and geochemical signatures of Mn/Sr and Rb/Sr ratios, aluminum and silica contents, and δ13C and δ18O values, were measured with multiple collector-inductively coupled plasma–mass spectrometric techniques. The chemostratigraphy of 87Sr/86Sr ratios of the drilled samples displays a smooth curve and a large positive anomaly just below the PC/C boundary between the upper part of Baimatuo Member of the Dengying Formation and the lower part of the Yanjiahe Formation. The combination of chemostratigraphies of δ13C and 87Sr/86Sr indicates that the 87Sr/86Sr excursions preceded the δ13C negative excursion, and suggests that global regression or formation of the Gondwana supercontinent, possibly together with a high atmospheric pCO2, caused biological depression. © 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Keywords: Sr isotope; Precambrian–Cambrian boundary; δ13C, chemostratigraphy; Gondwana

1. Introduction Recent paleontological studies have clarified the emergence of two types of animals in the Ediacaran of the Neoproterozoic, especially after the Marinoan glaciation (e.g. Sprigg, 1947; Glaessner and Wade, 1996; Brasier and Antcliffe 2004; Xiao et al., 1998; Li et al., 1998; Chen et al., 2000; McCall, 2006; Meert and Lieberman, 2008-this issue). The Ediacaran-type fauna (Vendobionts) appeared in Australia, Siberia, Mackenzie Mountains, Western USA and Newfoundland, whereas cnidarians, sponges and possible bilaterians appeared in South China (Li et al., 1998; Chen et al., 2002; Xiao et al., 2000; Chen et al., 2004a,b; Bengtson and Budd 2004; Komiya et al., 2008b–this issue). The PC/C boundary is one of the most important intervals for the evolution of life, because it is characterized by ⁎ Corresponding author. Tel.: +81 3 5734 2618; fax: +81 3 5734 3538. E-mail address: [email protected] (Y. Sawaki).

the decline or extinction of the Ediacaran fauna (e.g. Brasier et al., 1994; Landing, 1994; Brasier, 1995). However, the phenomena and origin of the biological vicissitudes around the PC/C boundary are still ambiguous, because of the scarcity of well-preserved outcrops around the PC/C boundary and of the restriction to some sections in South China, Siberia, Mongolia, Morocco, Newfoundland and Death Valley (Brasier, 1992; Landing, 1994). Many chemostratigraphies through the PC/C boundary have been reported. However, their application to decoding the evolution of surface environment and biological activity is restricted, because many chemostratigraphies of δ13C and δ18O were reported, but no detailed multiple chemostratigraphy of C, O and Sr isotopes was demonstrated. Shields and Veizer (2002) suggested that the radiogenic Sr isotope ratio suddenly increased in the Neoproterozoic based on the compilation of Sr isotope compositions through geologic time. Fig. 1 shows chemostratigraphies of Sr isotopes of the Neoproterozoic to Cambrian sequences in Siberia, Mongolia,

1342-937X/$ - see front matter © 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2007.11.002

Y. Sawaki et al. / Gondwana Research 14 (2008) 134–147 Fig. 1. Comparison of chemostratigraphies of δ13C and 87Sr/86Sr ratios across the Precambrian–Cambrian boundary in (a) Mongolia, (b) Namibia, (c) Siberia and (d) South China (modified after Brasier et al., 1996; Kaufman et al., 1993; Derry et al., 1994; Wang et al., 2002). 135

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Namibia and South China (Derry et al., 1994; Brasier et al., 1996; Kaufman et al., 1993; Wang et al., 2002). Many previous works showed that 87Sr/86Sr ratios had risen from 0.7066 at ca. 590 Ma up to 0.7085 at the PC/C boundary (Melezhik et al., 2001). However, 87Sr/86Sr measurements around the PC/C boundary are still sporadic and insufficient for detailed discussion of surface environmental change and its influence on biological activity around the PC/C boundary. We undertook drilling through the PC/C boundary in South China, where there is a complete sequence from the Neoproterozoic to Cambrian with many fossils in order to make chemostratigraphies of δ 13 C, δ 18 O, 87 Sr/ 86 Sr, and 88 Sr/86Sr ratios. The drill-sampling allows us to minimize the effect of secondary alteration and oxidation on the surface and to make a very continuous chemostratigraphy at intervals of millimeters. This work presents a detailed new chemostratigraphy in the Three Gorges Region in South China, and discusses the evolution of the ambient ocean, with respect to changes in biological activity, surface environment and activity of the solid earth. This is the first detailed 87 Sr/ 86 Sr chemostratigraphy in the Three Gorges Region in South China. 2. Geology of the Three Gorges Region 2.1. Geological setting Neoproterozoic–Cambrian rocks crop out widely in South China; the Three Gorges is located ca. 30 km west of Yichang along the Yangtze River. The succession contains many fossils of Neoproterozoic and Cambrian age (Fig. 2b). In the Three Gorges Region, shallow marine carbonates and deep sea black shales were deposited in the platform interior (Fig. 2a). The Neoproterozoic and Early Palaeozoic successions surrounding the Huangling Anticline are present in northwestern Yichang (Fig. 2b). The Neoproterozoic and Early Palaeozoic sections are especially well exposed along the Yangtze River, cutting through the southern part of the anticline. Since the recognition of the Yangtze Gorges area as a type locality of the Sinian System (Lee and Chao, 1924; Liu and Sha, 1963), the Sinian sections of the area have been intensively investigated. The Three Gorges section consists of the Doushantuo, Dengying, Yanjiahe, Shuijintuo and Shipai Formations in ascending order. The ca. 250 m thick Doushantuo Formation (Fm.) comprises four members: Member 1 (Cap Dolomite), Member 2 (black shale-dominated layers), Member 3 (dolostone-dominated layers) and Member 4 (black Shale) in ascending order. The ~ 5 m thick Member 1 is characterized by unusual sedimentary and diagenetic structures such as stromatactis-like structures, tepee-like structures, sheet cracks, and barite fans in a cap carbonate (Jiang et al., 2003; Zhou et al., 2004). This member is divided into three subsequences; a disrupted limestone/ dolostone layer, laminated limestone/dolostone layers, and a laminated silty limestone/dolostone layer in ascending order (Jiang et al., 2003). The age of the Doushantuo Fm. in the Yangtze Gorges area is constrained by zircon U–Pb ages of three ash beds by a conventional TIMS method. The U–Pb zircon dating ranges from 635.2 ± 0.6 Ma for the ash bed within

