Spatial and temporal variations in carbon and sulfur isotopic compositions of Sinian sedimentary rocks in the Yangtze platform, South China

Precambrian Research 97 (1999) 59–75 www.elsevier.com/locate/precamres

Spatial and temporal variations in carbon and sulfur isotopic compositions of Sinian sedimentary rocks in the Yangtze platform, South China R. Li a, *, J. Chen a, S. Zhang b, J. Lei a, Y. Shen a, X. Chen a a Institute of Geology, Chinese Academy of Sciences, Beijing 100029, People’s Republic of China b Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, People’s Republic of China Received 10 December 1998; accepted for publication 15 April 1999

Abstract Eight stratigraphic sections from the Yangtze Platform, South China were selected for a study of spatial and temporal variations in carbon and sulfur isotope compositions of Sinian sedimentary rocks. Carbon isotope compositions of carbonate from lower Sinian strata deposited between pulses of glacial diamictite range from −3.9 to −9.9‰; the more negative values recorded from Mn carbonates suggest that, in part, the carbon was derived from the oxidation of organic matter during early diagenesis. In the upper Sinian strata d13C varies with the facies. They are positive for the platform carbonates, whereas they are negative from about −3 to −12‰ for the carbonates formed in or near basin environments. It is evident that post-depositional alteration accounts for the strongly negative d13C values. However, some information on primary isotopic compositions can be obtained by evaluating sample quality. The primary d13C values of carbonates from the Doushantuo Formation are ca −3 to −4‰, and from the Dengying Formation as low as −6‰. d34S values for pyrite from the lower Sinian rocks are highly positive, and range from values near the coeval seawater sulfate up to ca +60‰. In contrast, d34S values for pyrite from the upper Sinina rocks are negative down to −27 to −30‰. The feature of sulfur isotopic composition for the lower Sinian rocks is consistent with the late Proterozoic sulfur isotopic record. It might be explained by the ‘supercontinent model’: many basins on the Yangtze block in a supercontinent tectonic background had limited or no access to the open ocean and the basin waters contained sulfate with considerably higher d34S values than the coeval seawater for a geologically long period. An alternative explanation is based on the sulfate-minimum zone (SMZ) modal. Because of the slow sinking of organic matter bacterial sulfate reduction would be important in the lower Sinian shallow water environment. Particularly in the SMZ, the residual sulfate might have been significantly enriched in 34S, so that pyrite produced in closed diagenetic system would yield very positive d34S values. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Paleogeography; Sinian; Stable carbon and sulfur isotopes; Yangtze block

* Corresponding author. Fax: +86-10-649-19140. E-mail address: [email protected] (R. Li) 0301-9268/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 3 0 1- 9 2 68 ( 9 9 ) 0 00 2 2 -4

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1. Introduction Over the last decade many papers have dealt with the isotopic composition of carbon in Neoproterozoic strata and their temporal variations (e.g. Knoll et al., 1986; Lambert et al., 1987; Aharon et al., 1987; Strauss et al., 1992; Narbonne et al., 1994; Iyer et al., 1995; Kaufman and Knoll, 1995). Due to the general absence of sedimentary sulfates, research on secular variations in the sulfur isotopic composition of coeval strata have focused on the isotopic composition of authigenic pyrite (e.g. Holser et al.,1988; Lambert and Donnelly, 1992; Hayes et al., 1992; Bottomley et al., 1992; Ross et al.,1995; Canfield and Teske, 1996; Strauss,1997). The isotope geochemistry of sedimentary rocks provides valuable information on Precambrian geochemical cycles and associated environmental changes, and also serves as an important tool for stratigraphic correlation (Holser et al., 1988; Knoll and Walter, 1992; Schidlowski and Aharon, 1992; Hayes et al., 1992; Strauss, 1993; Kaufman and Knoll, 1995; Knoll et al., 1995; Kaufman et al.,1997). The first study of secular variations in the carbon-isotopic composition of carbonates deposited near the Late Proterozoic (Sinian)/ Cambrian boundary on the Yangtze platform was by Hsu¨ et al. (1985), Lambert et al. (1987) published detailed data for upper Sinian carbonates at Yangtze Gorges near Yichang. Similarly Brasier et al. (1990) and Chen et al. (1992) have also reported results from Precambrian–Cambrian boundary carbonates from Meishucun, Yunnan Province and Maidiping, Sichuan Province. All the studied geological sections are located on shallow water carbonate platforms. In this study, however, we have selected sections from a range of sedimentary environments, from proximal to distal, in order to evaluate the role of facies on the carbon isotopic composition of carbonates and co-existing organic matter. Sulfur isotopic data has been published previously from lower Sinian manganese carbonate ore deposits in the Songtao area, Guizhou Province ( Wang et al., 1985; Liu et al., 1989) and the Minle area, Hunan Province ( Tang, 1990; Li et al., 1994). The d34S values for pyrite were found to be extraor-

dinarily positive, from 37 to 52‰ for the Songtao manganese carbonate ore beds (Liu et al., 1989) and from 47 to 59‰ for the Minle manganese carbonate ore beds ( Tang, 1990), much greater than coeval seawater sulfate (ca 20; Claypool et al., 1980; Strauss,1993; Ross et al., 1995). This paper extends the sulfur isotopic study to other ore deposits regionally, as well as through all Sinian strata, with the objective of defining d34S secular variations for pyrite in the Yangtze platform.

2. Geological setting The Yangtze block includes a broad area of South China, from Kunming to Shanghai. Eight sections were selected for study of the spatial and temporal variations in carbon and sulfur isotopic compositions of carbonate, organic matter and sulfides ( Fig. 1). Between 1050 and 850 Ma, southern China was composed of the Yangtze block in the north-west, the Cathaysia block in the southeast, and the South China Ocean in-between. Subduction of the South China ocean in the Jiangshan–Shaoxing area ca 850 Ma is believed to have resulted in collision of the Cathaysia and Yangtze blocks ( Fig. 2, Liu et al., 1994). Sinian

Fig. 1. The Yangtze block and locations of the studied geological sections.

R. Li et al. / Precambrian Research 97 (1999) 59–75

Fig. 2. Tectonic framework of South China for the Pre-Sinian time (after Liu et al., 1994).

strata were subsequently deposited in a remnant ocean basin to the present-day southwest. The stratigraphic correlation of geological sections is shown in Table 1. Since it was established in 1924, the Yangtze Gorges section has been the stratotype for Sinian stratigraphic correlation ( Wang et al., 1998), and the focus of both isotopic and paleontologic investigations (e.g., Lambert et al., 1987; Knoll and Walter, 1992; Xiao et al., 1998). The lower part of the succession is composed of the Liantuo, Gucheng, Datangpo and Nantuo formations, while the upper part is composed of the Doushantuo and Dengying formations (Cowie, 1985; Liu and Xu, 1994; Wang et al., 1998). The Liantuo Formation, composed of sandstones, lies unconformably above the Huangling Granite or metamorphic rocks of the Sandouping Group. The Gucheng Formation lies unconform-

