# Low Oxygen Levels in Early Neoproterozoic Shallow Seawater and Evolution Stasis of Crown-Group Eukaryotes

Anran Hou, Xiaoying Shi, Romain Guilbaud, Baozeng Xie, Longfei Sun, Lei Xu, Limin Zhou, Wei Liu, Xiqiang Zhou, Xinqiang Wang, et al.

<figure><img src="image_1.png" /></figure>

► To cite this version:

Anran Hou, Xiaoying Shi, Romain Guilbaud, Baozeng Xie, Longfei Sun, et al., Low Oxygen Levels in Early Neoproterozoic Shallow Seawater and Evolution Stasis of Crown-Group Eukaryotes. Journal of Geophysical Research: Biogeosciences, 2025, 130 (12), pp.e2025JG008835. 10.1029/2025JG008835. hal-05400558

HAL Id: hal-05400558

https://hal.science/hal-05400558v1

Submitted on 5 Dec 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>
Low oxygen levels in early Neoproterozoic shallow seawater and
evolution stasis of crown-group eukaryotes

Anran Hou¹,², Xiaoying Shi¹,², Romain Guilbaud³, Baozeng Xie², Longfei Sun²,
Lei Xu⁴, Limin Zhou⁵, Wei Liu⁵, Xiqiang Zhou⁶,⁷, Xinqiang Wang¹,², Dongjie
Tang¹,⁴*

¹State Key Laboratory of Geomicrobiology and Environmental Changes, Frontiers
Science Center for Deep-time Digital Earth, China University of Geosciences
(Beijing), Beijing, China

²School of Earth Sciences and Resources, China University of Geosciences (Beijing),
Beijing, China

³Géosciences Environnement Toulouse, CNRS, Toulouse, France

⁴Institute of Earth Sciences, China University of Geosciences (Beijing), Beijing,
China

⁵National Research Center for Geoanalysis, Beijing, China

⁶State Key Laboratory of Lithospheric and Environmental Coevolution, Institute of
Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

⁷College of Earth and Planetary Sciences, University of Chinese Academy of
Sciences, Beijing, China

Corresponding author: Dongjie Tang (dongjtang@126.com)

Key Points:

* Carbonate I/(Ca+Mg) and Ce anomaly were used to reconstruct the redox conditions of early Neoproterozoic shallow seawater

* The results reveal that the Tonian carbonates from the Xuhuai region of North China record suboxic shallow marine conditions
26

27

28

* Low oxygen and nutrient-poor conditions together hindered the sustained evolution of crown-group eukaryotes
## 29 Abstract

30 Crown-group eukaryotes diversified rapidly in the late Mesoproterozoic (ca. 1.1–1.0
31 Ga), but their evolution was slowed in the early Neoproterozoic (ca. 1.0–0.8 Ga), with
32 a significant episode of diversification occurring again around 0.8 Ga. Previous
33 studies suggest nutrient (nitrogen and phosphorus) scarcity in the seawater may have
34 delayed eukaryotic evolution during this period. However, the influence of marine
35 redox conditions on the evolution, either directly or indirectly, remains unclear.

36 Applying integrated approaches of sedimentology, mineralogy and geochemistry, we
37 analyze the shallow marine carbonates from the Zhaowei, Niyuan, and Jiudingshan
38 formations of the Huaibei Group (ca. 1.0–0.95 Ga) in the southeastern North China to
39 constrain the redox conditions of the early Neoproterozoic seawater. Microscopic
40 observations revealed abundant water-column carbonate mud (“whiting”) precipitates
41 in these formations, indicating moderately oxygenated surface waters. Geochemical
42 results show that the I/(Ca+Mg) values of the carbonates are mostly (98%) below the
43 Precambrian background baseline (0.5 µmol/mol), without significant negative Ce
44 anomaly (0.82 ± 0.06, n = 44). These findings support low-oxygen conditions in the
45 shallow waters. The combination of low oxygen conditions and nutrient scarcity
46 likely delayed the Neoproterozoic diversification of early crown-group eukaryotes.
47 This study provides valuable insights into marine redox conditions in Earth’s middle
48 age and their potential impact on early eukaryotic evolution.

