The molybdenum (Mo) isotope system, together with trace metal geochemistry have been taken as the indicator of the redox conditions in ancient paleo-envi- ronments  -  . The rate and completeness of removal of Mo to sediment differs among 1) oxic (O2 concentration in bottom waters, [O2] > 2 mLO2/LH2O), 2) suboxic to weakly euxinic ([O2] < 0.2 mLO2/LH2O, [H2S] < 11 μm), 3) strongly euxinic ([H2S] > 11 μm) in water-column redox facies    . A model for interpreting primary redox environments from the δ98/95Mo of organic carbon rich mudrock deposit has been defined  . In fully oxic conditions, Mo would be mainly adsorbed to Mn-oxides, and little Mo enrichment below sediment- water interface  . In suboxic and weakly euxinic conditions, Mo isotopic fractionation might be between that of oxic and strongly euxinic sediments   . In anoxic seawater and pore fluids, Mo is converted to the particle reactive thiomolybdate anion (x = 0 to 3)   . In euxinic sediments ([H2S] > 11 μm), Mo is turned into tetrathiomolybdate and δ98/95Mo has heavy isotope value closer to that of modern ocean water (2.3‰)   .
Trace elements, such as Mo, U, V, etc., are highly sensitive to redox changes in the water column and are highly enriched in the reducing sediment, potentially making them and their ratios for important proxies for paleo-redox conditions    .
The early Cambrian represents a unique period in Earth history characterized by global environment and biological changes   . Lower Cambrian organic-rich black shales have been discovered in North America, southern Australia, parts of European, and Asia      . A possible global ocean anoxia event could be occurred during the early Cambrian period, and numerous geochemical proxies (such as C, S isotopes), also support this suggestion.
In China, the Lower Cambrian sedimentary strata, containing black shales and cherts, occur on the Yangtze and Tarim Platforms       . Previous research has been mainly focused on the Yangtze Platform about the paleo-environment during the early Cambrian period       . However, there is no equivalent research on the sedimentary sequence of the Tarim Platform. In this study, we investigated black rock series (containing phosphorite, phosphorous chert, chert, and carbonaceous shale) from the Lower Cambrian Sugetbrak section, Akesu-Wushi area, Tarim Basin. The joint application of δ98/95Mo, TOC concentrations and trace element geochemistry attempts to contribute to an understanding of the paleo-environment of Early Cambrian seawater on the Northwest China.
2. Geological Setting and Sampling
The Tarim Basin, located within the Xinjiang Uygur Autonomous Region of northwestern China, is one of the largest hydrocarbon-bearing intracontinental basins in the world  . The Tarim Basin is surrounded by the orogenic belts of Tienshan Mountains to the north, the western Kunlun Mountains to the south, and the Central-Southern Altyn Tagh Mountains to the southeast  . The strata of the Cambrian in the Tarim Basin consist of the Yurtus, Xiaoerblaq and Wusonger Formations in an ascending order  . The Yurtus Formation unconformably overlies the dolomite of the Ediacaran Qigeblaq Formation, and conformably underlies the trilobite-bearing limestone of the Xiaoerblaq Formation  . Lithologically, the Yurtus Formation is composed of basal black rock series and limestone above those basal rocks. The black rock series are over- and underlain by limestones, suggesting a shallow marine environment  . The black rock series contain bedded black chert with dark colored phosphatic granule layers, and carbonaceous shale (Figure 1(c)).
The Sugetbrak section ,which is one of the well-known sections of the Akesu-Wushi area, is located in the northern margin of the Tarim Basin (Figure 1(a), Figure 1(b))      According to paleontological studies in this area, the Asteridium-Heliosphaeridium-Comasphaeridium (AHC) acritarch assemblage zone, which was restricted to the Meishucunian Stage  , was found in the black rock series at the base of the Yurtus Formation.
The measured thickness of the Sugetbrak section is 2.06 m. This section consists of interbedded chert-phosphorite assemblages (1.04 m) and the carbonaceous shale (1.02 m) of the Early Cambrian Yurtus Formation in an ascending order (Figure 1(c)). A total of 12 chert-phosphorite samples and 4 carbonaceous shale samples were collected from the lower Yurtus Formation in the Sugetbrak
Figure 1. (a) Position of the study area. Modified after reference  ; (b) geological setting of the studied section and the tectonic setting. Modified after reference  ; (c) lithostratigraphy and the sampling horizons of the Sugetbrak section.
section (Figure 1(c)).
