Multiple Proxies on the Paleoenvironment of the Early Cambrian Marine Black Rock Series in the Tarim Basin , NW China : Molybdenum Isotope and Trace Element Evidence

The early Cambrian carbonaceous shale and laminated chert-phosphorite assemble (the black rock series) are widespread at the northwest margin of the Tarim Basin, Northwest China. In combination with previously reported data, we present stable molybdenum isotope (δMo), TOC, and redox-sensitive trace elements to evaluate the sedimentary conditions in early Cambrian water column during the deposition of the black rock series in the Tarim Basin. Redox variation was documented based on enrichment factors (MoEF, VEF, and UEF) and redox-sensitiv elements ratios (Ni/Co, V/Cr, δU), etc. During the early Cambrian, there was transgressive event, and the sea level continues to rise. In the basal Cambrian, laminated chert-phosphorite assemble with low TOC concentrations suggest the oxic sedimentary condition in a restricted basin. Light Mo isotope values and redox sensitive elements enrichment in the carbonaceous shale layer indicate lack oxygenic sedimentary condition, and was suboxic/anoxic conditions during the transgressive phase. The hydrothermal fluids from the open ocean affected the whole deposition process of the black rock series.

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 [15] [16] [17].
The early Cambrian represents a unique period in Earth history characterized by global environment and biological changes [18] [19].Lower Cambrian organic-rich black shales have been discovered in North America, southern Australia, parts of European, and Asia [7] [20] [21] [22] [23].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.
Previous research has been mainly focused on the Yangtze Platform about the paleo-environment during the early Cambrian period [7] [26] [28] [29] [30] [31].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 δ 98/95 Mo, TOC concentrations and trace element geochemistry attempts to contribute to an understanding of the paleo-environment of Early Cambrian seawater on the Northwest China.

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 [23].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 [32].Wusonger Formations in an ascending order [33].The Yurtus Formation unconformably overlies the dolomite of the Ediacaran Qigeblaq Formation, and conformably underlies the trilobite-bearing limestone of the Xiaoerblaq Formation [33].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 [23].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)) [23] [34] [35] [36] [37] According to paleontological studies in this area, the Asteridium-Heliosphaeridium-Comasphaeridium (AHC) acritarch assemblage zone, which was restricted to the Meishucunian Stage [35], 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 con-

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.

Trace Element and TOC Analysis
A routine HR/ICP-MS method was used.For each analysis, a

Analytical Results
The δ 98/95 Mo values, TOC, and trace element concentrations (such as Mo, U, and V, et al.) and their ratios are presented in Table 1.

Mo Isotope
Mo isotopic data are presented in Table 1, and their stratigraphic trend is shown in

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  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).

Evaluation of Detrital Input
Detrital, biogenic, and hydrogenous fractions show three independent concentrations of sediments [45].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 Th 4+ states, and largely independent of factors such as source area and grain size [16].Therefore, it is used in the present study to monitor detrital input [46].The lack of correlation between Th concentrations and U, and V ratios suggests no detrital influence on redox-sensitive elements (Figure 3(a),
Based on this criterion, all samples fall into the anoxic region (Table 2) [43] [44].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.
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 [9] [15] [41] [54] [55].Figure 2 reveals that the samples in the carbonaceous shale beds fall into the suboxicanoxic 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  of depositional conditions, due to the disturbance of the system by the substitution of V and U into apatite [11].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 [16].The REE results indicate that the lower Yurtus Formation shales are highly influenced by hydrothermal inputs [23] [27].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 [27].These significantly negative Ce anomalies are generally ascribed to organic matter derived from organisms in the euphotic zone [26].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 [56] [57] [58].In addition, no sulfide was found at basal of the Yurtus Formation in the studied area.Thus, the bottom water during the early  [56] 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.

