Geochemical Characteristics and Their Marine Environmental and Organic Source Implications for the Lower Cambrian Shales in Guizhou Province, South China

In this study, we conducted systematic analysis of total organic carbon (TOC) content, lipid biomarkers, carbon isotopes of kerogen (δCkerogen), and mineral composition in Niu-ti-tang (Є1n) and Pa-lang (Є1p) shales from Guizhou Province in order to provide a better understanding of the organic sources and marine environmental condition during deposition of the Lower Cambrian shales of South China. The results show that a broad variety of lipid biomarkers, such as n-alkanes, pristane, phytane, terpanes, hopanes, and steranes, are in these shales, which suggests a significant contribution of various paleobios with bacterial microorganisms and algae thriving under a low-salinity and stable anoxic environment. The negative δCkerogen value (minimum −36.4‰) and occurrence of pyrite (1–7.5%) and carbonates (2.4–57.3%) indicate that bacterial sulfate reduction prevailed under anoxic conditions during deposition of the Lower Cambrian shales. Moreover, the difference in mineral and δCkerogen composition between Є1n and Є1p shales might imply significant changes in primary production and paleoocean environments due to sea-level rise, as shown by the higher average TOC content in Є1p shales (2.52%) compared to that in Є1n shales (1.79%). The covariances of TOC content and mineral and δCkerogen composition suggest that the Є1p shales might have been deposited under a higher sea level associated with high primary productivity, compared to Є1n shales. Thus, high primary productivity driven by sea-level rise is suggested to be the main controlling factor on organic matter enrichment in Є1p shales under stable anoxic conditions.


Introduction
The Early Cambrian was a critical interval in geological history and featured profound oceanic and biotic changes, i.e., "Cambrian Explosion" [1,2]. During this critical time interval, organic-rich black shale was deposited during global sealevel rise and oceanic anoxic events [3][4][5][6][7]. The formation of organic-rich black shales is a complicated issue due to the interplay of various atmospheric, oceanographic, and biological factors and therefore has been an important research target for understanding life evolution and geochemical conditions of the Early Cambrian ocean.
Meanwhile, a set of organic-rich Lower Cambrian black shales in South China, e.g., Niu-ti-tang (Є1n) and Qiongzhu-si Formations (Є1q), is widely distributed throughout the Yangtze Platform. They have been identified as essential source rocks for shale gas exploration and development in this region because of their considerable thicknesses, large distribution area, relatively high total organic carbon (TOC) content, and thermal maturity [8][9][10]. Over the past decades, organic-rich black shales in the Yangtze region have attracted interest due to their significant geological and economic importance as shale gas [11][12][13][14][15]. In particular, shale reservoir characteristics and gas potential, including their TOC content, pore size distribution, and methane sorption capacity, have been extensively determined [16,17]. However, there are still many challenges in promoting shale gas exploration and development in China. One of the primary challenges includes the poor understanding regarding the organic matter enrichment mechanisms of marine shales in South China.