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ably above the Liantuo Formation, and consists of sandstones and conglomerates with tillites (Liu, 1991; Wang et al., 1998). The Chunmu Formation in the Minle and Xiangtan sections and the Liangjiehe Formation in the Songtao section ( Table 1) consists of brown and gray siltstones and sandstones. They lie disconformably above the slightly metamorphosed Banxi Group. Similar to the Gucheng Formation, the Chunmu and Liangjiehe formations contain tillites (No. 405 Geological Team and Laboratory Center of Geological Survey Bureau, Hunan Province, 1984; Tang, 1990; Liu, 1991; Xue et al., 1993). The Datangpo and Minle formations in the Minle, Xiangtan, Xiushan and Songtao sections are composed of black shales in the lower part and gray siltstones and shales in the upper part. Manganese carbonate ore beds in the Minle, Xiangtan, Xiushan and Songtao sections and Mn-bearing limestones in the Yangtze Gorges section are interlayered with black shales near the base of the Datangpo and Minle formations. Thickness of the Datangpo and Minle formations varies (e.g. <10 m in the Yangtze Gorges and hundreds of meters in the Minle, Songtao and Xiushan sections). The lower Sinian Nantuo Formation with tillites is extensively distributed in the Yangtze area. Thickness of the continental tillites in the Yangtze Gorges and Songlin-Tanshanbao (Fig. 1) varies greatly, from a few meters to 120 m (Liu, 1991). They are poorly sorted with angular boulders >1 m. Thickness of the Nantuo Formation in Xiushan (Fig. 1) is quite stable and approaches a few hundreds of meters. It includes marine glacial mudstones, siltstones and sandsones with rafted boulders (Liu and Xu, 1994). The upper Sinian Doushantuo formation mainly comprises carbonates, phosphorites, siltstones and shales. It lies disconformably above the Nantuo Tillites ( Ye, 1989; Liu, 1991; Liu and Xu, 1994) and lies conformably beneath the Dengying Formation in the Yangtze Gorges, Songlin–Tanshanbao, Minle, Songtao and Xiushan sections and the Liuchapo Formation in Jishou and Yuanling sections. Doushantuo Formation is the most important P-bearing strata of the Sinian–lower Cambrian period. The base of this formation is a transgressive carbonate bed with a thickness of a few meters

Niutitang Formation Dengying Formation Doushantuo Formation Nantuo Formationg

Songlin–Tanshanbaob

Banxi Group

Nuititang Formation Dengying Formation Doushantuo Formation Nantuo Formationg Datangpo Formation Liangjiehe Formationg

Songtaof Niutitang Formation Dengying Formation Doushantuo Formation Nantuo Formationg Datangpo Formation

Xiushanb,f

a Cowie (1985). b Liu and Xu (1994). c Wang et al. (1998). d Geological Team and Laboratory Center of Geological Survey Bureau Hunan Province (1984). e Tang (1990). f Liu (1991). g With tillites.

Lower Cambrian Shuijingtuo Formation Upper Sinian Dengying Formation Doushantuo Formation Lower Sinian Nantuo Formationg Datangpo Formation Gucheng Formationg Liantuo Formation Pre-Sinian Sandouping Group

Yangtze Gorgesa,b,c

Table 1 Stratigraphic correlation of the Sinian Strata

Banxi Group

Niutitang Formation Dengying Formation Doushantuo Formation Nantuo Formationg Minle Formation Chunmu Formationg

Minle, Xingtand,e

Niutitang Formation Liuchapo Formation Doushantuo Formation Nantuo Formationg

Jishouf

Niutitang Formation Liuchapo Formation Doushantuo Formation Nantuo Formationg

Yuanlingf

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after Nantuo glacial period. It is widely distributed and always lies disconformably above the Nantuo Tillites. So, the Nantuo Tillites and the Doushantuo basal carbonate bed can be the key marker for lower/upper Sinian stratigraphic correlation (Liu, 1991; Tang, 1998). The Dengying Formation consists of dolostones, limestones, and siliceous rocks. The uppermost part contains basal Cambrian shelly fossils ( Xiao et al., 1998 and references therein), and lies disconformably beneath the black shales of the lower Cambrian Shuijingtuo and Niutitang formations in the Yangtze Gorges and Songlin–Tanshanbao sections (Liu, 1991; Wang et al., 1998). The Liuchapo Formation is composed of siliceous rocks with black shales. It lies conformably beneath the black shales of the lower Cambrian Niutitang Formation in the Xiushan, Jishou and Yuanling sections (Liu, 1991). Black shales at the base of the Niutitang Formation, frequently containing phosphorus nodules are also used as a marker for regional stratigraphic correlation (Liu, 1991). The regional change in the lithofacies of the lower Sinian strata is poorly understood because of limited distribution. A simplified map of the lithofacies and paleogeography of the Minle/ Datangpo Formation, compiled by Zhao (1996), is shown in Fig. 3. On the other hand, the lithologies of the upper Sinian strata are better exposed and change greatly with the sedimentary environment. The lithofacies of the Doushantuo Formation is composed of carbonates, phosphorites, siltstones, shales and siliceous rocks. They were deposited in the environments from platform to basin ( Ye, 1989). The platform carbonates and phosphorites are mainly distributed in the western part of the Yangtze platform, including the Weng — an area where the phosphorites of the late Neoproterozoic Doushantuo Formation preserve an exceptional record of multicellular life from just before the Ediacaran radiation of macroscopic animals ( Xiao et al., 1998). Shallow water sedimentary structures, such as wavy and cross bedding, washout and desiccation cracking can be found frequently in these platform rocks. The basin facies is mainly distributed in the southeast part of the Yangtze platform and composed of finely laminated shales and siliceous rocks. Major

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lithofacies of the transitional belt, which includes the Xiushan, Jishou and Yuanling sections in Fig. 1, between the platform and basin are the even bedded, micritic carbonates and phosphorites, and the finely laminated shales. In the Dayong area of this belt, about 100 km to the north of the Yuanling section, they have an intrabed slide and convolute bedding sedimentary structure ( Ye, 1989). The thickness of the Doushantuo Formation varies from hundreds of meters of the platform facies to ten of meters of the basin facies. The lithofacies and paleogeography of the Dengying Formation, shown in Fig. 4, is similar to the Doushantuo Formation ( Ye, 1989; Liu and Xu, 1994). On the platform, represented by stratigraphic sections at Yangtze Gorges near Yichang and at Songlin–Tanshanbao, sedimentary carbonates are composed of dolomite with microbialaminated, grainy and micritic textures. Siliceous rocks could be deposited on the platform environment. They are lenticular or irregularly banded and organic-poor. In contrast, the siliceous rocks with black shales of the Liuchapo Formation in the Yuanling and Jishou sections are considered to have formed in or near basin environments. They are organic-rich and laminated. In the Xiushan section, the Dengying Formation contains limestones, black shales and black siliceous rocks ( Fig. 5), deposited in a deeper-water environment at the transition belt between platform and basin ( Ye, 1989; Liu and Xu, 1994). The limestones, with a thickness of ca 9 m, are micritic and centimeter-thick rhythmic deposits with horizontal bedding, capped by a 0.5 m thick muddy dolostone layer. The black shales and siliceous rocks have sedimentary features similar to those of the Liuchapo Formation in the Yuanling and Jishou sections. In Xiushan and other places (e.g. Dayong as mentioned before) of the transitional belts, the rocks of Dengying Formation have intrabed slide and convolute bedding sedimentary structures, and in the Dayong area they contain fossils such as sponge spicule ( Tang et al., 1980; Ye, 1989). The thickness of the Dengying Formation varies from hundreds meters to about one thousand meters in the platform. However, normally the Liuchapo Formation ranges from tens of meter to about one hundred meters in the basin ( Ye, 1989).

