Redox Conditions of the Late Ediacaran Ocean on the Southern Margin of the North China Craton

: Previous studies have revealed dynamic and complex redox conditions of the late Ediacaran ocean. Integrated analyses of Ediacaran successions on different continents can help to better understand global ocean redox conditions. In this study, we used iron and redox-sensitive trace elements (RSTEs) geochemical analyses to present the detailed redox conditions of the late Ediacaran Dongpo Formation on the southern margin of the North China Craton (NCC). Paleoredox reconstruction reveals a dominantly anoxic late Ediacaran ocean punctuated by multiple transient oxygenation events across the southern margin of the NCC. These transient oxidation events in the NCC may have contributed to the appearance of the Ediacaran tubular fossil Shaanxilithes . Based on the assumption that local iron speciation data in a global framework can track the mean and variance of paleoredox conditions through time, we additionally analyzed about 3300 new and published iron speciation data from ﬁne-grained clastic rocks to infer the global redox change in Ediacaran–Cambrian oceans. Our statistical analyses indicated dynamic Ediacaran marine redox conditions and stepwise early–middle Cambrian ocean oxygenation. The appearance and rise of the Ediacaran biota and the diversiﬁcation of metazoans corresponded temporally with the middle Ediacaran global ocean oxygenation and the early–middle Cambrian stepwise oceanic oxygenation, respectively. Our results highlight the coevolutionary relationship between ocean redox conditions and early animals.


Introduction
Because animals require free oxygen (O 2 ), understanding the cause-and-effect relationship between O 2 and life is important for deducing the early evolution of Earth's environments and animals [1]. The oxygenation of the Earth's atmosphere proceeded in two major episodes near the beginning of the Proterozoic Eon, known as the 'Great Oxidation Event' or GOE [2], and its end, known as the 'Neoproterozoic Oxidation Event' or NOE [3,4]. During the NOE, the Ediacaran period was a critical interval in the late Neoproterozoic, which witnessed the breakup of the Rodinia supercontinent, the climatic recovery after the 'Snowball Earth', and the assembly of Gondwanaland [5]. To better understand the ocean redox conditions under the impact of the NOE, previous studies have revealed the highly fluctuating and heterogeneous redox conditions of the global marine environment throughout the Ediacaran-Cambrian interval [6][7][8]. Presumably, the protracted rise of oxygen in the ocean and atmosphere during the late Ediacaran could oxygenate basins regionally, one after another [9]. Biological changes might have happened within these oxygenated basins under suitable environmental conditions. The relationship between environmental conditions and the evolution of early animals can be suggested by regionally, one after another [9]. Biological changes might have happened within these oxygenated basins under suitable environmental conditions. The relationship between environmental conditions and the evolution of early animals can be suggested by comprehensive studies of paleontological, petrologic, geochemical, geochronological, and sequence stratigraphic analyses of global Ediacaran successions [10][11][12][13].
As mentioned above, integrated analyses of Ediacaran successions on different continents can help to better understand the global ocean redox conditions. The North China Craton (NCC) was suggested as being isolated at low latitudes of the Northern Hemisphere during the Ediacaran Period [14][15][16][17] (Figure 1A). The paleoredox conditions of the late Ediacaran ocean in the NCC are not well understood. Thus, detailed paleoredox reconstruction from successions on the NCC can fill in the blanks in global Ediacaran studies. To explore the marine redox structure of the NCC during the late Ediacaran, we analyzed iron-speciation and redox-sensitive trace metals for the fine-grained clastics of the Dongpo Formation from one fossiliferous Ediacaran section on the southern margin of the NCC. Combining this analysis with fossil record of the same section, we try to provide new insights into changes in marine redox conditions across the late Ediacaran and the potential links to early animal evolutions.