Recently, studies have been conducted to reconstruct the paleoenvironment and paleoproductivity of the Lower Cambrian marine shales in South China and to constrain the main factors controlling the formation of organic-rich black shales [3,5,6,[13][14][15][18][19][20][21][22][23][24][25][26][27]. Of these studies, considerable advances have been made in our understanding of the organic matter enrichment mechanisms of these Lower Cambrian shales and shale gas resources, which play a critical role in delineating shale gas exploration and development targets in South China [23]. However, the organic matter enrichment mechanisms of marine shales in this area is complex, and the depositional environments during different time intervals of the Early Cambrian could have resulted in markedly different organic matter enrichment mechanisms [24]. For instance, Huang et al. [6] tried to unravel the oceanic redox condition changes within a sequence stratigraphy framework of the Є1n (~528-521 Ma) shales of the Yangtze Platform by performing high-resolution analyses of iron speciation and redox-sensitive trace elements (Mo and U). They concluded that anoxic-ferruginous and euxinic bottom waters with moderately strong restriction resulted in high production and good preservation of organic matter. On the other hand, Wang et al. [7] proposed that paleoproductivity was the main factor controlling the high organic content of the Є1q black shales in the Sichuan Basin due to the covariation among P org , Cu bio , and TOC. Liu et al. [22] also proposed that productivity was the dominant factor controlling organic matter preservation within the black shales of South China. Jin et al. [21] applied multiple paleoproductivity, paleoredox, upwelling/restriction, and terrigenous flux proxies on these shales and suggested that a combination of primary productivity, redox conditions, and terrigenous fluxes driven by sea-level change may play a key role in organic matter accumulation on the western Yangtze Platform during the Early Cambrian. As a whole, the marine redox conditions and biogeochemical cycling in the Early Cambrian ocean changed spatiotemporally due to paleogeographic, paleoclimatic, and eustatic sea-level variations [6,15,20]. Therefore, the organic matter enrichment mechanisms of marine shales in this area are still not fully understood. In addition, previous studies focused mainly on inorganic geochemical ratios, i.e., iron speciation and redox-sensitive trace elements, of the Lower Cambrian organic-rich black shales of the Yangtze Platform [6,21]. The lack of studies linking bulk organic geochemical, lipid biomarkers, and mineral composition to decipher the mechanisms driving organic matter accumulation hinders our understanding of the spatiotemporal evolution of primary productivity and redox conditions of paleooceans, in particular, what factors control the enrichment of organic matter.
The north region of the Guizhou Province was located in the southeast Yangtze Platform (Figures 1(a) and 1(b)).
Massive black shales were deposited with a thickness of more than 100 m under global sea-level rise and oceanic anoxic events during the Meishucunian and Qiongzhusian Ages [4,28]. These shales exhibit significant lateral and vertical variation in both geochemical and mineral compositions (Figure 1(c)). They were formed through multiple tectonic events, which significantly influenced the primary productivity and environmental conditions of the paleoocean [29]. In this study, we collected samples from a continuous core (ZK601 core) covering the Lower Cambrian Є1n and Pa-lang (Є1p) shales in northern Guizhou Province, which provides the opportunity to decipher the geochemical characteristics of organic-rich black shales and their marine environmental and organic source implications. This study presents geochemical evidence including lipid biomarker composition and carbon isotopes of kerogen (δ 13 C kerogen ), together with bulk geochemical parameters (i.e., TOC content) and mineral composition, to decipher the biodiversity, primary productivity, and paleoocean environment during the Early Cambrian. The particular focus of this study is to shed new light on the predominant controlling factors for organic matter accumulation in the Lower Cambrian shales from Guizhou Province, South China, which will benefit delineating shale gas exploration and development targets in the northern Guizhou area.

Materials and Methods
2.1. Geological Background and Sampling. During the Early Cambrian, water depths in the Guizhou area shifted from shallow to deep from west to east, forming significant stratigraphic variations [30]. The Lower Cambrian system in the Guizhou area can be divided according to its stratigraphic and biota differences into the Yangtze region, a transition region, and the Jiangnan region from west to east [31]. The ZK601 core samples of the Lower Cambrian shale were collected from a mineral exploration drill core in the Tong-ren district of northern Guizhou Province (Figures 1(a) and 1(b)), a geological transition region between the Yangtze Platform and the Cathaysia Platform with a restricted deep water marine environment which prevailed during the deposition of these Lower Cambrian shales. The Lower Cambrian strata in this region can be divided into the Є1n, Є1p, and Qing-xu-dong Formations [32]. A stratigraphic correlation of cores was conducted by the Guizhou Bureau of Geology and Mineral Exploration and Development based on the regional geology, lithology, and biostratigraphy [33][34][35][36][37]. The lithology of these shales has been described in detail in our previous study [28]. In brief, the Є1n is mainly composed of carbonaceous and siliceous shale, and the Є1p consists of silt shales, calcareous shales, siliceous shales, and argillaceous limestones (Figure 1(c)). The core profile covers the lower part of the Є1n and part of the overlying Є1p shales with a total depth of 397.8 m. The shale samples were measured for total organic carbon (TOC) content, pyrolysis parameters, mineral composition, biomarker signatures, and δ 13 C kerogen composition. 2 Lithosphere

Total Organic Carbon, Pyrolysis, and Mineral
Composition Analysis. The TOC contents were measured using a Leco C230 Carbon/Sulfur analyser after carbonates of each powder sample were removed with 5% HCl. The instrument was automatically programmed, and the combustion temperature was set to about 1500°C. The TOC values were calculated according to the peak area of CO 2 generated from the combustion of organic matter. The pyrolysis analyses were determined using a Rock-Eval 6 Standard analyser. Mineral composition analyses were carried out on a Bruker D8 Advance X-ray diffractometer (XRD). The relative mineral percentages were estimated semiquantitatively based on the intensity of specific reflections, the density, and the mass sorption coefficient of the identified mineral phases. Those data (Table S1 and S2) were published by our team in previous studies [28,38].
