Constraints on the Organic Matter Accumulation of Lower Cambrian Niutitang Shales in the Middle Yangtze Region, South China

The lower Cambrian Niutitang shales, as one of target intervals with the greatest potential for shale gas exploration and development, have attracted much attention. Nevertheless, the organic matter enrichment mechanisms of the lower Cambrian Niutitang shales need further study, especially in the hydrothermal active zone. In this study, samples from ND1 well in western Hubei Province, middle Yangtze region, South China, were investigated for the controlling factors of organic matter accumulation of Lower Cambrian Niutitang shales by detailed petrographic, mineralogic, and geochemical proxies. The results show that hydrothermal activity and sea level fluctuation controlled the redox conditions and paleoproductivity of seawater and ultimately controlled the organic matter accumulation of Niutitang formation. In the Niu-1 member, the intense hydrothermal events lead to a suboxic to anoxic environment, which is conducive to the organic matter preservation. However, low sea level strengthens the restriction of water mass and reduced nutrient upwelling into the shelf, leading to decreased marine primary productivity, which was ultimately responsible for depleted organic matter accumulation in the Niu-1 member. In the Niu-2 member, the anoxic-euxinic environment and high paleoproductivity, driven by continuous hydrothermal activity and rising sea level, were the main factors controlling the enrichment of organic matter. In the Niu-3 member, the dysoxic to oxic condition plus low primary productivity, caused by the disappearance of hydrothermal activities and sea-level fall, resulted in the unfavorable organic matter accumulation. The results of this paper enrich the model of organic matter enrichment in the lower Cambrian black shale in the middle Yangtze region.


Introduction
Organic matter-rich shales are dark-colored rocks with organic matter and silt-to clay-sized mineral grains [1]. Owing to the enrichment of organic matter, shales have been considered as important source rock with great hydrocarbon-generating potential [2]. However, the dominant factors that permitted organic matter to accumulate and preserve in black shales are still controversial [3]. As the organic matter enrichment is a very complex physical and chemical process, its mechanisms have been widely discussed for many years [4][5][6][7][8][9][10][11]. It's likely, however, that no single factor can be responsible for organic accumulation, and there may be several factors that lead to the enrichment of organic-rich sediments in each sedimentary environment [12]. Ocean redox conditions, primary productivity and terrigenous detrital input, etc., were considered to play a significant role in the organic accumulation of marine shales [1,4,7,11,[13][14][15][16]. In recent years, the influence of hydrothermal activities on the enrichment of organic matter has attracted much attention [4,[17][18][19][20][21].
As one of the greatest potential target intervals for the exploration and development of shale gas, the Lower Cambrian Niutitang (or correlative Shuijingtuo) shales have been paid much attention, whereas previous studies were mostly focused on lithofacies, depositional environment, and pore structure [22][23][24][25][26][27][28][29][30]. Studies on organic accumulation of Lower Cambrian Niutitang Formation mainly focus on the black shales deposited in deep water settings in the middle-lower Yangtze Block, such as Xiangxi area and Guizhou Province [2,4,19,31,32]. However, the research work in shallow-water settings is relative insufficient, such as in western Hubei. Moreover, it is still controversial whether the development of Lower Cambrian Niutitang shales in western Hubei is influenced by the hydrothermal fluid [4,33]. Hence, ND1 well in western Hubei is selected in this paper to elucidate the organic enrichment mechanisms of Lower Cambrian Niutitang shales in the middle Yangtze region, South China.