Geological Background and Sampling
The NCC is one of the major Precambrian cratons in China, with a maximum age of ~3.8 Ga and an area of ~3 × 10 6 km 2 [20][21][22][23]. The Meso-Neoproterozoic strata are mainly deposited within the Xiong'er, Xuhuai, Yanliao, and Zhaertai-Bayan Obo-Huade basins [18,19,24,25] (Figure 1B). Widespread Cambrian transgressive successions disconformably overlie on these Meso-Neoproterozoic strata in the NCC, known as the 'Great Unconformity' [26]. Owing to the mid-late Neoproterozoic uplift of the NCC, the late Neoproterozoic strata in most of this continent were denuded. Following the subsequent northward transgression, the Ediacaran successions were mainly deposited on the southern margins of the NCC ( Figure 1B), such as the Zhengmuguan and Dongpo formations on the southwestern margin [27][28][29], the Luoquan and Dongpo formations on the southern margin [30,31], and the Fengtai Formation on the southeastern margin.

Geological Background and Sampling
The NCC is one of the major Precambrian cratons in China, with a maximum age of~3.8 Ga and an area of~3 × 10 6 km 2 [20][21][22][23]. The Meso-Neoproterozoic strata are mainly deposited within the Xiong'er, Xuhuai, Yanliao, and Zhaertai-Bayan Obo-Huade basins [18,19,24,25] (Figure 1B). Widespread Cambrian transgressive successions disconformably overlie on these Meso-Neoproterozoic strata in the NCC, known as the 'Great Unconformity' [26]. Owing to the mid-late Neoproterozoic uplift of the NCC, the late Neoproterozoic strata in most of this continent were denuded. Following the subsequent northward transgression, the Ediacaran successions were mainly deposited on the southern margins of the NCC ( Figure 1B), such as the Zhengmuguan and Dongpo formations on the southwestern margin [27][28][29], the Luoquan and Dongpo formations on the southern margin [30,31], and the Fengtai Formation on the southeastern margin.
The studied Luoquancun section is located at Luoquan Village, Mangchun Town, Linru County, in west Henan Province, on the southern margin of the NCC. It is the stratotype section of the Ediacaran Luoquan and Dongpo formations. In this section, the Ediacaran Luoquan Formation is conformably overlain by the Ediacaran Dongpo Formation and disconformably overlies the sandstone of the Mesoproterozoic Beidajian Formation. The Luoquan Formation ranges from 20 to 200 m in thickness on the southern margin of the NCC and has a thickness of~190 m in the study section. Here, the Luoquan Formation comprises upper stratified diamictites and lower massive diamictites ( Figure 2C,E). Still higher, the Shaanxilithes-bearing Dongpo Formation is disconformably overlain by the trilobite-bearing sandstone of the Cambrian Xinji Formation (Figure 2A). The Dongpo Formation is made up of shale/siltstone ( Figure 2B,D). The finding of the late Ediacaran fossil Shaanxilithes in the Dongpo Formation gives convincing evidence of the Ediacaran age of the Dongpo Formation [32] and correlates it with these fossiliferous formations, such as the Tuerkeng Formation of the southwestern NCC [33], Dengying Formation of South China [34], and Zhoujieshan Formation of Qaidam Block [35]. Previous researchers have well studied the biostratigraphy, lithostratigraphy, and detrital zircon geochronology of the Dongpo Formation [26,31] and the sedimentary environments and glacial characteristics of the Luoquan Formation [22,30,[36][37][38]. Such studies have suggested that the Luoquan Formation was deposited in a lacustrine glacial or continental glacial to proglacial marine environment and that the overlying Dongpo Formation formed in a shallow marine environment [24,30,38]. In this study, we used iron and redox-sensitive