Member 1 through 632.5 ± 0.5 Ma for the ash in Member 2 to 551.1 ± 0.7 Ma for the ash from Member 4 (Condon et al., 2005). The ages of the last two ash beds were also determined by SHRIMP dates, which are comparable with those of previous works (Yin et al., 2005; Zhang et al., 2005). In summary, the Doushantuo Formation represents more than 80 million years, or roughly 90% of the Ediacaran Period. The Dengying Formation in the Yangtze Gorges area corresponds to the top 10% of the Ediacaran Period. Its thickness varies from ~ 240 to 850 m (Zhao et al., 1988). It is composed of three Members; Hamajing, Shibantan and Baimatuo Members in ascending order. The Hamajing Member, ca. 23 m thick, is characterized by massive intraclastic and oolitic dolomitic grainstone. The Shibantan Member, 100– 160 m thick, has dark gray, thin-bedded limestones. The algal fossil, Vendotaenia antiqua, Ediacaran macrofossil, Paracharnia dengyingensis, possible Planolites-like trace fossils, and sponge spicules were found in the Shibantan Member (Sun, 1986; Zhao et al., 1988; Steiner et al., 1993). The Baimatuo Member consists of 40–400 m thick massive micritic and recrystallized dolomite, and in some areas an erosional surface was found at the top (Zhu et al., 2003). The tubular fossil, Sinotubulites, which may represent the earliest shell-producing metazoan, was found in the lower part of the Baimatuo Member (Chen et al., 1981). The fossils from the middle and upper parts of the Dengying Formation are referred to as the “Xilinxia Biota” (Chen et al., 1981; Ding et al., 1992). The Yanjiahe Fm. is also composed of an alternation of black limestone, and black dolostone, clastic sediments and black shale. The Yanjiahe Fm. contains the key Small Shelly Fossils (SSFs), Protohertzina anabarica and Anabarites trisulcatus for Stage 1 and Aldanella for Stage 2 (proposed by Zhu et al., 2007 and Babcock and Peng, 2007). So, the PC/C boundary is located within the Yanjiahe Formation. The Shuijintuo and Shipai Formations mainly consist of black shale, clastic sediments and a few carbonates. Trilobites in the upper part of the Shuijintuo and Shipai formations indicate the Atdabanian stage. 2.2. Stratigraphy of the Wuhe-Gaojiaxi section The Ediacaran System in the Yangtze Gorges area was deposited on the Western Hubei platform. The Wuhe-Gaojiaxi section in the SW of Zigui near Yichang, Hubei Province (Fig. 2b, c), is one of the best known sections in the Yangtze Gorges region (e.g. Chen, 1987). The section is located on a north-faced cliff, which is exposed along the paved road from Zigui to Gaojiaxi in the south of Sandouping. On the unconformity above the Huangling Granite, the section comprises the Liantuo, Nantuo (Marinoan-aged tillite), Doushantuo, Dengying, Yanjiahe, Shuijintuo and Shipai formations in ascending order. We have undertaken drilling in this area since 2005 from the Marinoan glaciation to the early Cambrian. Site 3 is about 150 m thick extending from the top of the Baimatuo Member through Yanjiahe and Shuijintuo Formations to Shipai Formation (Fig. 2d). The drill core contains the uppermost part of the Baimatuo Member, which is composed of a 5–10 m thick massive white to

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Fig. 2. (a) Simplified paleo-geographic map of the Yangtze Platform around 600 Ma (modified from Zhu et al., 2003). Large circles indicate our study areas. (b) Geological map of the Yangtze Gorges area, South China, showing our study area along the Yangtze River. (c) Detailed geological map of the Wuhe-Anjiahe region, Three Gorge area. Sites 1–4 represent our drilling positions. (d) Schematic cross-section along an F–G line in panel c.

