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VOI. 42 N O . 6 SCIENCE IN CHINA(Series D) December 1999

Organic carbon isotopes of the Sinian and Early

Cambrian black shales on Yangtze Platform, China *

LI Renwei (%iiE#)', LU Jialan ( p %$ ! ) 2 ,ZHANG Shukun (% ?&!* 3 and LEI Jiajin (S$U )) '

(1 . Institute of Geology, Chinese Academy of Sciences, Beijing 100029, China; 2. Guangzhou Institute of Geochemistry,Chinese Academy of Sciences, Guangzhou 510640, China;3. Institute of G eomechan ics, Ch inese Academy of

Geological Sciences, Beijing100081 , China)

Received July 9 , 1998; revised February 19, 1999

Abstract Organic matter of the Sinian and early Cambrian black shales on theYangtze Platform belongs to the lightcarhon groupof isotopes with the 6 I3cvalues from - 27% to - 35 % , which are lower than those of the contempomne-ously depositedcarbonates and phosphorites. A carbon isotope-stratified paleooceanographic model caused by upw ellingis proposed, which canbe used not only to interpret the characteristicsof organic carbon isotopic compositions of theblack shales, but also to interpret the paleogeographic difference in the organic carbon isotope compositions of varioustypes of sedimen tary rocks.

Keywords : black shales, stable carbon isotopes, paleooceanography, h e Sinian, th e Early Cambrian.

The Sinian and Early Cambrian black shales are extensively distributed on Yangtze Platform. As

an important reservoir of carbon owing to the huge quantity of organic matter, they have an influence

on the global biochemical cycle and the evolution of the biospherer']. Many major phosphorites, Mn

and Ba ore deposits that occur in the Sinian and Early Cambrian strata in China are intimately related

to the black shales in originL2] In addition, the abundant noble and rare metals in the Early Cambri-an black shales, such as V , Mo , Ni , Co , Cr , Se , U and AU[~-'] , have important economic val-uesL6]. Geologists have already studied the black shales, but still lack sufficient knowledge about

their stable isotope geochemistry. This paper reports on the organic carbon isotope compositions and

reconstructs the paleooceanographic conditions for the formation of the black shales.

1 Geological setting

The black shales dealt with in this paper include those from the Early Sinian Datangpo, the Late

Sinian Doushantuo and Dengying, and the Early Cambrian, which are distributed in such areas as

Minle, Yuanling, Taojian-Yiyang and Dongkou counties in Hunan Province, Zunyi and Qingzhen

counties in Guizhou Province, and Xiushan County in Sichuan Province (fig. 1) . The Early Sinianblack shales outcrop mainly in Hunan , Guizhou and Sichuan provinces. However, the thickness var-

* Project supported by the National Natural Science Foundation of China (Grant No.49472114) and the Open Laboratory ofOr-ganic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.

1 ) Wu Chaodong, Upper Sinian-Lower Cambrian black shale series and the related ore mineralization,Ph. D . Dissert&wn,

1997, Institute of Geology, Chinese Academy of Sciences.


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596 SCIENCE IN CHINA (Series D) Vol. 42

ies from greater than 20 m in Minle to only a fewmeters in Xiushan. Many important Mn carbonateores, e . g. the Minle and Xiangtan ores in Hunan

Province, the Datangpo ore in Guizhou Province andthe Xiushan ore in Sichuan Province, occur in theEarly Sinian black shale series. The heavy sulfur iso-tope ( 3 4 ~ ) bounds in pyrite from the Mn carbonateores and black shales, with 8 3 4 ~ alue high , up to+ 60% . Possibly, they formed in isolated, closed

basins far from the open ocean on a geological settingof the late Proterozoic supercontinent[7981 There are

Fig. 1 . Studied area in this paper. 1 , Xiushan; 2 ,two sets

ofblack shales in the sinian DoushantuoYuanling; 3 , Yiyang-Taojian ; 4 , Dongkou ; 5 , Minle; 6 , Formation. The thickness of the early Doushantuo