trace elements (RSTEs) geochemical analyses of the Ediacaran Dongpo Formation to further decipher the redox conditions of the late Ediacaran ocean on the southern margin of the NCC. totype section of the Ediacaran Luoquan and Dongpo formations. In this section, acaran Luoquan Formation is conformably overlain by the Ediacaran Dongpo Fo and disconformably overlies the sandstone of the Mesoproterozoic Beidajian For The Luoquan Formation ranges from 20 to 200 m in thickness on the southern m the NCC and has a thickness of ~190 m in the study section. Here, the Luoquan Fo comprises upper stratified diamictites and lower massive diamictites ( Figure 2C higher, the Shaanxilithes-bearing Dongpo Formation is disconformably overlain by lobite-bearing sandstone of the Cambrian Xinji Formation (Figure 2A). The Dong mation is made up of shale/siltstone ( Figure 2B,D). The finding of the late Ediacar Shaanxilithes in the Dongpo Formation gives convincing evidence of the Ediacara the Dongpo Formation [32] and correlates it with these fossiliferous formations, the Tuerkeng Formation of the southwestern NCC [33], Dengying Formation o China [34], and Zhoujieshan Formation of Qaidam Block [35]. Previous research well studied the biostratigraphy, lithostratigraphy, and detrital zircon geochron the Dongpo Formation [26,31] and the sedimentary environments and glacial cha tics of the Luoquan Formation [22,30,[36][37][38]. Such studies have suggested that t quan Formation was deposited in a lacustrine glacial or continental glacial to pr marine environment and that the overlying Dongpo Formation formed in a shal rine environment [24,30,38]. In this study, we used iron and redox-sensitive trace e (RSTEs) geochemical analyses of the Ediacaran Dongpo Formation to further deci redox conditions of the late Ediacaran ocean on the southern margin of the NCC.  Here, we focus on the Dongpo Formation, which is made up of shale/siltstone. The shale is made up of clay minerals and organic matter, and the siltstone consists of clay minerals, fragment quartz, and organic matter. In the study section, the total thickness of the Dongpo Formation is~70 m, and most of the lamina are less than 1 mm. We collected thirty-six fresh samples from the Dongpo Formation for thin section and geochemical analyses.

Geochemical Methods
Thirty-six silty shales (LQC 11-46) of the Dongpo Formation were carefully collected at the Luoquancun section on the southern margin of the NCC. Redox-sensitive trace elements (RSTEs) were analyzed using a PerkinElmer Elan 9000 type inductively coupled plasma mass spectrometer (ICP-MS) at the Laboratory of ALS Chemex (Guangzhou). The enrichment factors (EFs) of the RSTEs were calculated as [El/Al (sample) /El/Al (reference) ], where the reference is the values of average shale [39].
Iron (Fe) geochemical analyses were carried out at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences at Wuhan (SKLBEG, CUG). Iron contents in carbonates (Fe carb ), oxides (Fe ox ), and magnetite (Fe mag ) were measured following the standard iron sequential extraction procedure illustrated in [40], and iron in pyrite (Fe py ) was determined using the chromium reduction method [41]. Fe carb , Fe ox , and Fe mag were sequentially extracted using sodium acetate, sodium dithionite, and oxalic acid-sodium oxalate. The extracted iron speciations were diluted and measured via atomic absorption spectrometry (AAS) with a relative standard deviation (RSD) lower than 5%. Fe py was calculated based on pyrite-derived sulfur content, which was extracted and precipitated as Ag 2 S. Sample powders were reacted with HCl and CrCl 2 to convert pyrite sulfur to H 2 S and recovered as Ag 2 S in silver nitrate traps.