Geologic Setting
By ca. 530 Ma, the South China Craton was positioned near the equator (Figure 1(a); [34,35]). During early Neoproterozoic, the amalgamation of the Yangtze and Cathaysia blocks formed the South China Block [32,36,37]. Afterwards, the South China Block gradually separated into Yangtze and Cathaysian Block again during 850-820 Ma,  [34,35]). (b) Paleogeographic map of the Yangtze Block during the Ediacaran-Cambrian transition (after [32]). (c) Stratigraphic column of ND1 well about Cambrian Niutiang Formation. 2 Lithosphere due to intense rifting [37,38]. The Yangtze block underwent an intense extensional (rifting) tectonism during the E-C transition [36,39,40,41], as indicated by the existence of a hydrothermal eruption system related to extensional faults along the southern margin of the middle Yangtze Block. After that, the Yangtze Block was composed of carbonate platform, narrow intraplatform basin, and open siliceous basin from NW to SE during the E-C transition (Figure 1(b); [32]). This sedimentary facies (or lithofacies) configuration lasted until the whole platform was almost completely flooded during the Niutitang Formation deposition [39]. Therefore, the E-C successions of the Yangtze Block across the platform-basin transect are quite variable ( Figure 2). As shown in Figure 2, in the upper Ediacaran, the dolomite-dominated Dengying Formation covers platform and slope margin; however, slope and basinal settings are covered by the chert-dominated Liuchapo Formation, which is equivalent to Dengying Formation [39,40,42,43]. Towards the basin, the Liuchapo Formation appears as a diachronous unit, straddling the E-C boundary [42,43]. The lowermost Cambrian succession overlies the upper Ediacaran succession unconformably in shallow-water setting or conformably in deep-water setting ( Figure 2). The Yanjiahe and Yangjiaping Formations constitute the lowermost Cambrian strata in the platform successions, which change into the upper Liuchapo Formation towards the basin. These formations are further overlain by the widely distributed Niutitang (or correlative Shuijingtuo) Formation, the basal age of which is 522 Ma (Figure 2; [39,40,43]).
The ND1 well, located in Niejiahe Town, Yichang City, Hubei Province, comprises the upper part of the Dengying Formation (about 10 m) and the whole Niutitang Formation with a total thickness of 140 m (Figure 1(c)), representing intraplatform basin deposition. The Dengying Formation, underlying the Niutitang Formation, is composed of light gray dolomites. The Niutitang Formation is dominated by siliceous shale with a Ni-Mo sulfide ore layer at the bottom and can be subdivided into three unites from bottom to top (Figure 1(c)): (1) Niu-1 member, characterized by abundant barites and pyrites; (2) Niu-2 member, locally rich in pyrite and calcite vein; and (3) Niu-3 member, which enriches calcite veins and bands.

