Marine C, S and N Biogeochemical Processes in the Redox-Stratified Early Cambrian Yangtze Oceanc

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research-article10.1144/jgs2014-054Marine C, S and N biogeochemical processes in the redox-stratified early Cambrian Yangtze oceanC. Cai, L. Xiang, Y. Yuan, X. He, X. Chu, Y. Chen &, C. XuXXX10.1144/jgs2014-054C. Cai et al.Biogeochemical processes in Early Cambrian 20152014-054

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Research article Journal of the Geological Society Published online April 8, 2015 doi:10.1144/jgs2014-054 | Vol. 172 | 2015 | pp. 390–406

Marine C, S and N biogeochemical processes in the redox-stratified early Cambrian Yangtze ocean

C. Cai1*, L. Xiang2, Y. Yuan1, X. He3, X. Chu1, Y. Chen1 & C. Xu1 1 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2 State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Nanjing 210008, China 3 Hangzhou Research Institute of Geology, PetroChina, Hangzhou 310023, China * Correspondence: [email protected]

Abstract: The temporal change of redox conditions of the Yangtze ocean has been revealed by investigating the Ediacaran– Cambrian transition section at Zhalagou, South China. During the earliest Cambrian, cherts and shales were deposited under an anoxic and ferruginous bottom water with significantly increasing total organic carbon and P contents, and negative shift in kerogen δ13C values in the lowest part of the section. Euxinic bottom water conditions occurred during the earliest Cam-

brian Stage 2, with the surface water dominated by N2 utilization by cyanobacteria or sulphur bacteria leading to negative kerogen δ15N values. During Stage 3, dissolved oxygen and sulphate concentrations were significantly increased, and thus the oxidized surface water and the redox transition zone overlying a euxinic bottom water may have been expanded, resulting in an increase in kerogen δ15N increasing to 2–4‰, a decrease in pyrite δ34S decreasing to as low as –24.6‰ and differences in δ34S values between kerogen and pyrite as high as 37‰. This period coincided with the abrupt appearance of large-body metazoans. Thus, the expanding oxic surface water may have reinforced the evolution of animals or vice versa. Interestingly, kerogen δ34S values show negative relationships to FePy/FeHR ratios and pyrite sulphur contents, indicating that they can be used to reflect redox conditions, with the lightest values being obtained from euxinic environments. Received 23 May 2014; revised 12 December 2014; accepted 13 December 2014

Dramatic biota evolution occurred during the Early Cambrian with anoxic and sulphidic conditions during Stage 2 in the inner shelf the disappearance of late Ediacaran soft-bodied biota and the and slope environments as recorded in the Shatan and Songtao sec- occurrence of the Cambrian explosion. During the Early Cam- tions (Goldberg et al. 2007; Guo et al. 2007; Xu et al. 2012). This brian, small shell fossils, sponges and arthropods, and trilobites sulphidic water column did not extend to the deeper lower slope and other metazoan fossils appeared, and the abundance and diver- and basin environment as found in the Tianping and Lijiatuo sec- sity of fossils were then rapidly increased. Redox conditions for tions (Cai et al. 2012; Wang et al. 2012a). the Early Cambrian Yangtze ocean are crucial to understand these However, Fe speciation measurements do not focus on the changes. Oxygen has been considered as the most likely extrinsic surface water, where the N cycle is most active. Consequently, the factor for the evolution of large, metabolically active animals N cycle in ocean chemistry must be evaluated directly (Busigny (Knoll & Carroll 1999); thus increasing oxidization of deep ocean et al. 2013; Godfrey et al. 2013). In sedimentary rocks, nitrogen is + has been considered to lead to the evolution of large-body metazo- mostly preserved as organic nitrogen and as fixed NH4 substitut- ans (Wang et al. 2012a) although the oxidization may not neces- ing for K+ in phyllosilicates. Nitrogen isotopes can provide a sarily result from an increase in atmospheric oxygen (Lenton et al. record of specific biosignatures and are sensitive to environmental 2014). redox changes during Earth history (Beaumont & Robert 1999; The global C, S, Fe, N and P cycles are intimately linked Godfrey & Falkowski 2009; Busigny et al. 2013; Cremonese et al. through biotic and abiotic processes, which are controlled by 2013, 2014; Godfrey et al. 2013; Thomazo & Papineau 2013). The marine redox and finally by atmospheric oxygen concentration nitrogen cycle for the late Archaean ocean has been proposed to

(Berner 1989; Algeo & Ingall 2007). Carbon, S, Fe and N specia- include N2 fixation, denitrification, nitrification and anammox and + tion, and chemical and isotopic compositions have been shown to NH4 assimilation into organic matter (Fig. 1). It is considered that have great potential for palaeoenvironmental and biogeochemical the ‘normal’ or oxygenated marine sedimentary N has a δ15N range reconstructions (Canfield et al. 1986, 2008; Canfield & Teske from +2 to +6‰, and is a product of equilibrium between nitrate

1996; Poulton & Canfield 2005; Godfrey & Falkowski 2009; assimilation, N2 fixation and denitrification. Under anoxic condi- Li et al. 2010; Godfrey et al. 2013). tions, nitrate is limited and thus dissolved atmospheric N2 is uti- Based on Fe speciation and Mo isotopic composition, the slope lized by diazotrophs in the water column; sedimentary organic to basin environment below storm wave base in south China dur- matter is expected to have an isotopic value averaging zero ing the earliest Cambrian (Fortunian) was considered as sulphidic (Beaumont & Robert 1999; Kuypers et al. 2004). The δ15N values (Canfield et al. 2008; Wille et al. 2008). However, the strata have can be negative in euxinic conditions (Cremonese et al. 2013)

been considered to date to about 20 Ma after the Ediacaran– where chalcophiles such as Mo and Fe, which are essential to N2 Cambrian transition (Jiang et al. 2009). More recently, the earliest fixation (Anbar & Knoll 2002), may have been precipitated as sul- Cambrian lower slope to basin environment has been accepted as a phides. However, positive δ15N signatures have been reported redox stratified water column with a thin oxygenated (oxic or dys- from the Late Archaean ferruginous ocean (Busigny et al. 2013) oxic) upper layer overlying an anoxic and ferruginous lower layer and sulphidic Mesoproterozoic ocean in the Animikie Basin as supported by several geochemical proxies (Cai et al. 2012; (Godfrey et al., 2013), as well as fully oxic ocean (Cremonese Wang et al. 2012a). The underlying water chemistry changed to et al. 2013; Busigny et al. 2013), and thus may record very