Qingzhen ; 7 , Zunyi .black shales are less than one meter in the Xiushan

area, and a few meters in Yuanling area ( e . g. at Sanjiaoping Village). The late Doushantuo blackshales are relatively thick, even greater than 10 m in the Zunyi area (e . . the Son& Village) andYuanling area ( e . g. the Sanjiaoping Village) . The Doushantuo is a major P-accumulated timeCg1,and many phosphorites are closely related to the Doushantuo black shales in origin. In the Yuanling

area, a few meters of phosphorites are continuously superimposed on the early Doushantuo blackshales. Phosphorites of low grade are intercalated in the late Doushantuo black shales in the Zunyiarea. The black siliceous rocks intercalating black shales were deposited later during the Dengyingstage of the Late Sinian. Some Chinese geologists call it Liuchapo Formation. Owing to enrichment oforganic matter, the siliceous rocks are considered for the study of stable carbon isotopes in this paper,as well. The Early Cambrian black shales are extensively distributed in southeastern China includingthe Yangtze Platform, with an area of at least 500 000 km2. Its thickness is over one hundred metersin the Qingzhen, Taojing and Dongkou areas. The early Cambrian is another P-accumulated time inthe geological history of chinaLg1 and the deposited phosphorites are closely related to the contempo-raneous black shales in origin of as well. The Late Sinian and Early Cambrian black shales weremainly deposited in outer shelf-slope basin .

So far, we lack precise date on the geological age of the Sinian and Early Cambrian blackshales. The Rb-Sr isochron age of the Mn carbonates ores taken from Minle area is (728 * 28)~ a ' " ] , which would approach the age of the Early Sinian black shales. The Sm-Nd isochron age ofthe phosphorites taken from the upper part of Doushantuo Formation is (645.5 * 23.6) ~ a [ " ] whichwould approach the age of the black shales of the late Doushantuo.

2 Analytical methods and results

Inorganic minerals, such as carbonates and silicates, were removed from rocks using HCVHFby heating before the organic carbon isotopes of the rocks were analyzed. Due to the very low solubleorganic matter that can be identified from the Rock-Eval pyrolytic measurements: the So and S1 valuesare near zero[51 and the high contents of organic carbon of the black shales (table I ) , the organicmatter obtained from the above method is similar to kerogen of the rocks in composition. The stable


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No. 6 ORGANIC CARBON ISOTOPES OF BLACK SHALES, CHINA 597

isotope analyses of organic carbon were completed by the Laboratory Center of Institute of Petroleum

Exploration and Development, Beijing , using PDB standard.

Table 1 Data on stable carbon isotopic and organic geochemical analyses of the Sinian and Early Cambrian sedimentary rocks

TOC H/C 6 "c,, 6 '3C,d.Area Strata Sample Lithology A

(wt%) (atomic) (460) (460)

~ 8 4 BSH 6 .28 - 31.4~ 5 5 P-BSH 11 -58 0.29 - 31.2~ 5 2 BSH 10.96 - 34.4

G d n ~ 4 7 ~ ~ 4 0 L 0.03-0.20 - 8 . e - 8 . 5

%d ~ 3 1 BSH 2.66 0.30 -3 4.1

~ 7 3 D 0 .66 - 5 .2 28.9Yuanling -I B50 BSH 9.86 - 33.2

B49 BSH 10.16 0.31 - 33.3x47 BSH 10.47 -3 3. 7%1 B41 SR 1.13 - 32.7

B39 SR 1.21 - 30.8%d B34 BSH 7.25 0.33 -3 3.7

B23 P 0.98 0.38 - 24.2B9 BSH 2.30 - 35.087 MD 1.14 - 32.5B3 D 0.06 -4 .8 27.7

Yiyang- -t Y 6 BSH 10.00 0.19 - 32.9Taojian T37 P-BSH 21.40 0.15 - 31.9Dongkou - I D9 BSH 9.65 0.17 -27.1

