Organic Matter Enrichment Mechanisms in the Lower Cambrian Shale: A Case Study from Xiangandi #1 Well

: In order to investigate the effect of primary productivity, organic matter dilution, and preservation on the accumulation of organic matter, geochemical data, and proxies of primary productivity, clastic inﬂux, and redox conditions were obtained for organic-rich shales in the Cambrian Niutitang Formation. The primary productivity (total organic carbon [TOC], Mo, P, Ba, and Babio) and redox (Ni/Co, V/Cr, U/Al, and Th/U) proxies suggest the organic-rich shales were deposited in anoxic-euxinic conditions during periods of high primary productivity. Pyrite in the Niutitang Formation comprises spherical framboids, which also indicate that anoxic bottom waters were present during organic matter deposition. High primary productivity enhanced the organic C ﬂux into the thermocline layer and bottom waters, which lead to the development of anoxic bottom waters owing to O 2 consumption by microorganisms and organic matter degradation. The anoxic bottom waters were beneﬁcial for the preservation of organic matter. In addition, Ti/Al ratios correlate well with TOC contents throughout the Niutitang Formation, indicating that clastic input increased the burial rate and prevented organic matter degradation during deposition. Therefore, the accumulation of organic matter in the Niutitang Formation was controlled mainly by primary productivity rather than bottom-water redox conditions.


Introduction
Variations of organic matter accumulation in sediments are relatively large, as a result of a complex interaction between productivity, preservation, and dilution of organic matter [1][2][3].The primary productivity associated with different nutrient and organic matter types can provide important insights into the organic matter in shales and mudstones [4][5][6][7].Dilution of organic matter is a complex function of sedimentation rates [8,9] and biogenic alteration of inorganic matter [2,10].The redox conditions of seawater also have a significant effect on the preservation of organic matter and can prevent its oxidation and destruction in sediments [11][12][13][14][15]. Organic matter enrichment in shales has been explained by two distinct models, i.e., the primary productivity and preservation models [5][6][7][8][9][10][11][12].
The Lower Cambrian Niutitang Formation in the Middle and Upper Yangtze regions consists mainly of bedded siliceous shale, laminated shale, muddy siltstone, and sand-stone [16].The third-order sequence of the Niutitang Formation is a transgressive systems tract (TST), consisting mainly of siliceous and calcareous shales, and a highstand systems tract (HST), consisting mainly of argillaceous and silty mixed shales.The absence of a lowstand systems tract (LST) can be attributed to rapid transgression at the Ediacaran-Cambrian boundary [17,18].Organic matter contents vary in the different sedimentary facies and decrease from the deep-water shelf to the shallow-water shelf and Minerals 2024, 14, 183 2 of 14 tidal flat facies [19].Owing to negligible fractionation during sediment transport and deposition, geochemical data and proxies obtained from the clastic sediments provide important insights into their palaeoenvironments during deposition [20][21][22][23].Shields and Stille discussed the effects of post-depositional diagenesis on rare earth elements (REEs) in the basal Cambrian phosphorites of the Meishucun section [24].Li et al. and Zhou et al. proposed that seawater made a greater contribution to REE contents than hydrothermal inputs in the Niutitang Formation phosphatic, siliceous, and silty mixed shales [25,26].Several studies have investigated the geochronology and geochemistry of the Niutitang Formation and the processes responsible for organic matter enrichment [27][28][29][30].However, few studies have investigated the effects of primary productivity and organic matter dilution on organic matter accumulation.In this study, the geochemical compositions of the Niutitang Formation shales were used to determine variations in primary productivity, organic matter dilution and preservation in order to develop a better understanding of organic matter accumulation in organic-rich shales.