3. Samples Preparation and Analytical Methods
Fresh samples were directly selected indoors. The samples were washed with deionized water and ground to 200-mesh using an agate mortar for chemical analysis. Trace elements, TOC, and Mo isotope were analyzed in this study.
3.1. Trace Element and TOC Analysis
A routine HR/ICP-MS method was used. For each analysis, a 50 mg sample of 200-mesh powder was accurately weighed and placed into a Teflon dissolving can. The sample was leached with 1 mL of HF at 150˚C and boiled to dryness to remove carbonate and calcium phosphate minerals. Then, the residues were fully dissolved in 1.0 mL of HF and 0.6 mL of HNO3. The mixture was placed into a Teflon dissolving can and heated at 190˚C for at least 96 h. The solution was evaporated into an emulsion to remove excess HF. The residue was dissolved in 1 mL of concentrated HNO3 and evaporated to an emulsion (this step was repeated twice). The residue was dissolved in 1.6 mL of HNO3 and heated at 140˚C for 3 h to 5 h, and then transferred into a 50 mL centrifuge tube. The resultant heated residue was mixed with 1 mL of 500 ppb Rb internal standard, diluted to 50 mL, and analyzed by HR/ICP-MS at the State Key Laboratory for Mineral Deposits Research of Nanjing University. The analytical precision of elemental concentrations was generally better than 5%.
The same set of 200-mesh powder samples was analyzed for TOC composition. The powder samples were reacted with 1mol/L HCl in a water bath at 50˚C for at least 48 h (adding 1mol/L HCl twice) until no further reaction was observed. The solutions were washed with deionized water until pH 7, and the residue was dried and ground to 200-mesh. Samples were analyzed using a FLASH EA1112 elemental analytical instrument at the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences.
3.2. Mo Isotope Analyses
A detailed description of the analytical techniques is given in Zhang et al. (2009) and Wen et al. (2010, 2011)    , and only a brief summary is presented here. Sample powders with an equivalent of >100 ng were oxidized at 600˚C for 8 h, and then transferred to a Teflon beaker. Samples were digested using a mixture of HF and HNO3 (1:2) at 100˚C for at least 16 h until the samples were completely dissolved. An improved anion/cation exchange resin double- column procedure was used to separate Mo from natural samples  .
The Mo isotopic measurements were performed at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, using an Isoprobe MC-ICP-MS. To correct isotopic drifts of the instrument, a sample-standard bracketing technique was employed  . The working Mo standard solution was prepared from a newly standard Mo solution (NIST SRM 3134, Merck, JMC, and Aldrich). For data presentation the δ98/95Mo ratio is used. The external standard reproducibility is at or below 0.1‰ for the δ98/95Mo ratio. The value of δ98/95Mo is defined by the following Equation (1).
4. Analytical Results
The δ98/95Mo values, TOC, and trace element concentrations (such as Mo, U, and V, et al.) and their ratios are presented in Table 1.
4.1. Mo Isotope
Mo isotopic data are presented in Table 1, and their stratigraphic trend is shown in Figure 2. The measured δ98/95Mo values of the samples vary from −0.17‰ to 1.56‰. We obtained one negative shift at the base of the Yurtus formation (N1, sample SG0904-2, 3, 4, 5, 6), one positive shift (P1) immediately with one negative shift in the carbonaceous shale layer (N2) (Figure 2).
4.2. Trace Elements and TOC
The trace element results and their selected ratios are shown in Table 1. Enrichment factors are calculated by normalizing each trace element to aluminium (Al) in concentration, which is assumed to represent the detrial influx, and
Figure 2. Stratigraphic distribution of elemental ratios and Mo isotopic compositions.
Table 1. The δ98/95Mo values, TOC, and redox sensitive element data and ratios of the Sugetbrak section, Tarim Basin.
“/” means not measured.
then comparing these ratios to the normal shale   . The data of standard shale are from McLennan et al. (1984)  . The EF values of several redox sensitive elements (such as U, Mo, and V) are given in Table 1. The MoEF values are from 6.22 to 438.46 (average 146.53), UEF values are from 5.73 to 1431.41 (average 182.03), and VEF values are from 3.15 to 159.89 (average 60.66).