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light gray dolostone layer with faint internal layering at a scale 13 cm to 1 m. The bottom boundary of the Yanjiahe Fm. is a thin black mud layer. The lower part of the Yanjiahe Fm. is an alternation of gray dolostone and organic-rich black shale that contains much pyrite. The upper part of the Yanjiahe Fm. is an alternation of black limestone and black shale. The Shuijintuo Fm. mainly consists of black shale interbedded with a small amount of black limestone. The black shale layers contain many limestone nodules, up to ca. 1 m across. The Shipai Fm. also mainly consists of calcareous black shale. According to paleontological studies in this area (Chen, 1984; Qian, 1999), the small shelly fossils (SSFs), P. anabarica and A. trisulcatus, first appeared at a horizon 11.7 m above the Dengying/Yanjiahe Formation boundary. These fossils were reported from the Nemakit-Daldynian in Siberia. This horizon was correlated with the lower part of Stage 1 of the Lower Cambrian by Babcock and Peng (2007). At 2.7 m below the Yanjiahe/Shuijintuo Formation boundary, another SSF assemblage with Aldanella occurs. This genus was reported from the Tommotian in Siberia, thus the assemblage is correlated with Stage 2 of the Lower Cambrian. Zhu et al. (2003) reported the occurrence of Early Cambrian trilobites from the upper part of the Shuijintuo Formation. These trilobites are components of the Zone that corresponds to Stage 3 or 4. 2.3. Petrography of the studied rock samples We carefully conducted microscopic observations, and estimated the involvement of clastic and carbonaceous materials, and post-depositional oxidation, dissolution and alteration. We detected three distinct types of carbonate rocks at Site 3. First, gray dolostones of the Baimatuo Member mainly consist of anhedral dolomite with minor amounts of organic materials. The grain sizes of dolomite are irregular. We found thin (ca. 10 µm width) dolomite veins, a few clastic minerals (e.g. plagioclase and quartz) and a few pyrites. The edges of dolomite are white, and ooid-like structure remain (Sample No. S36006). Second, the dolostone in the lower part of the Yanjiahe Fm. is mainly composed of many euhedral dolomites with minor amounts of clay minerals (micas). Grain sizes of dolomite are irregular and up to 70 µm maximum. We found a few veins, a few clastic minerals and a few pyrites, some of which are euhedral. The edges of a few pyrites consist of altered magnetites (Sample No. S35001-A), but the edges of most pyrites are unaltered. Dolomites are brownish in places due to the presence of iron hydroxide along the grain boundaries of some slightly altered samples. The distribution of organic material is homogeneous and micritic dolomites remain. The third type comprises lime mudstones from the upper part of the Yanjiahe Fm. to the bottom of the Shipai Fm., which contains fine-grained anhedral calcite grains with minor amounts of clay minerals (micas) and detrital minerals (e.g. plagioclase and quartz). The size of the detritus is less than 20 µm across. The distribution of organic material is homogeneous. We found a few small calcite veins and a few

pyrites, almost all of which are anhedral. All pyrites are still preserved even at their rims. Some ooids and small sponge spicules are present in these limestones (Sample No. S33111). The distribution of detrital minerals is less than 2% in all carbonate samples. 3. Sample preparation and analytical methods Rock powders were prepared by micro-drilling of small holes millimeters across in thin slabs of the drill core samples of Site 3 to avoid visible altered parts and veins of carbonate and quartz in the slabs. The scraped parts are devoid of veins of carbonate and quartz, and brownish parts based on observations with the unaided eye and the microscopic; very small amounts of tiny detritus are possibly involved, because they are scattered over those parts. The powders were dissolved in 2 M acetic acid at 70℃ for 24 h to avoid dissolving the detrital silicate minerals. Dissolved samples were evaporated and the residues were also dried. The dried residues were re-dissolved in 2 M nitric acid. We obtained Sr, Mn, Al, Si and K abundances with an inductively coupled plasma-optical emission spectrometer (ICP-OES, LEEMANS Labs. Ink., Prodigy) at the Center for Advanced Materials Analysis in the Tokyo Institute of Technology. Rb and Sr contents were also determined by an inductively coupled plasma mass spectrometer (ICP-MS) at the Tokyo Institute of Technology, which is a ThermoElemental VG PlasmaQuad 2 quadrupole-based ICP-MS equipped with an S-option interface (Hirata and Nesbitt, 1995). The elemental abundances were obtained by calibration of peak intensities of sample solutions by an analytical standard solution, NIST 987 (Hirata et al., 1988). Sr was chemically separated from coexisting matrix elements (e.g. K, Mg, Ca, and Fe), and Rb to avoid influence of an isobaric interference on 87Sr using a chromatographic technique (Ohno and Hirata, 2006; Ohno et al., submitted for publication). In this study, the samples dissolved in 2 M nitric acid were loaded onto ca. 0.25 ml of preconditioned Sr Spec column (i.d. 6 mm, height 10 mm, particle 50–100μm). After major elements were removed by 5 ml of 7 M nitric acid and 3 ml of 2 M nitric acid, Sr were eluted by 5 ml of 0.05 M nitric acid. Sr isotope compositions of 86 Sr, 87 Sr and 88 Sr were measured with a MC-ICP-MS (Nu plasma 500, Nu Instrument Ltd, Wrexham, Wales) at the Tokyo Institute of Technology. The operation conditions, including the torch position, Ar gasflow rates and lens settings, were adjusted so as to maximize the signal intensity of 88Sr (Ohno and Hirata, 2006; Ohno et al., submitted for publication). Details of the instrument and the operating parameters are summarized in Table 1. The axial faraday collector was used to measure 87 amu. A correction of the mass discrimination effect is necessary for MC-ICP-MS measurements in order to obtain precise and accurate isotopic data. A typical mass discrimination effect of Sr observed in MC-ICP-MS measurements is 2–3%/amu. In this study, the mass discrimination effect was corrected by two correction techniques based on an exponential law (Russell et al., 1978). One is an internal correction technique, which

Y. Sawaki et al. / Gondwana Research 14 (2008) 134–147 Table 1 Details of the instruments and the operation parameters (1) MC-ICP-MS instrument Nu instruments Nu plasma (2) ICP ion source ICP Power Argongas flow rates Cooling Auxiliary Nebulizer (3) Mass spectrometer Ion energy Extraction Analysis mode Ion detection Typical transmission Integration time Scan settled time Number of cycles Total analysis time (4) Mass bias correction Internal correction External correction