2.3. Extraction, Separation, and Biomarker Analysis. Clean analytical procedures were followed during sample preparation and analysis [39]. The solvents, including n-hexane, dichloromethane (DCM), and methanol (MeOH), are of HPLC grade. All glassware was heated in an oven at 400°C for 4 h before use. Filter paper, alumina, and silica gel were extracted by DCM before use. The surfaces of the bulk samples were cleaned in order to eliminate possible contamination and crushed to less than 100 mesh powders for further treatment. The methods for extraction and separation followed those of [40]. Briefly, powder samples were subjected to Soxhlet extraction for 72 h with a mixture of DCM and MeOH (9 : 1, v/v). Activated copper were added for removing elemental sulfur. After the removal of the solvent using an evaporator, the extractions were dissolved in n-hexane   to remove asphaltenes. Alumina/silica gel column chromatography was used to separate the resultant maltene fraction into saturated hydrocarbons, aromatic hydrocarbons, and polar compounds, using n-hexane, a mixture of n-hexane and DCM (4 : 1, v/v), and a mixture of DCM and MeOH (1 : 1, v/v) as elution solvents, respectively. Saturated hydrocarbons were analysed using an Agilent HP6890 gas chromatograph (GC) with a flame ionization detector and a Thermo Finnigan Gas Chromatography Mass Spectrometer (GC-MS). The analytical conditions were identical to those of [40]. In brief, a DB-5 fused silica column (30 m × 0:25 mm i:d:× 0:25 μm film thickness) was used. The samples were injected in the splitless mode; N 2 was the carrier gas at a constant flow mode. The oven temperature was initially set at 80°C (held for 5 min) and programmed to increase at 3°C/min to 290°C (held for 10 min). An electron impact ion source of 70 eV was used for GC-MS, and the MS full scan range was m/z 50-600.

Stable Carbon Isotope Analysis.
The δ 13 C kerogen composition was measured by an Isotope Ratio Mass Spectrometer (Finnigan Delta Plus XL). Before analysis, each powdered sample was treated repeatedly with diluted HCl and HF for the removal of carbonate and silicate minerals, then washed with distilled water until a neutral state was attained, and finally freeze-dried. After that, approximately 2 mg of each sample was loaded into a clean tin capsule for analysis. The results were reported in the standard delta notation as δ 13 C vs. VPDB. Instrument performance was routinely checked using a standard, and the reported δ 13 C values represented the average values of two parallel measurements with an error of less than 0.5‰.

Results and Discussion
3.1. Bulk Organic Geochemical Parameters. The TOC content and pyrolysis data are cited from previous studies and summarized in Table S1 [28,38]. In brief, the TOC content ranges from 0.29% to 6.52% and averages 2.52% for the Є1p shales; for the Є1n shales, the values vary from 0.07% to 7.34% and average 1.79%. The TOC contents > 2 % are mainly within depth ranges of 290 m to 360 m, and the Є1p shales display a higher average TOC content than the Є1n shales. The TOC values in this study are comparable to those of the Є1q shales (0.87-7.21%) in the Sichuan Basin [7].