Samples and Method
The Xiangandi #1 well (XAD 1 well) is located near Anhua, in central Hunan Province (Figure 1).The Niutitang Formation overlies the chert successions of the Liuchapo Formation and contains mainly black organic-rich shales intercalated with gray limestones.The 53 samples from the Niutitang Formation and 2 samples from the Liuchapo Formation were selected from XAD 1 well for major and trace elements analysis (Figure 1c).Cambrian boundary [17,18].Organic matter contents vary in the different sedimentary facies and decrease from the deep-water shelf to the shallow-water shelf and tidal flat facies [19].Owing to negligible fractionation during sediment transport and deposition, geochemical data and proxies obtained from the clastic sediments provide important insights into their palaeoenvironments during deposition [20][21][22][23].Shields and Stille discussed the effects of post-depositional diagenesis on rare earth elements (REEs) in the basal Cambrian phosphorites of the Meishucun section [24].Li et al. and Zhou et al.
proposed that seawater made a greater contribution to REE contents than hydrothermal inputs in the Niutitang Formation phosphatic, siliceous, and silty mixed shales [25,26].
Several studies have investigated the geochronology and geochemistry of the Niutitang Formation and the processes responsible for organic matter enrichment [27][28][29][30].However, few studies have investigated the effects of primary productivity and organic matter dilution on organic matter accumulation.In this study, the geochemical compositions of the Niutitang Formation shales were used to determine variations in primary productivity, organic matter dilution and preservation in order to develop a better understanding of organic matter accumulation in organic-rich shales.

Samples and Method
The Xiangandi #1 well (XAD 1 well) is located near Anhua, in central Hunan Province (Figure 1).The Niutitang Formation overlies the chert successions of the Liuchapo Formation and contains mainly black organic-rich shales intercalated with gray limestones.The 53 samples from the Niutitang Formation and 2 samples from the Liuchapo Formation were selected from XAD 1 well for major and trace elements analysis (Figure 1c).Argon ion polished thin sections, taken perpendicular to the bedding orientation, were observed using a Quanta FEG 450 scanning electron microscope.The micrographs were employed to study the morphology of the minerals.Total organic carbon (TOC) content was determined using the carbon/sulfur analyzer.Major element concentrations of SiO 2 , Al 2 O 3 , CaO, TFe 2 O 3 , K 2 O, MgO, MnO, Na 2 O, P 2 O 5 , and TiO 2 were determined using an AxiosmAX X-ray fluorescence spectrometer following experimental details of Li et al. [25].The relative precision of major-element concentrations in the Lower Cambrian Niutitang Formation shales was <±5%.The trace elements concentrations of Mo, Ba, Cu, Ni, Co, V, Cr, and U were determined using an ELEMENT XR plasma mass spectrometer (National Research Center for Geoanalysis, Chinese Academy of Geological Sciences, Beijing, China) following experimental details of Zhou et al. [26], with a relative precision of <±5%.Element enrichment factors (EF element ) were defined as follows: EF element = (element/Al) sample /(element/Al) average shale , where (element/Al) average shale refers to the data from an average shale [31].The major elements concentrations, trace elements concentrations, and TOC content of the Niutitang Formation shales are presented in Tables 1 and 2.

Primary Production
Zircon U-Pb geochronology has yielded an age of 536.5 ± 5.5 Ma for the Liuchapo Formation sedimentary rocks [32].Zircon U-Pb and Re-Os geochronology has yielded ages of 521-514 Ma for the Niutitang Formation sedimentary rocks, corresponding to the Tommotian of the Early Cambrian [27,28].The Liuchapo and Niutitang formations predate the radiation of vascular plants in terrestrial ecosystems, and terrigenous organic C made a negligible contribution to their TOC contents.Although sponge spicule, acanthomorphic acritarch, and scyphozoa fossils are present in the shales, biomarker analysis of the Niutitang Formation shales indicates that planktonic algae were the main primary producers [30].Relative to average shale [31], Niutitang Formation and Liuchapo Formation sediments have less average Al2O3 contents (average Al2O3 = 6.99%), indicating minor clay minerals.Relative to average shale [31], sedimentary rocks of the Niutitang