Redox Conditions of the Late Ediacaran Ocean on the Southern Margin of the NCC
The iron and RSTEs geochemical data for the studied samples are listed in Table S1 (Supplementary Materials) and summarized in Figure 3. Based on observations of different iron phases in fine-grained siliciclastic rocks, iron geochemistry is commonly used as a paleoredox proxy to distinguish the local bottom water-column redox state of oxic, ferruginous (anoxic with dissolved ferrous Fe), and euxinic (anoxic with dissolved sulfide) [42,43]. Highly reactive iron (Fe HR ) includes iron that has been transformed to pyrite (Fe py ) and iron that can react with H 2 S to form pyrite in the water column or during early sediment diagenesis, such as carbonate-associated iron (Fe carb ), ferric oxides (Fe ox ), and magnetite iron (Fe mag ) [44]. The other poorly or non-reactive iron (Fe U ) consists of iron in clay minerals and residual silicate-bound irons which are basically unreactive towards H 2 S during deposition and early diagenesis [44]. In this scheme, Fe HR = Fe carb + Fe ox + Fe mag + Fe py and Fe T = Fe HR + Fe U . The ratios of Fe HR /Fe T in modern and ancient sediments provide a threshold of 0.38 for the upper limit of oxic marine sediments, with Fe HR /Fe T values exceeding this threshold indicative of anoxia [42,43]. Under anoxic conditions (Fe HR /Fe T > 0.38), the ratios of Fe py /Fe HR can further distinguish the water column redox state as ferruginous (Fe py /Fe HR < 0.7-0.8) or euxinic (Fe py /Fe HR > 0.7-0.8). Because some non-redox processescan mute iron augmentation [45] and record lower Fe HR /Fe T ratios [43], a lower Fe HR /Fe T threshold value of <0.22 was suggested to interpret the oxic conditions, and Fe HR /Fe T values between 0.22 and 0.38 should be applied with caution [42,46]. Moreover, iron geochemistry is not appropriate for samples with Fe T < 0.5 wt.%, which usually yield elevated Fe HR /Fe T values [47]. In this study, the total iron of all samples had an average weight of 3.41% with a minimum Fe T value of 2.92% (LQC43), which exceeds the minimum request for iron geochemistry analysis, indicating that the Fe-speciation for these samples is suitable for redox reconstruction. Because the Fepy fraction is typically transformed into immobile iron oxides preserved near the original pyrite during chemical weathering and the Fepy/FeHR and FeHR/FeT show no statistical correlation despite pyrite weathering [48,49], the FeHR/FeT is thought to be basically unaffected by modern oxidative weathering and can be used as a redox proxy despite evidence of secondary oxidative weathering [49]. The FeHR/FeT for the Dongpo Formation of the Luoquancun section ranges from 0.19 to 0.49 (mean = 0.33), fluctuating around the upper threshold of 0.38, and in combination with low Fepy/FeHR ratios (~0), it revealed rapid oscillations between anoxic-ferruginous and oxic local bottom water column redox conditions ( Figure 3). Considering that a part of the samples with FeHR/FeT ratios below 0.38 was plotted between 0.22 and 0.38, which also could be indicative of anoxic conditions, the measured iron speciation data, therefore, probably suggested a dominantly anoxic late Ediacaran ocean punctuated by multiple transient oxygenation events across the southern margin of the NCC.
The alternative redox proxy of total iron to aluminum (FeT/Al) can provide additional information on redox patterns. In the case of FeT/Al, crustal values of 0.5 ± 0.1 typically indicate oxic conditions and sediments deposited beneath anoxic water columns are specifically enriched in total iron, giving rise to Fe/Al enrichments above this threshold [10,45]. Therefore, the ratio of FeT/Al is widely used as an alternative redox proxy [10,44,[50][51][52]. Samples from the Dongpo Formation give FeT/Al ratios from 0.35 to 0.61 (mean = 0.44), oscillating around a mean of 0.5. Samples with elevated FeT/Al ratios (>0.5), coupled with high FeHR/FeT ratios (>0.38), point to anoxic-ferruginous bottom water-column conditions. On the other hand, those samples with lower FeT/Al ratios (<0.5) are positively associated with lower FeHR/FeT ratios (<0.22-0.38), suggesting more oxygenated corresponding water column redox conditions (Figure 3).