In this study, T max values vary from 450°C to 599°C, suggesting varied maturities. Thus, it is necessary to acquire other effective maturity indices, i.e., bitumen reflectance and vitrinite equivalent reflectance. Since there is a lack of vitrinite particles in marine source rocks of the Lower Paleozoic, bitumen reflectance (R b ) is widely used to calculate the vitrinite equivalent reflectance (VR oequiv ) of kerogen [41,42]. The calculated VR oequiv values of representative samples are ranged from 2.37 to 3.64% [38]. Meanwhile, the measured Raman reflectance ( Rmc Ro) values for representative shales vary from 2.92% to 3.54% [38]. These results are generally consistent and suggest that these Lower Cambrian shales show high to overmaturity [43]. The clay minerals in the studied samples consist mainly of illite further suggesting that the Є1n and Є1p shales have entered the late diagenetic stage with a high to overmaturity [44][45][46][47][48].
The TOC content is usually used to represent the relative content of organic matter in rocks, but it needs to be used with caution when it comes to highly matured rocks [44,49]. In fact, the TOC content of the highly matured rocks represent the residual carbon or dead carbon; therefore, the original TOC (TOC o ) would be underestimated. It is well documented that the measured residual TOC of these highly matured shales from the Early Cambrian is far from the TOC o [44]. Meanwhile, previous research showed that there is a simple formula to recover the TOC o , and the recovery coefficients of the Lower Cambrian shales from South China are largely between 3.0 and 3.4 [49]. Assuming a recovery coefficient of 3.0, the TOC o of these Lower Cambrian shales reach 22%. In addition, the plot of S 1 versus TOC content reflects that all of the studied shales from the Lower Cambrian are characterized by autochthonous hydrocarbons (Figure 2(a)) [50,51]. Pyrolysis parameters, such as T max versus hydrogen indices (HI), are often used to indicate kerogen types. As is shown in Figure 2(b), the kerogen type of the studied samples is type III, which contradicts geological fact that no vitrinite particles existed in the Lower Cambrian shales [41,42]. For example, Tian et al. [44] recovered the original kerogen type in the Lower Cambrian Є1n shales in the Micangshan-Hannan Uplift, SW China, and suggested a mixture of kerogen types I and II, which are more aligned with the geological fact and previous evidence [29,42].
Overall, the bulk organic geochemical parameters suggest that these Lower Cambrian shales have a high TOC o content (maximum 22%) with high to overmaturity, and contain a mixture of original kerogen types I and II, with average TOC content of the Є1p shales (2.52%) being higher than that of the Є1n shales (1.79%).

Mineral
Composition. The mineral composition of these shales are reported in previous studies and compiled in Table S2 [28,38]. These Lower Cambrian shales consist mainly of clay minerals, quartz, feldspar, carbonates, and pyrite. The Є1n shales consist mainly of quartz (mean: 64.9%), while the Є1p shales consist mainly of clay minerals (>50%) with lower quartz content (mean: 30.9%) ( Figure 3). The Є1n and Є1p shales all show poor correlations between TOC and quartz/clay mineral content (Figure 4), indicating that quartz and clay mineral have an insignificant effect on the enrichment of TOC. The Є1n and Є1p shales have variable carbonate contents (of 2.4-57.3%), which are dominated by dolomite and calcite, respectively. The pyrite content of Є1n and Є1p shales is largely less than 5%, with the Є1p shales being slightly higher. The different characteristics of mineral composition between Є1n and Є1p shales demonstrate that the paleomarine environment (i.e., water depth and sedimentary facies) might fluctuate. A previous study suggested that high quartz content is generally interpreted to develop in a shallow water environment, while high clay mineral content and low quartz content suggest deep water conditions [52]. Therefore, the heterogeneity of these shales 4 Lithosphere might be controlled by changes in sea level during the Early Cambrian when an extensive marine transgression occurred in the Upper Yangtze Platform [3-7, 21, 28, 29, 52], with the Є1p shales deposited under a higher sea level, compared to Є1n shales. Iron speciation, including total iron (Fe T ) and highly reactive iron (Fe HR ), carbonate iron (Fe carb ), (oxyhydr)oxide iron (Fe ox ), magnetite iron (Fe mag ), and pyrite iron (Fe