27.12 MHz 1.35 kW forward, b5 W ref. 13 l/min 0.7 l/min 0.99 l/min 4000 V 2400 V Static Analogue by Faraday 80 V/μg g− 1 5s 2s 40 cycles/run 200 s/run 88

Sr/86Sr Zr/90Zr

91

provides the radiogenic 87Sr/86Sr isotope ratios using the nonradiogenic 87Sr/86Sr isotope ratio of the international convention value determined by Nier (1938) as 0.1194 (Ohno and Hirata, 2006; Ohno et al., submitted for publication). The other correction technique is an external correction using Zr, which corrects only the mass discrimination effect in a mass spectrometer (Ohno and Hirata, 2006; Ohno et al., submitted for publication). Zr of NIST987 was added into both sample and analytical standard solutions. As a result, we obtained two isotopic ratios of 87Sr/86Sr and 88Sr/86Sr ratios. The 88Sr/86Sr isotope ratios were expressed as the relative deviation from the ratio of an isotopic standard reference material (NIST NBS987) in terms of delta notations (δ). δ88 Sr ¼ ½ð88 Sr=86 SrÞsample =ð88 Sr=86 SrÞNIST −1  1000 The detailed correction techniques are discussed in Ohno et al. (submitted for publication). The initial values of radiogenic 87 Sr/86Sr isotope ratios are calculated from the depositional ages, Rb/Sr ratios and a half-life of 87Rb of 4.88 × 1010 years. 4. Results The chemical and isotopic compositions are summarized in Table 2 and Figs. 3–5. 4.1. Post-depositional alteration The Baimatuo Member consists of pale gray and white thickbedded dolostone with abundant dissolution structures. The isotopic compositions of Sr in marine carbonates are susceptible to alteration. The processes of alteration may include early diagenetic transformations and late diagenetic fluid reaction. Alteration through interaction with clay minerals and groundwater with radiogenic isotopic compositions would increase

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87

Sr/86Sr ratios, whereas interaction with fluids from a juvenile volcanic source would decrease the 87Sr/86Sr values. Generally speaking, our 87Sr/86Sr values increase due to post-depositional alteration. Previous workers cited 18O-depletion, a very high manganese content, a high Mn/Sr ratio and a conjugate decrease of δ13C and δ18O, as characteristics of diagenetic alteration by meteoric water (Brand and Veizer 1980, 1981; Banner and Hanson 1990; Jacobsen and Kaufman 1999). In this paper, we used the Mn/Sr ratio, δ18O values and correlation between δ13C and δ18O; these methods were also used by Yoshioka et al. (2003). Detailed investigation of δ13C and δ18O values in this section indicates that δ13C of carbonate rocks is well-preserved as a primary value, and there is no conjugate decrease of δ13C and δ18O (Fig. 3, Ishikawa et al., 2008–this issue). The Mn/Sr ratio is often used for discrimination of diagenetic alterations, and the maximum Mn/Sr ratio of apparently unaltered samples ranges from 1 to 3 (Kaufman et al., 1993; Brasier et al., 1996; Kennedy et al., 1998; Jacobsen and Kaufman, 1999). All of the limestones have low Mn/Sr ratios, ranging from 0.160 to 0.672, and moderate δ18O values (Table 2 and Fig. 4). The geochemical signature of the low Mn/Sr ratio and moderate δ 18 O values, as well as our petrographic observations, suggests that the influence on the limestone samples by secondary alteration through interaction with meteoritic fluids is insignificant. Dolostone samples in the Yanjiahe Fm. have relatively high Mn/Sr ratios from 1.25 to 5.82 and slightly low Sr concentrations from 51 to 189 ppm (Table 2). Dolostone samples in the Dengying Fm. also possess slightly high Mn/Sr ratios from 0.713 to 2.012 and low Sr concentrations from 52 to 94 ppm. However, no correlations among 87Sr/86Sr, Mn/Sr and δ18O are obvious (Fig. 4). The meteoric diagenesis model (Banner and Hanson, 1990; Jacobsen and Kaufman, 1999) showed that dolostone with very high Mn/Sr ratios from 10 to 40 might suffer isotopic modification of δ13C and δ18O ratios through interaction with meteoric fluids (Banner and Hanson, 1990). Previous workers suggested a criteria of Mn/Sr b 3 for unaltered dolostone samples (Kaufman et al., 1993; Brasier et al., 1996; Kennedy et al., 1998; Jacobsen and Kaufman, 1999). Generally speaking, carbonate rocks precipitated from anoxic waters have a high Mn/Sr ratio. REE compositions of black dolostones of the Yanjiahe Fm. and gray dolostones in the Dengying Fm. suggest they were deposited under anoxic conditions (Komiya et al., 2008a–this issue). As a result, we selected dolostone samples with Mn/Sr b 3 as the least altered samples. The lack of correlation between δ13C and δ18O also suggests that the secondary isotopic modification of 87Sr/86Sr composition is also a minimum. 4.2. Influence of detrital components Involvement of detrital materials influences Sr isotope compositions, because terrigenous detritus has radiogenic Sr isotopic compositions. Especially, the involvement of clay minerals and feldspar originating from continental crust significantly increases the 87Sr/86Sr ratios, because they contain