Primary Production
Zircon U-Pb geochronology has yielded an age of 536.5 ± 5.5 Ma for the Liuchapo Formation sedimentary rocks [32].Zircon U-Pb and Re-Os geochronology has yielded ages of 521-514 Ma for the Niutitang Formation sedimentary rocks, corresponding to the Tommotian of the Early Cambrian [27,28].The Liuchapo and Niutitang formations pre-date the radiation of vascular plants in terrestrial ecosystems, and terrigenous organic C made a negligible contribution to their TOC contents.Although sponge spicule, acanthomorphic acritarch, and scyphozoa fossils are present in the shales, biomarker analysis of the Niutitang Formation shales indicates that planktonic algae were the main primary producers [30].Relative to average shale [31], Niutitang Formation and Liuchapo Formation sediments have less average Al 2 O 3 contents (average Al 2 O 3 = 6.99%), indicating minor clay minerals.Relative to average shale [31], sedimentary rocks of the Niutitang and Liuchapo formations have lower Al 2 O 3 contents (average = 6.99%), indicative of the presence of only minor clay minerals.The TOC contents of mudstones or shales reflect a relatively small fraction of the primary production due to photosynthesis in the photic zone of the surface ocean [31].Most organic material generated by primary production sinks from the ocean surface into the thermocline and deep ocean, reaches the sediment-water interface, undergoes decomposition, and is lost [33,34].Despite this decomposition and diagenesis, TOC contents can still be used to estimate the palaeo-primary productivity [5,22].
TOC contents reach a minimum value in the upper Liuchapo Formation (ULF) and then exhibit a slightly increasing trend (Figure 3).The increasing trend of TOC contents terminates at the marlstones of the base of the Niutitang Formation (BNF), in which TOC contents decrease to 2%.The TOC contents continue to increase in the lower Niutitang Formation (LNF) and subsequently decrease in the middle Niutitang Formation (MNF).The TOC contents remain uniform at 5% until a marked decrease in the marlstones of the upper Niutitang Formation (UNF).
Minerals 2024, 14, x FOR PEER REVIEW 7 of 15 Formation (LNF) and subsequently decrease in the middle Niutitang Formation (MNF).
The TOC contents remain uniform at 5% until a marked decrease in the marlstones of the upper Niutitang Formation (UNF).
Molybdenum is present as MoO4 2− in seawater and is rarely incorporated into natural minerals.Organic matter can scavenge Mo from the MoO4 2− in seawater [35,36], or Mo can be captured as Fe-Mo-S cluster compounds that are present due to H2S [37,38].Owing to the positive relationships between TOC and Mo contents, Mo provides important insights into primary productivity [39].
Molybdenum exhibits a distinctive increase at the boundary between the Liuchapo and Niutitang formations and then decreases to ~10 ppm in the marlstones in the BNF (Figure 2).Molybdenum then increases (with slight fluctuations) and peaks in the LNF.Molybdenum decreases back to ~45 ppm in the MNF.Subsequently, Mo contents remain uniform, until a distinctive decrease in the marlstones of the UNF.In general, Mo contents correlate with TOC contents throughout the Niutitang Formation (Figure 4a).Molybdenum is present as MoO 4 2− in seawater and is rarely incorporated into natural minerals.Organic matter can scavenge Mo from the MoO 4 2− in seawater [35,36], or Mo can be captured as Fe-Mo-S cluster compounds that are present due to H 2 S [37,38].Owing to the positive relationships between TOC and Mo contents, Mo provides important insights into primary productivity [39].
Molybdenum exhibits a distinctive increase at the boundary between the Liuchapo and Niutitang formations and then decreases to ~10 ppm in the marlstones in the BNF (Figure 2).Molybdenum then increases (with slight fluctuations) and peaks in the LNF.Molybdenum decreases back to ~45 ppm in the MNF.Subsequently, Mo contents remain uniform, until a distinctive decrease in the marlstones of the UNF.In general, Mo contents correlate with TOC contents throughout the Niutitang Formation (Figure 4a).Phosphorus is present in dissolved and particulate form in seawater and is a prominent nutrient for microorganisms [40].Remineralised P after burial is preferentially retained and precipitated in sediments under oxygenated bottom-water conditions but not under O2-depleted conditions, which is attributable to differences in P fixation associated with redox cycling [41].Although P accumulation mechanisms differ from those of TOC, P is also a proxy for primary productivity [42].
A prominent increase in P contents occurs across the boundary between the ULF and BNF, where P contents increase from 0.012 to 0.445% (Figure 2).Subsequently, P contents remain relatively high throughout the LNF, with slight fluctuations.Contents of P decrease in the overlying MNF, continue to decrease in the grey marlstones in the UNF, and then increase slightly.
Barium is derived mainly from three sources: terrigenous detrital input, hydrothermal fluids, and benthic organisms [43].The substantial amount of Ba transported from the ocean surface zone to the sediment-water interface along with decaying organic matter is termed biogenic Ba [44].In order to correct for the presence of detrital Ba, Dymond et al. proposed a method for determining the content of biogenic Ba (Babio) [45]: where (Ba/Al)detr is the detrital Ba fraction estimated from the upper continental crust.
Both Ba and Babio exhibit similar trends in the Niutitang Formation, and the contents of Ba and Babio are almost the same (Figure 2).A prominent increase in Ba and Babio contents occurs across the boundary between the ULF and BNF.Subsequently, Ba and Babio contents remain relatively high throughout the LNF and then decrease.Barium and Babio contents continue to decrease in the interval of grey marlstones in the UNF.However, the different Ba and Babio contents in the interval of grey marlstones in the UNF may be indicative of a prominent contribution from detrital Ba.
In summary, multi-proxy data for the primary productivity in the LNF are higher than those of the overlying grey-black shales in the MNF and UNF.The primary productivity increased throughout the LNF and subsequently decreased in the MNF and UNF.In addition, the primary productivity reached a minimum during deposition of the grey marlstone intervals at the base and top of the Niutitang Formation.