## 49 Plain Language Summary

50 Oxygen is essential for all eukaryotic life. The evolution of early crown-group
eukaryotes during the early Neoproterozoic was notably slow. Whether this was
51 influenced by the redox conditions of the oceans during this period remains unclear.
52 To address this issue, we analyzed the sedimentology, mineralogy, and geochemistry
of early Neoproterozoic carbonates from the Zhaowei, Niyuan, and Jiudingshan
formations in the Xuhuai region, located on the southeastern margin of the North
53 China Craton. Our results suggest that oxygen levels in the shallow waters during the
54 time were low. The low-oxygen coupled with nutrient-poor environments may have
55
slowed the early evolution of crown-group eukaryotes. These findings provide new insights into the chemical conditions of the Early Neoproterozoic oceans and how they have affected the evolution of early eukaryotes.

## 1. Introduction

The Neoproterozoic era has witnessed a profound transformation of Earth surface systems, particularly regarding the evolution of eukaryotes (Anderson et al., 2024; Cohen & Kodner, 2022; Tang et al., 2024). During this period, eukaryotes gradually outcompeted prokaryotes and became the dominant contributors to primary production in the ocean (Brocks et al., 2023), with multicellular eukaryotic algae fossils reported from various continents over the world (Brocks et al., 2017). The appearance of large, morphologically diverse acanthomorphic acritarchs marked a further development and differentiation of eukaryotes in both cellular structures and ecological functions (Liu et al., 2014; Tang et al., 2015). The emergence of metazoans, represented by the Ediacaran biota, marked the first appearance of large, structurally complex organisms in Earth's history (Chen et al., 2013; Narbonne, 2005). However, the evolution of crown-group eukaryotes did not follow a pattern of continuous progress throughout the Neoproterozoic. After a short period of rapid diversification at the end of the Mesoproterozoic (Butterfield, 2000; Gibson et al., 2018; Li et al., 2023; Loron et al., 2019; Tang et al., 2020), many crown-group eukaryotic organisms experienced an evolutionary stasis in the early Neoproterozoic (ca. 1.0–0.8 Ga), with low abundance and diversity (Cohen & Macdonald, 2015; Knoll et al., 2006; Tang et al., 2024). However, the potential causes for this evolutionary stasis have received little attention.

Previous studies have suggested a long-term deficiency of nitrogen (Kang et al., 2023) or phosphorus (Guilbaud et al., 2020) in early Neoproterozoic oceans. Both nitrogen and phosphorus are essential nutrients for marine life, and have played a critical role in regulating primary productivity (Howarth, 1988). The Huaibei Group (ca. 1.0–0.8 Ga) is characterized by consistently low δ¹⁵N values, indicative of low nitrate availability in the seawater (Kang et al., 2023). Meanwhile, widespread anoxic
and iron-rich (ferruginous) oceans (Guilbaud et al., 2015) limited phosphorus recycling from the sediment, as the phosphorus released during organic matter degradation may adsorb and reprecipitate with ferric (oxyhydr) oxides, leading to oligotrophic conditions which resulted in limited primary productivity, carbon burial and, eventually, oxygen production (Guilbaud et al., 2020). Oceanic redox conditions can also directly influence eukaryotic evolution (Runnegar 1991; Zhang et al., 2018). Studies of modern marine eukaryotes show that oxygen concentrations below ~10.8 µM (~2.7% Present Atmospheric Level; PAL) fail to meet the demands necessary for the diversification of crown-group eukaryotes (Mills et al., 2024). Previous studies indicate a widespread oceanic anoxia during the early Neoproterozoic. For instance, iron speciation studies on the carbonates of the Jiuliqiao and lower Sidingshan formations of the Feishui Group (Guilbaud et al., 2015; Sun et al., 2025a), as well as the Shijia and Wangshan formations of the Huaibei Group (Sun et al., 2025a) in the Xuhuai region of North China, suggest that the shallow waters were largely ferruginous. Uranium (U) isotope studies of carbonates from the Xuhuai region have been interpreted as evidence of widespread shallow marine anoxia during the early Tonian (Zhang et al., 2022a). However, a subsequent study suggests that the extremely low U isotope values in the Tonian shallow marine carbonates may reflect an intermittent expansion of short-term anoxic water masses during the ongoing oceanic oxidation process, rather than an extreme shallow marine anoxia (Clarkson et al., 2023). This is further supported by large fractionations of U isotopes during the partial oxidation of anoxic sediments, whereby the light isotopes remain in solution (Guilbaud et al., 2025). We note that, however, the behavior of U isotope at the transition between oxic and ferruginous settings remains largely unknown (Cole et al., 2020; Gilleaudeau et al., 2025). Thus, the uncertainty on early Neoproterozoic shallow seawater redox conditions has limited our understanding of how environmental conditions influenced the evolution of crown-group eukaryotes.