The V/Cr ratios range from 0.34 to 5.71 in the chert-phosphorite beds, and from 2.20 to 8.04 in the carbonaceous shale layers; Ni/Co ratios are between 2.71 and 28.9 in the chert-phosphorite assemblages and from 13.4 to 24.0 in the carbonaceous shale beds (Table 1). Furthermore, δU (δU = 2U × (U + Th/3)) values reveal a little more consistency, and are from 1.76 to 2, indicate anoxic sedimentary conditions   .
The TOC concentrations in the laminated chert-phosphorite assemble are from 00.7% - 0.94%, and from 0.66% to 2.79% in the carbonaceous shale layer (Table 1).
4.3. Evaluation of Detrital Input
Detrital, biogenic, and hydrogenous fractions show three independent concentrations of sediments  . Monitoring of the detrital fraction (i.e., crustal) is necessary to assess the enrichment of redox-sensitive elements relative to their detrital component. Th is stable in the water column, and occurs permanently in the insoluble Th4+ states, and largely independent of factors such as source area and grain size  . Therefore, it is used in the present study to monitor detrital input  . The lack of correlation between Th concentrations and U, and V ratios suggests no detrital influence on redox-sensitive elements (Figure 3(a), Figure 3(b)).
5.1. Redox Sensitive Elements Geochemistry
Trace elements (e.g., Mo, U, and, V et al.) and their ratios (including Ni/Co, V/Cr, and δU) have been widely used as indicators for paleoredox conditions
Figure 3. Cross-plot of various parameters cited from the Yurtus black rock series unit. Th correlates with V (a), and U (b).
    . In anoxic waters, the Mo, U, and V values of the sediments may become highly concentrated, compared to their concentrations in sediments formed under oxygenated water columns    . U enrichment is minor in oxic-suboxic environments, where U is present mainly as U (VI) in the form of chemically unreactive uranyl carbonate complex    . Under anoxic or euxinic conditions, U (VI) can be reduced to U (IV) as the insoluble uranium dioxide(UO2) or less soluble uramium fluoride complexes    . Just like U, Mo and V is also considered sensitive to conditions around the redox boundary during early diagenesis, and is preferentially enriched in sediments underlying anoxic or euxinic waters    . The ratios of MoEF (average 146.53), VEF (average 60.66), and UEF (average 182.03) (EF = Enrichment Factor) reveal the relative enrichment than that of average shale, which indicate the sedimentary environment may not be oxygenic environment (Table 2)    .
Wignall (1994a, b) proposed to use the δU (δU = 2U × (U + Th/3)) index to distinguish the sedimentary environment of shales, with δU > 1 representing anoxic environment and δU < 1 for normal marine sedimentary environment. Based on this criterion, all samples fall into the anoxic region (Table 2)   . Therefore, this criterion is not suitable for environment analysis of our data. It needs more rigorous criteria to constrain the sedimentary environment of the black shale series we studied.
Previous studies have established standard values for trace element ratios to distinguish oxic, suboxic and anoxic conditions (Table 2)      . Although different redox indicators have diverse threshold to indicate the redox conditions, there is general trend that V/Cr and Ni/Co ratios increase with decreasing oxygenation levels in water columns      . Figure 2 reveals that the samples in the carbonaceous shale beds fall into the suboxic- anoxic sedimentary conditions area, and most of chert-phosphorite samples are also deposit in suboxic-anoxic sedimentary conditions. The same patterns could also be observed from Ni/Co vs. Mo and V/Cr, δU vs. Ni/Co and V/Cr (Figure 4). However, the lithology of the basal Yurtus Formation is kinds of phosphatic rocks, U enrichment, V enrichment, and V/Cr ratio could not be used as indicators
Table 2. Redox classification of the depositional environment.
Figure 4. Crossplots of trace element ratios as paleoredox proxies. (a) Mo vs. Ni/Co; (b) V/Cr vs. δU; (c) V/Cr vs. Ni/Co; (d) Ni/Co vs. δU. Ranges for Ni/Co and V/Cr are from Jones et al. (1994)  ; range for δU is from Wignall (1994a, b)   .
of depositional conditions, due to the disturbance of the system by the substitution of V and U into apatite  .