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Fig. 3. Lithofacies and paleogeography for the early Sinian Minle stage (after Zhao, 1996) 1, Land or rise; 2, sedimentary district boundary; 3, manganese carbonate ore deposits (or mineralization sites); 4, sections selected for the study of sulfur isotopes. I, Littoral sandstones and conglomerates; II, deeper-water manganiferous mudstones; III, shallow marine siltstones and mudstones; IV, deep sea flysh sandstones and mudstones.

The absolute age of Sinian strata is poorly constrained (e.g. Cowie, 1985; Wang, 1985; Liu, 1991; Liu and Xu, 1994; Xiao et al., 1998). Tuffs in the underlying Liantuo Formation yield a U– Pb zircon age of 748±12 Myr ( Xiao et al.; 1998 and references therein). Samples from manganese carbonate ore beds in the Minle Ore District yielded a Rb–Sr isochron age of 728±27 Ma (Geological Team and the Laboratory Center of Geological Survey Bureau, Hunan Province, 1984). The age of Doushantuo Formation is ca 570±20 Myr ( Xiao et al., 1998) based on chemostratigraphic correlation of Sinian d13C variations with equivalents in Namibia that are well constrained by radiometric determinations (Grotzinger et al., 1995).

3. Analytical methods Whole rock samples that were analyzed for carbon, oxygen and sulfur isotopes, their localities,

and lithologies are listed in Table 2. All samples were pulverized to <200 mesh sizes. The CO 2 from carbonates for mass spectrometric determination was evolved by the reaction in vacuo with anhydrous H PO at 25°C for 25 h for limestones, 3 4 72 h for dolostones, and for 72 h at 50°C for manganese carbonate rocks. The d13C and d18O values are presented relative to the PDB standard. The samples for organic carbon isotopic analyses were treated using HCl/HF to remove inorganic minerals, such as carbonates and silicates. Organic matter obtained was mixed with CuO and sealed in evacuated quartz tubes for combustion at 850°C. Recovery and purification of the released CO 2 allowed quantitative and isotopic analysis. Pyrite is present in a finely disseminated form in most rocks and could therefore not be extracted manually. Instead, a chemical extraction procedure modified from Canfield et al. (1986) has been utilized (Chu et al., 1993). Following the removal of the HCl-soluble sulfide fraction, pyrite in the

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Fig. 4. Lithofacies and paleogeography for the late Sinian Dengying stage (after Ye, 1989; Liu and Xu, 1994) 1, Land; 2, sedimentary district boundary; 3, facies boundary; 4, phosphorite deposits. I , Carbonate platform; I , evaporite and carbonate platform; I , 1-1 1-2 2-1 phosphorite-bearing carbonate platform; I , platform-basin sediments: carbonates and black shales; I , basin sediments: black 2-2 2-3 shales and organic-rich siliceous rocks; II , shallow marine sandstones and mudstones; II , deep sea flysh sandstones and mudstones. 1 2

Fig. 5. Spatial and temporal variations of d13C values (‰PDB) for carbonates from the upper Sinian strata. $, Least-altered samples in the Songlin–Tanshanbao, Xiushan, Jishou and Yuanling sections. Symbols and Formation names as in Fig. 6.

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Table 2 Isotope compositions of carbon, oxygen and sulfur of the Sinian rocks from the Yangtze block Localities

Sample

Strata

Lithologies

Xiushan

Y-96 Y-95 Y-93 Y-90 Y-89 Y-88 Y-87 Y-85 Y-84 Y-55

Nt Nt Nt Nt Nt Nt Nt Nt Nt Nt

ML L ML L ML ML ML BSh BSh BSh

Y-25 Y-23 Y-21 Y-52 Y-48 Y-47 Y-46 Y-45 Y-44 Y-43 Y-42 Y-41 Y-40

Dn Dn Dn Dn Dn Dn Dn Dn Dn Dn Dn Dn Dn

ML L L BSh MD L L L L L L L L

Y-39 Y-36 Y-33 Y-32 Y-31 Y-73 Y-5

Du Du Du Du Du Du Na

CSL CSL SL Ms BSh D Ss

Y-3 Y-2 Y-1-1 Y-1-2

Mi Mi Mi Mi

SL BSh Mn carb. ore Mn carb. ore

M-45 M-43 M-39 M-36 M-35 M-29 M-28 M-26 M-24 M-23 M-22 M-17 M-16 M-15 M-10

Mi Mi Mi Mi Mi Mi Mi Mi Mi Mi Mi Mi Mi Mi Mi

D DSL BSh BSh BSh Mn Mn Mn Mn BSh Mn Mn Mn Mn D

Minle

d34S (CDT ‰) 2.5 6.5

12.0 −2.8 0.3 8.1

−6.9 −26.5 −29.8

22.6a −27.1 −22.2 −5.9

d 13C carb. (PDB ‰) 1.7 2.4 −0.3 2.3 2.4 2.2 1.9

d18O carb. (PDB ‰) −13.5 −13.1 −12.0 −13.7 −13.4 −14.2 −14.0

−6.0 −8.2 −8.9

−14.1 −14.6 −15.7

−7.6 −8.3 −8.2 −8.1 −8.0 −8.2 −8.4 −8.2 −8.5

−8.6 −14.0 −14.3 −14.0 −14.7 −14.7 −14.5 −14.6 −14.7

−10.5 −10.9

−13.5 −15.3

−5.2

−7.5

19.5 30.5 21.3 41.6a 45.1a 16.1

49.1 59.1 57.6 54.2 52.5 60.3 47.8

d13C Org. (‰)

0.10

−26.2

6.28 11.58

−31.4 −31.2

0.61 0.04 0.03 10.96 0.83 0.10 0.20 0.03 0.03

−35.7 −34.4

0.03 0.07 0.03 0.02 0.08 0.14 0.25 2.66 0.06 0.16

−34.1

0.26 2.50

−3.9 −7.4

−6.7 −10.5

17.8 54.7 56.4 57.2

TOC (mgC/g)

−8.3 −9.4

−8.7 −7.4

−9.6

−7.0

−9.2 −9.3 −8.5 −7.6 −7.2

−6.6 −6.2 −10.1 −10.3 −4.4

0.05 0.11 3.80 2.40 3.43 4.05 4.08 2.20 3.23 4.58 2.85 2.65 5.10 2.58 0.74

−33.2

−34.4 −34.7 −34.9 −30.0

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R. Li et al. / Precambrian Research 97 (1999) 59–75 Table 2 (continued ) Isotope compositions of carbon, oxygen and sulfur of the Sinian rocks from the Yangtze block Localities

d34S (CDT ‰)

Sample

Strata

Lithologies

M-9 M-8 M-6 M-5 M-4 M-3

Ch Ch Ch Ch Ch Ch

SL SL SL D DS DS

23.5 24.1

M-1

P-S

Slt

39.1

Jishou

H-16 H-15 H-13 H-4 H-3 H-2

Du Du Du Du Du Du

D L L D D D

Yuanling

B-49 B-47 B-41 B-39 B-34 B-30 B-28 B-26 B-25 B-9 B-7 B-5 B-3

Nt Nt Lu Lu Du Du Du Du Du Du Du Du Du

BSh BSh SR SR BSh D D D MD BSh D Pyritic SL D

TB-41 TB-39 TB-38 TB-37 SD-22 SD-20 SD-19

Dn Dn Dn Dn Dn Du Du

D D D D D D DSL

D-47 D-32 D-36 D-37 D-42 D-45

Da Da Da Da Da Da

BSh Mn carb. Mn carb. Mn carb. Mn carb. BSh

ore ore ore ore

36.8a 57.6 57.8 53.4 48.6 54.4

X-5 X-4 X-2 X-1

Mi Mi Mi Mi

BSh Mn carb. ore BSh Mn carb. ore

63.8 60.8 58.6 43.4

Songlin–Tanshanbao

Songtao

Xiangtan

d 13C carb. (PDB ‰)

d18O carb. (PDB ‰)

