Re–Os geochronology of the Cambrian stage-2 and-3 boundary in Zhijin County,Guizhou Province,China

Shuaichao Wei?Yong Fu,2?Houpeng Liang?Zhihua Ge?Wenxi Zhou?Guangzhe Wang

The Ediacaran-Cambrian(E–C)transition is an important interval in geological history.During this interval,Ediacaran soft-body biota were replaced by small shelly fossils(SSFs),after which occurred the major phase of the‘Cambrian Explosion,’ regionally evidenced by the Chengjiang Biota(Hou et al.2004;Zhu 2010).The Global Boundary Stratotype Section and Point(GSSP)E–C boundary is marked by the first appearance of trace fossilTreptichnus pedum,or by SSFs in the platform facies of the Yangtze Block(Brasier et al.1994;Landing 1994;Buatois et al.2013).Currently,the first occurrence of SSFs has been constrained by a zircon U–Pb age of 539.4± 2.5 Ma from bentonite(Compston et al.2008),but their extinction time remains controversial.Meanwhile there is only a rough estimated age of 525 Ma for the Chengjiang Biota(Zhu et al.2003).Therefore,a reliable radiometric age of the Chengjiang Biota was sought in this study.Furthermore,the oceanic anoxic event(OAE),formed by the expansion and/or intensification of the global or local anoxic ocean waters during a relatively short time interval,occurred globally during the E–C transition,having great influence on the co-evolution of environment-life processes on Earth’s surface.Hence,the timing of the OAE is crucial to the understanding of several problems.

In the past,multiple isotopic dating methods,such as Rb–Sr,K–Ar,Sm–Nd,and U–Pb dating,have been applied to constrain the depositional age of sedimentary strata(Dickin 2005),but these methods are limited by the availability of suitable dating materials.In contrast,the Re–Os dating methodology can be well-applied in organicrich rocks,i.e.,black shale,because these rocks are high in Re–Os.Because the closure temperature of the Re–Os isotopic system is relatively high(Dickin 2005;Li et al.2014),with negligible diagenetic alteration,Re–Os has been widely used in dating sedimentary rocks(Cohen 2004;Hannah et al.2004;Selby and Creaser 2005;Kendall et al.2009;Li et al.2014).On the other hand,the Os isotope is an effective tracer for the paleo-ocean environment,being particularly sensitive to hydrothermal activity(Cumming et al.2012;Fu et al.2016).In this study,we utilized the Re–Os isotopic system to investigate the black shale at the base of the Niutitang Formation from the Gezhongwu section in the Zhijin area,Guizhou Province.This paper aims to:(1)constrain the ages of OAE and of the subsequent occurrence of the Chengjiang Biota in the Early Cambrian using the Re–Os isochron age;(2)explore the environmental background of the Early Cambrian OAE and subsequent Cambrian Explosion in light of the initial Os isotopic ratios.

1 Geologic setting

From the Early Ediacaran,large-scale carbonate platform systems have been established on paleo-highs in the Yangtze Block(Jiang et al.2009).However,in the late Ediacaran,enhanced extensional tectonism caused block tilting and differential subsidence across faulted blocks,resulting in intense subaerial exposure and erosion on the uplifted ridges of platforms,apparently forming an unconformity with respect to the overlying Lower Cambrian sediments.In subsequent large-scale transgression,the carbonate platforms were completely flooded or submerged,thereby forming a fine-grained siliciclastic shelf basin setting(Yeasmin et al.2016)(Fig.1a,b).The study area thus experienced an environmental change from dolomitic-phosphatic shelf to fine-grained(muddy)siliciclastic shelf.

The Gezhongwu area,about 14 km east of Zhijin City,is located on the southeastern wing of the Dayuan anticline,across which the Ediacaran,Cambrian,Carboniferous,Permian,and Triassic systems crop out(Fig.1c).The lithostratigraphic units of the Ediacaran and Lower Cambrian include the Dengying,Gezhongwu and Niutitang Formations,in ascending order(Fig.2);simple descriptions are provided below.