Phosphorite genesis needs a favorable environment in the bottom water. Upwelling hydrothermal fluid and transgressive events could generate the proper conditions for the formation of phosphate nodules  . The REE results indicate that the lower Yurtus Formation shales are highly influenced by hydrothermal inputs   . The phosphatic rocks and cherts of the basal Yurtus Formation exhibit oxic conditions with significantly negative Ce anomalies, and Ce/Ce* is in the range of 0.37 - 0.48  . These significantly negative Ce anomalies are generally ascribed to organic matter derived from organisms in the euphotic zone  . However, the TOC concentrations of phosphatic rock and cherts are relatively low (Table 1, <0.94 wt %). Therefore, organic matter may not be the main trace element source for the phosphorites and cherts. The formation of phosphorite, especially phosphate nodules, in siliciclastic depositional systems might be related to high-energy hydrodynamic regimes, reworking and redeposition    . In addition, no sulfide was found at basal of the Yurtus Formation in the studied area. Thus, the bottom water during the early Cambrian could not be anoxic or euxin. Study on the Liuchapo Formation, southeastern Chongqing proposed by Li et al. (2015) further indicates the oxygen sedimentary conditions in early Cambrian  These consistent results from different areas demonstrate that oxic bottom seawater widespread in the whole basin around the Ediacaran-Cambrian boundary, and the following transgressive event induced an anoxic basin at the beginning of the early Cambrian.
5.2. Mo Isotopic Geochemistry
Mo isotope fractionation patterns have been used to reconstruct the redox state of the Earth’s atmosphere and oceans   . The δ98/95Mo isotope composition of the black rock series from the Sugtebrak section, is from −0.17‰ to 1.56‰, and reveal two negative anomalies (N1 and N2) and one positive recovery (P1) over a few meters of stratigraphy thickness only (Figure 2, Table 1). Several mechanisms could contribute to the variabilities of Mo fraction, including: 1) the changes of the redox conditions (such as oxic, anoxic and strongly euxinic water-column). In oxygenated seawater, Mo is present as, and Mo isotope signature has light Mo isotopic composition (δ98/95Mo = −0.7‰), which represent a large (~3‰) negative fractionation relative to the modern seawater composition (δ98/95Mosw) of ~+2.3‰    . A growing realization that Mo isotopes could also be fractionated during removal in intermediate redox environment where O2 is scarce but H2S is not abundant which is so-called suboxic or anoxic setting  ; 2) low-temperature hydrothermal systems. Mo may have been fractionated by precipitation of hydrothermal Fe-Mn oxides     ; and 3) influx of riverine Mo in restricted ocean basins not fully connected to the global ocean circulation.
In the Sugtbrak section, the δ98/95Mo ratios show lighter than that present-day euxinic sediments (2.3‰)  , and heavier than typical of Mo adsorbed onto Mn oxides (−0.7‰)   , and fall into the range of suboxic sediments (Figure 5). The pattern of δ98/95Mo curve has good relationships with trace element ratios (V/Cr and Ni/Co) trends (Figure 2). δ98/95Mo negative anomaly (N1 and N2) could correspond to the suboxic-anxoic conditions which V/Cr and Ni/Co ratios indicated, and the δ98/95Mo positive shift (P1, Figure 2) also could relative with the oxic-suboxic conditions. Wen et al. (2015) reported δ98/95Mo values of early Cambrian black shales, which have a wide range of δ98/95Mo values (0.27‰ - 1.79‰) in carbonaceous shales and cherts (interval 1), and are from 0.11‰ to1.70‰ in black shale layer (interval 2) in the Zunyi section, in South China  . Combining with Fe abundance in samples, redox elements characters, and δ98/95Mo values, the redox conditions changed from anoxic and ferruginous in interval 1, and euxinic in interval 2 have been concluded  . Comparison with Zunyi section, no sulfide layer was occurred in the Sugetbrak section during the early Cambrian period. At the same time, the Early Cambrian sediments were affected by upwelling oceanic hydrothermal fluids   . So the variation of Mo isotope in this study might not be merely connected with the suboxic or anoxic paleo-environment.
Figure 5. Mo isotope composition of present-day seawater sources and variations of reducing sediment in Earth history   .
Overprint by fractioned Mo from low-temperature hydrothermal systems onto sediments could also result in the variabilities of Mo isotope composition   . REE and Os isotope of the black shales give the clues of the hydrothermal overprint   , indicating that the hydrothermal fluid might affect the variability of Mo isotope and the Mo enrichment in our study. At the end of Neoproterozoic, the southern Tianshan Ocean began to pull-apart because of the breakup of the Rodinia suppercontinent   . The northern Tarim passive continental margin and southern Tianshan Ocean were developed in the Cambrian  . There are no deep faults developed on the northern Tarim passive continental shelf and no possibility to form local hydrothermal activity. The hydrothermal fluid in the northern Tarim continental shelf might come from extensional setting during the pattern of transgression and a rapid regional sea- level rise in the early Cambrian  .