Clastic Influx
Most of Ti occurs in clay minerals in sediments, and Ti normalized to Al (Ti/Al ratios) represents the detrital non-aluminosilicate input [23,46].The Ti/Al ratios decrease to minimum values in the marlstones in the BNF.Subsequently, the Ti/Al ratios have relatively high values (with slight fluctuations) in the LNF (Figure 5), decrease slightly in the MNF, and increase in the UNF.The Ti/Al ratios return to relatively low values in the marlstone intervals of the UNF before increasing again.Phosphorus is present in dissolved and particulate form in seawater and is a prominent nutrient for microorganisms [40].Remineralised P after burial is preferentially retained and precipitated in sediments under oxygenated bottom-water conditions but not under O 2 -depleted conditions, which is attributable to differences in P fixation associated with redox cycling [41].Although P accumulation mechanisms differ from those of TOC, P is also a proxy for primary productivity [42].
A prominent increase in P contents occurs across the boundary between the ULF and BNF, where P contents increase from 0.012 to 0.445% (Figure 2).Subsequently, P contents remain relatively high throughout the LNF, with slight fluctuations.Contents of P decrease in the overlying MNF, continue to decrease in the grey marlstones in the UNF, and then increase slightly.
Barium is derived mainly from three sources: terrigenous detrital input, hydrothermal fluids, and benthic organisms [43].The substantial amount of Ba transported from the ocean surface zone to the sediment-water interface along with decaying organic matter is termed biogenic Ba [44].In order to correct for the presence of detrital Ba, Dymond et al. proposed a method for determining the content of biogenic Ba (Ba bio ) [45]: where (Ba/Al) detr is the detrital Ba fraction estimated from the upper continental crust.
Both Ba and Ba bio exhibit similar trends in the Niutitang Formation, and the contents of Ba and Ba bio are almost the same (Figure 2).A prominent increase in Ba and Ba bio contents occurs across the boundary between the ULF and BNF.Subsequently, Ba and Ba bio contents remain relatively high throughout the LNF and then decrease.Barium and Ba bio contents continue to decrease in the interval of grey marlstones in the UNF.However, the different Ba and Ba bio contents in the interval of grey marlstones in the UNF may be indicative of a prominent contribution from detrital Ba.
In summary, multi-proxy data for the primary productivity in the LNF are higher than those of the overlying grey-black shales in the MNF and UNF.The primary productivity increased throughout the LNF and subsequently decreased in the MNF and UNF.In addition, the primary productivity reached a minimum during deposition of the grey marlstone intervals at the base and top of the Niutitang Formation.