58.4 41.8 −5.2 −7.9 −8.4

−6.1 −10.1 −5.5

−2.9 −9.5 −9.9 −4.7 −4.1 −3.5

−2.0 −13.0 −13.0 −7.4 −7.2 −6.0

19.0 22.6 8.7 14.9

−5.3

TOC (mgC/g) 0.93 1.24 1.18 0.12

d13C Org. (‰)

−32.0

10.16 10.47 1.13 1.21 7.25

−33.3 −33.7 −32.7 −30.8 −33.7

−35.0 −32.5

−11.6 −12.1 −5.6 −4.4

−6.7 −4.9 −6.9 −8.3

−7.5

−9.0

2.30 1.14

−4.8

−8.5

0.06

0.5 1.5 1.1 1.0 2.0 0.7 1.2

−10.6 −2.9 −8.2 −6.9 −3.1 −1.8 −3.2

0.03

16.4

0.31 0.04

−27.7

2.73 3.13 3.75 3.80 5.05 1.91

a Pure pyrite; TOC, total organic carbon; carb., carbonates; org., organic carbon. Stratigraphy: Ch, lower Sinian Chunmu Formation; Da, lower Sinian Datangpo Formation; Dn, upper Sinian Dengying Formation; Du, upper Sinian Doushantuo Formation; Lu, upper Sinian Liuchapo Formation; Mi, lower Sinian Minle Formation; Na, lower Sinian Nantuo Formation; Nt, lower Cambrian Niutitang Formation; P-S, Pre-Sinian. Lithology: BSh, black shale; CSL, calcareous siltstone; D, Dolostone; DS, dolomitic sandstone; DSL, dolomitic siltstone; L, limestone; MD, muddy dolostone; ML, muddy limestone; Mn carb. ore, manganese carbonate ore deposit; Ms, mudstone; SL, siltstone; Slt, slate; SR, siliceous rock; Ss, sandstone.

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rocks was completely digested by CrCl , the sulfur 2 being released as H S trapped as CdS in cadmium 2 acetate solution, and precipitated by AgNO solu3 tion as Ag S. Organic carbon was determined by 2 a LECO analyzer after the removal of carbonates by HCl. The trace elements of the whole rocks were determined by ICP-ES.

4. Results 4.1. Carbon and oxygen isotopic compositions of carbonates Owing to the paucity of carbonates, only three samples, M-3, M-4 and M-5 from the glaciogenic lower Sinian Chunmu Formation at Minle were collected and analyzed ( Table 2). From microscopic observations, the dolomite in the M-5 sample is detrital, as are quartz and feldspar and its d13C value is −5.2 ‰. The M-3 and M-4 samples are gray dolomitic sandstones and contain pyrite. They have more negative d13C values, −8.4‰ for M-3 and −7.9‰ for M-4. The carbonates of the lower portion of the Minle Formation are dolomites (M-10, M-43 and M-45) and manganese carbonates, such as rhodochrosite and kutnohorite (M-15 to M-29). Compared especially to the dolomite M-45 that has a d13C value of −3.9, d13C values for the manganese carbonates are more negative and range from −7.6 to −9.6. For the upper Sinian Doushantuo carbonates from the Xiushan, Jishou and Yuanling sections, d13C values are negative and vary from −2.9 to −12.1‰; d18O values range from −2.0 to −15.3‰ ( Table 2). However, for manganiferous dolostones at the base of the Doushantuo Formation (e.g. Y-73 in the Xiushan section, B-3 in the Yuanling section, H-2, H-3 and H-4 in the Jishou section) d13C values are relatively consistent and less negative, from −3.5 to −5.2‰ ( Table 2 and Fig. 5). The upper Sinian Dengying carbonates in Xiushan section have negative d13C values as well, ranging from −6.0 to −8.9‰ (Fig. 5 and Table 2). The corresponding d18O values of these carbonates are from −14.0 to −15.7‰. d13C values of platform dolostones from Songlin–Tanshabao are positive

from +0.5‰ near the Sinian–Cambrian boundary to +2‰ at a greater depth in the section, similar to the same interval at the Yichang section (Lambert et al., 1987). 4.2. Organic carbon isotopic composition In order to avoid the influence of post-depositional contamination, only the samples containing organic carbon >1 mg C/g sample were selected for organic carbon isotopic analysis ( Kaufman and Knoll, 1995). The results are listed in Table 2. The d13C value of the siltstone sample (M-6) from the Lower Sinian Chunmu Formation is −32.0‰. d13C values for the dolostones, Mn carbonates and black shales from the Lower Sinian Minle Formation range from −30.0 to −34.9‰. d13C value of the Upper Sinian Doushantuo dolostone (B-7) and black shales, B-9 and B-34 in the Yuanling section and Y-31 in the Xiushan section are similar, the former is −32.5 ‰ and the latter ranges from −33.7 to −35.0‰. A d13C value obtained from the sample Y-25 of the Dengying limestone is −35.5‰. The siliceous rocks of the upper Sinian Liuchapo Formation in the Yuanling section have d13C values of −30.8‰ (B-39) and −32.7‰ (B-41). Kerogen analyzed by Tang (1995) from the Yuanling and Jishou sections have d13C values ranging from −34.0 to −35.1‰. 4.3. Sulfur isotopic composition More than 40 samples from the lower Sinian formations were selected for sulfur isotopic analysis. All d34S values for pyrite are highly positive ( Table 2 and Fig. 6). At Minle, these commence with +24.1 and +23.5‰ in the lower Chunmu Formation, and rise rapidly through +41.8 and up to +58.4‰ in the upper Chumu Formation. Pyrite from the manganese carbonate ore deposits and closely associated (or interbedded) black shales of the Minle Formation have the heaviest d34S values. They range from +47.8 to +60.3‰ with an average of +54.9‰ for the samples from the Minle section and from +48.6 to +57.8‰ with an average of +54.4‰ for the samples from the Songtao section. Similar values for pyrite from

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Fig. 6. Spatial and temporal variations of d34S values (‰CDT ) for pyrite from the Sinian strata. The dash-line in the Xiushan and Minle sections represents the average d34S value of coeval seawater sulfate (Claypool et al., 1980; Strauss, 1993; Ross et al., 1995). Formations: Ch, lower Sinian Chunmu; Da, lower Sinian Datangpo; Dn, upper Sinian Dengying; Du, upper Sinian Doushantuo; Lu, upper Sinian Liuchapo; Mi, lower Sinian Minle; Na, lower Sinian Nantuo; Nt, lower Cambrian Niutitang; P-S, Pre-Sinian. Lithologies: Bsh, black shale; D, dolostone; L, limestone; MD, muddy dolostone; Mn, manganese carbonate deposit; Ms, mudstone; P, phosphorite deposit; PN, phosphorite nodule-bearing black shale; Ps, pebbly sandstone; SL, siltstone; Slt, slate; SR, siliceous rock; Ss, sandstone; UC, unconformity.

manganese ore beds in the Minle area have been reported by Tang (1990) and in the Songtao area by Liu et al. (1989). Although the samples from the Xiangtan and Xiushan sections are limited, they too have a heavy sulfur isotopic composition ( Table 2 and Fig. 6). Stratigraphically above the manganese carbonate ore deposits and the closely associated (or interbedded) black shales, the d34S values decrease upward to +17.8 and +16.1‰. Similar trends are evident in the Xiushan and Songtao sections ( Table 2), where d34S values drop from +45.1 to +21.3‰ in the Xiushan section, and from +57.8 to +36.8‰ in the Songtao section.