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phosphate from an anoxic basin has positive feedbacks on marine productivity (Algeo & Ingall 2007). The increase in primary productivity is expected to result in a decrease in selectivity of 12C and 13C, and thus to change the organic matter δ13C value. In the present study, we performed a series of analyses on Si, P and Ba elements, Fe speciation, S isotopes of pyrites, and C, S and N isotopes of organic matter (or kerogen) from cherts, shales and mudstones from the Lower Cambrian in the Zhalagou section, South China. The aims of this study were (1) to determine redox conditions of water in the palaeohigh around a deep basin during the Early Cambrian using multi-proxies, (2) to understand the nitrogen cycle in oxygenated surface water overlying anoxic and sulphidic or ferruginous water, and thus (3) to show whether δ15N values can be used to explain the N cycle without independent constraints from other proxies; also, (4) to determine what controlled total organic carbon (TOC) or organic productivity, (5) to describe sulphur occurrence and show whether kerogen δ34S is related to redox conditions, and (6) to discuss the possible link of evolution Fig. 1. The biogeochemical nitrogen cycle (Godfrey & Falkowski of redox conditions to the Cambrian explosion. 2009). Geological setting different N biogeochemical cycles. For example, no extreme N limitation has been proposed to explain positive and stable The Zhalagou section is located about 5 km east of Sandu County δ15N values (c. 5.3‰ at inner shelf and c. 3.9‰ at outer shelf sites) in Guizhou Province, South China (Fig. 2). The Yangtze Block for sediments deposited under sulphidic conditions in the evolved from a rift basin to a passive continental margin basin dur- Mesoproterozoic redox-stratified water column in the Animikie ing the Ediacaran–Cambrian transition (Wang et al. 2003). Basin (Busigny et al. 2013). Thus, interpretation of the N isotopes Metazoans began to diversify and expand during the Early in terms of the N biogeochemical cycle requires independent con- Cambrian. The lowest part of the Cambrian was stratigraphically straints on the redox structure of the ocean using other proxies correlated using assemblages of small shelly fossils (Steiner et al. such as iron speciation and sulphur isotopes. 2007). Traditionally, the Cambrian System was divided into the Organic matter is expected to have δ15N values lower than bulk Early, Middle and Late Cambrian series. The Early Cambrian in – + – sediment if inorganic N (NO3 , NH4 , or NO2 ) is not fully con- the Yangtze Block was further subdivided into four stages: in sumed, or if N2 fixation is high relative to other forms of N assim- ascending order, the Meishucunian Stage, Qiongzhusian Stage, ilation. Thus, a roughly parallel changing trend was observed Canglangpuan Stage and Longwangmiaoan Stage (Yin 1996; (Kump et al. 2011). The δ15N values will be similar when produc- Xiang et al. 1999). Based on the small shelly fossils record, the tivity consumes all inorganic N (Godfrey & Falkowski 2009; Meishucunian Stage is subdivided into four biozones in shallow Godfrey et al. 2013). deposits and could be further correlated to the Early Cambrian Sulphur occurs mainly as sulphates, pyrite, organic sulphur and strata in other parts of the world. The basal boundary of the elemental sulphur in sedimentary basins. Reduced sulphur is Qiongzhusian Stage is characterized by the first appearance of tri- mainly derived from bacterial reduction of sulphate in a water lobites (Steiner et al. 2004, 2007). In contrast to a conventional body or shallow sediments. Bacteriogenic sulphide is then precipi- threefold subdivision, the Cambrian System has recently been sub- tated as FeS or/and FeS2, or incorporated into organic matter to divided into four series with 10 stages. The former Lower Cambrian form organically bound sulphur with a small portion from assimi- is subdivided into two series, the Terrenenvian and Series 2. The latory sulphate reduction (Werne et al. 2003, and references Terrenenvian corresponds roughly to the Meishucunian Stage in therein). Organic sulphur shows an increase in its content and has the Yangtze Block, and includes the Fortunian Stage and Stage 2, a greater contribution from bacteriogenic sulphide with increasing or the former Nemakit–Daldynian Stage and Tommotian Stage in sediment depths, resulting in a negative shift in δ34S values as Siberia. The base of the Qiongzhusian Stage (or Yu’anshan Fm) found in many cases such as in Soap Lake, USA (Tuttle et al. corresponds to the base of Stage 3 of Series 2, or the former 1990) and Solar Lake, Sinai, Epypt (Aizenshtat & Amrani 2004). Atdabanian in Siberia (Babcock & Peng 2007; Kouchinsky et al. However, in a Scottish fjord, the δ34S of the sedimentary organic 2012). fraction was found to shift towards lower, more bacteriogenic, val- The Yangtze Block was deposited in environments from shal- ues with depth in the profile, without any increase in the size of low-water carbonate and phosphorite platform to slope and deep this sulphur pool. Thus, the organic sulphur fraction in this case basin during the Fortunian Stage (or Early–Middle Meishucunian) was concluded to be open to interaction with bacteriogenic sul- (Zhu et al. 2003). As a result of transgression at the end of the phide without the occurrence of net addition (Bottrell et al. 2009). Fortunian Stage, widespread inner shelf platform carbonate rocks Organic sulphur has δ34S values generally heavier than the asso- gave way to mudstones and shales during the Stage 2 (Goldberg ciated pyrite. 32S-rich reduced sulphur was considered to preferen- et al. 2007; Jiang et al. 2012). tially precipitate as pyrite and the residual 34S-rich sulphur to be The Zhalagou section consists of, in ascending stratigraphic incorporated into organic matter after reactive Fe was consumed order, the Ediacaran Dengying Fm, Lower Cambrian Laobao Fm, (Tuttle et al. 1990; Tuttle & Goldhaber 1993; Aizenshtat & Amrani Zhalagou Fm and Middle Cambrian Duliujiang Fm. The Dengying 2004; Cai et al. 2005). However, the balance of evidence now indi- Fm is composed of micrite dolomites, capped by a karstic surface, cates that both the Fe and organic matter pools have a spectrum of and shows a discontinuous contact with the overlying Laobao Fm. reactivity toward sulphide, so the timing of the two processes over- The Laobao Fm is 5 m thick, and consists mainly of thin cherts laps significantly (Bottrell et al. 2009). beds intercalated with shales with thin phosphorite-bearing hori- P is a redox-sensitive element and its cycle is closely linked to zons on the basal parts. The cherts are rich in organic matter, bar- the production of molecular oxygen, and the remineralization of ium and redox-sensitive trace elements. The Laobao Fm is covered Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 392 C. Cai et al.

Fig. 2. (a, b) Location map of the Zhalagou sections in Guizhou Province, South China; (c) geological sketch map of theZhalagou section. conformably by the overlying Zhalagou Fm. The Zhalagou Fm is A third-order sequence, however, is not discernible owing to the 85 m thick, and is composed of black, medium- to thin-bedded silt- nature of the condensed sediments. bearing mudstone, shale and carbonaceous mudstones with horizontal lamination. Shales and mudstones in the lower parts of the Sampling and methods formation contain abnormally high organic matter and are locally called stone coal layers. Slumps developed in the upper part with Forty-eight black shales, cherts and mudstones were sampled from the occurrence of incomplete sponge fossils. In the top 15 m of the the Zhalagou section, with 10 from the Laobao Fm at smaller sam- Zhalagou Fm, abundant sponge spicules were found in limestone pling intervals, 37 from the Zhalagou Fm and one from the Middle lenses and mudstone in this study and by Yang et al. (2010). The Cambrian Duliujiang Fm (Figs 2 and 3). These samples were selected Zhalagou Fm is overlain conformably by the Duliujiang Fm lime- without considering TOC and lithology, but less weathered samples stone and yellow–green mudstone and siltstone. were preferentially chosen. Rock blocks of 1.5–2 kg were sampled During the latest Ediacaran, the Zhalagou section was deposited and subsequently trimmed to remove the weathered surface, and pul- in a shallow-water environment, as indicated by the Dengying Fm verized (200 mesh, <74 μm in size) in an agate shatter box. micrite dolomites and the occurrence of the exposed surface at the Concentrations of total sulphur (TS) and total organic carbon top. The Zhalagou section has been defined as a palaeo-uplift in (TOC) were measured with a LECO CS-200C030 system at the southeastern Guizhou during this period by Steiner et al. (2001), Experimental Research Center of the Wuxi Research Institute of Zhu et al. (2007) and Jiang et al. (2012). This uplift may have Petroleum Geology of SINOPEC. Analytical errors were ≤±0.1%. developed during the deposition of the upper part of the Doushantuo Elements Si, P and Ba were analysed by X-ray fluorescence (XRF). Fm and/or the lower part of the Dengying Fm (Zhu et al. 2007). It The ocean redox conditions were determined by the Fe specia- may have been part of the Jiangnan arc system and acted as the tion. The highly reactive Fe (FeHR) is broadly apportioned into base of subsequent platform carbonate rocks (Steiner et al. 2001). four pools: carbonate Fe (Fecarb), oxide Fe (Feox), magnetite Fe As the result of the transgression during the earliest Cambrian, the (Femag) and pyrite Fe (FePy). The sum of these pools represents uplift was gradually drowned and subsequently formed an isolated the total concentration of highly reactive Fe (FeHR = Fecarb + palaeotopographic high (Fig. 3). The palaeohigh may have water Feox + Femag + FePy). The content of total iron, carbonate-asso- depths below the storm wave base and thus weak hydrodynamic ciated iron phases (Fecarb), iron oxides (Feox) and magnetite- conditions, as indicated by (1) the distribution of horizontal lami- associated iron phases (Femag) were analysed following the nation and fine-grained sediments in the Zhalagou Fm and (2) the procedure of Poulton & Canfield (2005). domination of adjacent areas by a deep-water environment (Zhu Pyrite iron (FePy) was extracted following the chromium reduc- et al. 2003; Chen et al. 2009; Jiang et al. 2012). The entire tion method (Canfield et al. 1986). The Ag2S precipitate was Early Cambrian sequence of the Zhalagou section has been pro- weighed to determine the content of pyrite sulphur. The S-isotope posed by Mei et al. (2006) to represent a second-order sequence. of Ag2S was analysed at the Institute of Geology and Geophysics, Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 Biogeochemical processes in Early Cambrian 393

Fig. 3. Simplified palaeogeographical map of the Yangtze Platform during the Fortunian; modified from Steiner et al. (2001), Zhu et al. (2003, 2007), Goldberg et al. (2007), Chen et al. (2009) and Jiang et al. (2012). Locations 1–14 mark the sections mentioned in this study: 1, Shatan (Goldberg et al. 2007; Guo et al. 2007); 2, Three Gorges (Jiang et al. 2012); 3, Ganzipin (Chen et al. 2009); 4, Songtao (Goldberg et al. 2007; Canfield et al. 2008); 5, Longbizui (Wang et al. 2012a); 6, Tianzhu (Wang & Li. 1991); 7, Silikou (Chang et al. 2010, 2012); 8, Zhalagou (this study); 9, Zhongnancun (Mao et al. 2002); 10, Gezhongwu (Wen et al. 2011); 11, Laolin (Li et al. 2009); 12, Xiaotan (Zhou et al. 1997; Och et al. 2013); 13, Meishucun (Shen et al. 1998; Wen et al. 2011); 14, Nangao (Yang et al. 2007).