Clastic Influx
Most of Ti occurs in clay minerals in sediments, and Ti normalized to Al (Ti/Al ratios) represents the detrital non-aluminosilicate input [23,46].The Ti/Al ratios decrease to minimum values in the marlstones in the BNF.Subsequently, the Ti/Al ratios have relatively high values (with slight fluctuations) in the LNF (Figure 5), decrease slightly in the MNF, and increase in the UNF.The Ti/Al ratios return to relatively low values in the marlstone intervals of the UNF before increasing again.
ratios.The low Ti/Al ratios in the marlstone intervals of the BNF and UNF are attributable to dilution by carbonate and correspond to increases in CaCO3 contents.
Silica is derived mainly from terrigenous detrital and biogenic silica inputs.Previous studies have proposed that excess Si contents in the BNF and LNF are attributable to the generation of biogenic silica [25].Despite the effects of biogenic silica, the higher Si/Al ratios and minimum Ti/Al ratios in the LNF are thought to represent relatively high sea level [5].With a rise in relative sea level, terrestrial clastic input could then weaken or cut off, resulting in the production of carbonate.
Total organic C contents increase with the sedimentation rate until a threshold value, after which the sedimentation rate dilutes the organic matter accumulation [9].Clastic inputs increase the burial rate and prevent organic matter degradation [47] or provide more particle sites for organic matter adsorption [48].Accordingly, Ti/Al ratios correlate well with TOC contents throughout the Niutitang Formation, apart from where dilution occurred during deposition of the marlstone intervals.The Ti/Al ratios correlate with the TOC contents, but with some notable exceptions in the marlstone intervals of the BNF and UNF, where high Ti/Al ratios correspond to low TOC contents.This might be explained by the changes in Si/Al ratios and CaCO 3 contents.The Si/Al ratios reach maximum values with prominent fluctuations in the LNF and subsequently decrease in the MNF and UNF (Figure 5).Where the Si/Al ratios begin to increase in the BNF, the Ti/Al ratios exhibit a small increase and then decrease to low ratios.The low Ti/Al ratios in the marlstone intervals of the BNF and UNF are attributable to dilution by carbonate and correspond to increases in CaCO 3 contents.

Redox Conditions
Silica is derived mainly from terrigenous detrital and biogenic silica inputs.Previous studies have proposed that excess Si contents in the BNF and LNF are attributable to the generation of biogenic silica [25].Despite the effects of biogenic silica, the higher Si/Al ratios and minimum Ti/Al ratios in the LNF are thought to represent relatively high sea level [5].With a rise in relative sea level, terrestrial clastic input could then weaken or cut off, resulting in the production of carbonate.
Minerals 2024, 14, 183 9 of 14 Total organic C contents increase with the sedimentation rate until a threshold value, after which the sedimentation rate dilutes the organic matter accumulation [9].Clastic inputs increase the burial rate and prevent organic matter degradation [47] or provide more particle sites for organic matter adsorption [48].Accordingly, Ti/Al ratios correlate well with TOC contents throughout the Niutitang Formation, apart from where dilution occurred during deposition of the marlstone intervals.

Redox Conditions
Cobalt and Cr are soluble cations in oxygenated seawater, whereas Co and Cr are transported into sediments as authigenic sulphides under anoxic conditions [49].Because authigenic sulphides of Co and Cr are scarce, detrital/terrigenous inputs contribute most Co and Cr in sediments [28].Vanadium and Ni are precipitated from seawater under O 2 deficient conditions, which are associated with organic matter decay [4].Therefore, Ni/Co and V/Cr ratios are proxies of redox conditions (Figure 4b,c).Ratios of Ni/Co and V/Cr both increase (with fluctuations) across the ULP-BNF boundary and through the LNF and then decrease in the MNF and UNF (Figure 6).Relative to the MNF and UNF, the higher Ni/Co and V/Cr ratios in the LNF are indicative of an O 2 -depleted depositional environment.

Organic Matter Accumulation Mechanism of Organic-Rich Shales
The redox and productivity proxies suggest that the black, organic-rich shales of the LNF were deposited under anoxic (or even euxinic) conditions during periods of high primary productivity.The grey-black, organic-poor shales of the MNF and UNF were deposited under suboxic conditions during periods of relatively low primary productivity.
Euhedral pyrite is either precipitated directly or formed indirectly by framboidal precursor regrowth, indicative of limited FeS2 saturation and intense late diagenesis [53].Framboidal pyrite in marine sediments has two main mechanisms of formation: (1) under oxic-dysoxic bottom-water conditions, in which diagenetic, framboidal pyrite grows in anoxic pore waters in sediments [54]; and (2) under euxinic bottom-water conditions, in Uranium occurs as dissolved U 6+ in oxygenated seawater, and dissolved U 6+ is reduced to insoluble U 4+ under O 2 -depleted conditions, resulting in U enrichment in pelagic and hemipelagic sediments [50].Algeo and Maynard proposed that organic matter accelerated the scavenging rate of U in sediments [49].Ratios of U/Al are 6-137 × 10 −4 in the LNF and 0.7-6.9× 10 −4 in the MNF and UNF.Ratios of U/Al increase across the Liuchapo-Niutitang formation boundary and remain relatively high (with fluctuations) throughout the LNF.Subsequently, the U/Al ratios exhibit a decrease in the MBF and UNF (Figure 6).The higher U/Al ratios of the LNF indicate the bottom waters were anoxic.
Thorium occurs as insoluble Th 4+ in seawater and is unaffected by the redox conditions [51].Wignall and Twitchett proposed that Th/U ratios generally increased with increasing oxygenation from 02 for anoxic conditions to 2-8 for oxic conditions [52].Ratios of Th/U decrease across the Liuchapo-Niutitang formation boundary, remain relatively low with fluctuations in the LNF, and subsequently increase in the MNF and UNF (Figure 6).The lower Th/U ratios in the LNF relative to the MNF and UNF are indicative of anoxic bottom waters.
In order to discriminate euxinic from anoxic depositional conditions, Mo was used to recognise redox changes during deposition of the Niutitang Formation.Under euxinic conditions, Mo is precipitated preferentially as Fe sulphides owing to the presence of free H 2 S [49].If only U and V are scavenged and enriched without Mo, free H 2 S did not exist in the seawater; however, if U, V, and Mo exhibit concurrent enrichments, then euxinic conditions existed with free H 2 S [22].Uranium, V, and Mo are all enriched in the LNF, indicating that euxinic conditions existed during deposition of the LNF.