Sampling and Methods
In order to elucidate the organic enrichment mechanisms of Lower Cambrian Niutitang shales in South China, shale samples from the high-quality drill core of Niutitang Formation in ND1 well were determined for total organic carbon (TOC) content, mineral composition, and major and trace elements. TOC content was analyzed by vario EL III CHNOS elemental analyzer. Initially, 2~3 g powdered shale samples were immersed with 5% hydrochloric acid (HCl) in beaker for 48 hours (h) to remove inorganic carbon. Then, the samples pretreated with HCl were washed into the centrifuge tube with deionized water. Centrifuge was used to separate the solid and liquid until all samples were washed into the centrifuge tube. Finally, samples were oven-dried for elemental analyzer test.
X-ray diffraction (XRD) analysis was carried out at the State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences (Wuhan). Seventeen shale samples were grounded to less than 200 mesh powders for mineralogical composition measurement. Dmax-2500 X-ray diffractometer was used in this study to  Figure 2: Simplified lithostratigraphic framework and nomenclature of the Ediacaran-Lower Cambrian in the studied area (after [39]).
3 Lithosphere scan the samples at a step of 0.5°in the range of 3°-90° [44]. The mineral composition was calculated semiquantitatively according to the peak area of the individual mineral on the X-ray spectra.
Twenty-nine shale samples from ND1 well were analyzed to measure the content of major and trace elements in the Laboratory of Regional Geological and Mineral Investigation Institute, Hebei. Utilizing the Axios max X-ray fluorescence (XRF) spectrometer, major elements were determined by the method of glass melting based on the standard GB/T 14506. . Trace elements were examined by X Serise 2 Inductively Coupled Plasma Mass Spectrometry (ICP-MS) based on the standard GB/T 14506. . Before analysis, the powdered samples were oven-dried for 1-2 h at 105°C, then digested in a sealed container with 1 ml HF and 0.5 ml HNO 3 . Later on, 5 ml HNO 3 was added into the insol-uble residue, and then the resulting solution was heated for 3 hours and diluted to 25 ml at last. After that, ICP-MS analysis was carried out.
Element enrichment factors (EF) were calculated by the formula where ðX/AlÞ sample and ðX/AlÞ PAAS represent the ratio of elements X and Al in samples of this study and in the Post-Archean Australian Shale (PAAS) [45]. Excess Ba (Ba ex ), which is total Ba (Ba total ) minus detrital Ba, were estimated by the following equation: Ba ex = Ba total − Al sample × ðBa/AlÞ PAAS ð2Þ [20]. (Ba/Al) PAAS is the Ba/Al ratio of PAAS [45], which is a constant of 0.0077. Ba total and Al sample are        (Figure 3(b)), meanwhile, calcites in abundance increase compared to Niu-1 and Niu-2 member (Figures 4(p) and 4(q)). The mineral compositions of the Niutitang Formation in ND1 well are present in Figure 5 and Table 1, which show significant mineral composition variation and notable heterogeneity of each unit. Quartz contents range from 30% to 93% and have an increasing trend from the Niu-1 member to Niu-2 member and then a decreasing trend from the Niu-2 member to Niu-3 member ( Figure 5). K-feldspar contents range from 0 to 35% and only enrich in the Niu-1 member with an average of 25% ( Figure 5). Conversely, there is no albite in the Niu-1 member. Albite appears in the Niu-2 member and has a slightly increasing trend upwards ( Figure 5), with an average content of 6% and 11% in the Niu-2 member and Niu-3member, respectively. Calcite has a similar trend to albite ( Figure 5), the contents of which are nil in the Niu-1 member, while mean 5% and 13% in the Niu-2 member and Niu-3 member, respectively. Dolomite is enriched in the Niu-1 member, the highest content of which is up to 31%, and decreased sharply in the Niu-2 member and Niu-3member ( Figure 5). Pyrite contents have a generally decreasing trend upwards, in addition to the high value in partial samples of the Niu-2 member ( Figure 4). The trend of clay mineral contents is stable in the Niu-1 member, while fluctuates greatly in the Niu-2 member and gradually increases upwards in the Niu-3 member ( Figure 5). In addition, the average content of clay minerals in Niu-1, Niu-2, and Niu-3 member is 15%, 11%, and 15%, respectively ( Table 1).
The total organic carbon (TOC) has a similar trend to quartz and is extremely concentrated in the Niu-2 member, ranging from 3.27 wt% to 15.05 wt% in the Niu-2 member,  Table 2. SiO 2 and Al 2 O 3 are the dominant major elements, which would be expected from their mineral compositions. The highest content of SiO 2 occurs in the Niu-2 member with an average value of 64.28%, and the average content of SiO 2 in Niu-1 and Niu-3 member is 50.48% and 50.81%, respectively. Al 2 O 3 displays a converse trend with SiO 2 , and its content is obviously lower in the Niu-2 member (mean 7.12%) than the Niu-1 member (mean 13.77%) and Niu-3 member (mean 10.92%). The contents of Fe 2 O 3 are relatively higher in the Niu-1 member (mean 4.75%), which may be due to the widely distribution of pyrites in the Niu-1 member, whereas lower in Niu-2 (mean 3.03%) and Niu-3 member (mean 4.04%). The similar trend is followed by MgO and K 2 O, which is consistent with the relative enrichment of dolomite and K-feldspar in the Niu-1 member. However, the average content of CaO accounts for 11.27% in the Niu-3 member, obviously higher than the value of 4.29% in the Niu-1 member and 4.26 in Niu-2 member, which may be owing to the high content of calcite in Niu-3 member. Similarly, Na 2 O contents are relatively higher in the Niu-3 member (mean 1.12%) and lower in the Niu-2 (mean 0.63%) and Niu-1 member (mean 0.17%), which is consistent with the trend of albite. The contents of TiO 2 , MnO, and P 2 O 5 almost account for <1%.

Trace Elements.
The selected trace element concentrations are tabulated in Table 3. As shown in Figure 6, there is a close correlation between TOC and trace elements. The ratio of V/Cr, U/Th, Ni/Al, Cu/Al, Zn/Al, P/Al, and the enrichment factors of Mo and U (Mo EF and U EF ) are abnormally high in the Niu-2 member, and by contrast, they display very low in the Niu-1 and Niu-3 member, except for the bottom of the Niu-1 member ( Figure 6). Moreover, they tend to gradually decrease upward in the Niu-3 member ( Figure 6), while the concentrations of Ba ex are abnormally high (avg. 7543 ppm) in the Niu-1 member, but gradually decrease upward, with average Ba ex concentrations of 1883 ppm and 935 ppm in the Niu-2 and Niu-3 member, respectively (Table 3, Figure 6).