140

Table 2 Isotopic and geochemical data Formation

Depth

Rock name

Dissolve 87Sr/86Sr Error a rate (%) measured

Rb/Sr

Age (Ma)

87

Sr/86Sr δ88Sr Error Sr initial (ppm)

Mn Mn/Sr Al Si K Rb δ13C b δ18O b (ppm) (ppm) (ppm) (ppm) (ppm) (‰ vs.PDB) (‰ vs.PDB)

30802 31504D 33111 33202 33503 33509U 33509 M 33509L-A 33509L-B 33611 33612 34303-A 34303-B 34803 34905 35001-A 35001-B 35102 35110 35202 35204 35206 35207 35208 35306 35403 35409 35509 35601 35707 36006 36402

Shipai Fm. Shuijintuo Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Yanjiahe Fm. Dengying Fm. Dengying Fm. Dengying Fm. Dengying Fm. Dengying Fm. Dengying Fm. Dengying Fm. Dengying Fm. Dengying Fm.

4.50 22.40 79.14 80.60 93.85 95.45 95.57 95.68 95.68 97.95 98.35 105.95 106.01 112.57 114.65 116.68 116.68 119.74 121.24 121.10 121.31 121.46 121.52 121.62 125.12 128.17 129.58 132.38 133.94 137.03 142.52 145.91

Limestone Limestone Limestone Limestone Limestone Limestone Black shale Limestone Limestone Dolomite Dolomite Dolomite Dolomite Black shale Dolomite Dolomite Dolomite Dolomite Dolomite Black shale Dolomite Dolomite Dolomite Dolomite Dolomite Dolomite Dolomite Dolomite Dolomite Dolomite Dolomite Dolomite

92.93 75.80 94.35 90.49 83.08 79.13 27.10 76.34 77.47 97.12 96.33 61.44 70.65 37.04 72.46 70.66 69.89 58.43 59.65 25.51 44.25 86.24 89.52 90.00 99.82 99.45 98.57 99.55 97.15 78.30 95.51 95.84

0.0013 0.0008 0.0030 0.0031 0.0058 0.0056 0.0115 0.0060 0.0116 0.0078 0.0049 0.0310 0.0369 0.1146 0.0493 0.0415 0.0495 0.1110 0.1009 0.1382 0.1137 0.0202 0.0236 0.0257 0.0037 0.0022 0.0036 0.0031 0.0007 0.0086 0.0016 0.0017

N510 N510 N510 N510 N510 N510 N510 N510 N510 N510 N510 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540 N540

0.70873 0.70888 0.70878 0.70896 0.70864 0.70878 0.70996 0.70887 0.70874 0.70910 0.70896 0.70979 0.70970 0.71163 0.71081 0.71072 0.71062 0.71219 0.71199 0.71687 0.71111 0.71014 0.70998 0.71018 0.70971 0.71023 0.70936 0.70935 0.70926 0.70873 0.70899 0.70903

259.9 368.1 131.6 53.9 83.6 54.1 335.6 37.6 77.2 415.0 423.9 391.1 560.5 626.8 242.4 107.6 299.4 183.8 184.1 176.1 105.8 136.6 133.9 134.9 105.5 121.5 100.3 70.3 71.5 59.3 60.8 59.3

0.70876 0.70890 0.70884 0.70902 0.70876 0.70890 0.71020 0.70899 0.70898 0.70926 0.70906 0.71046 0.71050 0.71412 0.71188 0.71162 0.71169 0.71460 0.71418 0.71987 0.71358 0.71058 0.71049 0.71074 0.70979 0.71028 0.70944 0.70942 0.70928 0.70892 0.70902 0.70907

N.D. = no data. a 2σ standard error of the mean. b 13 δ C and δ18O analyses from Ishikawa et al. (2008–this issue).

0.00001 0.00001 0.00003 0.00022 0.00002 0.00001 0.00004 0.00005 0.00001 0.00004 0.00006 0.00001 0.00003 0.00003 0.00003 0.00008 0.00003 0.00007 0.00006 0.00002 0.00004 0.00006 0.00001 0.00004 0.00001 0.00001 0.00001 0.00004 0.00001 0.00004 0.00001 0.00001

N.D. N.D. N.D. N.D. 0.27 N.D. 0.17 N.D. 0.24 0.36 0.33 N.D. 0.31 0.35 0.41 N.D. 0.32 N.D. 0.37 0.27 0.41 0.33 0.29 0.25 0.41 0.54 0.44 0.51 0.24 0.45 0.27 0.45

0.09 0.05 0.08 0.09 0.07 0.07 0.02 0.07 0.02 0.05 0.03 0.03 0.05 0.01 0.06 0.02 0.04 0.03 0.08 0.09 0.04 0.05 0.07

471.3 1717.6 195.8 158.2 523.4 304.5 917.2 234.4 460.5 154.4 144.7 67.2 147.4 100.4 102.5 51.0 103.7 62.4 92.6 189.2 40.2 109.3 102.7 92.1 52.4 76.1 64.9 94.3 56.4 83.2 56.5 53.3

0.551 0.214 0.672 0.341 0.160 0.178 0.366 0.160 0.168 2.688 2.929 5.821 3.804 6.245 2.366 2.112 2.887 2.943 1.989 0.931 2.631 1.250 1.303 1.465 2.012 1.596 1.545 0.746 1.267 0.713 1.076 1.113