py ), has been widely used in fine-grained siliciclastic sediments to reconstruct redox conditions of the water column [6]. For instance, a Fe HR content of >38% of the total sedimentary iron pool generally indicates anoxic water conditions, whereas a much lower proportion of Fe HR commonly suggests an oxic water setting [53]. Fe py /Fe HR ratios > 0:8 are considered to mark anoxic conditions [54]. As shown in Table S2, pyrite was identified in most samples, particularly in the Є1p shales, with contents ranging from 1% to 7.5%. Although iron speciation data was not available in this study, the dominance of pyrite, which is the only iron mineral detected in these shales, can also reflect anoxic water conditions. In addition, framboidal pyrite has been identified in these shales [38], which is considered to be the result of rapid nucleation and crystallization under a fluid environment with high iron and high sulfur content. The presence of framboidal pyrite has been used as an indicator of redox conditions [55][56][57]. It is inferred that the precipitation of pyrite was commonly induced by organiclastic sulfate reduction (OSR) (2CH 2 O + SO 4 2− ⟶ 2HCO 3 − + H 2 S) and sulfatedriven anaerobic oxidation of methane (SO 4 -AOM) (CH 4 + SO 4 2− ⟶ HCO 3 − + HS − + H 2 O) in marine sediments [58]. Sulfate reduction, carried out by sulfatereducing bacteria, has been regarded as the dominant anaerobic biogeochemical process in marine sediments since sulfate is the most abundant dissolved electron acceptor in seawater [59]. The sulfur occurs mainly in three forms, including the sulfate dissolved in sea water, sulfate precipitated in ancient evaporite deposits, and sulfide in marine detrital deposits (dominated by pyrite) [60]. The sulfur cycle begins with sulfate in the oxidized state, whose initial reduction product is H 2 S (ΣH 2 S ⟶ H 2 S + HS − + S 2− ). It can react with active iron to form intermediate products such as FeS and Fe 3 S 4 and finally transform into authigenic pyrite (FeS 2 ) under anoxic conditions [61,62]. It is noteworthy that elemental sulfur, the intermediate product of pyrite formation, was observed in bitumens of the studied shales [40], which further confirms the occurrence of OSR and/or SO 4 -AOM. In addition, bacteria sulfate reduction is responsible for an increase in alkalinity that favors the precipitation of authigenic carbonates, including calcite and dolomite [63,64]. The Є1n and Є1p shales contain different carbonates (dolomite and calcite, respectively), which might be the result of different depositional settings and a difference in dominated microbial communities [63,64].  [29,65,66]. The n-alkanes in the Є1n and Є1p shales range from C 14 to C 27 and exhibit dominant short-chain n-alkanes with a unimodal distribution (peaked at n-C 17 , n-C 18 , and n-C 23 ) (Table S3; Figure 5). The C 21 -/C 22+ ratios vary from 0.93 to 5.21, indicating lower aquatic organism input [67]. Notably, Yamada et al. [66] proposed that the short-chain n-alkanes extracted from the Lower Cambrian shales in the Three Gorges area of South China originated primarily from the phytyl groups of chlorophylls in phototrophs. Pristane (Pr) and phytane (Ph) are present in relatively high abundance, with phytane always being more abundant than pristane. Previous studies found that pristane and phytane have multiple sources, including cyanobacteria, purple sulfur bacteria, and archaea [40]. An obvious unresolved complex mixture hump (UCM) associated with the short-chain n-alkanes is observed ( Figure 5), which is common in biodegraded hydrocarbons [68]. Nevertheless, the abundant n-alkanes relative to isoprenoids indicate that the biomarkers experienced only slight biodegradation since n-alkanes are more prone to biological degradation than branched alkanes.
A complete series of terpanes, including tricyclic terpanes (TT) and pentacyclic triterpanes, were detected both in the Є1n and Є1p shales ( Figure 6). In these shales, the pentacyclic triterpanes are in higher abundance than tricyclic terpanes, with the C 19-24 TT/(C 19-24 TT + αβC 30 Hop) generally less than 1.0 (Table S3). Tricyclic terpanes are dominated largely by C 21 TT and C 23 TT. Tricyclic terpanes are detected commonly in crude oil and source rock extracts and are generally believed to originate from bacteria [69] and higher terrestrial plants [70]. Considering that no vitrinite particles existed in the Lower Cambrian shales [41,42], we suggest that bacteria are the likely source input.