Dl BSH 5.80 0.13 - 28.2Qingzhen - , 4567 BSH 1.74 0.36 - 32.9

Zunyi *I TB47 BSH 4.75 0.32 - 31.9TB45 BSH 5 -08 -33.3TB42 P- BSH 14.83 0.33 - 34.4

%dn TB39 D + 1.5% dn SD22 D 0.31 -27.7 + 2.0 29.7

SD20 D + 0. 7% d SDll BSH 3.35 - 30.5

SD4 P-BSH 3.25 0.41 - 31.9Minle 21 M45 D 0.05 -3 . 9 29.3

M36 BSH 2.4 -33.2

M26 Mn 2.20 - 34.4M22 Mn 2.85 - 34.7M15 Mn-BSH 2.58 - 34.9MI0 MD 0.74 - 30.0M6 SL 1.18 - 32.0M5 D 0.12 -5 . 2 26.8

Stable carbon isotope analyses of carbonate rocks of the Sinian and Early Cambrian age on

Yangtze Platform were needed for comparison with the organic carbon isotopes of the black shale. Sam-

ples reacted with anhydrous H3 O4 at 250C for 24 h for limestone, and for 72 h for dolostone. Theresults are represented by PDB standard.

Elemental analysis of the isolated organic matter of the black shale samples was made in the

Open Laboratory of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The data


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used to check the primary memory of the organic carbon isotope compositions willbe discussed later.The data on the stable carbon isotopes of the black shales and some carbonate rocks of the Sinian

and Early Cambrian on Yangtze Platform are listed in table1 . The distribution of the samples in

stratigraphic profile is shown in fig. 2 . 6 values of organic carbon( 6 l3 c,,. ) of the Early Sinianblack shales and the Mn carbonate ores related to the black shales (M15-M36 in table1 ) in Minle

Yuanling

XiushanZunyi

I... ...I 3

I_._[ 4

SDl 1

SD 4

Fig. 2 . Profiles of the late Sinian and Early Cambrian strata on Yangtze Platform. Lithology: 1 , Lime-stones; 2 , dolostones; 3 , siltstones; 4 , silty shales; 5 , siliceous rocks; 6 , black shales; 7 , phospboms

nodulebearing black shales ; 8 , phosphorite beds. Strata: 43, , the Lower Cambrian; dn, DengyingFormation ; 1 , 1,iuchapo Formation ; d , Doushantuo Formation.

area are similar, with a range from - 33 -28 0 to - 34.9% . The 6 l3cO,. value of the early Sinianorganic-rich rock (M 6) in age is - 3 2 .0 % 0 . The Late Sinian Doushantuo black shales (Y 31 , B9,B34, SD11 and SD4 in table 1 ) have 6 ' 3 ~ 0 r g .alues with a range from - 30.5% to - 35.0% , ndthe organic-rich siliceous rocks of the Late Sinian Liuchapo (B1 and B39 in table1 ) have 6 13c,,,.values with a range from - 30.8%0 to - 32 .7 % . Tang has obtained similar data on organic carbonisotopes of the contemporaneous siliceous rocks,with a range of 6 l3 COT.values from - 29.3% to- 35.3%') . The 6 l 3 C,,. values of the Early Cambrian black shales are from - 27. l%o to- 34.4%0. Compared to black shales the carbonate rocks and phosphorites are clearly enrichedwith

heavy carbon ( l 3 c ) . For example, the Dengying dolostone from Zunyi area (SD22) and the EarlyCambrian limestone from Xiushan area have6 l3cO rg. alues - 27.7960 and - 26.2%, respectively.The 6 ' 3 ~ , , , , value of the Doushantuo phosphorites from Yuanling area is- 24 .2%0 .

1 ) Tang Shimng, Sedimentology and sedimentary geochemistry of the Cambrian and Sinian siliceous rocks in Hunan and Sichuan

provinces, Ph . D . Dissertation , 1995 , Institute of Geology, Chinese Academy of Sciences.


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tope compositions signals a sedimentary environmental change of paleoocean in the phosphorites depo-

sition.