Discussion
5.1. Hydrothermal Activity. Hydrothermal activity is believed to have an important effect on the organic matter enrichment, by affecting water redox condition and biological productivity [4,20,21,46]. Several evidences manifest that there may be active hydrothermal activities in the study area during the deposition of Niutitang Formation.
Moreover, there are abundant barites distributed in the Niu-1 member (Figure 3(a)). Ba may originate from detrital, biogenic, or hydrothermal sources [48]. As presented in Figures 8(a) and 8(b), there is no correlation between Ba ex and Al as well as Ba ex and Ni, ruling out that Ba ex is sourced from detrital or biogenic origin. Thus, the only source of Ba is hydrothermal input. Consequently, barites in the Niu-1 member are of hydrothermal origin. The concentrations of Ba ex in Niu-1 and Niu-2 member are in the range of 1391 to 21001 ppm (mean 7543 ppm) and 516 to 3021 ppm (mean 1883 ppm), respectively, which are much higher than that in the Niu-3 member (range from 502 to 2048 ppm, mean 935 ppm) (Table 3, Figure 6). This indicated that the intensity of hydrothermal activity is the strongest in the Niu-1 member and then weakened upwards from Niu-2 to Niu-3 member.

Insights from Geochemical Indices. Ce anomalies versus
Nd concentrations and Y anomalies ((Y/Ho)n) can be used to distinguish different types of sediments without considering their mineral compositions [21]. The (Y/Ho)n ratios of Niu-1, Niu-2, and Niu-3 member range from 1.25-1.84, 1.16-1.42, and 0.93-1.32, respectively ( Table 4). Most of the samples present positive Y anomalies, suggesting that the Niutitang Formation in ND1 well was affected by hydrothermal activity to varying degrees. As shown in Figure 9(a), most samples in Niu-1 and Niu-2 member fall into the hydrothermal zone and the intersection zone of hydrothermal and diagenesis, while samples in the Niu-3 member fall into the diagenetic zone in the δCe-Nd discriminant diagram. As presented in Figure 9(b), all samples in Niu-1 and Niu-2 member and most samples in the Niu-3 member fall into the hydrothermal zone. These results indicate that during the deposition of Niu-1 and Niu-2 member shales, they were 8 Lithosphere    10 Lithosphere mainly affected by hydrothermal activity, which may also be influenced by diagenesis, while Niu-3 member shales were mainly affected by diagenesis. As previous studies reported, there are Eu anomalies in the Ediacaran-Cambrian black shale in Guizhou and Hunan provinces, and these anomalies are cited as evidence of hydrothermal input [42,47,50,51]. In ND1 well, samples in Niu-1 and Niu-2 member present pronounced to moderate positive Eu anomalies with δEu in the range of 1.93-7.35 and 1.23-3.39, respectively, while samples in the Niu-3 member display slightly positive Eu anomalies with δEu in the range of 1.04-1.46 (Table 4, Figures 6 and 7). The variation of δEu in ND1 well in this study is consistent with that in GMD-1well reported by [19]. Wu et al. [19] concluded that strong hydrothermal activities existed in the shelf area during the deposition of black shales, owing to the strong positive Eu anomaly in the lower-middle unites of Niutitang Formation. Therefore, the Eu anomalies also further prove the above conclusion: there was a strong hydrothermal activity during the deposition of the Niu-1 member, and then the hydrothermal activity weakened in Niu-2 member and disappeared in the Niu-3 member.
Much attention must be paid to explain the Eu anomalies measured by ICP-MS, because of the interference of various barium-containing compounds. The correlation between δEu and Ba ex can be used to determine whether the Eu anomalies are interfered by the Ba content. As shown in Figure 8(c), although δEu is positively correlated with Ba ex in the Niu-1 member, there is no correlation between δEu and Ba ex in Niu-2 and Niu-3 members. This illustrates  In summary, there was a strong Ba-rich hydrothermal activity in the shelf region of Yangtze Platform during Niu-1 member deposition, and the hydrothermal activity weakened in the Niu-2 member and gradually disappeared in Niu-3 member.
In ND1 well, V/Cr and U/Th ratios of Niu-1 member samples are mostly in the range of 0.71-2.46 and 0.23-0.95, respectively, except for several samples at the bottom with high values (Table 3), and may be indicating that Niu-1 member was deposited in dysoxic to oxic conditions ( Figure 6). However, The Mo EF /U EF ratios are 0.3-1 times that of modern seawater in the Niu-1 member, indicative of suboxic to anoxic conditions ( Figure 10). The existence of abundant barites in the Niu-1 member suggested that the water environment at that time was oxygen poor at least, because barite was formed in a weakly reduced environment  Figure 9: Different types of sediments discriminant plots (after [52]). 12 Lithosphere [57,58]. Therefore, the Niu-1 member shales were deposited in a suboxic to anoxic environment. Niu-2 member yields V/Cr and U/Th ratios in the range of 1.13-33.18 (mean 14.27) and 1.08-19.06 (mean 9.13), respectively (Table 3), which are abnormally high, suggesting strong anoxia conditions ( Figure 6). Mo EF and U EF display similar variation trend with V/Cr and U/Th ratios, which highly accumulated in the Ni-Mo sulfide ore layer and Niu-2 member (Figure 6), suggesting the Ni-Mo sulfide ore layer and Niu-2 member were deposited in anoxic to sulfidic water conditions. Besides, the Mo EF /U EF ratios of most samples in Niu-2 member are 1-3 times that of modern seawater, also indicating that the Niu-2 member was deposited in anoxic to sulfidic water conditions ( Figure 10).
Niu-3 member shows low V/Cr ratios (range 1.7-2.41) and U/Th ratios (range 0.31-1.21) (Table 3), revealing dysoxic to oxic environment. The enrichment factors of Mo and U in Niu-3 member significantly decreased compared with the Niu-2 member (Figure 6), reflecting that the water environment inthe Niu-3 member was gradually oxygenated. As presented in Figure 10, the Mo EF /U EF ratios in the Niu-3 member is too scattered to effectively indicate the deposition environment at that time. In summary, the Niu-3 member was deposited in a dysoxic to oxic environment.
As shown in Figure 6, the TOC content varies with redox conditions, which is extremely concentrated in the Niu-2 member, stating that anoxic-euxinic water condition is more conducive to the preservation of organic matter.