The pentacyclic triterpanes were dominated by αβC 30 hopane followed by αβC 29 hopane, and the relative abundance of homohopanes decreased with increasing carbon number ( Figure 6), with the αβC 29 /αβC 30 Hop values from 0.66 to 0.85. The hopanes were present from C 27 to C 35 , sug-gesting a high contribution of bacteria-derived organic matter, such as cyanobacteria, methanogenic bacteria, and other autotrophic microorganisms [71][72][73][74][75][76]. Gammacerane, commonly considered to be derived from tetrahymanol living in the interface of the chemocline, was detected in low abundances, with the Ga/C 30 Hop ratios from 0.12 to 0.33. 25-Norhopanes were also detected. The C 28 (Table S3). It is generally believed that 25-norhopanes are marks of biodegradation and/or microbial activities [68].
A series of steranes were recognized in these samples,  (Table S3). For regular steranes and diasteranes, 20R αααC 27 sterane and 20R αααC 29 sterane are the maxima peak compounds. The relative abundance of the regular steranes is often applied to identify the dominant source of organic matter. As shown in Table S3, the percentages of C 27 , C 28 , and C 29 steranes among the total of C 27 -C 29 steranes were in the range of 29-47%, 22-29%, and 31-48%, respectively, with the C 29 slightly higher than C 27 . It is proposed that C 27 steranes are commonly derived from animals (i.e., zooplankton) and some phytoplankton (i.e., red algae, green algae) [77,78]. C 28 steranes are mostly from phytoplankton (i.e., diatoms and haptophytes) instead of zooplankton [78], and C 29 steranes are generated by both vascular plant and algal sources (i.e., green microalgae) [78,79]. The high abundance of C 29 steranes in the studied shales could be derived from algae (i.e., green microalgae). C 30 4α-methyl-24-ethyl steranes were detected in low amounts, suggesting a minor dinoflagellate input [80]. Compared to terpanes, steranes show lower concentrations with the sterane/ hopane ratios (St/Hop) from 0.15 to 0.45 (Table S3), indicating a higher contribution of prokaryotic bacteria over algae [68].

Lithosphere
Lipid biomarkers are not only used as marks for organic source input identification but also used as indicators for environment conditions. The ratios of Pr/Ph, Pr/n-C 17 , and Ph/n-C 18 are often used in the analysis of sedimentary conditions. However, these parameters may be altered by source input, maturation, and biodegradation [68]. Generally, the Pr/Ph ratio < 0:8 indicates anoxic conditions. In this study, the low values of Pr/Ph (0.30-0.85) in the studied shales, combined with the cross-plot of Pr/n-C 17 and Ph/n-C 18 ratios, suggest a strong anoxic environment with considerable marine organic matter input (Figure 8(a)    7 Lithosphere reducing marine shale environment (Figure 8(b)). High abundances of gammacerane and C 35 hopane are typically observed in evaporitic or high-salinity environments [81], which were not observed in these samples. Their low Ga/ C 30 Hop values (0.12-0.33) and minor abundance of C 35 hopane reflect a moderately saline environment [82]. The diasterane/regular sterane ratio is also used as an indicator of a sedimentary environment because oxic conditions are favorable for diasterane formation. At comparable levels of thermal maturity, it can be used to reflect the redox condition, with high values indicating an oxic environment [68]. However, considering the high thermal maturity of these shales, the abundance of diasteranes in this study with Dia/ Reg C 27 St ratios around 0.6 likely suggests an anoxic water column.