Chinese Academy of Sciences (IGGCAS) on a Finnigan Delta S give the total residual kerogen sulphur. Dissolved iron was measured gas source mass spectrometer. Sulphur isotope results are gener- at pH < 2, using atomic absorption spectrometry, to determine the ally reproducible within ±0.3‰. maximal residual pyrite content in the kerogen after the chromium About 500 g ground samples were treated with hot 6N HCl to reduction (assuming that all Fe occurs as pyrite in the kerogen). further dissolve carbonate minerals. A mixture of 6N HCl and 40% Organic sulphur in the kerogen was subsequently calculated by sub- HF, and then 6N HCl were added to the samples after HCl treat- traction of the pyrite sulphur from the total residual kerogen sulphur. ment. After dilution by distilled water and centrifugation, the remaining kerogen was separated from the residue (precipitate) Results using heavy liquids (KBr + ZnBr) with densities of 2.0–2.1 g cm−3. Organic C, S and P concentrations Organic carbon isotopes were analysed on the kerogen samples. The kerogen and CuO wire were added to a quartz tube, and com- SEM observation shows that the samples contain chert, barite, busted at 500 °C for 1 h and then 850 °C for 3 h. Isotopic ratios pyrite, phosphorite and clay minerals. This result is supported by were analysed on a Finnigan MAT-253 mass spectrometer at chemical compositions of bulk-rocks (unpublished data). IGGCAS, and expressed in standard delta notation relative to The TOC content ranges from 0.24 to 12.84% (n = 48) with sig- Vienna Peedee Belemnite (VPDB) standard with an analytical nificantly higher values in the shale interlayered with chert inter- error of ≤0.06‰. vals in the Laobao Fm and the basal parts of the Zhalagou Fm Nitrogen isotopic analyses on the kerogen samples were con- (Table 1). The TS abundance of the whole studied intervals ranges ducted using a modified elemental analysis-isotope ratio mass from 0.11 to 5.4%, averaging 2.1% (n = 48) with no significant dif- spectrometry (EA-IRMS) system (CEEA1112 C/N/S Analyser ference in the Laobao Fm and the overlying Zhalagou Fm. There interfaced with a DELTA plus XL mass spectrometer) at are positive correlations between TOC and TS for the Laobao Fm 2 Guangzhou Institute of Geochemistry, Chinese Academy of with R = 0.75, but not for the Zhalagou Fm (Fig. 4a) and positive Sciences (Li & Jia 2011). The measurement results were corrected correlation between TOC and total organic sulphur content for all samples analysed with R2 = 0.92 (Fig. 4b). for the procedural N2 blank and normalized using three-point cali- bration. The analytical error for δ15N analyses is ±0.5‰. Organic C/P molar ratios are in the range of 6–1026 (n = 10, The method for analysis of sulphur isotopes of kerogen was Table 1) for the Laobao Fm with an average of 300, and 65–420 reported by Cai et al. (2009). Pyrite was removed from the kero- (n = 15) with an average of 233 for the Zhalagou Fm. The ranges are close to that of 46–533 from European Middle and Upper gen by adding a mixture of hot 6N HCl and CrCl2 to the ground dry Cambrian organic matter-rich marine sediments (Algeo & Ingall kerogen under a nitrogen cushion with gas flow carrying the H2S 2007, and references therein). Average values are in the range of to a trap where it was recovered as Ag2S. Excess acids and acid- soluble salts were removed from the residual kerogen by water- 117–144 for samples from Newfoundland and New Brunswick washing. About 2 h later, the residual kerogen was collected and and 387–444 for samples from Sweden and Norway. There reground to expose new pyrite surfaces, and the whole procedure is a positive logarithmic correlation between P2O5 and TOC 2 was repeated once more. with R = 0.69, and high P2O5 and TOC in the basal Laobao Fm Aliquots of 0.5–1.5 mg of dry kerogen were taken to analyse for (Fig. 4c). the H, N and C contents by EURO3000 with an analytical precision of ±0.5%. A given weight (between 350 and 900 mg) of kerogen Iron speciation after the treatment was combusted in a Parr bomb apparatus at c. In the whole section, all FeHR/FeT ratios are >0.38 (Table 2). 25 atm oxygen to oxidize organically bound sulphides to sulphate. The ratios are <0.7 in the Laobao Fm and lower Zhalagou Fm, Dissolved sulphate was then precipitated as BaSO4 and weighed to and >0.7 in the basal (Ni and Mo sulphides bed) and the upper Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 394 C. Cai et al.

Table 1. Contents of total organic carbon (TOC), different sulphur species, Si, P and Ba, and organic matter δ13C values

Sample TOC TS TOC/Spy δ13C SiO P O C /P Ba (ppm) S Spy TOS S /TS Spy/TS TOS/TS org 2 2 5 org BaSO4 BaSO4 no. (%) (%) (‰) (%) (%) (%) (%) (%)

ZN48 0.24 2.99 0.3 −31.5 ZN47 0.77 1.21 0.6 −30.2 70.6 0.07 65 1536 0.04 1.16 0.01 0.03 0.96 0.01 ZN46 1.82 2.00 1.2 −30.7 67.8 0.03 359 1779 ZN45 2.23 3.48 1.3 −30.1 63.9 0.10 132 3285 ZN44 1.88 3.01 0.7 −31.7 63.6 0.08 139 3455 ZN43 2.91 2.76 1.5 −30.7 62.1 0.08 215 3266 0.08 2.64 0.05 0.03 0.96 0.02 ZN42 3.29 2.85 2.0 −31.3 61.8 0.05 389 3440 0.08 2.72 0.05 0.03 0.95 0.02 ZN41 2.13 2.90 0.9 −31.9 62.7 0.09 140 3220 0.08 2.79 0.04 0.03 0.96 0.01 ZN40 2.20 2.42 1.0 −31.0 63.0 0.05 260 2809 ZN39 2.26 3.22 1.2 −32.2 61.7 0.07 191 2806 ZN38 1.13 2.81 0.8 −30.3 65.6 0.03 223 2888 ZN37 1.14 0.99 1.0 −30.9 71.1 0.04 169 6811 0.15 0.82 0.02 0.15 0.83 0.02 ZN36 2.13 2.62 1.2 −31.1 60.6 0.03 420 2530 0.06 2.52 0.04 0.02 0.96 0.01 ZN35 1.71 0.65 5.0 −32.4 ZN34 6.86 0.29 103.2 −33.2 ZN33 6.77 0.25 73.5 −33.8 ZN32 7.44 0.11 147.1 −33.3 ZN31 7.57 0.56 111.8 −33.3 ZN30 6.95 1.73 7.4 −34.2 ZN29 6.33 2.66 2.6 −34.1 ZN28 9.57 1.15 30.8 −34.0 ZN27 5.92 1.72 3.7 −34.2 ZN26 9.10 1.86 6.1 −34.1 ZN25 8.59 2.39 7.0 −34.1 ZN24 4.32 1.90 2.4 −34.4 ZN23 6.03 2.02 3.1 −33.7 ZN22 2.21 2.10 1.1 −34.2 ZN21 1.70 0.61 4.4 −34.3 ZN20 5.77 1.92 3.2 −34.1 ZN19 4.68 1.69 3.5 −34.1 ZN18 6.86 3.01 2.3 −33.7 ZN17 7.68 1.82 4.5 −33.5 ZN16 2.49 1.84 4.7 −33.2 ZN15 7.60 1.97 4.0 −33.3 ZN14 7.59 1.59 8.1 −33.2 ZN13 7.25 1.40 5.9 −34.5 ZN12 6.81 3.48 3.5 −35.6 61.1 0.17 237 9584 0.22 2.73 0.53 0.06 0.78 0.15 ZN11 10.21 3.62 6.4 −35.2 61.1 0.18 336 4212 0.10 2.64 0.87 0.03 0.73 0.24 ZN10 7.06 2.92 12.9 −35.2 65.7 0.18 232 79139 2.00 0.63 0.29 0.68 0.22 0.10 ZN09 2.31 0.66 15.2 −35.8 80.2 0.02 683 17510 0.47 0.01 0.20 0.71 0.02 0.30 ZN08 3.28 3.59 3.9 −36.0 63.7 0.20 97 42014 2.75 0.66 0.18 0.77 0.18 0.05 ZN07 3.47 0.47 21.3 −36.2 89.6 0.02 1027 14004 0.35 0.01 0.27 0.74 0.02 0.58 ZN06 11.13 4.57 10.9 −35.3 40.7 2.4 27.4 114125 2.63 1.18 0.76 0.58 0.26 0.17 ZN05 2.33 1.34 4.2 −35.7 88.9 0.02 689 31881 0.35 0.82 0.18 0.26 0.61 0.13 ZN04 3.36 1.51 9.0 −35.8 82.4 0.19 105 39254 0.98 0.31 0.21 0.65 0.21 0.14 ZN03 12.84 5.40 12.4 −35.2 31.9 4.97 15 39713 3.25 1.25 0.90 0.60 0.23 0.17 ZN02 2.30 0.61 10.5 −35.1 86.0 0.11 124 14273 0.34 0.09 0.18 0.56 0.14 0.30 ZN01 8.34 2.68 15.2 −33.4 43.4 8.90 6 20620 0.48 1.59 0.61 0.18 0.59 0.23

Spy, pyrite S; SBaSO4, S present asBaSO4; Corg, organic carbon; Corg/P in molar ratio; TS, total sulphur in a rock; TOS, total organic sulphur in a rock.