Primary Productivity.
Productivity is one of the controlling factors of organic matter enrichment in marine sediments [2,59]. Numerous geochemical proxies have been considered as indicators of paleoproductivity, such as the stable isotopes of carbon and nitrogen and TOC content as well as the concentrations of selected trace metals (P, Cu, Ni, Zn, and Ba) [19,[60][61][62]. In this work, we applied Ba ex , TOC ,and the ratio of nutrient elements to aluminum (P/Al, Cu/Al, Ni/Al, and Zn/Al) to characterize the primary productivity, and the dilution effects of other components can be eliminated by using Al as denominator [63,64].
As presented in Figure 6, Ni/Al, Cu/Al, Zn/Al, and P/Al ratios are abnormally high in the Ni-Mo sulfide ore layer and Niu-2 member, while display relatively low in the Niu-1 and Niu-3 member, suggesting that the primary productivity was much higher during the Ni-Mo sulfide ore layer and Niu-2 member deposition than that during the deposition of the Niu-1 and Niu-3 member. TOC display similar variation trend with the ratio of nutrient elements to aluminum. This covariation between TOC and these nutrient elements states that primary productivity plays a significant role in the organic matter enrichment in Niutitang shales, while Ba ex concentrations display different pattern of stratigraphic variation, which are much higher in Niu-1 member (average 7543 ppm) and gradually decrease upward (average 1883 ppm in Niu-2 member and 935 ppm in Niu-3 member). Ba may originate from detrital, biogenic, or hydrothermal sources, and only biogenic Ba (Babio) fraction can be used to characterize the primary productivity [16,48,61,65,66]. The poor correlation between Ba ex and P (Figure 8(a)) may suggest that Baex is not originate from biogenic source [16]. Hence, Ba ex cannot be used as a reliable indicator of paleoproductivity in this study.