In summary, a broad variety of lipid biomarkers including n-alkanes, pristane, phytane, tricyclic terpanes, C 27 -C 35 hopanes, gammacerane, 25-norhopanes, C 21 -C 22 diginanes, C 27 -C 29 regular steranes and diasteranes, and C 30 4αmethyl-24-ethyl steranes is detected in the Є1n and Є1p shales. However, the composition and distribution of these biomarkers are similar in the Є1n and Є1p shales, which likely resulted from high thermal maturity. The biomarker signatures in this study are generally consistent with those    [83][84][85][86]. In addition, our previous study suggests that the biomarkers in the studied samples can provide in situ evidence [40]. Therefore, the molecular evidence reveals a significant contribution of various paleobios with bacterial microorganisms and algae prosperous in a low-salinity and anoxic environment of the Є1n and Є1p shales but failed to decipher variations of primary productivity between the Є1n and Є1p shales because of high thermal maturity.
3.4. δ 13 C Composition of Organic Matter. The δ 13 C value of organic matter is generally controlled by the organic carbon fractionation process during photosynthesis, organic matter depositional rates, and postdepositional alteration through thermal alteration and diagenesis [15,87]. Based on the significant variation in δ 13 C kerogen ratios (−36.4‰ to −29.8‰) and similar thermal maturity of these shales, we suggest the δ 13 C kerogen values undergo minimal postdepositional alteration [88][89][90][91]. Therefore, the variation of carbon isotopic composition in this study is controlled mainly by the evolution of source organisms and geochemical conditions of the paleoocean [87].
Based on the rules of photosynthesis fractionation, enhancing primary productivity over a period time likely led to higher carbon isotope values by reducing the preferential selectivity of 12 C during photosynthesis [89,92]. In this study, the Є1n and Є1p shales show different δ 13 C kerogen values. The Є1n shales are depleted in 13 C and have minimum δ 13 C kerogen values of −36.4‰, which gradually increase towards the Є1p shales with δ 13 C kerogen excursion of 6.6‰ (Table S3). The positive excursion of δ 13 C kerogen values is consistent with an average TOC increasing from 1.79% (Є1n shales) to 2.52% (Є1p shales). This means that higher primary productivity might account for the positive isotope values and higher TOC contents in the Є1p shales compared to the Є1n shales.
On the other hand, previous studies suggest that the organic carbon isotopes of marine biomass and marine sedimentary organic matter are generally higher. For instance, organic matter derived from marine algae has a typical δ 13 C range of −23‰ to −16‰ with an average of −19‰ [93]. The marine phytoplanktons in the Black Sea also have positive δ 13 C values (−25‰) [94]. Hayes et al. [89] reported an average δ 13 C value of −28‰ for marine sedimentary organic matter over the past 800 Ma. Therefore, the increase in the contribution of 13 C-depleted organisms to bulk organic matter may greatly contribute to the negative δ 13 C kerogen values because of the prevailing anoxic water conditions in the Є1n and Є1p shales. It is generally accepted that chemoautotrophic bacteria and prokaryotic organisms have the most negative carbon isotopic composition [68]. For example, the carbon isotopic values of organic matter derived from chemoautotrophic bacteria vary from −45‰ to −34‰ [72,73], nitrifying bacteria have an isotopic composition of about −38‰ to −36‰ [94], and sulfatereducing bacteria and methanogenic bacteria are characterized by negative δ 13 C values of about −90‰ to −40‰ [89,92]. Abnormal depletion of 13 C in organic matter during the Early Cambrian is widely observed [15,29,83]. The negative δ 13 C values of these samples, particularly in the Є1n shales, suggest a significant contribution from chemoautotrophic bacteria and other prokaryotic organisms as revealed by biomarker evidence, especially sulfate-reducing bacteria. The widely distributed pyrite and high carbonate contents are consistent with the speculation that bacterial sulfate reduction might have prevailed under anoxic conditions during deposition of these Lower Cambrian shales, whereas the shift to higher isotopic values in the Є1p shales reflects higher primary productivity, which is also consistent with the higher average TOC content in the Є1p shales.