The sulfur isotopic composition of pyrite changes rapidly at the lower/upper Sinian boundary (Fig. 6 and Table 2), from the d34S value of +19.5‰ of the uppermost part of the Nantuo Formation to −5.9‰ of the lower part of Doushantuo Formation in the Xiushan section. Further up the section, pyrite d34S values are very negative with the exception of the sample Y-36 that has a d34S value of +22.6‰, similar to that of coeval seawater sulfate (Claypool et al., 1980; Strauss, 1993, 1997; Ross et al., 1995). In the Yuanling section, the drop in d34S values is from +16.4 to −5.3‰ at the lower/upper Sinian boundary ( Table 2 and Fig. 6).

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5. Discussion 5.1. Carbon isotopic composition Before discussing the geological significance of the carbon isotopic data, the preservation of isotope memory in the Sinian sedimentary rocks needs to be considered. For organic carbon, thermally-driven losses of hydrocarbons are by far the most important cause of large, postdepositional positive shifts in d13C (Hayes et al., 1983). Strauss et al. (1992a,b) suggest that samples with H/C atomic ratios of 0.2 or lower should be excluded from consideration. Some data on the elemental analysis of kerogen were reported by Tang (1995). The H/C atomic ratio of one Liuchapo siliceous rock from the Jishou section is 0.31 and the average of the H/C atomic ratios of the Liuchapo siliceous rocks from Yuanling is 0.24. Similar results are reported here ( Table 2). It is thus deduced that the diagenetic alteration of the organic carbon isotopic compositions, at least for the upper Sinian rocks analyzed in this report, is not serious. Aharon and Liew (1992), Derry et al. (1992), Veizer et al. (1992a,b) and particularly more recently Kaufman and Knoll (1995) summarized the criteria for evaluating sample quality of Proterozoic carbonates. They are based on the petrographic, chemical and isotopic tests. For example, Aharon and Liew (1992) suggest that the ‘early’ dolomicrites which show detailed preservation of the primary fabrics of the sediments (e.g. cryptalgal laminites, fenestrae, ooids, calcareous algae) preserve the isotopic memory for the Krol dolostones, whereas the ‘late’ diagenetic dolospars which show void-filling and destructive fabrics yield anomalous 18O and 13C-depleted d values. The elemental concentration of Mn, Sr and Fe are important for understanding the degree of alteration associated with meteoric diagenesis and dolomitization. Under the influence of meteoric fluids, Sr is expelled from marine carbonates while Mn is incorporated. Kaufman and Knoll (1995) suggest that carbonates (both limestones and dolostones) with Mn/Sr<10 commonly retain near primary d13C abundance. Oxygen-isotopic compositions are sensitive indicators of diagenesis, with a

decrease in d18O values often resulting from isotopic exchange with meteoric or hydrothermal fluids. It seems that d18O values of limestones <−5‰ represent some degree of oxygen isotopic alteration and samples with values <−10‰ are considered to be unacceptably altered ( Kaufman and Knoll, 1995). In cases where individual sedimentary units show wide variations in C-isotopic abundance and d13C plots against d18O as a straight line of positive slope, only samples with the most enriched 13C and 18O compositions can be considered as potentially unaltered. Data on the isotopic compositions of co-existing carbonate and organic carbon is very useful for distinguishing diagenetic effects. Diagenetic processes can alter the isotopic composition of either carbonate carbon or organic carbon, but no process is currently known that alters both signals by the same magnitude in the same direction ( Knoll et al., 1986; Kaufman and Knoll, 1995; Kaufman et al., 1997). If Dd is co-varied stratigraphically and smoothly, the isotopic composition of both the carbonate carbon and organic carbon should be primary. The dolomitic siltstones (M-3 and M-4) and detrital dolostone (M-5) from the Lower Sinian Chunmu Formation have negative d13C values from −8.4 to −5.2‰ ( Table 2). Because the Chunmu Formation is glaciogenic, it is unlikely that they could provide some information on the isotopic composition of coeval seawater in the deposition. Manganese carbonates in the Lower Sinian Minle Formation have d13C values ranging from −7.6 to −9.6 ‰, which are negatively correlated with manganese contents [c=−0.918, Li et al. (1994)], similar to the Jurassic manganese carbonates deposited at Molango, Mexico and Urkut, Hungary (Okita et al., 1988; Polgari et al., 1991). It seems likely that the manganese carbonate deposits all formed under similar environmental conditions (Okita and Shanks, 1992), probably by early-diagenetic reduction of Mn-oxides via oxidation of organic matter in near-surface anoxic sediments. The range of negative carbon isotopic results for the manganese carbonates in the Molango and Urkut deposits suggests a mixed seawater and organic source for the carbon (Okita et al., 1988; Polgari et al., 1991). In contrast,

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unmineralized calcite samples from the same deposit have d13C values near 0‰, within the estimated range of the Jurassic seawater (Polgari et al., 1991). Following this mode, it seems likely that the more negative values for rhodochrosites in the Minle Formation (e.g. M-22, M-24 and M-28) are also attributable to a greater contribution of organic carbon in the carbonates. In fact, the unmineralized dolomite sample (M-45) in the lower part of the Minle Formation is relatively 13 C enriched with a d13C value of −3.9‰. We consider that this value may better approximate the primary carbon isotopic composition of basin sea water. (See below.) A positive correlation between d18O and d13C values in carbonates from the Doushantuo Formation in the Xiushan, Yuanling and Jishou sections (Fig. 7, Table 2) suggests the effects of post-depositional alteration. In this case only the most 18O and 13C-enriched samples (e.g. H-2 and H-16 with d13C values, ca −3 and −4) are therefore considered as ‘least altered’. The carbonates of the Dengying Formation in the Xiushan section have very negative d18O values (ranging from −8.6 to −15.7; Table 2), which suggests a serious degree of post-depositional alteration. However, we still consider that they preserve some information on the primary carbon isotopic composition. The reasons are as follows: 1. Petrographically, the carbonates are micritic without secondary carbonates observed under

Fig. 7. Plot of d13C (‰PDB) versus d18O (‰PDB) for carbonates of the upper Sinian Doushantuo strata. $, Samples from Yuanling; +, samples from Jishou; 6, samples from Xiushan.