3 . 3 Paleooceanographic depositionary environments and origin of the Sinian and Early Cambrian black

shales

So far, the origin of the Sinian and Early Cambrian black shales has been a question without a

satisfactory answer. It is discussed here through the data of stable carbon isotopes. Sinian and Early

Cambrian sedimentary organic matter was derived from the photosynthetic aquatic phytoplankton. The

carbon isotope compositions were determined both by the forms and compositions of the available car-

bon dioxide in the ocean and by the mechanism of photosynthesis[1~18~-201. n discussing the causes

for the anomalous carbon isotope compositions of the 1-amorphous kerogen and the organic matter of

Jurassic Toarcian black shales, several suggestions have been proposed, such as ( ) special planktonspecies, (ii) a change in atmospheric C 0 2 , (iii) bacterial reworking of organic matter, and ( iv ) spe-

cial paleooceanographic environments. However, only the fourth suggestion could provide a reasonable

explanation[18320-221 Generally, the 6 13C values of dissolved inorganic carbon in oceans equilibrated

with atmospheric C02 are close to 0-1%. In anaerobic basins , such as the Jurassic studied by KUS-

pert (1982), organic matter is degraded by sulphate-reducing bacteria, and the oxidized organic car-

bon is much more enriched in 12c han normal oceanic inorganic carbon and has the 6 13cvalue ofca . - 25560. Due to restriction of sea water, light carbon accumulates in the deeper water. Once itescapes to the photic zone by convection and diffusion and is in turn taken up by planktons, the pro-

duced organic matter is 12c-rich in stable carbon isotopesLm1 In the restricted epicontinental basins ofshallow water with depth less than 200 m , C02 produced by the microbial degradation of organic mat-

ter can be the source for phytoplanktons. Applying this oceanographic model, Lewan (1986) has ex-

plained the origin of the I-amorphous organic matter[lE1 The occurrence of the Late Sinian and Early

Cambrian black shales on Yangtze Platform is closely related with siliceous rocks and phosphorites in

the spatial and temporal distributions[91 Furthermore, Ba is enriched in the black shales and barite

ores occur in the black shale strata1) . According to the characteristic sedimentary association of C-Si-P and c - s ~ - B ~ [ ~ ~ ] " )it is reasonable to suggest that the Late Sinian and Early Cambrian black shales

formed in an upwelling setting of high organic productivity. The ocean could become stratified in thestable carbon isotopes during long lasting periods in such an upwelling setting (fig. 3 ) . In the oxygenminimum zone (OMZ) , as shown in fig. 3 , the C0 2 derived from the bacterial degradation of large

amounts of organic matter (CH20) under reducing conditions, having a low 6 13c value to - 25%o,was a 12C-rich inorganic carbon reservoir, compared with that of the surficial ocean. Applying this

oceanographic model to the Yangtze Platform, the characteristics of stable carbon isotope compositions

for the Late Sinian and Early Cambrian sedimentary rocks, including the black shales, can be under-

stood.

Generally, the photosynthesis in oceans occurs above a depth of ca. 200 m. In the platform set-ting where carbonates deposited, such as A shown in fig. 3 , photosynthesis of surficial ocean could

not be influenced by the oxygen minimum zone (OMZ) . However, in the outer shelf and the slope

1 ) Wang Zhongcheng, Sedimentation of Early Cambrian stratiform barite and w itherite in South China bioorganism and palmma-

rine environment, Ph . D . Dissertation , 1997, Institute of Geology, Chinese Academy of Sciences.


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No. 6 ORGANIC CARBON ISOTOPES OFBLACK SHALES, CHINA 60 1

near it ( B , C , D in fig. 3) , photosynthesis occurred not only directly in the upper OMZ, but also inthe surficial ocean influenced by the OMZ, where the inorganic carbon reservoir had markedly less6 13c value than that in the platform due to the upward convection and diffusion of the 12c-richC%

from the OMZ. As shown in table 1 , he Sinian and Early Cambrian sedimentary rocks have similar Avalues ( = 6 l3cCmb. 6 13Co,. ) . We suggest that the carbon isotope fractionation in photosynthesisin the ocean was not changed greatly. Then, the synthesized organic matterin the outer shelf and ad-jacent slope would have markedly negative6 13C values, compared with those in the platform setting.This sedimen tary record has been kept in the Late Sinian and Early Cambrian rocks. For example, the6 13c,,. value of SD22 in the Zunyi area is ca . - 28% , but the 6 13c,,. va lues of the black shalesin Xiushan, Yuanling and Taojian area located in the outer shelf and adjacent slope are as lowas- 34%.