1038 138 591 0.62 2308 468 842 1.40 693 334 442 0.59 602 311 470 0.49 1546 580 809 3.06 1372 1555 1211 1.71 10050 4125 16249 10.54 1188 359 537 1.40 2400 1003 263 5.32 1242 168 1716 1.21 959 167 1764 0.71 1122 246 2570 2.08 2707 644 2246 5.44 13642 1356 22152 11.50 3642 662 1027 5.05 1242 100 696 2.11 6308 904 5948 5.13 4581 92 2390 6.93 6598 1822 5293 9.34 24913 13598 16411 26.16 5340 6065 4251 4.57 2296 704 621 2.21 2262 344 565 2.42 2715 407 584 2.37 311 24 432 0.20 326 3 452 0.17 416 60 438 0.24 396 b3 245 0.29 294 b3 265 0.04 409 165 215 0.72 443 b3 256 0.09 303 b3 251 0.09

N.D. N.D. −8.57 −2.60 2.46 N.D. N.D. 4.24 4.24 −3.48 −3.18 −6.73 N.D. −3.49 −2.49 −2.47 −2.47 −0.69 0.01 0.15 0.24 0.50 0.66 0.66 0.94 1.46 1.77 1.17 1.35 1.10 0.52 0.15

N.D. N.D. −6.57 −7.34 −7.19 N.D. N.D. −6.69 −6.69 −5.46 −5.14 −4.76 N.D. −7.17 −5.58 −5.57 −5.57 −5.42 −5.75 −5.74 −6.74 −5.08 −4.87 −5.52 −5.39 −6.01 −4.80 −4.90 −4.87 −5.87 −5.26 −5.15

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Sample number

Y. Sawaki et al. / Gondwana Research 14 (2008) 134–147 Fig. 3. Lithostratigraphic column and chemostratigraphies of δ13C, δ18O, 87Sr/86Sr and δ88Sr of drilled core samples in the Three Gorges area, South China. The δ13C and δ18O values are from Ishikawa et al. (2008–this issue). Small white squares in 87Sr/86Sr indicate possibly altered or detritus-bearing samples, evident by our screening procedure.

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Fig. 4. Isotopic and elemental cross-plots of drill core samples. (a) 87Sr/86Sr vs. Mn/Sr diagram along with compositional fields of dolostones by previous workers (Kaufman et al., 1993; Derry et al., 1994), 87Sr/86Sr vs. δ18Ocarb diagram, δ18Ocarb vs. Mn/Sr diagram, and δ18Ocarb vs. δ13Ccarb diagram.

high Sr contents. Because clay minerals and feldspars contain higher Al, Si, K and Rb contents than carbonate minerals, we checked the correlation between 87Sr/86Sr ratios and Al, Si, K and Rb contents for estimating the influence of a detrital component. There are no correlations between 87Sr/86Sr ratios and Al, Si, K and Rb contents in the limestone samples of the Yanjiahe Fm. and dolostone samples of the Dengying Fm., indicating an

insignificant influence and involvement of clay minerals and feldspar on 87Sr/86Sr (Fig. 5). On the other hand, dolostones samples from the Yanjiahe Fm. have a faint correlation between 87 Sr/86Sr ratios and Al, Si, K and Rb contents. Petrographic observations show that the modal abundances of detrital materials are so low, and dissolution of samples in acetic acids minimizes the involvement of silicate detrital materials. As a result, the influence of the detrital minerals is

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Fig. 5. Isotopic and elemental cross-plots of 87Sr/86Sr ratios and Al, Si, K and Rb contents of drill core samples. The solid lines represent a compositional path of contaminants (see text for details). The numbers along the black triangles indicate abundance of contaminated materials (%).

considered insignificant. However, we calculated the degree of influence of detrital minerals on Sr isotopic compositions. Microscopic observations show that detrital minerals are mainly plagioclase, quartz, clay minerals and mica. A granitic composition was applied to the calculation as a proxy, because a mixture of the detrital minerals is similar to granite in composition. Although a wide distribution of the juvenile crust in South China at that time is unlikely, we used the composition of a juvenile granite, which contains 67.2, 15.2, and 1.26% and

17.5 ppm in SiO2, Al2O3, K2O and Rb contents (Whalen et al., 1987) and 0.720 in 87Sr/86Sr ratio to estimate the maximum isotope effect by crustal contamination (Fig. 5). Fig. 5 shows that contamination of granite hardly affects the 87 Sr/86Sr ratio. However, it is possible that selective contamination of minerals with a high Sr concentration significantly changes the 87Sr/86Sr ratio. Some dolostone samples in the Yanjiahe Fm. have very high Rb/Sr ratios (N 0.1), which is equivalent to the ratios of the black shale samples. The