Although the above analysis of iron geochemical data indicated anoxic-ferruginous depositional conditions for a portion of our samples, no significant RSTE enrichments (e.g., Mo, V, U) were observed in such samples (Figure 3). Samples with high FeHR/FeT Because the Fe py fraction is typically transformed into immobile iron oxides preserved near the original pyrite during chemical weathering and the Fe py /Fe HR and Fe HR /Fe T show no statistical correlation despite pyrite weathering [48,49], the Fe HR /Fe T is thought to be basically unaffected by modern oxidative weathering and can be used as a redox proxy despite evidence of secondary oxidative weathering [49]. The Fe HR /Fe T for the Dongpo Formation of the Luoquancun section ranges from 0.19 to 0.49 (mean = 0.33), fluctuating around the upper threshold of 0.38, and in combination with low Fe py /Fe HR ratios (~0), it revealed rapid oscillations between anoxic-ferruginous and oxic local bottom water column redox conditions ( Figure 3). Considering that a part of the samples with Fe HR /Fe T ratios below 0.38 was plotted between 0.22 and 0.38, which also could be indicative of anoxic conditions, the measured iron speciation data, therefore, probably suggested a dominantly anoxic late Ediacaran ocean punctuated by multiple transient oxygenation events across the southern margin of the NCC.
The alternative redox proxy of total iron to aluminum (Fe T /Al) can provide additional information on redox patterns. In the case of Fe T /Al, crustal values of 0.5 ± 0.1 typically indicate oxic conditions and sediments deposited beneath anoxic water columns are specifically enriched in total iron, giving rise to Fe/Al enrichments above this threshold [10,45]. Therefore, the ratio of Fe T /Al is widely used as an alternative redox proxy [10,44,[50][51][52]. Samples from the Dongpo Formation give Fe T /Al ratios from 0.35 to 0.61 (mean = 0.44), oscillating around a mean of 0.5. Samples with elevated Fe T /Al ratios (>0.5), coupled with high Fe HR /Fe T ratios (>0.38), point to anoxic-ferruginous bottom water-column conditions. On the other hand, those samples with lower Fe T /Al ratios (<0.5) are positively associated with lower Fe HR /Fe T ratios (<0.22-0.38), suggesting more oxygenated corresponding water column redox conditions (Figure 3).
Although the above analysis of iron geochemical data indicated anoxic-ferruginous depositional conditions for a portion of our samples, no significant RSTE enrichments (e.g., Mo, V, U) were observed in such samples (Figure 3). Samples with high Fe HR /Fe T ratios (>0.38) and low RSTE enrichments (EF < 1) are also found in the Doushantuo Formation from the inner-shelf Jiuqunao and Jiulongwan sections in South China, which were interpreted as a consequence of the different sensitivities of iron speciation and RSTE to high-frequency redox variation in the Ediacaran ocean [53]. In theory, Mo enrichments require sulfide to convert soluble molybdate to thiomolybdates [54], but sulfide in the investigated sections is fairly low. Other redox-sensitive trace metals (e.g., V, U), which do not need sulfidic environments, usually exhibit low enrichments in the Proterozoic anoxic units [50,55]. It has been regarded as a consequence of widespread reducing sinks [50,56] or the small size of the corresponding oceanic trace metal reservoirs [57,58]. Compared with those classic Phanerozoic anoxic units, it is hard to separate the low Proterozoic enrichments from background levels [59]. Therefore, the absence of trace metal enrichments in our samples does not necessarily point to oxic conditions.

The Link between Ocean Redox Conditions and the Evolution of Early Animals
The present paleoredox reconstruction from the Luoquancun sections provides evidence of several marine shelf oxidation events in an anoxic late Ediacaran ocean along the southern margin of the NCC. However, the timing and magnitude of these oxidation events in the NCC and their temporal relationship with successions in other continents are uncertain. Previous studies have revealed a dominantly anoxic Ediacaran-early Cambrian ocean punctuated by multiple oceanic oxygenation events (OOEs) in the early (OOE1,~632 Ma), middle (OOE2,~580 Ma), and late (OOE3,~560-541 Ma) Ediacaran and early Cambrian (OOE4) [55,60] (Figure 4C). Along with these OOEs, large-magnitude δ 13 C excursions ( Figure 4B) and key macrofossil assemblages ( Figure 4A), including Avalon (~575-560 Ma), White Sea (~560-550 Ma), and Nama (~550-540 Ma) assemblages, occurred during the Ediacaran Period [61]. Because the studied sections are dominated by siliciclastic rocks that render a spotty δ 13 C chemostratigraphic record, the correlation of the studied sections with δ 13 C excursions in other Ediacaran successions is not straightforward [62].