2 Samples and methods

In view of the high Re and Os contents of black shale,samples were mainly selected from black(carbonaceous)shale in the lower part of the Niutitang Formation.

All samples were collected from a 2 m-thick drill core with sampling spacing of 10 cm,ensuring sufficient expansion of the Re–Os isochrones and a precise Re–Os isotopic age.To avoid potential diagenetic alteration,samples containing hydrothermal veins and pyrites were excluded.

The selected samples were carefully cleaned,then put in a drying oven for 24 h at a constant temperature of 60°C.Bulk sample was grinded to 200 mesh using a mortar,and the mortar was carefully cleaned by alcohol between samples to avoid contamination.Approximately 0.5 g of sample powder was precisely weighed,and then transferred by neck funnel to a pipe with length 20 cm and diameter 2 cm at the bottom of the Carius.After using 3 mL 15-mol/L hydrochloric acid to transferthe185Re–190Os mixed diluent into the Carius tube being frozen with liquid nitrogen,we added 5 mL 15-mol/L HNO3and 1 mL 30%H2O2.The Carius tube was heated at 230°C for 24 h after being liquefied with petroleum gas and oxygen flame(Du et al.2001;Qu and Du 2003).After reaction,we opened the frozen Carius tube at the narrow neck,added ultrapure water to 25 mL,and used 5 mL ultrapure water,the absorption solution,to separate and enrich the Os by direct distillation(Li et al.2010).After completing the distillation of Os,we turn the distilled residue in the Carius tube into a 150-mL Teflon beaker,and put it on the heating plate until nearly dry.We used acetone to extract Re in 10-mol/L NaOH alkaline medium(Li et al.2009).After acetone drying,we added 2 mL ultrapure water.Lastly,we used the X-series inductively coupled plasma–mass spectrometer(ICP-MS)to measure the isotopic ratio.Re and Os measurement accuracies were 0.1%and 0.5%,respectively.

3 Results

The Re–Os abundances and isotopic data for the Niutitang black shale are shown in Table 1.In this experiment,the blanks of Re and Os were-0.001 and 0.0002 ng/g,respectively—significantly lower than the Re and Os contentsin samples,which ranged from 21.27 to 312.78 ng/g and from 0.455 to 7.789 ng/g,respectively.The187Re/188Os ratio varied between 102.30 and 306.43,and the187Os/188Os ratio between 1.72 and 3.51.Thus,even samples with lower Re and Os contents were not affected by the blanks.Thus,the errors of187Re/188Os and187Os/188Os ratios mainly derived from mass spectrometer,ratio of diluent,balanced reaction between diluent and sample,and reproducibility of sample data.The corrected data were analyzed by isoplot,generating a Re–Os isochron(Ludwig 2001)with a decay constant of 1.666×10-11a-1for187Re(Smoliar et al.1996;Ludwig 1999,2001).The Jinchuan standard samples from the Re–Os Laboratory of National Research Center were used as the standards in this study.The measured Re and Os contents were 39 and 16.49 ng/g,respectively,and the ratios of187Re/188Os and187Os/188Os were 12 and 0.338,respectively,consistent with the recommended ratios.The Re–Os isotope age of the black shale samples was 522.9±8.6 Ma with initial187Os/188Os of 0.826±0.026(MSWD=0.52).

Fig.1 a Geographic location of the Yangtze Platform;b paleogeographic map of the Yangtze Platform in the Early Cambrian(modified from Yeasmin et al.2016);c geologic sketch map of Nayong-Zhijing in Guizhou.Note the sampling location

4 Discussion

4.1 Re–Os isotopic dating of black shale

The Re–Os isotopic system can effectively reflect the depositional age of black shale,and is mainly affected by two factors.The first is the sources of Re and Os in black shale.During the E–C transition,the global OAE(Leggett 1980;Wu et al.1999,2000)caused a rapid reduction of seawater Os.With the upward migration of the redox boundary,Re and Os oxides were reduced and entered into the organic phase via adsorption of organic matter and/or sulfide generated by reaction with H2S.As a result,the Re–Os isotopic closed system was formed.Given the Re and Os contents of typical black shales in other ages(Fig.3),we suggest that the Re and Os in the black shale derived from seawater.