Terrigenous Clastic
Input. The major elements Al and Ti have been considered as indicators of detrital flux, since they are commonly immobile during diagenesis [36,67,68]. However, only when there is a positive correlation between Al and Ti can they be used as indicator of terrigenous clastic influx, because Al and Ti may be influenced by other factors independent of terrestrial flux, such as pH and authigenic clay minerals [67,69,70]. Our results show that the variation patterns of Al and Ti are similar (Figure 6), and there is a strong positive correlation between them (R 2 = 0.83; Figure 11(a)), suggesting that both Al and Ti are mainly sourced from terrigenous detrital. Detrital flux depends on the relative sea level, which shifts the site of sedimentation landward during transgression and basinward during regression [68]. Al and Ti exhibit a decreasing trend from Niu-1 to Niu-2 member and an increasing trend from Niu-2 to Niu-3 member, reaching the lowest value in Niu-2 member ( Figure 6). This result appears to suggest that the input of terrigenous clastic reduced from Niu-1 to Niu-2 member, while enhanced from Niu-2 to Niu-3 member. Meanwhile, it also implies that the sea level rose first from Niu-1 to Niu-2 member and then fell from Niu-2 to Niu-3 member.
The terrigenous clastic influx plays a role on organic matter accumulation by diluting the OM [36,[71][72][73]. Both Al and Ti show moderate negative correlations with TOC (Figures 11(b) and 11(c)). Moreover, the content of Al and Ti was elevated in the Niu-1 and Niu-3 member, with lower TOC contents in these intervals, revealing that clastic dilution was disadvantageous for OM accumulation.
5.5. Controls on the Accumulation of Organic Matter. Consensus has been reached on the factors controlling the enrichment of organic matter, ocean redox conditions, primary productivity, and terrigenous detrital input or their  Figure 10: Crossplot of Mo EF versus U EF (modified from [56]). 13 Lithosphere combinations [1,4,7,11,[13][14][15][16]. In addition, hydrothermal activity is also believed to have an important effect on the organic matter enrichment, by affecting water redox condition and biological productivity [4,17,20,21,46]. Taken paleoredox conditions, primary productivity, terrigenous input, and hydrothermal activity together, an organic matter enrichment model is established for the study area ( Figure 12).
As indicated by the V/Cr, U/Th, and Mo EF /U EF ratios, Niu-1 member shales were deposited in a suboxic to anoxic environment. And there was an intense hydrothermal activity in this period, as evidenced by the concentrations of Ba ex , positive Eu anomalies as well as abundant barites. Nevertheless, the sea level was low during the deposition of the Niu-1 member, as indicated by elevated Al and Ti contents. Therefore, hydrothermal events may be the main causes of dysoxic to anoxia depositional environments in the Niu-1 member. A large number of reducing gases (e.g., H 2 S, CH 4 , H 2 , and CO) released during hydrothermal activity can lead to a benthic anoxic condition [74,75]. In addition, Ba-rich hydrothermal fluid, accompanied by abundant nutrient elements, vented along the deep-seated faults in shelf and basinal regions, resulting in nutrientrich bottom waters. Nevertheless, shallow water may cause stronger water mass restriction and weaker nutrient upwelling into the shelf [33,76]. Thus, metallic elements (e.g., Ba, Fe, Cu, Zn, and Ni) carried by hydrothermal fluid may not be brought to the seawater surface, but directly deposited in situ when they were exposed to abundant H 2 S, forming a Ni-Mo sulfide ore layer and metallic sulfide minerals such as barites and pyrites. Therefore, owing to lack of nutrients in surface water, the marine primary productivity during Niu-1 member was low. In summary, the enrichment of organic matter in the Niu-1 member was primarily controlled by low primary productivity (Figure 12(a)), which is further supported by the generally low Ni/Al, Cu/Al, Zn/Al, and P/Al ratios and similar variation trend of TOC and these ratios ( Figure 6).
During the deposition of Niu-2 member, the sea level rose, as evidenced by the relatively lower Al and Ti contents (Figure 6), and hydrothermal activity was still proceeding, but its intensity was weakened compared with Niu-1 member, as revealed by Figure 6. The rising sea level and continuous hydrothermal activity further enhanced the hypoxia of seawater, forming an anoxic-euxinic environment [18,33,78], as evidenced by the abnormally high V/Cr and U/Th ratios as well as Mo EF and U EF values (Figures 6 and 10). Moreover, hydrothermal activity was commonly accompanied by the release of abundant nutrients (N, P, Si, Fe, and Zn), resulting in nutrient-rich bottom waters [4,18,19]. The increase of seawater depth may lead to an increase in water mass circulation and nutrient upwelling to the shelf [78], which carried the nutrient-rich bottom waters to the photic zone, resulting in the phytoplankton bloom ( Figure 12(b)). Therefore, the marine primary productivity is significantly improved in this period, consistent with the abnormally high Ni/Al, Cu/Al, Zn/Al, and P/Al ratio ( Figure 6). Subsequently, after these phytoplankton die, their bodies fall on the seafloor as "sea snow," and the decomposition of these organic matter during the deposition process likely consumed a large amount of oxygen in the water column, thereby promoting the spread of euxinic environment. The anoxic-euxinic environment was conducive to the burial and preservation of organic matter. In conclusion, continuous hydrothermal activity and rising sea level resulted in strong reducing bottom waters and high paleoproductivity, which jointly controlled the enrichment of organic matter in the Niu-2 member (Figure 12(b)).
Through Niu-3 member, the sea water was shallow, dominated by dysoxic to oxic conditions and unaffected by hydrothermal events, as shown in Figures 6 and 12(c). The disappearance of hydrothermal activities and sea level fall led to a significant reduction of nutrients supplied to the surface of seawater and a decline in primary productivity [20,67]. In addition, they also weakened the reducibility of bottom water, which was not conducive to the organic matter preservation. Thus, the low primary productivity combined with dysoxic to oxic condition controls the unfavorable organic matter accumulation in the Niu-3 member (Figure 12(c)).
In summary, the redox and paleoproductivity of seawater, which finally controlled the organic matter accumulation, depend on hydrothermal activity and sea level fluctuation. In addition, the phenomenon that the Niu-1 member with strong hydrothermal activity but low TOC content and Niu-2 member with weak hydrothermal event but high TOC content found in our paper is consistent with that reported in Tarim Basin by [74]. 14 Lithosphere