The occurrence of the potential late Ediacaran index fossil Shaanxilithes from the Dongpo Formation probably constrains its depositional age to be late Ediacaran (~550-541 Ma) [31,35]. These fossils were found to be within the top of the Dongpo Formation, where oxygenations were persistent. Morphological and taphonomic reconstructions show that the body plan of the tubular fossil Shaanxilithes is close to the contemporaneous well-known skeletal fossil-Cloudina, which is a globally distributed eumetazoan and is well-preserved in the middleupper part of the Dengying Formation in South China, the lower part of the Birba Formation in Southern Oman, and the lower Nama Group in southern Namibia [32,51]. Integrated data for I/[Ca + Mg], Ce/Ce*, and Fe-speciation analysis from the Nama Group indicated that the Nama Basin experienced intermittent anoxia and that the Nama skeletal communities were abundant in well-oxygenated environments but absent from oxygen-impoverished regions [51,63,64]. Such a phenomenon supports the prominent role of oxygen availability in controlling the distribution of Ediacaran skeletal metazoan communities [63,64]. Expanded anoxic conditions, recorded by δ 238 U and δ 34 S, occurred after the first appearance of the skeletal fauna of the Nama Assemblage, demonstrating that the decline of the Ediacaran biota was unlikely to be driven by the expansion of anoxia in the Nama Basin [7,65]. Widespread oceanic anoxia and dynamic redox conditions at both temporal and spatial scales were also recorded in the fossiliferous Ediacaran successions in South China [8,66]. The Ediacaran biotas, as recorded in the Nama Assemblage, are spatiotemporally consistent with the episode of oceanic oxygenation, suggesting that the global oxygenations may have promoted the diversification of the Ediacaran biota [60,67,68].
Therefore, the iron geochemical data in this study revealed a dominantly anoxic late Ediacaran ocean punctuated by multiple transient oxygenation events across the southern margin of the NCC. This result reinforces previous studies' findings, based on analyses of C-S-N isotopes and trace elemental enrichments, that the Ediacaran Period was characterized by the pulsed oxygenation of a predominantly anoxic global ocean [5,69,70]. The new Fe HR /Fe T and Fe T /Al data presented here suggest transient oxidation of shallow oceans during the deposition of the fossiliferous part of the Dongpo Formation. These transient oxidation events in the NCC may have contributed to the appearance of the Ediacaran tubular fossil Shaanxilithes. Taken together, this study from the NCC combined with previous data from other Ediacaran successions (e.g., the Nama Group in Namibia, the Dengying Formation in South China, etc.) confirms a dominantly anoxic ocean punctuated by pulsed oxygenation events during the late Ediacaran and confirms that global oceanic oxygenation events may have contributed to the rise of the Ediacaran biota.

Statistical Analysis of Global Iron Speciation Data from the Ediacaran to Middle Cambrian
Although iron-based proxies only represent local redox conditions, these proxies still have important global implications when analyzed collectively and statistically [71]. Based on the assumption that local iron speciation data in a global framework can track the mean and variance of paleoredox conditions through time, we additionally integrated our iron speciation data from the NCC with published data from correlative sections in other continents for a global perspective on redox tracers. Following this template, we developed a data set of about 3300 new and published iron speciation data with Fe T > 0.5 wt.% from fine-grained clastic rocks to infer the global redox change in Ediacaran-Cambrian oceans. Each sample was assigned a depositional environment and established age from its original literature or based on age constraints and sedimentation rates (Table S2, Supplementary Materials). The samples were binned into six relative age bins based on the major geological timescales. The Ediacaran was subdivided into the lower Ediacaran (635-580 Ma), middle Ediacaran (580-560 Ma), and upper Ediacaran (560-541 Ma), based on subdivision models in [62]. The Cambrian age bins were followed by the lower-middle Cambrian series as the Terreneuvian (541-521 Ma), Series 2 (521-509 Ma), and the Miaolingian (509-497 Ma).