Sea level

I \ Upwelling

Fig. 3 . Paleooceanographic environments for the formation of the Late Sinian and Early Cambrian black shales.

The change of the stable carbon isotope compositions of carbonates deposited on Yangtze Platformduring the Late Sinian and Early Cambrian supports the paleooceanographic model shown in fig.3 .

Th e 6 l3cCarb.alues of Y47-Y40 deposited in the outer shelf are around- 8% (table 1 ) . But inthe platform setting, where the surficial ocean was not influenced by the OMZ and had high organicproductivity due to the rich nutrients carried by upwelling, the6 13C values of the inorganic carbonreservoir of the surficial ocean increased a s the light carbon (1 2 c ) was selectively taken up from seawater by the flourishing phytoplankton through photosynthesis. This sedimentary record has indeedbeen kept in the Late Sinian carbonate rocks. For example, the6 13~,,,a value of SD22 from Zunyi

area is + 2.0% (table 1 , and it ranges from + 4% to + 7% in the Yangtze Gorge area'"] .We have no evidence to use the upwelling model described above for explaining the origin of the

Early Sinian and even the Late Sinian Doushantuo black sha les. However, the mechanism proposedby Kuspert (1982) and Lewan etal. (1986) is worthy to be The Early Sinian black

shales and related Mn carbonate ores formed in isolated, closed basins far from open ocean. Organicmatter was degraded by sulphate reduc ing bacteria under anaerobic conditions leading to the formationof black s ha les. Although the formation of Mn carbonate ores was not under the same anae robic condi-


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602 SCIENCE IN CHINA (SeriesD) Vol. 42

tion, it depended upon the degradation of organic matter by Mn reducing bacteria: CHzO + Mn O2 =

Mn2+ + C02 + H ~ o [ ~ ] .he 12c-rich C02 derived in these two cases could be the carbon sources forphotosynthesis of organic matter, and then light carbon isotope was enriched in the organic matter of

the Early Sinian black shales and related Mn carbonate ores.

4 Conclusion

This paper studies the organic carbon isotopes of the Sinian and Early Cambrian black shales on

Yangtze Platform. The organic carbon of these rocks is rich in light carbon (12c) . 6 13c alues of theEarly Sinian black shales and related Mn carbonate ores are from - 33%0 to - 35%0, the Late Sinian

Doushantuo black shales from - 31% to - 35%0, the Late Sinian Dengying black siliceous rocksfrom - 31% to - 33%o, and the Early Cambrian black shales h m - 27% to - 34%. Possibly,the Late Sinian and Early Cambrian black shales on Yangtze Platform formed in an upwelling setting of

high organic productivity, and the paleoocean was stratified in the stable carbon isotopes. The light

carbon isotope was markedly enriched in the inorganic carbon reservoir of the oxygen minimum zone,

and the difference of organic carbon isotope compositions in the rocks deposited in various sedimentary

environments could be interpreted as the extent of influence of the oxygen minimum zone upon the

photic zone. Compared with the carbonate rocks deposited in platform setting, the black shales formed

in the outer shelf and adjacent slope basins had markedly lower 6 13c,,. values. The difference of sta-

ble carbon isotope compositions in carbonates deposited in various sedimentary environments supports

this conclusion. The 6 l3cCarb. alues of carbonates deposited in outer shelf basin are negative. How-

ever, they are positive for the carbonates deposited in the platform setting. The Early Sinian black

shales and related Mn carbonate ores formed in the restricted basins where the carbon isotopes were

stratified, as well.

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