Organic Matter Accumulation Mechanism of Organic-Rich Shales
The redox and productivity proxies suggest that the black, organic-rich shales of the LNF were deposited under anoxic (or even euxinic) conditions during periods of high primary productivity.The grey-black, organic-poor shales of the MNF and UNF were deposited under suboxic conditions during periods of relatively low primary productivity.
Euhedral pyrite is either precipitated directly or formed indirectly by framboidal precursor regrowth, indicative of limited FeS 2 saturation and intense late diagenesis [53].Framboidal pyrite in marine sediments has two main mechanisms of formation: (1) under oxic-dysoxic bottom-water conditions, in which diagenetic, framboidal pyrite grows in anoxic pore waters in sediments [54]; and (2) under euxinic bottom-water conditions, in which syngenetic framboidal pyrite grows in anoxic bottom waters and sinks into the sediments [55].Framboidal pyrite dominates over euhedral pyrite in the Niutitang Formation sediments (Figure 7), indicating the framboidal pyrite could be an indicator of the redox environment.The mean diameter and size distribution of framboidal pyrite in the Niutitang Formation sediments suggest the pyrite formed under euxinic conditions [55].Based on organic biomarker analysis, the main primary producers were planktonic algae [35].Planktonic algae might have produced mineralised tests that sank into the sediments.Owing to tectonic subsidence in the Huanan rift and a relative rise in sea level [22], the accommodation space increased during deposition of the LNF.Because of the rapid transgression at the Ediacaran-Cambrian boundary, the Niutitang Formation is divided into transgressive and highstand systems tracts [21].During deposition of the LNF, the relative sea level rose to the maximum flooding surface and the seasonal mixing could not affect the bottom waters [25,26].Biomarker analyses show that nutrients were derived from planktonic algae [30], and upwelling provided nutrients for primary producers based on the occurrence of phosphorites and P enrichment.Productivity proxies (Mo and Babio) show that primary productivity was higher in the ocean surface zone during deposition of the LNF shales.The high primary productivity enhanced the organic C flux into the chemocline layer and bottom waters, leading to the development of anoxic bottom Owing to tectonic subsidence in the Huanan rift and a relative rise in sea level [22], the accommodation space increased during deposition of the LNF.Because of the rapid transgression at the Ediacaran-Cambrian boundary, the Niutitang Formation is divided into transgressive and highstand systems tracts [21].During deposition of the LNF, the relative sea level rose to the maximum flooding surface and the seasonal mixing could not affect the bottom waters [25,26].Biomarker analyses show that nutrients were derived from planktonic algae [30], and upwelling provided nutrients for primary producers based on the occurrence of phosphorites and P enrichment.Productivity proxies (Mo and Ba bio ) show that primary productivity was higher in the ocean surface zone during deposition of the LNF shales.The high primary productivity enhanced the organic C flux into the chemocline layer and bottom waters, leading to the development of anoxic bottom waters owing to O 2 consumption by microorganisms and organic matter degradation.In addition, clastic inputs increased the burial rate and prevented organic matter degradation.The anoxic bottom waters were beneficial for the preservation of organic matter in the LNF shales (Figure 8).into transgressive and highstand systems tracts [21].During deposition of the LNF, the relative sea level rose to the maximum flooding surface and the seasonal mixing could not affect the bottom waters [25,26].Biomarker analyses show that nutrients were derived from planktonic algae [30], and upwelling provided nutrients for primary producers based on the occurrence of phosphorites and P enrichment.Productivity proxies (Mo and Babio) show that primary productivity was higher in the ocean surface zone during deposition of the LNF shales.The high primary productivity enhanced the organic C flux into the chemocline layer and bottom waters, leading to the development of anoxic bottom waters owing to O2 consumption by microorganisms and organic matter degradation.In addition, clastic inputs increased the burial rate and prevented organic matter degradation.The anoxic bottom waters were beneficial for the preservation of organic matter in the LNF shales (Figure 8).
During deposition of the grey-black MNF and UNF shales, relative sea level fell and primary productivity decreased.Seasonal mixing may have reached the bottom waters and, in addition, low primary productivity would not have formed anoxic bottom waters.The low primary productivity and suboxic bottom waters would have resulted in the relatively organic-poor MNF and UNF shales.During deposition of the grey-black MNF and UNF shales, relative sea level fell and primary productivity decreased.Seasonal mixing may have reached the bottom waters and, in addition, low primary productivity would not have formed anoxic bottom waters.The low primary productivity and suboxic bottom waters would have resulted in the relatively organic-poor MNF and UNF shales.