3.5. Controls on Organic Matter Enrichment. The causes of organic matter enrichment and their relationship with paleoenvironmental changes and primary productivity in the Yangtze Platform of South China are still debated [4-6, 14, 15, 18-20, 22-27]. Of these studies, several mechanisms have been proposed to control the formation of organic-rich black shales, including primary productivity, preservation conditions, sea-level change, terrigenous detrital supply, paleoclimate change, sedimentation rates, tectonic activity, and hydrothermal activity [22][23][24][25][26][27]. Among those mechanisms, higher organic carbon fluxes caused by high primary productivity, and anoxic conditions favoring organic matter preservation, have been proposed as two major factors for increasing the accumulation of organic matter in these shales. But there is no consensus on the controlling factors of organic matter enrichment in this region. The primary reasons include (1) spatiotemporal differences caused by paleogeographic, paleoclimatic, and eustatic sea-level variations [6,7,14,20,21] and (2) the lack of comprehensive studies on organic and inorganic geochemical evidence [27]. Thus, the reconstruction of primary productivity and preservation conditions on the basis of multidisciplinary evidence in different regions of South China is critical for understanding the formation of organic-rich black shales in the Early Cambrian.
In this section, we summarize the depth variation of TOC content, δ 13 C kerogen values, mineral composition, and key biomarker parameters of the Lower Cambrian Є1n and Є1p shales in Figure 9. As indicated by the mineralogical evidence, the Є1p shales might have been deposited under higher sea levels compared to the Є1n shales. And primary productivity was higher during this period, as evidenced by the TOC content and δ 13 C kerogen values. The covariance of mineralogical and organic geochemical evidence suggests that sea-level rise may be the main driver of higher primary productivity in the Є1p shales. This is consistent with the viewpoint that the rise in sea level could have stimulated the material exchange and brought rich nutrients and increased the primary productivity, which would result in high TOC content, positive δ 13 C kerogen values, and anoxic bottom conditions [15]. It is notable that the role of sealevel change on organic matter accumulation has only recently been taken into consideration. Recent geochemical and mineralogical studies have emphasized that sea-level change is the controlling factor on the redox conditions and primary productivity of marine shales [23,25,26]. 9 Lithosphere There is consensus that the increase in water depth may result in an increase in water mass circulation and nutrient upwelling, which carried the nutrient-rich bottom waters to the photic zone, leading to high primary productivity and strongly reducing bottom waters [25,26]. Meanwhile, these shales all show a poor correlation between depth variations of TOC contents and varied biomarker indicators (i.e., St/Hop, Pr/Ph, Ga/C 30 Hop) (Figure 9), which might be the result of alteration associated with thermal maturity during the deposition of these Lower Cambrian shales. The anoxic conditions favored organic matter preservation and resulted in high TOC contents [20,21]. However, the stable anoxic environment within the Є1n and Є1p shales illustrate that anoxic conditions should not be the dominant factor controlling the enrichment of organic matter. Overall, we suggest that high primary productivity driven by sea-level rise was the main factor controlling organic matter enrichment in the Є1p shales on the premise of the stable anoxic condition.

Conclusions
The biodiversity, primary productivity, and paleoocean environment have been deciphered based on lipid biomarker composition and δ 13 C kerogen values, along with bulk geochemical parameters (i.e., TOC content, T max ratio) and mineral composition through the Lower Cambrian Є1n and Є1p shales from Guizhou Province, South China. The composition and distribution of a broad variety of lipid biomarkers in these shales, including n-alkanes, pristane, phytane, terpanes, hopanes, and steranes, suggest that various paleobios prospered in a low-salinity and stable anoxic environment, and bacterial microorganisms combined with algae are major source inputs. The significant negative δ 13 C kerogen values (minimum of −36.4‰) and the presence of carbonate and pyrite reflect bacterial sulfate reduction (OSR and/or SO 4 -AOM) which might have prevailed under anoxic conditions. The distinct mineral composition, TOC content, and δ 13 C kerogen values in the Є1n and Є1p shales indicate that the Є1p shales might be deposited under a higher sea level associated with high primary productivity compared with Є1n shales. High primary productivity driven by sea-level rise is interpreted to be the main factor controlling organic matter enrichment in the Є1p shales, instead of stable anoxic conditions.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.  The TOC contents and mineral composition were adopted from [28,38]. Notes: Pr: pristane; Ph: phytane; TT: tricyclic terpane; Ga: gammacerane; St: sterane; Hop: hopane. 10 Lithosphere Lithosphere