The second factor is diagenetic alteration.(Creaser et al.2002)suggested that the Re–Os isotope system in black shale could not be destroyed at temperatures as high as 650°C as indicated by the simulation experiment.Therefore,the influence of regional tectonic metamorphism and hydrothermal activity would be negligible on the Re–Os system.Jiang and Li(2010)used the thermoelectric coefficient method of pyrite to obtain a formation temperature of 100–240 °C from the coarse-grained sulfide(pyrite)of the Lower Cambrian black shale series in the Tian’eshan section of Zunyi,Guizhou.In addition,the formation temperatures of fracture- filled quartz and calcite crystals in the black shales measured by the homogenization temperature of fluid inclusions were 126–230 °C and 113–153 °C,respectively.In this light,the diagenetic temperature was significantly lower than the disclosure temperature limit of the Re–Os system,suggesting that the isotopic system of the Zhijin samples was well preserved with a good closure,so that the Re–Os isochron age can effectively reflect the depositional age of black shales.Despite the existence of errors of instrument and technique,we believe the samples have the same initial isotopic composition.

Fig.2 Ediacaran–Cambrian boundary succession in the Gezhongwu section,Zhijin.(Niutitang Formation:6.Dark gray to black carbonaceous mudstones intercalated with siltstone layers.～15 m thick.5.Dark gray to black mudstone/shale,containing siliceous–phosphatic concretions,and anomalously high concentrations of Mo,Ni,and V.～15 m thick.Conformity.Gezhongwu Formation:4.Dark purplish medium-bedded dolomitic bioclastic phosphorite,siliceous phosphorite,and phosphatic dolostone intercalated with liver-colored cellophane.12.8 m thick.3.Varicolored siliceous phosphorite with angular rubbles/breccias of dolostone,phosphorite,and chert.11.6 m thick.Unconformity:Dengying Formation:2.Maolongjing Member:light-gray to gray thin-bedded phosphatic micro-to fine-crystalline dolostone.2.6 m.1.Algae dolostone Member:light-gray to gray thin-to thick-bedded algal dolostone and laminated siliceous dolostone.3.2 m thick.Base not observed.)

Table 1 Re–Os abundances and isotope data

Fig.3 Re–Os content of typical black shale in the world.1:Re–Os contents in pyrite in black shale in South Africa(Hannah et al.2004);2:Re–Os contents in sedimentary rock of Mesoproterozoic in Taoudeni basin,Mauritania(Rooney et al.2010);3:Re–Os contents in black shale in Britain(Cohen et al.1999);4:Re–Os contents in metalliferous layer in Hunan and Guizhou Provinces(Fu et al.2016);5:Re–Os contents of siliceous shale in Hunan and Guizhou Provinces(Fu et al.2016);6:Re–Os contents of black shale in Hunan and Guizhou Provinces;7:Re–Os contents of Neoproterozoic black shale in the South of Australia and Northwest of Tasmania(Kendall et al.2009).Data are from Cohen et al.1999;Kendall et al.2009;Hannah et al.2004;Rooney et al.2010

Previous Re–Os studies mainly focused on the polymetalliferous layer in the Niutitang Formation.The polymetalliferous layers have much higher Re and Os contents than organic-rich rocks(with Re content up to ppm level,and Os content of several hundred ppb)(Li et al.2002;Mao et al.2002;Xu et al.2011).