1.
Redox and primary productivity proxies suggest that the black, organic-rich shales in the Niutitang Formation were deposited in anoxic-euxinic conditions during periods of high primary productivity; 2.
Framboidal pyrite indicates that anoxic bottom waters existed during organic matter deposition.The high primary productivity enhanced the organic C flux into the thermocline layer and bottom waters, which formed the O 2 -depleted bottom waters; 3.
Ti/Al ratios correlate with TOC contents throughout the Niutitang Formation, indicating the clastic input enhanced the burial rate and prevented organic matter degradation during deposition of the Niutitang Formation.

Figure 1 .Figure 1 .
Figure 1.Simplified geological map in the southeast Yangtze Platform margin and lithological profile of XAD 1 well section.(a) Location map of the studied area in China, (b) Simplified geological map of Anhua County, Hunan Province, (c) lithological profile and sample sits of XAD 1 well Figure 1.Simplified geological map in the southeast Yangtze Platform margin and lithological profile of XAD 1 well section.(a) Location map of the studied area in China, (b) Simplified geological map of Anhua County, Hunan Province, (c) lithological profile and sample sits of XAD 1 well.
sediments.Enrichment factors of major elements indicate that SiO 2 , T Fe 2 O 3 , CaO, and P 2 O 5 are enriched in the Niutitang Formation shales, whereas MgO, Na 2 O, K 2 O, MnO, and TiO 2 are not markedly enriched (Figure 2).

Figure 2 .
Figure 2. Enrichment factors (EF) of major elements in the Niutitang Formation shales relative to average shale.

Figure 2 .
Figure 2. Enrichment factors (EF) of major elements in the Niutitang Formation shales relative to average shale.

Figure 3 .
Figure 3.The primary production proxies (TOC, Mo, P, Ba, and Babio) in the Niutitang Formation.Figure 3. The primary production proxies (TOC, Mo, P, Ba, and Ba bio ) in the Niutitang Formation.

Figure 3 .
Figure 3.The primary production proxies (TOC, Mo, P, Ba, and Babio) in the Niutitang Formation.Figure 3. The primary production proxies (TOC, Mo, P, Ba, and Ba bio ) in the Niutitang Formation.

Figure 4 .
Figure 4. Crossplots of Mo, Ni/Co, and V/Cr with TOC for the Niutitang and Liuchapo Formation.(a) Mo and TOC, (b) Ni/Co and TOC, (c) V/Cr and TOC.

Figure 4 .
Figure 4. Crossplots of Mo, Ni/Co, and V/Cr with TOC for the Niutitang and Liuchapo Formation.(a) Mo and TOC, (b) Ni/Co and TOC, (c) V/Cr and TOC.

Figure 8 .
Figure 8. Conceptual model showing the organic matter enrichment mechanisms of the Niutitang Formation shales.

Figure 8 .
Figure 8. Conceptual model showing the organic matter enrichment mechanisms of the Niutitang Formation shales.

Table 1 .
The major element concentrations (%) of the Niutitang Formation shales.

Table 2 .
The trace element concentrations (ppm) and TOC contents (%) of the investigated shales.