13 part of Zhalagou Fm, and show dramatic change between the δ Ckero values and TOC for the Zhalagou Fm and for the Laobao Laobao and Zhalagou Fms, and samples 35 and 36 (Fig. 5). Fm, respectively (Fig. 6). All the samples with TOC > 6% have 13 13 δ Ckero values lighter than –33‰; however, similarly light δ Ckero 13 values are found to occur in part of the samples with TOC ranging Organic matter δ Ckero variation and its relationship to TOC from 2 to 4%. As shown in Figure 6, the δ13C values of the Early Cambrian kero Sulphur species, pyrite δ34S and organic matter samples show lower values (–36.2 to –33.4‰) than those (–31.3 to py δ34S values –32.0‰) of the Ediacaran ones. A negative shift of 2.8‰ in kero 13 δ Ckero occurred at the beginning of the Laobao Fm. A positive Main sulphur species include sulphur present as pyrite, barite and excursion of 3.0‰ appeared at the transition between the upper organically bound sulphur in kerogen. Positive correlations occur Laobao Fm and the basal part of the Zhalagou Fm. No significant between total sulphur (TS) and total Fe (FeT) for the Laobao Fm variation occurs in the lower Zhalagou Fm (samples 11–35; with correlation coefficient R2 of 0.43, but no correlation for the

Fig. 5). Roughly negative and positive relationships occur between Zhalagou Fm (Fig. 7a). Assuming that all Ba is present as BaSO4, Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 Biogeochemical processes in Early Cambrian 395

Pyrite S has contents of 0.01–1.59% in the Laobao Fm (Table 1), which are significantly lower than the main range of 0.8–2.8% in the Zhalagou Fm with four lower values <0.1% in samples ZN31 to ZN34 in the middle part of the section. Pyrite S accounts mainly for 2–26% of total sulphur with values up to 61% in the Laobao Fm, and increases to 73 and 78% in the basal Zhalagou Fm, and to 83–96% in the upper part (samples ZN36 to ZN47) (Fig. 5). Barite S shows contents of 0.3–3.4% with an average of 1.36% in the Zhalagou Fm, and decreases to 0.1–0.2% in the basal Zhalagou Fm, and to <0.003% in the upper Zhalagou Fm (Table 1). Barite S accounts for 24–77% of total sulphur in the Laobao Fm, and for 2.1–3.4% (n = 13) in the Zhalagou Fm with two abnor- mal data of 6.4% and 15.0%. In the Laobao Fm, in total sulphur, total organic sulphur (TOS)

contents [100% × (TS – SBaSO4 – Spy)/TS] range from 18 to 90% with an average of 35% (n = 10). TOS contents decrease from 53% and 87% in the basal Zhalagou Fm to 1–5% in the upper Zhalagou Fm. TOS accounts for 5–30% of total sulphur in the Laobao Fm. The values decrease from 15 and 24% in the basal Zhalagou Fm to 1–2% in the upper Zhalagou Fm. It has been found that Laobao Fm samples have higher barite S and organic sulphur, and lower pyrite sulphur than those of the Zhalagou Fm. 34 Pyrite shows a wide range of δ Spy values from –24.6 to +15.1‰ (n = 25) and increases from –1.8 to 1.2‰ in the basal Laobao Fm to 10.3‰ in the middle, and then decreases to –9.4‰ at the boundary between the Laobao Fm and Zhalagou Fm. The value rises dramatically to 10.9‰ in the basal Zhalagou Fm, drops to negative values, reaching the most negative value of –24.6‰ at the top of the Zhalagou Fm, and rises to 11.5‰ at the boundary of the Zhalagou Fm and overlying formation. This changing trend resembles the trend of FePy/FeT, with high vales (>0.7) corre- 34 sponding to negative δ Spy values (Fig. 5). Interestingly, there is a roughly negative relationship between 34 13 δ Spy and δ Ckero values for the Laobao Fm, but such a relationship does not occur for the Zhalagou Fm or for all the samples analysed (Fig. 8). 34 δ Skero values of organic matter show a narrow range from 9.0 34 to 18.3‰ (n = 24), most of which are heavier than δ Spy values 34 with differences up to 37.2‰. The δ Skero values show no correla- 34 tion with δ Spy (Table 3), and negative correlations with FePy/FeHR ratios and pyrite sulphur contents with correlation coefficient R2 of 0.50 and 0.57, respectively (Fig. 9a and b). A 34 negative relationship occurs between the δ Skero values and 34 34 2 (δ Skero − δ Spy) with R of 0.97 (Fig. 9c).

Organic nitrogen content and isotopic composition Kerogen shows lower H/C atomic ratios of 0.3–1.0 and C/N of 65–84 in the Laobao Fm and the basal Zhalagou Fm than in the upper Zhalagou Fm, for which the respective values are 1.0–1.9 13 and 79–107 (Table 3). Kerogen C/N ratio and δ Ckero value do not 15 Fig. 4. Relationship of TOC to total sulphur (TS), total organic sulphur show a correlation with δ Nkero (Fig. 10a and b). 15 (TOS) and P2O5. δ Nkero shows a rapid decrease from 4.0 to 6.9‰ in the lower Laobao Fm, through 1.4–2.3‰ in the upper Laobao Fm, to –1.4‰ and –0.4‰ in the basal Zhalagou Fm (Fig. 5). It remained relative total pyrite sulphur and organic sulphur is sulphur present as stable in the upper Zhalagou Fm (2.1–3.4‰), and fluctuates within

BaSO4 subtracted from total sulphur (TS – SBaSO4). The positive a small range at the top of the Zhalagou Fm. 2 relationships between FeT and (TS – SBaSO4), with R values of 0.73 for the Laobao Fm and of 0.91 for the Zhalagou Fm (Fig. 7b), Discussion indicate that most of FeT and (TS – S ) occurs as pyrite Fe and BaSO4 Regional stratigraphic correlation S, respectively. The best-fit lines for FeT vs (TS – SBaSO4) do not pass through the origin on the graph, indicating that part of the Biostratigraphic data such as small shelly fossil biozones are not sulphur may have been present probably as organically bound sul- available for this palaeohigh or the slope to basin environments, phur (TOS). thus other indicators such as a Ni–Mo–(PGE–Au) sulphides ore Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 396 C. Cai et al.

Table 2. Iron speciation data for the lower Cambrian from the Zhalagou section

Sample no. Lithology Height (m) FeT (%) Fecarb (%) Feox (%) Femag (%) FePy (%) FeHR (%) FeHR/FeT FePy/FeHR