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microscope, and quite pure with CaO contents of 46.30–49.62 wt% for the limestones. Because the carbonate carbon reservoir is large relative to the carbon in diagenetic fluids, d13C compositions are less liable to change than are d18O ( Xiao et al., 1997). 2. The Sr content in the Dengying limestones is quite high up to 200–218 ppm, and the Mn/Sr ratio is very low (from 1.2 to 2.4). 3. Dd in the co-existing or closely associated carbonate–organic carbon pairs for the Sinian rocks (e.g. Y-25, SD-22, Y-31 and Y-73, B-3 and B-7, B-9 and B-25, M-36 and M-45), that can be calculated from the data in Table 2, is relatively consistent. They range from 27.7 to 30.6 with an average of ca 29, similar to the values reported by Knoll et al. (1986), Strauss et al. (1992a) and Kaufman et al. (1997) for the late Proterozoic rocks. Dd in the co-existing carbonate-organic carbon pair for the only Lower Cambrian sample, Y-87, is 28.1, close to the Sinian average. It is reasonable to deduce that d13C value of the carbonate and organic carbon, −6.0 and −35.7 for the Dengying limestone Y-25 in the Xiushan section is close to the primary value. The negative d13C values for the cap dolostones above the Sinian tillites, for example, Y-73, B-3 and M-45 in Table 2 and Fig. 5 are considered to be close to primary for the same reason. In fact, Kaufman et al. (1997) reported that the cap carbonates above the terminal Proterzoic tillites in the Upper Windermere Supergroup, northwest Canada and the Polarisbreen Group, Spitsbergen are both characterized by consistently negative d13C values. The upper Sinian sections are located in different paleogeographic settings ( Fig. 4). It is evident that the carbon isotopic compositions for the Upper Sinian sedimentary rocks change greatly with the paleogeography ( Fig. 5). The Songlin– Tanshanbao section in the central Guizhou province and the Yichang section in the Hubei province represent the carbonate platform. Similar to many other Neoproterozoic shallow marine carbonates (e.g. Knoll et al., 1986; Tucker, 1986; Magaritz et al., 1986; Aharon et al., 1987), d13C values for the platform dolostones in the Yichang section are positive, and the best approximations of primary

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isotopic compositions average ca +3 in the Doushantuo Formation and +2.0‰ in the Dengying Formation ( Fig. 5, Lambert et al., 1987). The dolostones in the Songlin–Tanshanbao section have less positive d13C values, ranging from +0.7 to +2.0‰ from the top of the Doushantuo Formation to the middle of the Dengying Formation ( Table 2 and Fig. 5). These differences may be due to variable productivity in different platform environments, with the Yichang section located in a phosphorite-bearing area having its supposedly higher nutrient supply (Fig. 4). The higher productivity would cause the removal of 12C in organic matter which would be reflected by a corresponding decrease in 12C in seawater and derived carbonates. The sedimentary strata in the Xiushan, Jishou and Yuanling sections formed in or near basin environments (Tang et al., 1980; Ye, 1989; Liu, 1991; Liu and Xu, 1994), and are characterized by organic-rich, black shales and black siliceous rocks ( Fig. 4, Table 2). In addition, the siliceous rocks are deficient in carbonates. Tang (1995) analyzed the samples taken from the Jishou and Yuanling sections and found that the contents of CaO and MgO of whole rock are only 0.03– 0.61 wt% and 0.02–1.05 wt%, respectively. The Upper Sinian carbonates (e.g. Y-25, H-16 and B-25) in the Xiushan, Jishou and Yuanling sections have negative d13C values ( Table 2 and Fig. 5), which perhaps, reflect deeper water conditions although the exact mechanism is unknown. 5.2. Sulfur isotopic composition d34S values for pyrite from the Early Sinian strata in the Yangze block are highly positive ranging from +16.1 to +63.8‰ ( Table 2 and Fig. 6). This extraordinary degree of 34S enrichment is consistent with the known terminal Proterozoic sulfur isotopic record (Hayes et al., 1992; Lambert and Donnelly, 1992; Strauss, 1997). Overall, Proterozoic sedimentary sulfides exhibit a total d34S range of −32 to +58‰, with the preponderance of values between −10 and +20‰ (Hayes et al., 1992). Most of the highly positive d34S values are found in terminal Neoproterozoic sediments, contrasting sharply with the negative values recorded in modern marine sulfides

(ca −25‰)(Chambers, 1982). The highly positive d34S values amongst disseminated sulfides in Neoproterozoic sedimentary rocks, which in many cases exceed those of the d34S of coeval seawater sulfate, may be the result of the tectonic reorganization of continents. In particular the existence of a supercontinent (or a couple of megacontinents) with a plethora of intracontinental basins and platform environments with limited access to the open ocean may have resulted in extreme 34S enrichment. In such restricted settings, the mean rates of sulfate reduction may have exceeded those of sulfate replenishment, so that the waters of many basins contained sulfate with considerably higher d34S values than seawater for a geologically long period (Lambert and Donnelly, 1992; Hayes et al., 1992). This model may serve as an analog for the Yangtze block. About 1050 to 1000 Ma ago, South China was composed of the Yangtze and Cathaysia blocks separated by the ‘South China Ocean’ ( Fig. 2; Liu et al., 1994). The continental masses may have played an important role in assembly of the Rodinia supercontinent during the late Mesoproterozoic and early Neoproterozoic (Li et al., 1995). The Cathaysia block probably adjoined the northwestern Laurentia as part of an early Mesoproterozoic metamorphic belt prior to its suturing with the Yangtze block at ca 1.0 Ga ago. The Yangtze block itself may have been a continental fragment that was caught between Australia and Laurentia during the assembly of Rodinia. Suturing at ca 1.0 Ga joined the Australian craton, the Yangtze block and Laurentia to form the core of the supercontinent Rodinia (Li et al., 1995). Some basins on the Yangtze craton likely had limited or no access to the open ocean. An alternative explanation for the highly positive d34S values of the lower Sinian sulfides, particularly those in the Mn carbonates and associated black shales, is based on the research work and the model presented by Logan et al. (1995). They suggest that due to the slow sinking of organic matter, demands by heterotrophic organisms for oxygen and other electron acceptors in surface waters were high in Proterozoic oceans. To whatever extent oxygen escaped by evasion to the atmosphere, production of sulfide by planktonic sulfate-reducing bacteria

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is likely to have been important. In shallow water sediments where larger amounts of organic matter would be incorporated and particularly, where bioturbation was absent, sulfate-reducing bacteria would be cut off from supplies beyond those present in pore waters and, as consumption of the available sulfate approached completion, the d34S value of sedimentary sulfide would also approach that of seawater sulfate. This mechanism can be applied to explaining the sulfur isotopic compositions recorded in the rocks of the Chunmu Formation in the Minle section. We suggest that the Mn carbonate ores and associated black shales could have been deposited in areas of upwelling in the stratified basin, similar to the model proposed by Force and Cannon (1988). These carbonates have high organic contents ( Table 2), which is consistent with high organic productivity. Drawdown of sulfate in the water column would create a sulfate-minimum zone (SMZ ) analogous to the modern oxygen-minimum zone (Logan et al., 1995). In extreme cases (e.g. for Lower Sinian Mn carbonate formation) the residual sulfate in the SMZ might have already been significantly enriched in 34S. Where this zone impinged on coastal sediments, it would have supplied pre34S enriched sulfate to hetrotrophic communities. In addition, the Mn carbonates formed near the sediment–water interface, colonization there by sulfate-reducing bacteria would create a situation in which the first-formed, isotopically depleted sulfide would be preferentially lost by diffusion into the water column (Logan et al., 1995). It seems possible that Mn carbonate precipitation occluded porosity in the surficial sediments, thus establishing an effective barrier to the replenishment of sulfate to the sediments from overlying seawater (Okita and Shanks, 1992). We conclude therefore, that pyrite formed in this closed diagenetic system with a high ratio of organic matter to sulfate caused the phenomenal 34S enrichments of the Mn carbonate sulfides from the Minle Formation. d34S values of pyrite from Sinian strata in the Yangtze platfom are the highest thus far recorded in the Earth’s sedimentary record. Though both models described above are reasonable to some extent, we require a unique explanation for their origin. Further research is needed.