Mo Isotopic Geochemistry
Mo isotope fractionation patterns have been used to reconstruct the redox state of the Earth's atmosphere and oceans [4] [7].The δ 98/95 Mo 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 MoO − , and Mo isotope signature has light Mo isotopic composition (δ 98/95 Mo = −0.7‰),which represent a large (~3‰) negative fractionation relative to the modern seawater composition (δ 98/95 Mo sw ) of ~+2.3‰ [59] [60] [61].A growing realization that Mo isotopes could also be fractionated during removal in intermediate redox environment where O 2 is scarce but H 2 S is not abundant which is so-called suboxic or anoxic setting [14]; 2) low-temperature hydrothermal systems.Mo may have been fractionated by precipitation of hydrothermal Fe-Mn oxides [5] [62] [63] [64]; and 3) influx of riverine Mo in restricted ocean basins not fully connected to the global ocean circulation.
In the Sugtbrak section, the δ 98/95 Mo ratios show lighter than that present-day euxinic sediments (2.3‰) [61], and heavier than typical of Mo adsorbed onto Mn oxides (−0.7‰) [61] [65], and fall into the range of suboxic sediments (Figure 5).The pattern of δ 98/95 Mo curve has good relationships with trace element ratios (V/Cr and Ni/Co) trends (Figure 2).δ 98/95 Mo negative anomaly (N1 and N2) could correspond to the suboxic-anxoic conditions which V/Cr and Ni/Co ratios indicated, and the δ 98/95 Mo positive shift (P1, Figure 2) also could relative with the oxic-suboxic conditions.Wen et al. (2015) reported δ 98/95 Mo values of early Cambrian black shales, which have a wide range of δ 98/95 Mo 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 [7].Combining with Fe abundance in samples, redox elements characters, and δ 98/95 Mo values, the redox conditions changed from anoxic and ferruginous in interval 1, and euxinic in interval 2 have been concluded [7].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 [23] [27].So the variation of Mo isotope in this study might not be merely connected with the suboxic or anoxic paleo-environment.International Journal of Geosciences tensional setting during the pattern of transgression and a rapid regional sealevel rise in the early Cambrian [23].Kirschvink et al. (1997) argued that marine circulation was reorganized repeatedly during the early Cambrian [67].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 ( present-day dissolved river load (0.7‰) [69], and belong to the characters of continental margin suboxic sediments (from −0.7‰ to 1.6‰) [59].

Oceanic Sedimentary Environment during the Early Cambrian of the Tarim Basin
There is a long standing debate about the reason for significant changes in ocea- margin suboxic sediments (from −0.7‰ to 1.6‰) [59].At the beginning of the early Cambrian (Nemakit-Daldynian age), the black rock series was drowned by a transgressive event [33].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 [25] [72].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-phosphorite 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.

Conclusions
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.
Figure 1.(a) Position of the study area.Modified after reference [23]; (b) geological setting of the studied section and the tectonic setting.Modified after reference [37]; (c) lithostratigraphy and the sampling horizons of the Sugetbrak section.

Figure 2 .
Figure 2. Stratigraphic distribution of elemental ratios and Mo isotopic compositions.

Figure 3 .
Figure 3. Cross-plot of various parameters cited from the Yurtus black rock series unit.Th correlates with V (a), and U (b).

Figure 5 .
Figure 5. Mo isotope composition of present-day seawater sources and variations of reducing sediment in Earth history [5] [61].

Figure 6 .
Figure 6.The schematic model of deposition environment for the black rock series.

instrument at the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences. 3.2. Mo Isotope Analyses
Merck, JMC, and Aldrich).For data presentation the δ 98/95 Mo ratio is used.The external standard reproducibility is at or below 0.1‰ for the δ 98/95 Mo ratio.The value of δ 98/95 Mo is defined by the following Equation (1).
[39]ed 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 HNO 3 (1:2) at 100˚C for at least 16 h until the samples were completely dissolved.An improved anion/cation exchange resin doublecolumn procedure was used to separate Mo from natural samples[38].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[39].The working Mo standard solution was prepared from a newly standard Mo solution C. Y. Yao et al.DOI: 10.4236/ijg.2017.88055969 International Journal of Geosciences (NIST SRM 3134,

Table 1 .
[42]δ 98/95 Mo values, TOC, and redox sensitive element data and ratios of the Sugetbrak section, Tarim Basin.theseratios to the normal shale[15][41].The data of standard shale are from McLennan et al.(1984)[42].The EF values of several redox sensitive elements (such as U, Mo, and V) are given in Table1.The Mo EF values are from 6.22 to 438.46 (average 146.53), U EF values are from 5.73 to 1431.41 (average 182.03), and V EF values are from 3.15 to 159.89 (average 60.66).

Table 2 .
Redox classification of the depositional environment.
DOI: 10.4236/ijg.2017.88055974 International Journal of Geosciences 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

Table 1 )
[5]imilar to black shales of the Niutitang Formation (a strati-DOI: 10.4236/ijg.2017.88055976InternationalJournal of Geosciences graphic 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[5].At the same time, geophysics, geochemistry, and stratigraphic characters also support that the Tarim Basin is a kind of re-