ZN48 MudSt 91.2 5.59 0.22 0.29 0.04 0.72 1.26 0.23 0.57 ZN47 MudSt 89.8 2.21 0.41 0.51 0.03 1.11 2.06 0.93 0.54 ZN46 MudSt 86.5 2.32 0.20 0.24 0.01 1.68 2.13 0.92 0.79 ZN45 MudSt 82.2 3.10 0.22 0.13 0.00 1.53 1.88 0.61 0.81 ZN44 MudSt 78.0 3.08 0.22 0.16 0.00 2.45 2.83 0.92 0.87 ZN43 MudSt 74.2 2.83 0.28 0.40 0.01 1.68 2.37 0.84 0.71 ZN42 MudSt 69.5 2.96 0.27 0.36 0.00 1.44 2.07 0.70 0.70 ZN41 MudSt 65.0 2.76 0.22 0.09 0.00 2.13 2.44 0.88 0.87 ZN40 MudSt 61.3 2.56 0.23 0.24 0.00 2.01 2.48 0.97 0.81 ZN39 MudSt 58.6 3.02 0.25 0.24 0.00 1.68 2.17 0.72 0.77 ZN38 MudSt 55.0 2.74 0.27 0.37 0.00 1.23 1.87 0.68 0.66 ZN37 MudSt 52.5 1.98 0.21 0.47 0.00 0.89 1.57 0.79 0.57 ZN36 MudSt 50.6 2.62 0.25 0.50 0.02 1.60 2.37 0.90 0.68 ZN35 MudSt 47.0 1.28 0.37 0.47 0.03 0.30 1.18 0.92 0.26 ZN34 MudSt 46.0 0.49 0.02 0.18 0.03 0.06 0.29 0.58 0.20 ZN33 MudSt 45.8 0.68 0.03 0.30 0.05 0.08 0.46 0.68 0.17 ZN32 MudSt 45.0 0.87 0.16 0.55 0.04 0.04 0.80 0.91 0.06 ZN31 MudSt 43.2 1.01 0.09 0.21 0.03 0.06 0.40 0.39 0.15 ZN30 MudSt 42.0 4.43 0.22 0.89 0.06 0.82 1.99 0.45 0.41 ZN29 MudSt 40.5 6.72 0.14 0.86 0.04 2.10 3.14 0.47 0.67 ZN28 MudSt 39.0 4.63 0.18 1.54 0.15 0.27 2.14 0.46 0.13 ZN27 MudSt 37.0 4.03 0.17 0.33 0.04 1.41 1.96 0.49 0.72 ZN26 MudSt 35.5 3.70 0.18 1.78 0.04 1.31 3.31 0.90 0.40 ZN25 MudSt 34.0 6.10 0.15 1.40 0.02 1.08 2.66 0.44 0.41 ZN24 MudSt 32.5 3.79 0.19 0.52 0.06 1.59 2.36 0.62 0.68 ZN23 MudSt 31.0 5.67 0.12 0.62 0.04 1.70 2.47 0.44 0.69 ZN22 MudSt 29.0 5.58 0.13 1.19 0.07 1.73 3.11 0.56 0.56 ZN21 MudSt 27.0 2.47 0.15 1.61 0.06 0.34 2.15 0.87 0.16 ZN20 MudSt 25.0 4.08 0.11 0.62 0.03 1.57 2.34 0.57 0.67 ZN19 MudSt 21.5 4.77 0.28 1.00 0.05 1.17 2.50 0.52 0.47 ZN18 MudSt 19.0 5.23 0.21 0.76 0.04 2.57 3.58 0.68 0.72 ZN17 MudSt 17.0 4.61 0.15 0.78 0.06 1.50 2.49 0.54 0.60 ZN16 MudSt 14.8 1.81 0.23 1.00 0.05 0.46 1.74 0.96 0.27 ZN15 MudSt 12.2 4.97 0.12 0.37 0.04 1.66 2.20 0.44 0.76 ZN14 MudSt 10.0 2.52 0.24 1.21 0.04 0.82 2.31 0.92 0.35 ZN13 MudSt 8.4 3.34 0.13 0.73 0.03 1.08 1.96 0.59 0.55 ZN12 MudSt 7.0 2.99 0.24 0.27 0.01 1.70 2.22 0.74 0.77 ZN11 MudSt 5.8 3.35 0.27 0.28 0.01 1.49 2.05 0.61 0.73 ZN10 Shale 4.9 1.68 0.35 0.79 0.02 0.48 1.64 0.98 0.29 ZN09 Chert 3.9 1.38 0.35 0.76 0.03 0.13 1.27 0.92 0.10 ZN08 Shale 3.2 1.50 0.27 0.40 0.00 0.74 1.41 0.94 0.52 ZN07 Chert 2.6 1.23 0.34 0.71 0.01 0.14 1.20 0.98 0.12 ZN06 Chert 2.1 1.91 0.24 0.39 0.01 0.89 1.53 0.80 0.58 ZN05 Shale 1.7 1.25 0.24 0.33 0.00 0.49 1.06 0.85 0.46 ZN04 Chert 1.3 0.95 0.24 0.37 0.00 0.33 0.94 0.99 0.35 ZN03 Shale 0.7 1.82 0.25 0.50 0.02 0.91 1.68 0.92 0.54 ZN02 Chert 0.3 1.12 0.25 0.34 0.03 0.19 0.81 0.72 0.23 ZN01 Shale 0.0 2.34 0.46 0.66 0.03 0.68 1.83 0.78 0.37

MudSt, mudstone. layer, phosphate nodules, and geochronological and carbon iso- U–Pb zircon ages of a tuffaceous bed at the bottom of the ore bed tope stratigraphy have been used for correlation. (532.3 ± 0.7 Ma, Jiang et al. 2009; or 522.7 ± 4.9 Ma, Wang et al. A Ni–Mo–(PGE–Au) sulphides ore layer has long been recog- 2012b). The ages are close to a U–Pb SHRIMP age of 526.5 ± 1.1 Ma nized as a useful marker layer for stratigraphic correlation (Zhu analysed from single zircons from a bentonite layer within tuffa- et al. 2003). In the Xiaotan section, high Mo and V contents are ceous marl at the base of the Shiyantou Fm in the Meishucun sec- found to occur at the bottom of the Shiyantou Fm (or Stage 2) and tion, Yunnan Province (Compston et al. 2008). in the basal Yuanshan Fm (Stage 3). This sulphides bed in the In the studied section, a mudstone horizon rich in redox-sensi- lower Niutitang Fm is stratigraphically equivalent to the basal tive metal elements (such as Mo, U, V and Ni) and organic matter Shiyantou Fm, and has been found to occur along a narrow NE– is found at the bottom of the Zhalagou Fm (Xiang et al. 2012), and SW-striking belt >1600 km in length (Chen et al. 2009; Jiang et al. we tentatively place this layer as time-equivalent to the above- 2009, 2012; Wang et al. 2012a,b). Based on biostratigraphic data, mentioned Ni–Mo sulphide ore layer in the basal Shiyantou Fm this ore layer is considered to belong to Stage 2 (Tommotian stage; (Stage 2) (Fig. 11). The underlying Laobao Fm can be correlated Steiner et al. 2001). This hypothesis is corroborated by recently with the Laobao Fm in North Guangxi, and is stratigraphically determined sensitive high-resolution ion microprobe (SHRIMP) equivalent to the middle and upper portions of the Liuchapo Fm in Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 Biogeochemical processes in Early Cambrian 397

Fig. 5. Temporal variations in kerogen 13 δ Ckero, TOC, iron speciation, pyrite 34 34 and kerogen δ S, ∆ Sker-py and kerogen 15 34 δ N in the Zhalagou section. ∆ Sker-py is the sulphur isotope difference between kerogen and pyrite (see Fig. 11 for legend details). Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 398 C. Cai et al.

13 Fig. 8. Relationship between pyrite δ34S and kerogen δ13C values. Fig. 6. A crossplot of TOC and kerogen δ Ckero values from the Lower py kero Cambrian Laobao Fm and Zhalagou Fm compared with the Ediacaran Doushantou Fm. NW Guizhou. This is similar to the results of Chen et al. (2009) and Wang et al. (2012a,b), who placed the Ediacaran–Cambrian boundary within Liuchapo Fm cherts. The base of the upper Zhalagou Fm corresponds to the bottom of the Yu’anshan Fm (the Stage 3) in the Xiaotan section (Zhou et al. 1997; Steiner et al. 2004, 2007; Och et al. 2013). It is stratigraphically equivalent to the base of the Jiumenchong Fm overlying the Niutitang Fm in the Nangao section as indicated by the occurrence of trilobites Hupeidiscus and abundant sponge Diagoniella, Saetaspongia, Choia and Sanshapentella fossils in these strata (Yang et al. 2010). Thus, the lower part of the Zhalagou Fm was deposited during Stage 2 with the Ni–Mo-rich bed as the beginning of the Stage 2. Carbon isotope chemostratigraphy can provide further constraints on the stratigraphic divisions within this succession and 13 their correlation. δ Corg values generally vary in parallel with 13 δ Ccarb in Phanerozoic times (Shen et al. 1998), and are thus used in the slope and basinal sequences with lack of carbonate deposition to characterize the further stratigraphic divisions and their correlation with shallow-water equivalents in South China and elsewhere in the world (Chen et al. 2009; Jiang et al. 2012; Wang et al. 2012a; Guo et al. 2013; Shields-Zhou & Zhu 2013). This is 13 based on the assumption that δ Corg values reflect a primary signal and did not undergo significant post-depositional differential thermal alteration. In the studied section, the organic matter has equivalent vitrin-

ite reflectance (ERo) from 2.69 to 3.47% (He et al. 2012), and thus has not experienced metamorphism of greenschist or amphibolite 13 stage (Ro >4%). As a result, the δ Ckero values are expected to shift within 1–2‰ (Freudenthal et al. 2001; Galimov 2004). Moreover, the molar H/C ratios of organic matter vary between 0.30 and 1.95 with an average of 0.99 (n = 19, Table 3), similar to the range from 0.3 to 1.8 in the Yanwutan–Lijiatuo section in western Hunan Province (Guo et al. 2007), implying that the 13 δ Ckero values at this section and probably other South China sections were not significantly affected by thermal maturation. This 13 is because organic matter δ Ckero values have been shown to significantly change only when H was lost from the kerogen to result in molar H/C ratios of less than 0.2 (Strauss et al. 1992). Thus, 13 δ Ckero from this section can be used for the purpose of stratigraphic correlation. 13 Fig. 7. (a) Relationships between total sulphur (TS) and total Fe (FeT) In the studied section, there exists a large negative δ Ckero for the Laobao Fm and Zhalagou Fm; (b) relationship between FeT and excursion between the Laobao Fm and Dengying Fm. However, two large negative δ13C and/or δ13C excursions have been (TS – SBaSO4), assuming all Ba present in BaSO4 and thus pyrite sulphur carb kero and organic sulphur is total sulphur after subtraction of sulphur present found to occur within the Zhongyicun Member in the Xiaotan sec- as BaSO4 (TS – SBaSO4). tion (Fig. 11), and between the Zhujiaqing Fm and Dengying Fm, Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 Biogeochemical processes in Early Cambrian 399