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dolostone samples with a very high Rb/Sr ratio are also excluded as an influence by clay minerals and mica. As a result, the least altered samples with minor amounts of detrital materials were selected based on their Mn/Sr ratios and δ 13 C and δ 18 O compositions, together with microscopic observations and their Si, Al, K and Rb contents. 4.3. 87Sr/86Sr excursion around the Precambrian–Cambrian boundary We obtained 87Sr/86Sr ratios and δ88Sr values of the least altered carbonate rocks from the Dengying to the Shipai Formations in order to reconstruct the secular change in 87 Sr/ 86 Sr values of ancient seawater across the PC/C boundary. The carbonate rocks display a distinct positive excursion of 87 Sr/86Sr values around the Precambrian– Cambrian boundary from ca. 0.7087 in the upper part of the Baimatuo Member through 0.71081 at the bottom of the Yanjiahe Fm. to ca. 0.709 at the upper part of the formation (Fig. 3). The 87Sr/86Sr value increases suddenly from 0.70873 in the upper part of the Baimatuo Member with ca.137 m depth in the uppermost Ediacaran to the highest value 0.71081 at the bottom of the Yanjiahe Fm. (ca. 115 m deep). Subsequently, the 87Sr/86Sr value decreases gradually to ca. 0.709 through the PC/C boundary in the lower Yanjiahe Fm. from ca. 115 to 98 m depth. The 87Sr/86Sr values of the limestone are constant from the upper part of the Yanjiahe Fm., and range from 0.70864 to 0.70896 in the upper Yanjiahe Fm., 0.70888 in the Shuijintuo Fm. and 0.70873 in the Shipai Fm. It is noteworthy that 87Sr/87Sr and δ13C values show correlations. However, detailed observations indicate small differences between them. The beginning of the significant increase of 87Sr/87Sr precedes the start of the negative excursion of δ13C. δ13C changes from a negative to a positive excursion above the horizon of the maximum value of 87Sr/87Sr. However, the 87Sr/87Sr value simultaneously returns to a normal value at the end of the negative excursion of δ13C (Fig. 3). The Sr isotopic compositions define a relatively smooth curve. In addition, the Sr isotope compositions are very harmonically different to the excursion in δ13C, but apparently not with δ18O (Fig. 3). The 87Sr/86Sr ratios of the drill core samples are consistent with those of Wang et al. (2002, their Fig. 1d), who reported the values from the top of the Baimatuo Member and bottom of the Shuijintuo Fm. 5. Discussion 5.1. Comparison with global excursions The residence time of strontium in the oceans is from 2.5 to 4 million years at present (Palmer and Edmond, 1989; Hodell et al., 1990), much longer than both surface and thermohaline ocean circulations, ~ 103 years (Broecker, 1982). Therefore, Sr isotopic compositions are regarded to have been relatively homogeneous in the whole ocean. The 87Sr/86Sr compositions of carbonate rocks in the Three Gorges Region reflect the global

change of 87 Sr/ 86 Sr, because the Yangtze platform was connected with an open ocean in those days (Zhu et al., 2003). Fig. 3 shows a comparison between the radiogenic Sr isotopic excursion and the δ13Ccarb excursion (Ishikawa et al., 2008–this issue), and it shows that the positive excursion of 87 Sr/86Sr ratios corresponds to a negative δ13Ccarb excursion from a positive excursion 1 (P1) through a negative excursion 1 (N1) to a positive excursion (P2). Chemostratigraphies of both δ13C and 87Sr/86Sr values around the PC/C boundary were reported in Mongolia, Namibia, and Siberia (Fig. 1). Brasier and others (1996) reported chemostratigraphy and biostratigraphy from Neoproterozoic to early Cambrian sediments in southwest Mongolia. The carbon isotope chemostratigraphy displays an obvious negative excursion between P1 and N1, but the excursion between N1 and P2 is obscure compared with that in the Three Gorges (Fig. 1). Unfortunately, no 87Sr/86Sr isotopic compositions between P1and N1 are shown in the Mongolia section (Fig. 1). However, the 87Sr/86Sr values at the top of the Tsagaan Oloom Formation are about 0.7087 (Fig. 1). This is consistent with the values just below the onset horizon of the positive 87Sr/86Sr excursion in the Three Gorges area (Fig. 3). 87 Sr/86Sr values in Namibia show a small-scale but obvious positive excursion around the PC/C boundary horizon, defined by the presence of Trichophycus pedum (Kaufman et al., 1993), which is consistent with our data if the small positive anomaly corresponds to the start of the large positive excursion of 87 Sr/86Sr in the Three Gorges area. In Namibia the correlation of 87Sr/86Sr with δ13C is obscure possibly because a sequence from P-4 to P-5 is missing due to an unconformity (the definition of P-4 and P-5 is mentioned below). In Siberia a large positive anomaly is also present around the PC/C boundary (Derry et al., 1994). 5.2. Triggers of environmental change and their implications A compilation of our continuous, detailed chemostratigraphy of drill core samples in the Three Gorges area and the chemostratigraphies around the PC/C boundary in the three regions indicates that a positive 87Sr/86Sr excursion is present around the boundary, and probably correlates well with the δ13C excursion (Figs. 1 and 3), consistent with other recent compilations of chemostratigraphies around the PC/C boundary (Kennedy et al., 2006; Shields, 2007). A detailed observation of the chemostratigraphies of the δ13C, δ18O and 87Sr/86Sr ratios indicates the following correlations between δ13C and 87Sr/86Sr (Fig. 3). Firstly, δ13Ccarb rose to 1.77‰ in the upper part of the Dengying Fm. (Phase-1; P-1), consistent with the highest diversity of Ediacaran fauna at the end of the Neoproterozoic (Grotzinger et al., 1995). Afterwards, 87Sr/86Sr values began to increase gradually (P-2), and then δ13Ccarb dropped gradually (P-3). Just before the PC/C boundary, which is defined by the first appearance of small shelly fossils (SSFs), P. anabarica and A. trisulcatus (Chen, 1984; Qian, 1999), 87Sr/86Sr started to drop gradually and subsequently δ13Ccarb reached a minimum up to − 7‰ (N1) (P-4). Just after the PC/C boundary, δ13Ccarb increased up to about +5‰ and the 87Sr/86Sr still dropped to