The mean Fe HR /Fe T ratio in each time bin shows a dynamic variation in the Ediacaran time bins with mean Fe HR /Fe T ratios of 0.48, 0.39, and 0.51, respectively, and a progressive decline in the early-middle Cambrian with mean Fe HR /Fe T ratios declining from 0.60 in the Terreneuvian to 0.35 in the Miaolingian ( Figure 4D), indicating dynamic Ediacaran marine redox conditions and stepwise oceanic oxygenation during the early-middle Cambrian. For the Ediacaran, the lowest mean Fe HR /Fe T ratios (0.39), the lowest proportion of anoxic samples (0.40), and the lowest proportion of euxinic samples (0.07) appeared in the middle Ediacaran (580-560 Ma), which may reflect the highest global ocean oxygenation level in the Ediacaran. These results have important implications for the evolution of early animals. It has been suggested that the Ediacaran biota appeared in the middle Ediacaran [72], reached their maximum taxonomic diversity at~560 Ma, and then declined in the terminal Ediacaran stage (~550-541 Ma; Figure 4A) [67,[73][74][75]. Our statistical analyses demonstrate that the appearance and rise of the Ediacaran biota correspond temporally with the global ocean oxygenation in the middle Ediacaran (580-560 Ma). Coincidentally, the largest δ 13 C excursion (Shuram excursion, SE) in Earth history may also have occurred during this time period ( Figure 4B). The coupled oceanic oxygenation and rise of the Ediacaran biota in the middle Ediacaran are in agreement with previous studies based on analyses of Mo-U-S-Tl isotopes suggesting that a significant ocean oxygenation event may have happened during the SE and that the increase in global ocean oxygenation likely triggered the evolution of the Ediacaran-type biota [3,67,76,77].
Moreover, existing observations have revealed an anoxia-dominated global deep ocean and a highly redox-stratified shelf ocean during the Ediacaran-early Cambrian, in which a mid-depth euxinic (i.e., anoxic and H 2 S-bearing or sulfidic) water mass coexisted dynamically with the oxic surface and ferruginous deep waters [78]. Because early animals mainly or exclusively occupied continental shelves [79,80], the dynamically developed middepth euxinic waters in the shelf oceans may have extremely shaped the local ecosystems of early animals [66]. Our results suggest that the proportion of euxinic samples show variations of 0.14, 0.07, and 0.19, respectively, which reached their maximum proportion (0. 19) in the upper Ediacaran (560-541 Ma) ( Figure 4D). The decline of the Ediacaran biota may have been caused by the expansion of the mid-depth euxinic waters in the upper Ediacaran, whereas more detailed redox and paleontological studies are needed to prove this hypothesis. proportion (0.19) in the upper Ediacaran (560-541 Ma) ( Figure 4D). The decline of the Ediacaran biota may have been caused by the expansion of the mid-depth euxinic waters in the upper Ediacaran, whereas more detailed redox and paleontological studies are needed to prove this hypothesis.  Table S2), collected from [9,10,13,50,53,55,60,64,71,. The proportion of anoxic samples was calculated as the proportion of samples with FeHR/FeT > 0.38, and the proportion of euxinic samples was calculated as the proportion of samples with FeHR/FeT > 0. 38 and FePy/FeHR > 0.7. Whiskers represent standard errors. The ages for the Ediacaran are based on subdivision models in [62], and the Cambrian ages are based on the international chronostratigraphic chart.