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Acknowledgements The research work has been supported by the National Natural Science Foundation Committee of China (Grants Nos. 49573199 and 49472114). Chemical and carbonate isotopic analyses were completed in the Analytical and the Stable Isotope Laboratories of Institute of Geology, Chinese Academy of Sciences, Beijing. Organic carbon abundances and organic d13C were measured in the Laboratory Center of Institution for Petroleum Exploration and Development, Beijing. We thank J. Veizer and M. Schidlowski for their help in preparing the manuscript. Critical reviews by A.J. Kaufman and H. Strauss greatly improved this manuscript.

References Aharon, P., Liew, T.C., 1992. An assessment of the Precambrian/Cambrian transition events. In: Schidlowski, M.Golubic, A., Kimberley, M.M., McKirdy, D.M., Trudinger, P.A. ( Eds.), Early Organic Evolution: Implication for Mineral and Energy Resources. Springer-Verlag, Berlin, pp. 212–223. Aharon, P., Schidlowski, M., Singh, I.B., 1987. Chronostratigraphic markers in the end-Precambrian carbon isotope record of the Lesser Himalaya. Nature 327, 699–702. Bottomley, D., Veizer, J., Nielsen, H., Moczydlowska, M., 1992. Isotope composition of disseminated sulfur in Precambrian sedimentary rocks. Geochim. Cosmochim. Acta 56, 3311–3322. Brasier, M.D., Magaritz, M., Corfield, R., Luo, H., Wu, X., Ouyang, L., Jiang, Z., Hamdi, B., He, T., Frazer, 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. Canfield, D.E., Teske, A., 1996. Late Proterozoic rise in atmospheric oxygen concentration inferred from phylogenetic and sulphur-isotope studies. Nature 382, 127–132. Canfield, D.E., Raiswell, R., Westrich, J.T., Reaves, C.M., Berner, R.A., 1986. The use of chromium reduction in the analysis of reduced inorganic sulphur in sediments and shale. Chem. Geol. 54, 149–155. Chambers, L.A., 1982. Sulfur isotope study of a modern intertidal environment and the interpretation of ancient sulfides. Geochim. Cosmochim. Acta 46, 721–728. Chen, J., Zhong, H., Chu, X., 1992. Study of carbon isotope stratigraphy of Precambrian-Cambrian boundary in China. Chinese Sci. Bull. 37 (6), 540–542. in Chinese Chu, X., Zhao, R., Zang, W., Lei, J., Tang, Y., 1993. Extraction of various forms of sulfur and preparation of the samples

74

R. Li et al. / Precambrian Research 97 (1999) 59–75

for isotopic analyses in coals and sedimentary rocks. Chinese Sci. Bull. 38 (20), 1887–1890. in Chinese Claypool, C.E., Holser, W.T., Sakai, I.R., Zak, I., 1980. The age curves for sulfur and oxygen isotopes in marine sulfate and their mutual interpretation. Chem. Geol. 28, 199–296. Cowie, J.W., 1985. Continuing work on the Precambrian–Cambrian boundary. Episodes 8 (2), 93–97. Derry, L.A., Kaufman, A.J., Jacobsen, S.B., 1992. Sedimentary cycling and environmental change in the Late Proterozoic: evidence from stable and radiogenic isotopes. Geochim. Cosmochim. Acta 56, 1317–1329. Force, E.R., Cannon, W.F., 1988. Depositional model for shallow-marine manganese deposits around black-shale basins. Econ. Geol. 83, 93–117. Geological Team and Laboratory Center of Geological Survey Bureau Hunan Province, 1984. The Geological Features and Ore-forming Regularities of Minle Manganese Ore in Huayuan County, Hunan Province, China. No. 405 (an unpublished scientific report, in Chinese). Grotzinger, J.P., Bowring, S.A., Saylor, B.Z., Kaufman, A.J., 1995. Biostratigraphic and geochronologic constraints on early animal evolution. Nature 270, 598–604. Hayes, J.M., Kaplan, I.R., Wedeking, K.W., 1983. Precambrian organic geochemistry, preservation of the record. In: Schopf, J.W. (Ed.), Earth’s Earliest Biosphere: Its Origin and Evolution. Princeton University Press, Princeton, pp. 93–134. Hayes, J.M., Lambert, I.B., Strauss, H., 1992. The sulfur-isotope record. In: Schopf, J.W., Klein, C. (Eds.), The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press, Cambridge, pp. 129–132. Holser, W.T., Schidlowski, M., Mackenzie, F.T., Maynard, J.B., 1988. Biogeochemical cycles of carbon and sulfur. In: Gregor, C.B., Garrels, R.M., Mackenzie, F.T., Maynard, J.B. ( Eds.), Chemical Cycles in the Evolution of the Earth. Wiley, New York, pp. 105–174. Hsu¨, K.J., Oberhansli, H., Gao, J.Y., Shu, S., Haihong, C., Krahenbuhl, U., 1985. ‘Strangelove ocean’ before the Cambrian explosion. Nature 316, 809–811. Iyer, S.S., Babinski, M., Krouse, H.R., Chemale Jr, F., 1995. Highly 13C-enriched carbonate and organic matter in the Neoproterozoic sediments of the Bambui Group, Brazil. Precambrian Res. 73, 271–282. Kaufman, A.J., Knoll, A.H., 1995. Neoproterozoic variations in the C-isotopic composition of seawater: stratigraphic and biogeochemical implications. Precambrian Res. 73, 27–49. Kaufman, A.J., Knoll, A.H., Narbonne, G.M., 1997. Isotopes, ice ages, and terminal Proterozoic earth history. Proc. Natl. Acad. Sci. USA 94, 6600–6605. Knoll, A.H., Walter, M.R., 1992. Latest Proterozoic stratigraphy and Earth history. Nature 356, 673–678. Knoll, A.H., Hayes, J.M., Kaufman, A.J., Swett, K., Lambert, I.B., 1986. Secular variation in carbon isotope ratios from Upper Proterozoic successions of Svalbard and East Greenland. Nature 321, 832–838. Knoll, A.H., Grotzinger, J.P., Kaufman, A.J., Kolosov, P., 1995. Integrated approaches to terminal Proterozoic stratig-