Table 3. Kerogen C/N and H/C molar ratios, δ34S and δ15N values and the associated pyrite δ34S values

34 a 34 b 34 c 15 d d Sample δ Skero (‰) δ Spy (‰) Δ Skero-py (‰) δ Nkero(‰) (C/N)Kero (H/C)Kero ZN51 11.46 14.39 –2.92 0.75 106.8 1.42 ZN49 12.59 –24.58 37.17 2.75 101.3 2.87 ZN46 10.15 –10.92 21.07 1.34 84.1 1.48 ZN42 10.33 –4.19 14.52 2.14 92.4 1.59 ZN41 11.65 –3.27 14.92 – – – ZN39 10.21 –2.29 12.50 – – – ZN37 9.02 2.70 6.32 2.75 95.1 1.41 ZN35 8.99 –5.79 14.78 – – – ZN33 10.94 –0.10 11.04 3.39 79.5 1.04 ZN31 – –8.43 – 2.74 80.4 1.95 ZN30 24.45 15.09 –9.37 2.55 85.1 1.40 ZN29 10.67 10.95 –0.28 3.18 89.7 1.39 ZN12 9.56 –9.03 18.59 –0.42 67.3 0.86 ZN11 16.33 –9.42 25.75 –1.35 65.4 0.60 ZN10 10.01 8.14 1.87 1.98 67.1 1.00 ZN09 16.34 8.62 7.72 – – – ZN08 17.25 7.63 9.62 2.31 67.4 0.53 ZN07 18.31 10.27 8.04 – – – ZN06 14.17 10.27 3.90 1.35 68.3 0.50 ZN05 13.72 5.27 8.45 1.42 78.2 0.60 ZN04 16.98 6.49 10.49 6.85 83.7 0.55 ZN03 15.95 –1.80 17.75 6.65 75.4 0.46 ZN02 16.31 1.22 15.09 3.98 64. 8 0.36 ZN01 14.81 0.25 14.55 4.13 74.9 0.30

Note: a kero represents kerogen in short; b py is pyrite; c represents the δ34S difference between kerogen and pyrite; d is in molar ration.. the proposed Ediacaran–Cambrian boundary in South China (Zhou (März et al. 2008), and/or small amounts of pyrite may have been et al. 1997; Cremonese et al. 2013). The parallel unconformity oxidized during the exposure at the surface, as supported by the between the Laobao Fm and Dengying Fm may well indicate that similarity of δ34S values between pyrite S and NaCl-soluble sul- during the early Fortunian, the studied area may have been exposed phates (Goldberg et al. 2007). Similarly, sediments having FePy/ 13 at the surface without deposition. Thus, the δ Ckero excursion in the FeHR ratios between 0.7 and 0.8 are considered to have been depos- studied section is considered to correspond to the negative anomaly ited possibly beneath euxinic waters (Poulton & Canfield 2011). of carbonates and organic matter within the Zhongyicun Member in Iron speciation is established as a redox environment indicator the Xiaotan section (Fig. 11), within the Laobao Fm in the Nangao based on the study of shales and mudstones. Whether or not the section (Yang et al. 2007), and at the top of the Liuchapo Fm in the indicator can be used for study of the cherts depends on the process Ganzipin section and within the Liuchapo Fm (N2) in the Longbizui of formation of the cherts. The cherts were considered to have been section (Wang et al. 2012a; Guo et al. 2013). formed by (1) carbonate metasomatism, (2) redistribution and The subsequent positive excursion in the Ni–Mo sulphide layer of enrichment of SiO2 in the early diagenesis of mudstones (Murray the basal Zhalagou Fm is then proposed to be correlated with the et al. 1992) or owing to hydrothermal SiO2 contribution (Fan et al. positive anomaly between the Kuanchuanpo Fm and Guojiaba Fm 2013), or (3) primary precipitation (Ramseyer et al. 2013). It is clear (Goldberg et al. 2007), and within the Niutitang Fm in the Songlin, that iron speciation could not be used to reflect the redox environ- Ganzipin and Longbizui sections (Fig. 11; Guo et al. 2007; Chen ment of the cherts after metamorphism. Silicification and dilution et al. 2009; Wang et al. 2012a; Li et al. 2013; Shields-Zhou & Zhu processes may have changed Fe speciation to some degree, thus the 2013). The Ni–Mo sulphide layer corresponds to the basal Cambrian value of iron speciation to indicate redox conditions for cherts may Stage 2 in South China, about 20 Ma later than the Ediacaran– be slightly different from that for mudstones and shales. However, 13 some researchers (e.g. Takebe & Yamamoto 2003) have suggested Cambrian boundary (Jiang et al. 2009). The positive δ Ckero excursion between samples ZN35 and ZN36 is correlated with the positive that these processes have no significant effect on iron element anomaly at P5 within the Niutitang Fm in the Longbizui section migration, and that Fe speciation was controlled mainly by the water (Wang et al. 2012a) and between the Yu’anshan Fm and Shiyantou environment and redox state of the early diagenesis. The dilution Fm in the Xiaotan section (Zhou et al. 1997; Och et al. 2013). affects all iron speciation evenly, and thus is expected not to significantly change the ratios. This may be the reason why the trace ele- Iron speciation and phosphorus geochemistry ments and REE of the cherts can be used to determine the ancient redox conditions (Bolhar et al. 2005; Slack et al. 2007). The cherts FeHR/FeT ratio are used as proxy to distinguish an oxic (<0.38) have FeHR/FeT and FePy/FeHR ratios similar to those of the inter- from an anoxic (>0.38) bottom water environment (Raiswell & layered shales in this study. All FeHR/FeT ratios of the cherts are Canfield 1998). An anoxic condition is further classified as ferrugi- greater than 0.8, which are significantly higher than the threshold nous if FePy/FeHR < 0.8 or sulphidic if FePy/FeHR < 0.8. FeHR/ value of 0.38. These characteristics indicate that the cherts were very FeT > 0.38 and FePy/FeHR > 0.7 are also likely to result from an probably deposited under anoxic and ferruginous conditions. A sim- anoxic sulphidic condition because some Fe oxide minerals may ilar conclusion has been reached from the cherts from other contem- escape sulphidization during settling through the water column porary strata (Chang et al. 2010; Wang et al. 2012a). Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 400 C. Cai et al.

This may result from co-precipitation of Fe oxides and P in the water columns as a result of expanded oxic conditions in surface waters (Algeo & Ingall 2007). In the basal and upper Zhalagou Fm, most of the FePy/FeHR ratios are >0.7 with some <0.7. The features indicate that the bottom waters evolved to become dominated by euxinia during the deposition of Ni–Mo sulphides bed at the earliest Stage 2 and Stage 3, interrupted by ferruginous conditions (Fig. 3). In contrast, the bottom waters were dominated by anoxic and ferruginous conditions interrupted by euxinia during most of Stage 2. P contribution may have led to increase in primary productivity and thus TOC, resulting in a positive logarithmic relationship between P content and TOC (Table 1). The increase in primary productivity is expected to decrease selectivity of 12C and 13C, and thus organic matter should have relatively heavy δ13C values, as shown in the roughly positive correlation between TOC and 13 δ Ckero for the Laobao Fm (Fig. 6). However, from sample ZN1 in the basal Laobao Fm to sample ZN3 at 0.7 m, the kerogen δ13C value shows a negative shift from –33.4 to –35.2‰ with TOC increasing from 8.3 to 12.8%. The samples from the Laobao Fm 13 show higher TOC and more negative δ Ckero values than those from the overlying Zhalagou Fm and the underlying Ediacaran Doushantou Fm, respectively (Fig. 6). These characteristics may have resulted from two possibilities among others: (1) enhanced

CH4 fluxes during the Ediacaran–Cambrian transition and earliest Cambrian (Bartley et al. 1998; Kimura & Watanabe 2001; Chen et al. 2009); (2) overturn or upward expansion of an anoxic lower water mass (Brasier 1989; Hallam & Wignall 1999; Schröder & Grotzinger 2007).