In spite of relatively low Re and Os contents,black shale samples from the Gezhongwu section yield a good wholerock Re–Os isochron age of 522.9 ± 8.6 Ma for the basal Niutitang Formation (Fig.4a).Previous research has shown Re–Os isochron ages at the metalliferous layer in the basal Niutitang Formation in this area mainly concentrated in 542～521 Ma(Table 2).Because the isotopic age is mainly controlled by the two end-members on the isochrone,in order to further reduce the data error,this paper obtained an Re–Os isochron age(523 ± 6.5 Ma)of the metalliferous layer in the basal Niutitang Formation,which is consistent with the experimental results of Re–Os age(Fig.4b).A tuff layer that is widely observed below the poly-metalliferous layer has yielded zircon U–Pb ages of 532.3–522.3 Ma(Jiang et al.2009).The zircon U–Pb ages from a tuff bed in the Meishucun section in Yunnan Province,Bahuang,Panmen,and Taoying sections in Guizhou Province ranges from 526.5 to 522.3 Ma(Compston et al.2008;Wang et al.2012;Chen et al.2015a).At Songlin,Zunyi,an identical tuffaceous layer yielded two different zircon U–Pb ages at 532.3± 0.7 Ma(Xu et al.2011)and 518±5 Ma(Zhou et al.2008).Together with ages measured previously,the Re–Os isochron age obtained in this study(522.9±8.6 Ma)provides another useful constraint on the formation time of the Niutitang black shale in the Zhijin area.

4.2 Constraints on biological evolution

The Meishucun section in Yunnan Province used to be the standard candidate of the Precambrian-Cambrian boundary in South China(Qian and He 1996;Zhang et al.1997;Zhu et al.2003).However,it has been abandoned due to the presence of an unconformity between the Zhujiaqing and Dengying Formations.Even so,the Lower Cambrian succession at this section is complete,including,in ascending order,the Meishucun Stage(equivalent to the Fortunian Stage and Stage 2 of the global Terreneuvian Series)and Qiongzhusi Stage(equivalent to Stage 3 of the global Series 2).The Meishucun Stage consists of the Zhujiaqing and Qiongzhusi(or Helinpu)Formations.Of these,the former includes,in ascending order,the Daibu(DB),Zhongyicun(ZYC),and Dahai(DH)members;the latter comprises the Shiyantou and Yuanshan members(Fig.5).

Fig.4 a Re–Os isochron diagrams of black shale from the Niutitang Formation at Gezhongwu section in Zhijin;b Re–Os isochron diagrams of polymetallic sulfide bed from the Niutitang Formation in Guizhou and Hunan Provinces

Table 2 Re and Os isotope data of black shale in the Niutitang Formation in the Yangtze Platform

There are four biostratigraphic marker levels(Fig.5):Marker A,at the base of Daibu Member,represents the first appearance of the SSFs above which theAnabarites trisulcatus–Protohertzinaanabaricsassemblage (zone SSF1)appears in bed 3 of the basal Zhongyicun Member.Marker B,representative of SSF flourishing,is marked by the occurrence ofParagloborilus subglobsosus–Purella squamulosaassemblage(zone SSF2)in the top part of Zhongyicun Member,and by a volcanic ash layer(bentonite)(bed 5).Marker B was formerly used as the lower boundary of the Cambrian in China,but after 2004,the International Commission on Stratigraphy accepted the new division standard of four series and ten stages in the Cambrian,moving the basal boundary of the Cambrian downward(Peng 2006).Marker C is at the boundary between the Dahai and Shiyantou members,and represents a sedimentary environmental change from shallow water carbonate platform to organic-rich fine-grained siliciclastic(muddy)shelf.A volcanic tuff-originated bentonite layer around Marker C(Zhang 1997)yielded a zircon U–Pb age of 526.5±1.1 Ma(Compston et al.2008).Marker D,2.4 m above the base of the Yuanshan Member,marks the appearance of the earliest trilobites.A large number of fossils of Chengjiang Biota were found in the siltstone and mudstone in the middle part of the Yu’anshan Member or above Marker D(Chen et al.1994,1999;Shu et al.1999;Steiner et al.2001).The black shale at the base of the Yuanshan Member is rich in metals Cr,Ni,Mo,and V(Zhu et al.2003).