Kirschvink et al. (1997) argued that marine circulation was reorganized repeatedly during the early Cambrian  . Nevertheless, the regional restricted basin with occasional seawater replenishment and intermittent dominance by continental input into restricted basins via riverine transport was preferred in our study. Mo contents of black rock series are generally within dozens of ppm range (Table 1), similar to black shales of the Niutitang Formation (a stratigraphic equivalent of the Yuetus Formation) from the Dingtai profile investigated by Xu et al. (2012), who argued that the Dingtai profile represents a restricted basinal environment  . At the same time, geophysics, geochemistry, and stratigraphic characters also support that the Tarim Basin is a kind of regional restricted basin with passive continental margin during the early Cambrian period    . The remarkable variability of δ98/95Mo composition in the black rock series could be the expression the interplay between open seawater and continental input into restricted basins via riverine transport  . In the restricted basin, Mo influx is dominated by continental input, and is replenished by new seawater with heavy Mo restores the normal seawater situation  . The comparatively large number of samples with δ98/95Mo below average present-day dissolved river load (0.7‰)  , and belong to the characters of continental margin suboxic sediments (from −0.7‰ to 1.6‰)  .
5.3. Oceanic Sedimentary Environment during the Early Cambrian of the Tarim Basin
There is a long standing debate about the reason for significant changes in oceanic geochemistry during the Ediacran-Cambrian transition      . Our study researches the redox sensitive elements, TOC, and Mo isotope characters of the black rock series (cherts-phosphorite assemblages and carbonaceous shale) to evaluate the changes of sedimentary condition during the early Cambrian in the Tarim Basin. A schematic model of the depositional environment of the black rock series is shown in Figure 6. The sedimentary condition where black rock series deposited was the restricted basin and near the continental margin. And the δ98/95Mo values also reveal the characters of continental
Figure 6. The schematic model of deposition environment for the black rock series.
margin suboxic sediments (from −0.7‰ to 1.6‰)  .
At the beginning of the early Cambrian (Nemakit-Daldynian age), the black rock series was drowned by a transgressive event  . And the sea level stayed a relative low level with the first episode hydrothermal fluid. Oxic seawater conditions in combination with nutrition provided by upwelling hydrothermal fluids induced the formation of phosphorite. Trace element and Mo isotope data suggest that interbedded chert-phosphorite assembles were scavenged from seawater under restricted conditions. The inferred scavenging process could occurred under replenishment of nutrition to the restricted basins by upwelling oxidize seawater   . High current or wave energy in combination with oxic seawater did not favor the preservation of organic matter, thus , the TOC contents of the basal Yurtus Formation are quiet low (0.05% - 0.94%) in cherts-phospho- rite assemblages layer. When the relatively sea level increase, the early Cambrian shallow sea became lack oxygen and become suboxic/anoxic. In the suboxic/anoxic conditions of the bottom water, the organic matter that fell from the photic zone partly oxidized because the residual oxygen in the seafloor. Thus, the TOC concentrations of upper Yurtus Formation carbonaceous shale are low (1.15% - 2.79%). In combination with the suboxic/anoxic conditions, another episode of hydrothermal activities promoted the enrichment of trace elements.
Combination with previously report data and the analysis of the geochemical redox indicators, TOC concentration, and Mo isotope data from black rock series at early Cambrian in the Tarim Basin, we conclude the following:
1) During the early Cambrian period, the transgressive and upwelling hydrothermal fluids from the open ocean have taken lots of nutritions into the restricted basin. Oxic conditions and rich nutritious water induced the laminated chert-phosphorite sedimentary with low TOC concentrations. When the sea level continues to rise, the sedimentary conditions of bottom water became relative lack oxygen, where were fit for the sedimentary of the carbonaceous shale in the restricted basin.
2) The Mo isotope compositions of black rock series could not only affected by hydrothermal events, but also the changes of sea level, and marine conditions.
We are very grateful to the reviewers for their reviews and suggestions, which significantly improved the quality of the manuscript. This work was supported by National Natural Science Foundation of China (No. 41203023).
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