Additionally, for the early-middle Cambrian, all the mean FeHR/FeT ratios (0.6, 0.44, and 0.35), the proportion of anoxic samples (0.73, 0.43, and 0.36), and the proportion of euxinic samples (0.31, 0.17, and 0.01) show a progressive decline from Terreneuvian to Miaolingian, which may reflect stepwise oxygenation of the early-middle Cambrian ocean ( Figure 4D). Corresponding to the stepwise oceanic oxygenation, the number of animals rose dramatically ( Figure 4A). Previous studies in South China have found that the expansion of complex arthropod-dominated biotas and the regional increase in the diversity of basic metazoan body plans are consistent with the rising oceanic oxygen levels in the early-middle Cambrian [55]. Our study confirms that this coupled early-middle Cambrian ocean oxygenation and metazoan diversification should be a global phenomenon. Collectively, statistical analysis of global iron speciation data from the Ediacaran to the middle Cambrian revealed dynamic Ediacaran marine redox conditions and stepwise oceanic oxygenation during the early-middle Cambrian. Our findings reinforce previous studies suggesting that the coupled global ocean oxygenation and rise of the Ediacaran biota occurred during the middle Ediacaran and that stepwise oceanic oxygenation may have facilitated the evolution of animals during the early-middle Cambrian.  Table S2), collected from [9,10,13,50,53,55,60,64,71,. The proportion of anoxic samples was calculated as the proportion of samples with Fe HR /Fe T > 0.38, and the proportion of euxinic samples was calculated as the proportion of samples with Fe HR /Fe T > 0.38 and Fe Py /Fe HR > 0.7. Whiskers represent standard errors. The ages for the Ediacaran are based on subdivision models in [62], and the Cambrian ages are based on the international chronostratigraphic chart.
Additionally, for the early-middle Cambrian, all the mean Fe HR /Fe T ratios (0.6, 0.44, and 0.35), the proportion of anoxic samples (0.73, 0.43, and 0.36), and the proportion of euxinic samples (0.31, 0.17, and 0.01) show a progressive decline from Terreneuvian to Miaolingian, which may reflect stepwise oxygenation of the early-middle Cambrian ocean ( Figure 4D). Corresponding to the stepwise oceanic oxygenation, the number of animals rose dramatically ( Figure 4A). Previous studies in South China have found that the expansion of complex arthropod-dominated biotas and the regional increase in the diversity of basic metazoan body plans are consistent with the rising oceanic oxygen levels in the early-middle Cambrian [55]. Our study confirms that this coupled early-middle Cambrian ocean oxygenation and metazoan diversification should be a global phenomenon. Collectively, statistical analysis of global iron speciation data from the Ediacaran to the middle Cambrian revealed dynamic Ediacaran marine redox conditions and stepwise oceanic oxygenation during the early-middle Cambrian. Our findings reinforce previous studies suggesting that the coupled global ocean oxygenation and rise of the Ediacaran biota occurred during the middle Ediacaran and that stepwise oceanic oxygenation may have facilitated the evolution of animals during the early-middle Cambrian.

Conclusions
This study presents the detailed geochemical reconstruction of the fossiliferous Ediacaran section across the southern margin of the North China Craton. Iron and RSTEs geochemical data revealed a dominantly anoxic late Ediacaran ocean punctuated by multiple transient oxygenation events across the southern margin of the NCC. This is in agreement with previous studies suggesting that the Ediacaran Period was characterized via the pulsed oxygenation of a predominately anoxic global ocean. Moreover, the new Fe HR /Fe T and Fe T /Al data presented here suggest the transient oxidation of shallow oceans during the deposition of the fossiliferous part of the Dongpo Formation. These transient oxidation events in the NCC may have contributed to the appearance of the Ediacaran tubular fossil Shaanxilithes. To further assess the relationship between global ocean redox conditions and the evolution of early animals, we integrated our iron speciation data with published data from Ediacaran-middle Cambrian sections in other continents. Statistical analysis of global iron speciation data suggested dynamic Ediacaran marine redox conditions and stepwise oceanic oxygenation during the early-middle Cambrian. Integrated biotic and geochemical records suggested that the coupled global ocean oxygenation and rise of the Ediacaran biota occurred in the middle Ediacaran and that the stepwise early-middle Cambrian ocean oxygenation may have facilitated the evolution of animals.