Fig.5 Stratigraphic correlation of the E–C boundary successions between shallow-and deep-water settings in the Yangtze platform constrained by both U–Pb isotope ages and Re–Os isotope age data.(Meishucun section data from Compston et al.2008;Songlin section data from Xu et al.2011(Re–Os)and Jiang et al.2009(U–Pb);Bahuang and Panmen data from Chen et al.2015a;Taoying section data from Wang et al.2012).BYS Baiyanshao,XWTS Xiaowaitoushan;ZYC Zhongyicun;DH Dahai;SYT Shiyantou;YAS Yuanshan;LCP Liuchapo;DY Dengying;A:SSF appearance;B:SSF biodiversity;C:sedimentary facies change point;SSF1:Anabarites trisulcatus–Protohertzina anabarics assemblage zone;SSF2:Paragloborilus subglobsosus–Purella squamulosa assemblage zone;SSF3:Watsonella crosbyi assemblage zone

The division of Sinian-Cambrian boundary strata is based on the type-section Meishucun,which corresponds with the Gezhongwu section in Zhijin.The two sections share a common fossil assemblage,SSF1:Anabarites-Conotheca–Protohertzinaassemblage zone.However,due to environmental differences,different faunal members may exist in the same assemblage,i.e.,in SSF2.It has been suggested that theSiphogonuchites-Sachites–Lapworthellaassemblage zone at Gezhongwu is comparable to theParagloborilus subglobsosus–Purella squamulosaassemblage zone in the Zhongyicun Member of the Yuhucun Formation in the Meishucun.In addition,few fossils occur in the lower Niutitang Formation at Gezhongwu,so it is debatable to some degree whether it correlates with the Heilinpu Formation of the Qiongzhusi Stage in the Meishucun section.Our Re–Os isochron age of the black shale at the base of Niutitang Formation(522.9±8.6 Ma)corresponds(within the error range)with the zircon U–Pb age(526.5±1.1 Ma;Compston et al.2008)at the base of the Shiyantou Member of the Helingpu Formation.

4.3 Constraints on oceanic anoxic events

Fig.6 Mo isotope and δCe variations in the Early Cambrian.Molybdenum isotope composition of modern seawaters derived from Kendall et al.(2009)and Xu et al.(2012).Colored circles represent different data sources(Lehmann et al.2007;Xu and Lehmann 2010;Wen et al.2011;Xu et al.2012;Chen et al.2015b;Mao 2015;Gao et al.2016),similar to rectangle patterns

In South China,OAEs occurred from the Neoproterozoic to the Cambrian with a great thickness of black shales deposited.The changes in δ98/95Mo and δCe may reflect the occurrence of OAE(Fig.6).Based on research by Wen et al.(2011),the maximum Mo isotopic ratio of euxinic sedimentary rock may approximately represent the lower limit of seawater Mo isotopic ratio,and the Mo isotopic composition of seawater is affected by the oxic sediments in the sea floor,which are dominantly controlled by atmospheric oxygen and the redox conditions of the ocean(Anbar and Rouxel 2007).In addition,the cerium anomaly(δCe)can also be used as a tracer of paleo-environment(Wright et al.1987).Generally,δCe＜-0.1 reflects oxic aqueous conditions,while δCe＞-0.1 reflects a reducing(or anoxic)condition(Wright et al.1987).Through compiling δ98/95Mo and δCe datasets of phosphorite in the Gezhongwu Formation of the Meishucun Stage and the black shale in the Niutitang Formation of the Qiongzhusi Stage in the Lower Cambrian(Fig.6),we found that the maximum δ98/95Mo value of phosphorite approached the modern seawater Mo isotope value,but the maximum δ98/95Mo values at the base of overlying black shales showed a negative excursion and subsequent rapid increase.In addition, δCe values(＜-0.1)in the phosphorites of the Gezhongwu Formation indicate an oxic environment.In contrast,the δCe values(＞-0.1)in black shales of the Niutitang Formation suggest a reducing environment,constrained at 522.9 ± 8.6 Ma by Re–Os isochron dating.