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ca. 0.709 (P-5). In the upper part of the Yanjiahe Fm., δ13Ccarb has a positive plateau, and the 87Sr/86Sr values also became constant (P-6). After the δ13Ccarb positive excursion, δ13Ccarb decreased sharply down to about − 9‰ (P-7), possibly together with a faint positive excursion of 87Sr/86Sr. Namely, the excursions of Sr isotopes always preceded the δ13C excursions. The cause of the sudden increase in 87Sr/86Sr values is still obscure. Shields (2007) proposed the following possible causes to increase the 87Sr/86Sr ratio of seawater: (1) an increase in the 87 Sr/86Sr ratio of the rocks undergoing weathering, (2) a decrease in the input of mantle Sr, (3) a decrease in seafloor spreading rates, and a reversible increase of overall continental weathering rates. The assembly of the Gondwana supercontinent is apparently consistent with the high 87Sr/86Sr ratio of seawater, because the formation of a supercontinent suppresses seafloor spreading rates (Shields, 2007). However, the short duration of the positive excursion requires a reversible change just after the event, inconsistent with the relatively longterm tectonics. On the other hand, a black shale in the Yanjiahe Formation has a very high 87Sr/86Sr value, up to 0.720 (Table 2), consistent with the increase in the 87Sr/86Sr ratio of the rocks undergoing weathering. In addition, the sudden increase of atmospheric pCO2 in the Neoproterozoic to Cambrian would increase a ratio of silicate to carbonate weathering. The increase in atmospheric pCO2 possibly occurred just before the PC/C boundary due to decarbonation of ultrahigh temperature metamorphism (Santosh and Omori, 2008) of Southern Granulite Terrain, India (Brown, 2007). Although the model can accounts for the positive excursion and subsequent reversible decrease of 87Sr/86Sr in a relatively short term (e.g. Omori and Santosh, 2008), the evidence for sudden increase in atmospheric pCO2 is required in the future. A reversible increase in the overall continental weathering rates owing to increased rates of physical weathering due to tectonic uplift is the most plausible explanation (Shields, 2007). In a modern model, it is well known that the uplift of the Himalaya and the Tibetan Plateau increased continental weathering rates, and resulted in a radiogenic Sr isotope ratio of seawater higher (Richter et al., 1992). The formation and subsequent rifting of the Gondwana supercontinent possibly caused the higher continental weathering in the late Neoproterozoic. Especially, continental rifting at the break-up of the supercontinent can explain the sudden increase of 87Sr/86Sr and the subsequent abrupt decrease. Granitic material was selectively eroded and transported away at the beginning of continental rifting to the increased continental weathering rate. Also, mid-ocean ridge magmatism starts at the mature stage of continental rifting to increase the mantle Sr input. The shallow marine environment formed by continental rifting made a possible new niche for Cambrian biota to promote biological activity. Namely, the model of continental rifting and subsequent MORB magmatism accounts for both the reversible increase of continental weathering and the abrupt decrease of the 87Sr/86Sr ratio. In addition, global regression around the PC/C boundary (Brasier, 1995) and anoxic conditions due to rifting of the Gondwana supercontinent enhanced continental weathering.

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This model is also consistent, because the time scale of global regression is also comparable to the duration of the positive excursion of Sr isotopes as well as the negative excursion of δ13C (Ishikawa et al., 2008–this issue). The high 87Sr/86Sr values around the PC/C boundary suggest that inflow of continental materials extensively increased before the PC/C boundary. In this case, the high influx of continental materials would lead to higher nutrients and essential elements for life such as phosphorus and calcium in seawater and the removal of atmospheric pCO2 owing to carbonate precipitation. The former may have accelerated the evolution of life in the Cambrian. The observation that positive excursions of Sr isotopes precede δ13C negative excursions between P-2 and P-3 is inconsistent with an increase of erosional materials and consequent enhancement of organic burials, but is consistent with a mass extinction due to global regression. This fact supports the mass extinction at the terminal Neoproterozoic (e.g. Brasier, 1995). Moreover, it allows us to speculate an increase in primary productivity of the Ediacaran fauna due to a high nutrient influx causing O2-deficiency of seawater and the consequent mass extinction. 6. Conclusions (1) A positive 87Sr/86Sr excursion together with a negative δ C excursion of seawater was present around the PC/C boundary. The excursions of Sr isotopes always precede the δ13C excursions. The positive excursion of 87Sr/86Sr ratio is well explained by an increase in inflow of materials from the continental crust before the PC/C boundary. The precedence of the Sr isotopic change rather than the δ13C excursion indicates that an environmental change influenced life evolution, and the concomitant negative excursions in 87Sr/86Sr and δ13C is consistent with a mass extinction due to global regression rather than an increase of erosional materials and consequent enhancement of organic burial. This mass extinction might spread niche and relate to appearance of Cambrian-type life. The enhanced continental weathering just before the PC/C boundary led to higher nutrients, the essential elements for life such as phosphorus and calcium in seawater and the removal of atmospheric CO2. The former two factors may have accelerated the evolution of life in the Cambrian, whereas the last factor suppressed the higher continental weathering. 13

Acknowledgements We thank Takanori Kiguchi, Hiroshi Matsuo and Hitomi Tokimori for technical advice (ICP-OES) at the Center for Advanced Materials Analysis in the Tokyo Institute of Technology. We also thank Dr. Takahiro Wakabayashi for assistance (ICP-MS) in the acquisition of analytical data. This work was partly supported by grants for “secular variation of seawater composition (No. 16740284)”, and for “Coevolution of surface environment and solid earth from the Neoproterozoic Snowball Earth to Cambrian explosion events (No. 18740318)” from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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