(1) The CH4 from the enhanced fluxes during the period studied may have been oxidized aerobically (CH4 + 2O2 = CO2 + 2H2O) or 2− 2− − − anaerobically with SO4 (CH4 + SO4 = HCO3 + H2S + OH ) to 12 − 12 generate C-rich CO2 or HCO3 . Correspondingly, C-rich CO2 is expected to generate 12C-rich organic matter through photosynthesis. Consequently, negative δ13C shifts in organic matter and the shallow platform carbonates occur in the Ediacaran–Cambrian transition. The

CH4 may have been released from palaeo-gas pools as a result of hydrothermal fluid activity during deposition (Chen et al. 2009;

Bristow et al. 2011). Alternatively, mass release of CH4-hydrate stored in marine sediments during this period may be another source

of CH4 (Bartley et al. 1998; Kimura & Watanabe 2001). (2) Overturn or upward expansion of an anoxic lower water mass 12 − could deliver H2S, P and C-rich HCO3 and CO2 to shallow platform areas and overlying waters (Schröder & Grotzinger 2007). This may cause an upward movement of the oxygen-minimum zone (Brasier 1989; Hallam & Wignall 1999), thereby affecting the shallow-water inorganic carbon isotope record (Knoll et al. 1996; Kimura et al. 1997; Bartley et al. 1998; Schröder & Grotzinger

2007). The isotopically light CO2 may have resulted from organic matter remineralization (Knoll et al. 1996; Schröder & Grotzinger 2007; Ishikawa et al. 2013). Consequently, a negative carbon isotope excursion in organic matter is expected as a result of photosynthesis. This proposal is supported by the anoxia in the platform through to the slope to basin environment during the Fortunian

34 (Chen et al. 2009; Cai et al. 2012; Wang et al. 2012a), and coincides Fig. 9. Relationships of δ Skero value to (a) FePy/FeHR ratio and (b) 34 34 with the transgression in the Yangtze ocean (Mei et al. 2006). pyrite sulphur content, and (c) relationship between δ Spy and δ Skero 34 – δ Spy. Sulphur systematics Across the entire Zhalagou section, the Zhalagou Fm and Laobao Fm have FeHR/FeT ratios consistently >0.38 (Fig. 5), In the Zhalagou Fm, the total sulphide contribution to bulk sedi- indicating bottom water anoxia throughout. The bottom waters mentary S ranges mainly from 60 to 90% with organic sulphur were anoxic and ferruginous during the deposition of the Laobao (OS) <3% (Table 1). Combined with Fe speciation results, it can Fm as indicated by all FePy/FeHR ratios <0.7. However, FePy/ be concluded that most of the H2S generated from bacterial sul- FeHR shows some variation, with increase in Fe oxides and phate reduction (BSR) under sulphidic water columns may have been precipitated as pyrite S, and only small proportions were decrease in pyrite Fe. The sample ZN1 with the highest P2O5 of 8.9% shows the highest unsulphidized Fe oxide content of 0.96%. incorporated into organic matter in water columns or sediments. Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 Biogeochemical processes in Early Cambrian 401

open system and thus there is significant isotopic fractionation up to 30‰ during the transformation of sulphate to sulphide, and (2)

recycling of H2S by green and purple sulphur bacteria results in the generation of 32S-rich sulphur species of intermediate valences and even sulphate (Zerkle et al. 2009). These sulphur species in turn

may be reduced to even lighter H2S by subsequent bacterial sulphate reduction. Thus, pyrite and organic sulphur from euxinia are expected to have lighter sulphur isotopes than those from non- euxinic environments. Euxinic sediments have high FePy/FeHR 34 ratios or high pyrite sulphur contents; thus very light δ Skero values corresponding to high FePy/FeHR ratios and pyrite sulphur (Fig. 9a and b) may indicate that the kerogen has sulphur contributed

from H2S from euxinic water, probably from disproportionate H2S. In fact, some of the Zhalagou Fm samples (ZN36, ZN37 and ZN47) were not deposited in a euxinic environment. These samples and Laobao Fm samples were deposited under ferruginous water column conditions as indicated by Fe speciation. Under such

conditions, H2S was generated in the pore water of shallow burial fine sediments, which more closely represent closed systems (Cai et al. 2009) with limited sulphate and ferruginous supply. Limited sulphate supply and intense sulphate reduction fuelled by high organic carbon may have resulted in the conversion of most of the

sulphates to H2S. Consequently, as a result of environmental Rayleigh distillation, these samples and samples ZN6 to ZN10 34 from the Upper Zhalaogou Fm have pyrite δ Spy values from 8 to 15‰, significantly heavier than those of other Zhalagou Fm sam- 34 ples. However, lighter δ Spy values from –1.8 to 6.5‰ in the Lower Laobao Fm may indicate bacterial sulphate reduction just below the water–sediment boundary with sulphate supply from the overlying water body. Organic sulphur content and isotopic composition are seldom reported because it is hard to remove tiny pyrite particles com- pletely from organic matter. A method developed by Cai et al. (2009) was used and the results show that the Laobao Fm and basal Zhalagou Fm have much higher OS/Spy ratios than the middle and upper Zhalagou Fm (Table 1). This feature is explained as the result of higher TOC and perhaps limited Fe supply in sediments. 34 Organic matter in the studied section has δ Skero values compa- rable with those for counterparts from the Shatan and Songtao sections (Goldberg et al. 2007). However, our values show a much narrower range of 9.0–18.3‰ and are not so much correlated with 34 δ Spy values as those from Goldberg et al. (2007). One of the likely reasons is that contaminant pyrite was removed to a greater degree in this study, with pyrite S/total S <0.08 in the residual 34 kerogens (Cai et al. 2009). The δ Skero values in this section are similar to those from the Cambrian mudstones in the Tarim Basin (Cai et al. 2009) and other Lower Cambrian sections of the Yangtze Block (Cai et al. 2010). However, it is not clear why 34 δ Skero values have the above-mentioned range. The negative rela- 34 tionships of δ Skero values to FePy/FeHR ratios and pyrite S contents may indicate that the more H2S was precipitated as pyrite, the 34 lighter the δ Skero of organic matter. That is, the more open the 34 system is for H2S generation, the lighter the δ Skero values. A relatively open system with abundant sulphate supply may occur in water columns or at the water–sediment boundary with lower tem- Fig. 10. Relationships of δ15N values to (a) kerogen C/N atomic kero peratures (generally <20 °C). ratio, (b) C/N atomic ratio and (c) δ13C . kero kero It is not clear why organic sulphur has δ34S values heavier than the coexisting pyrite in the study area and other areas worldwide 34 This proposal is supported by the pyrite δ Spy values, which range (Aizenshtat & Amrani 2004). Possible explanations include the mainly from –3.3 to –9.4‰, being >35‰ lighter than values for following. contemporary seawater (c. 32‰, Claypool et al. 1980). The very 34 (1) Organic sulphur has higher proportions of incorporated light δ Spy values (–10.9‰, –24.5‰) in the basal and top Zhalagou Fm may have been generated from photic zone anoxia. It is gener- H2S contributed from early diagenesis than the associated pyrite. The system for H S generation in sediments during ally accepted that H2S and pyrite generated from photic zone 2 34 anoxia have light δ S values (Canfield & Thamdrup 1996; Bottrell early diagenesis is relatively closed, and thus the H2S is & Newton 2006). This is because (1) euxinic water columns are an expected to have δ34S values significantly heavier than the Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 402 C. Cai et al. al .

. (2013); Nangao section, Yang et Yang al . (2013); Nangao section,

al . (2009) and Pi et

al . (2006), Jiang et

al . (2007); Songlin section, Chen et

). al . (2012 a

. (2013); Shatan section, Goldberg et al . (2013); Shatan section, Goldberg

al . (2013) and Och et

. (2009); Longbizui section, Wang et Wang al . (2009); Longbizui section,

al . (2013), Li et

al . (1997), Cremonese et

(2007); Ganzipin section, Chen et o- and biostratigraphy and geochronological data. Data sources: Xiaotan section, Zhou Early Cambrian stratigraphic correlation across South China from the inner shelf to basin transect via integrated litho-, chem o- and biostratigraphy geochronological data. Data sources: Xiaotan section, Zhou Fig. 11. et Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 Biogeochemical processes in Early Cambrian 403

counterpart and resulting pyrite generated in a water col- a state of equilibrium between nitrate assimilation, N2 fixation and umn, a relatively open system. It is clear that bacteriogenic denitrification with a stabilizing δ15N values inside the range +2 to