5 Possible implication of the initial Os isotopic ratio

Through geologic time,silicate weathering could have influenced atmospheric CO2concentration, thereby changing the climate.Currently,the Sr isotope composition of seawater is a widely available tracer of riverine flux resulting from continental weathering relative to the hydrothermal flux into the ocean.The increase of87Sr/86Sr could have resulted from terrestrial weathering and/or enhanced orogeny,but there is generally a lack of widespread diagenetic-resistant materials available for Sr isotope analysis(Blum and Erel 1995;Derry and France-Lanord 1996;Quade and Ojha 1997).Os isotopic composition of seawater provides an alternative signature to trace the continental flux relative to hydrothermal flux,particularly for organic-rich sediments(Pegram et al.1992;Ravizza 1993;Peucker-Ehrenbrink et al.1995;Oxburgh 1998;Reusch et al.1998;Pegram and Turekian 1999).

In general,the Os isotopic ratio is controlled by the redox conditions of the medium and source of Os.Three main sources of Os in seawater include:(1)riverine input with a187Os/188Os ratio of 0.3–1.54;(2)mid-ocean ridge hydrothermal fluid with a187Os/188Os ratio of about 0.12,and with a non-radioactive characteristic(Sharma et al.2000);and (3)cosmic dust,having a comparable187Os/188Os ratio to the mantle(Yang et al.2005).The annual extraterrestrial Os supply is generally less than 17 kg(Levasseur et al.1999),accounting for quite a small part of the equivalent terrigenous supply.So the extrater-restrial input is not a significant source of Os in the ocean.In fact,cosmic dust could even be adsorbed in the submarine Fe–Mn crust,decreasing the Os isotopic ratio of the Fe–Mn crust(Peucker-Ehrenbrink 1996;Burton et al.1999;Sun et al.2006).The modern ocean yields a187Os/188Os ratio of 1.05–1.06,indicating mixing of different materials in seawater(Levasseur et al.1999).

Fig.7 Initial187Os/188Os ratios of black shale in Early Cambrian,South China.1)Ni–Mo metalliferous deposits in Zunyi and Zhangjiajie area(Li et al.2002);2)Ni–Mo metalliferous deposits in Zunyi(Mao et al.2002);3)metalliferous deposits in Dayong in Hunan Province(Jiang et al.2003);4)black shale under metalliferous deposits in Nayong(Fu et al.2016);5)metalliferous deposits in Hunan and Guizhou Province(Fu et al.2016);6)this study;7)Hunan and Guizhou Provinces(Xu et al.2011)

In this study,the initial187Os/188Os ratio from the basal black shale of the Niutitang Formation in the Zhijin area was 0.826±0.026(Table 1;Fig.7),falling within the range of 0.78–0.88 reported by other researchers(Li et al.2002;Mao et al.2002;Jiang et al.2007;Xu et al.2011;Fu et al.2016),but slightly lower than that of a continental source.This indicates that the shale was deposited in the shelf sea affected strongly by continental weathering flux and somewhat by hydrothermal flux during the Early Cambrian(Qiongzhusi epoch),resulting in the moderately high radioactive Os in the seawater.

During the Qiongzhusi epoch,owing to huge amounts of greenhouse gas released by sea floor hydrothermal venting or volcanic activity,atmospheric CO2may have increased rapidly,causing climate warming.On the other hand,the warm climate may have facilitated a large-scale sea-level rise,leading to the widespread transgression and marine anoxia under which organic-rich sediments could have been deposited as seen the widespread(basin-scale)Niutitang black shales over the antecedent Yangtze carbonate platform.

6 Conclusion

This study carried out Re–Os isotopic dating for the black shale in the Niutitang Formation at Gezhongwu,Guizhou and yielded an isochron age of 522.9±8.6 Ma,providing a radiometric time constraint on the onset of the Niutitang Formation and further facilitating stratigraphic correlation with the type section of E–C boundary strata in eastern Yunnan and elsewhere.

The initial Os isotope ratio of seawater during deposition of the Niutitang black shale was determined to be 0.826±0.026,suggesting moderately strong continental weathering flux and likely submarine hydrothermal release into the seawater.

AcknowledgementsThis research was supported by the China Geological Survey(DD20160346),the National Natural Science Foundation of China(41472089)and the Guizhou Geological Prospecting Fund(2016091).We would like to thank Prof.Daizhao Chen for his valuable reviews,the anonymous reviewers and the editor for their constructive comments.

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