raphy: an example from the Olenek Uplift, northeastern Siberia. Precambrian Res. 73, 251–270. Lambert, I.B., Donnelly, T.H., 1992. Global oxidation and a supercontinent in the Proterozoic : evidence from stable isotope trends. In: Schidlowski, M.Golubic, S., Kimberley, M.M., McKirdy, D.M., Trudinger, P.A. (Eds.), Early Organic Evolution: Implication for Mineral and Energy Resources. Springer-Verlag, Berlin, pp. 408–414. Lambert, I.B., Walter, M.R., Zhang, W., Lu, S., Ma, G., 1987. Paleoenvironment and carbon isotope stratigraphy of Upper Proterozoic carbonates of the Yangtze Platform. Nature 325, 140–142. Li, R., Chen, J., Zhang, S., Chu., X., Wang, X., 1994. The significance of organic matter in formation of the Minle manganese carbonate ore deposit, China. In: 14th International Sedimentological Congress, Abstracts H-11–H-12, Recife, Agosto. Li, Z.-X., Zhang, L., Powell, C.McA., 1995. South China in Rodinia: part of the missing link between Australia-East Antarctica and Laruentia? Geology 23, 407–410. Liu, B., Xu, X., 1994. Atlas of the Lithofacies and Paleogeography of South China. Science Press, Beijing. Liu, B., Xu, X., Pan, X., Huang, H., Xu, Q., 1994. Evolution of the Paleocontinents in South China and the Ore Genesis. Science Press, Beijing. in Chinese Liu, X., 1991. The Sinian System in China. Science Press, Beijing. in Chinese Liu, X., Wang, Q., Gao, X., 1989. Geology of Manganese Ore in Guizhou Province. Guizhou People’s Publishing House, Guiyang, China. in Chinese Logan, G.A., Hayes, J.M., Hieshima, G.B., Summons, R.E., 1995. Terminal Proterozoic reorganization of biogeochemical cycles. Nature 376, 53–56. Magaritz, M., Holser, W.T., Kirschvink, J.L., 1986. Carbonisotope events across the Precambrian/Cambrian boundary on the Siberian Platform. Nature 320, 258–259. Narbonne, G.M., Kaufman, A.J., Knoll, A.H., 1994. Integrated chemostratigraphy and biostratigraphy of the upper Windermere Supergroup (Neoproterozoic) Mackenzie Mountains northwestern Canada. Geol. Soc. Am. Bull. 106, 1281–1291. Okita, P.M., Shanks, W.C., 1992. Origin of stratiform sedimenthosted manganese carbonate ore deposits: examples from Molango, Mexico and Taojiang, China. Chem. Geol. 99, 139–164. Okita, P.M., Maynard, J.B., Spiker, E.C., Force, E.R., 1988. Isotopic evidence for organic matter oxidation by manganese reduction in the formation of stratiform manganese carbonate ore. Geochim. Cosmochim. Acta 52, 2679–2685. Polgari, M., Okita, P.M., Hein, J.R., 1991. Stable isotope evi´ rku´t manganese ore deposit, dence for the origin of the U Hungary. J. Sediment. Petrol. 61, 384–393. Ross, G.M., Bloch, J.D., Krouse, H.R., 1995. Neoproterozoic strata of the southern Canadian Cordillera and the isotopic evolution of seawater sulfate. Precambrian Res. 73, 71–79. Schidlowski, M., Aharon, P., 1992. Carbon cycle and carbon isotope record: geochemical impact of life over 3.8 Ga of

R. Li et al. / Precambrian Research 97 (1999) 59–75 Earth history. In: Schidlowski, M.Golubic, S., Kimberley, M.M., McKirdy, D.M., Trudinger, P.A. ( Eds.), Early Organic Evolution: Implications for Mineral and Energy Resources. Springer-Verlag, Berlin, pp. 147–176. Strauss, H., 1993. The sulfur isotopic record of Precambrian sulfater: new data and a critical evluation of the existing record. Precambrian Res. 63, 225–246. Strauss, H., 1997. The isotopic composition of sedimentary sulfur through time. Palaeogeography Palaeoclimatology Palaeoecology 132, 97–118. Strauss, H., Des Marais, D.J., Hayes, J.M., Summons, R.E., 1992a. The carbon-isotopic record. In: Schopf, J.W., Klein, C. (Eds.), The Proterozoic Biosphere. Cambridge University Press, Cambridge, pp. 117–127. Strauss, H., Des Marais, D.J., Hayes, J.M., Summons, R.E., 1992b. Proterozoic organic carbon-its preservation and isotope record. In: Schidlowski, M.Golubic, S., Kimberley, M.M., McKirdy, D.M., Trudinger, P.A. ( Eds.), Early Organic Evolution: Implications for Mineral and Energy Resources. Springer-Verlag, Berlin, pp. 203–212. Tang, S., 1990. Isotope geological study of manganese deposit in Minle area, Hunan Province. Acta Sedimentologica Sinica (4), 77–84. in Chinese Tang, S., 1995. Sedimentology and geochemistry of siliceous rocks in the southeastern part of Sichuang Province and the western and middle parts of Hunan Province, China. PhD. Thesis (Dissertation), Institute of Geology, Chinese Academy of Sciences, Beijing, China. Tang, T., 1998. Paleogeographic environment of the Sinian– Cambrian and the biomineralization of the Yangzhe area. In: Ye, L. (Ed.), Biomineralization and its Geological Bachground. China Ocean Press, Beijing, pp. 121–144. in Chinese Tang, T., Zhang, J., Yang, W., Qiu, J., Zhou, Y., 1980. The depositional environments of the carbonate rocks and the phosphorites from Upper Sinian in S.W. China. J. Stratigraphy (4), 264–272. in Chinese Tucker, M.E., 1986. Carbon isotope excursions in Precambrian/ Cambrian boundary beds, Morocco. Nature 319, 48–50.

75

Veizer, J., Clayton, R.N., Hinton, R.W., 1992a. Geochemistry of Precambrian carbonates: IV. Early Paleoproterozoic (2.25±0.25 Ga) seawater. Geochim. Cosmochim. Acta 56, 875–885. Veizer, J., Plumb, K.A., Clayton, R.N., Hinton, R.W., Grotzinger, J.P., 1992b. Geochemistry of Precambrian carbonates: V. Late Paleoproterozoic seawater. Geochim. Cosmochim. Acta 56, 2487–2501. Wang, H., 1985. Atlas of the Paleogeography of China. Cartographic Publishing House, Beijing. Wang, X., Yao, H., Chen, X., Meng, F., 1998. Field Trip Guide to the Stratigraphy of the Yangtze Gorges Area. Symposium on the Sequence Event and Combined Stratigraphy of Yangtze Gorges Area, Oct. 15–19, 1998, Yichang Institute of Geology, Hubei Province, China. Wang, Y., Wang, W., Zhu, S., Xie, Z., Chen, D., Zheng, S., Zhu, H., 1985. Sedimentary environments and the Mn concentration of the strata of Datangpo Group in the east part of Guizhou Province. Guizhou People’s Publishing House, Guiyang, China. in Chinese. Xiao, S., Yun, Z., Knoll, A.H., 1998. Three-dimensional preservation of algae and animal embryos in a Neoproterzoic phosphorite. Nature 391, 553–558. Xiao, S., Knoll, A.H., Kaufman, A.J., Leiming, Y., Yun, Z., 1997. Neoproterozoic fossils in Mesoproterozoic rocks? Chemostratigraphic resolution of a biostratigraphic conundrum from the Nouth China Platform. Precambrian Res. 84, 197–220. Xue, Y., Tang, T., Yu, C., 1993. Analysis of the Late Sinian paleogeography and paleoclimatology in the Yangtze region, Southern China. Biogenic Mineralization. Ocean Press, Beijing. Ye, L., Chief Compiler Ye, L., The Phosphorites of China. Science Press, Beijing. in Chinese Zhao, D., 1996. Geological features and biomineralized processes of the Sinian Datangpo epoch Mn-bearing formation. In: Ye, L. ( Ed.), Aspects of Biomineralization. Ocean Press, Beijing, pp. 261–277. in Chinese