H2S incorporation into organic matter and precipitation as +6‰ (modern oceanic values; Cremonese et al. 2013). The other – – pyrite overlap significantly (Bottrell et al. 2009). However, possible source of the NO3 or NO2 in oxic surface water is deri- + if organic sulphur has a smaller part of the incorporated vation from the oxidization of upward-migrated anoxic, NH4 - H2S from the water column and a higher proportion of bearing deep water. The transgression event as indicated by high P + H2S contributed from early diagenesis than the associated contents in the basal Laobao Fm may have enhanced NH4 supply pyrite, it is expected for the organic sulphur to have heavier and thus nitrification rates, leading to high primary productivity 34 – – δ S values than the coexisting pyrite. and high TOC. Subsequent partial denitrification of NO2 or NO3 + (2) Polysulphides and, less probably, elemental sulphur are to N2 or NH4 is expected to occur in the redox transition zones in proposed to have been incorporated into organic matter; the water column. If denitrification is not quantitative, residual 34 – 15 and these sulphur species have δ S values ranging between NO3 would become enriched in N (Sigman & Casciotti 2001; – those of bacteriogenic H2S and sulphate. Consequently, the Stüeken 2013). Assimilation of this residual NO3 by living organ- 15 15 organic sulphur is expected to be heavier than the associ- isms would result in N enrichment leading to the positive δ Nkero ated pyrite (Aizenshtat & Amrani 2004). values as recorded in sediments deposited from the Archaean and (3) Organic sulphur after the incorporation is enriched in 34S Mesoproterozoic redox layered ocean or Early Cambrian shallow by 4–5‰ relative to the precursor polysulphides and sul- water (Garvin et al. 2009; Godfrey & Falkowski 2009; Busigny phide as a result of isotope fractionation during the incor- et al. 2013; Cremonese et al. 2013; Stüeken 2013). 15 poration (Amrani & Aizenshtat 2004). The decrease in δ Nkero values from 1.4–6.9‰ in the Laobao Fm to the negative values (–1.4‰ and –0.4‰) in the basal (4) The incorporated H2S may have an origin by thermochemical reduction of barite (thermochemical sulphate reduction) Zhalagou Fm corresponds to the dramatic increase in FePy/FeHR by organic matter at temperatures >130 °C (Cai et al. 2003). ratio from 0.1 and 0.29 to 0.68 and 0.78, or a change from ferrugi- 34 nous to euxinic water. Similarly negative δ15N values to those Much higher barite and heavier δ Skero values in the Lao- kero bao Fm than in the Zhalagou Fm may support bacterial or of the basal Zhalagou Fm are found to occur in the stratigraphi- thermochemical reduction in a burial environment and sub- cally equivalent basal Shiyaotou Fm (N7 of Fig. 4b; Cremonese sequent incorporation. As an example, kerogen in sample et al. 2013), basal Niutitang Fm in Longbizui and between the ZN03 has a δ34S value of 16.0‰, much heavier than that of Xiaoyanxi Fm and Liuchapo Fm in the Lijiatuo section (Cremonese the coexisting pyrite (–1.8‰). et al. 2014). Interestingly, stratigraphically equivalent sulphide ores from the Huangjiawan mine site have a δ98/95Mo average 34 34 34 value of 1.06 ± 0.16‰ (Lehmann et al. 2007), similar to the aver- The negative correlation between (δ Skero − δ Spy) and δ Spy 34 age δ98/95Mo value of 1.20 ± 0.16‰ from the Dingtai profile (Fig. 9c) is explained by relatively small variation in δ Skero com- 34 (Guizhou province) and some other polymetallic sulphide ore pared with the large variation from –24.6 to 11.0‰ in δ Spy. deposits of Guizhou and Hunan Provinces, South China (Xu et al. δ15N 2012), characteristic of a euxinic environment. Thus, euxinia may kero have been widespread over the Yangtze Block during this period; As for carbon, it is important to determine if the N isotope composi- this is supported by the connection of the Yangtze Block to the tion reflects a signature of organisms living in the water column or open ocean (Xu et al. 2012). Euxinia is required for green or pur- was affected by late modifications related to post-depositional pro- ple sulphur bacteria to survive. Anaerobic bacteria (Pennock et al. cesses. Organic matter maturation may preferentially result in loss of 1996) are able to fix dinitrogen, ammonia and/or nitrite in their H relative to C and of N relative to C, and isotopically light 12C and tissues in the euphotic (sulphidic) anoxic zone with a fractionation 14N, thus resulting in an increase in H/C and C/N ratios, and δ13C kero ranging from 0 to –2‰ for bacteria using N2 as substrate and from 15 and δ Nkero values; thus correlations between the parameters are –25‰ to –8‰ using ammonia (Pennock et al. 1996; Ohkouchi expected. No such correlations occur in the Zhalagou section for all 15 et al. 2005). Thus, the small negative values of δ Nkero in the basal the samples analysed (Fig. 10a–c), indicating that δ15N values kero Zhalagou Fm are probably derived from N2 fixation by anaerobic may not have been changed significantly with maturation, and thus microorganisms including green or purple sulphur bacteria. N2 reflect primary signals (Ader et al. 2006; Boudou et al. 2008). This fixation may have served as the principal nitrogen source for the proposal is consistent with a study of the effect of thermal alteration local biota prior to or during sedimentation, nitrification may have 15 on δ Nkero values in organic matter, which shows that the fossil been inhibited owing to anoxic conditions (Klotz & Stein 2008), organic matter δ15N value appears not to be significantly affected by and thus no significant denitrification occurred. increasing thermal maturity (Ader et al. 1998) before the organic Thus, it can be found that kerogen deposited from a similarly matter reaches the greenschist metamorphic stage (Ro = 4–7%). stratified water with lower euxinia shows significantly different As proposed in the above section, the Laobao Fm and the mid- 15 δ Nkero values. The values are 2.1–3.4‰ in the upper Zhalagou dle and upper Zhalagou Fm were deposited under a euxinic or fer- Fm and –1.4‰ and –0.4‰ in the basal Zhalagou Fm. The differ- 15 ruginous lower water column with an overlying oxic or dysoxic ences are explained as follows. The small negative δ Nkero val- 15 water layer. The sediments show similarly positive δ Nkero values ues in the basal Zhalagou Fm may indicate that N2 fixers of 1.4–6.9‰ for the Laobao Fm and 2.1–3.4‰ for the upper dominated production in the photic zone with very thin or even 15 Zhalagou Fm. The similar values of δ Nbulk N have been explained no oxic surface water. This is because biotic H2S production − by deposition from an oxic water column from a ‘normal marine’ requires NO3 depletion because denitrifiers outcompete sulphate (oxygenated) inner shelf environment in the Xiaotan section reducers (Boyle et al. 2013). This explanation is consistent with

(Cremonese et al. 2013). Thus, it is necessary to explain the posi- the proposal of Wille et al. (2008) that H2S was released to the 15 tive δ Nkero values in the section. surface waters during this period, and thus oxygen was absent or In a redox layered ocean, ammonium was the stable form of N rare, resulting in no significant nitrification and denitrification. in underlying ferruginous and euxinic water during the deposition In contrast, in the upper Zhalagou Fm, upper oxic surface of the Laobao Fm and the middle and upper Zhalagou Fm, with water may have been expanded, thus an equilibration occurred – – + NO3 stable in oxic surface water. The NO3 in oxic surface water between nitrate assimilation, N2 fixation or NH4 oxidization and may have two sources. The first source is from N2 fixation, and is denitrification. Downloaded from http://jgs.lyellcollection.org/ at Chinese Academy of Sciences on May 5, 2015 404 C. Cai et al.

Implication for expanding oxic surface water TOC and P2O5 contents is considered to result in a negative shift in and Cambrian explosion kerogen δ13C values in the earliest Cambrian. During the earliest Stage 2, a euxinic bottom water with thin or no oxic surface water The positive excursion of 2.4‰ at the transition between the basal led to sediments having FePy/FeHR > 0.7, light pyrite δ34S values and the lower part of the Zhalagou Fm (between samples ZN12 of about –10‰ and large differences in δ34S values between and ZN14) is reproduced in the lower part of the Shiyantou Fm at kerogen and pyrite (Δδ34S). The surface water was dominated by the Xiaotan section in Yunnan Province (from –35.8 to –29.3‰; N2 utilization by cyanobacteria or sulphur bacteria and thus kero- Cremonese et al. 2013), in the lower part of the lower Pestrotsvet gen δ15N values show negative values. Dissolved oxygen and sul- Formation in Siberia (Kouchinsky et al. 2012), and in the lower phate concentrations were then significantly increased during part of the Lie de Vin Formation in Morocco (Maloof et al. 2010). Stage 3, and thus oxidized surface water and the redox transition Such consistent intrabasinal and interbasinal reproducibility across zone expanded, resulting in kerogen δ15N values increasing to large environmental and sedimentation-rate gradients indicates the 3–5‰, pyrite δ34S values as low as –24.6‰ and Δδ34S as high as global and primary nature of this geochemical signal. Similarly, 37‰. The expanding oxic surface water may have been one of the 13 the δ Ckero negative shift at the base of the Laobao Fm is also reasons for the oxidization of the deep ocean and thus reinforced global and primary, which corresponds to the negative shift within the evolution of animals during Stage 3 or vice versa. the Zongyicun member of the Zhujiaqing Fm in the Xiaotan section, and point N2 within the Liuchapo Fm in the Longbizui sec- Acknowledgements and Funding tion (Fig. 11; Wang et al. 2012a). This research was financially supported by China National Program on Key According to the accepted view of the global carbon cycle, the Basic Research Project (973 Program) (grant number 2011CB808805) and positive δ13C shifts in the lower Zhalagou Fm (between samples China National Funds for Distinguished Young Scientists (grant number kero 41125009).We thank Lianjun Feng (IGGCAS) for access to the stable isotope ZN12 and ZN14) and the middle and upper Zhalagou Fm (between laboratory and assistance in stable isotope analysis, and He Li (IGGCAS) for samples ZN35 and ZN38) indicate that rates of organic matter help in XRF analysis. Constructive and insightful reviews by G. Shields-Zhou deposition are higher than those of oxidative weathering. They and an anonymous reviewer helped us to improve this paper significantly. reflect high rates of net oxygen production (Hayes & Waldbauer 2006; Kump et al. 2011). 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