Conclusions
Petrologic, mineralogic, and multiple geochemical proxies have been used to elucidate the organic enrichment mechanisms of Lower Cambrian Niutitang shales in the middle Yangtze region, South China.  Figure 12: The organic matter accumulation models of Niutitang shales in studied area. (a) Niu-1 member was characterized by intense hydrothermal activity but low sea level. Low sea level caused strong water mass restriction and weak nutrient upwelling into the shelf, leading to the nutrients carried by hydrothermal fluid may not be brought to the seawater surface. Owing to lack of nutrients in surface water, marine primary productivity during Niu-1 member was low. In summary, the enrichment of organic matter in the Niu-1 member was primarily controlled by low primary productivity. (b) Niu-2 member was characterized by weakened hydrothermal activity and elevated sea level. The continuous hydrothermal activity and rising sea level formed an anoxic-euxinic environment. The rising sea level may increase the water mass circulation and nutrient upwelling to the shelf, which carried the nutrient-rich bottom waters to the photic zone, resulting in the phytoplankton bloom. Continuous hydrothermal activity and rising sea level resulted in strong reducing bottom waters and high paleoproductivity, which jointly controlled the enrichment of organic matter in the Niu-2 member. (c) Niu-3 member was dominated by dysoxic to oxic conditions with low primary productivity, owing to the disappearance of hydrothermal activities and sea-level fall. Palaeogeography is modified after [4,19,77].

Lithosphere
(2) Intense Ba-enriched hydrothermal activities occurred in the Niu-1 member, then weakened in Niu-2 member, and gradually disappeared in Niu-3 member, while the sea level rose first from Niu-1 to Niu-2 member and then fell from Niu-2 to Niu-3 member (3) The redox and paleoproductivity of seawater depend on hydrothermal activity and sea level fluctuation.
The intense hydrothermal events lead to a suboxic to anoxic environment in the Niu-1 member, which is conducive to the organic matter preservation. However, low sea level strengthens the restriction of water mass and reduced nutrient upwelling into the shelf, leading to decreased marine primary productivity, which was ultimately responsible for depleted organic matter accumulation in the Niu-1 member (4) In the Niu-2 member, continuous hydrothermal activity and rising sea level, resulting in an anoxiceuxinic environment and high paleoproductivity, controlled the enrichment of organic matter in the Niu-2 member (5) In the Niu-3 member, the redox condition sharply changed into dysoxic to oxic conditions, along with obviously declining primary productivity, which was attributed to the disappearance of hydrothermal activities and sea level fall. Therefore, the dysoxic to oxic condition plus low primary productivity was the main factors controlling the unfavorable organic matter accumulation in the Niu-3 member

Data Availability
The underlying data supporting the results of our study can be found in the manuscript we submit.

Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper. Lithosphere