This study investigates shallow marine carbonates from the Zhaowei, Niyuan, and Jiudingshan formations (ca. 1.0–0.95 Ga) within the continuous sedimentary
successions of the Xuhuai region, southeastern North China. By conducting sedimentological, mineralogical, and geochemical analyses, we aim to reconstruct the redox conditions of early Neoproterozoic shallow marine environments.

## 2. Geological Setting

The study area is located on the southeastern margin of the North China Craton (NCC; Fig. 1a), where the Meso-Neoproterozoic Huaibei Group unconformably overlies the metamorphic crystalline basement of the Paleoproterozoic Taishan Group. The Huaibei Group is mainly distributed in the northern part of Anhui Province, China, with its sedimentation dominated by a shallow marine carbonate platform connecting to the ocean in the south (Fig. 1b; Hu et al., 2016). The group, in stratigraphic order, can be divided into the Lanling, Xinxing, Jushan, Jiayuan, Zhaowei, Niyuan, Jiudingshan, Zhangqu, Weiji, Shijia, Wangshan, Jinshanzhai, and Gouhou formations (Fig. 1c). The group is unconformably overlain by the Early Cambrian Houjiashan Formation (Fig. 1c), with a stratigraphic gap of approximately 200-300 million years between them (Wan et al., 2019).

Our study focuses on a continuous ~600-meter-thick carbonate succession exposed at Qingtongshan, Xiazhai Village, Yongqiao District, Suzhou City, Anhui Province (Fig. 1d). The section spans four formations of the Huaibei Group: the upper Jiayuan, Zhaowei, Niyuan, and lower Jiudingshan formations (Fig. 2). These strata (Jiayuan and Zhaowei formations) are significant as they preserve leiospheric acritarchs and possible vase-shaped microfossils in the Zhaowei section, southern Jiangsu, but no crown-group eukaryotes (Xiao et al., 2014; Zang & Walter, 1992), offering valuable insights into early Neoproterozoic environmental and biological co-evolution. The Jiayuan Formation, exposed in the upper 10 meters, consists of sandy limestone (Fig. 2a), indicative of shallow subtidal deposition (Dou et al., 2014). The Zhaowei Formation, with a thickness of 185 meters, comprises two members. Member I is composed of gray, thick-bedded argillaceous dolomite-bearing limestone, with locally developed storm debris (Fig. 2b), and columnar stromatolites (Fig. 2c), suggesting a deposition of shallow subtidal environment. Member II is characterized
by medium- to thin-bedded gray argillaceous dolomite-bearing limestone, interbedded with calcareous shale (Fig. 2d), representing a deposition from shallow subtidal to intertidal zones (Lu et al., 2013). The Niyuan Formation, 410 meters thick, includes two members. Member I consists of medium-bedded gray argillaceous limestone and argillaceous dolomite-bearing limestone (Fig. 2e). The upper part of Member I contains molar tooth structures (Fig. 2f), columnar stromatolites (Fig. 2g), ooids (Fig. 2h), and storm debris (Fig. 2i), indicating a deposition from shallow subtidal to intertidal zones. The Member II comprises medium- to thick-bedded argillaceous limestone and argillaceous calcareous dolostone (Fig. 2j), transitioning upwards into argillaceous dolostone interbedded with yellow, thin-bedded calcareous shale (Fig. 2k), indicating a low-energy tidal flat deposition (Meng et al., 2023). The lower part of the Jiudingshan Formation is exposed, comprising only 58 meters of gray, thick-bedded argillaceous limestone that bears molar tooth structure and storm debris (Fig. 2l), pointing to a shallow subtidal deposition.

Previous nitrogen isotope (Kang et al., 2023) and uranium isotope (Zhang et al., 2022a) studies targeted equivalent formations in sections ~20 km northwest of our locality, spanning the southern Jiangsu-northern Anhui border. For the regional context, we compare our data with published iron speciation data: (1) from the temporally equivalent Jiuliqiao and lower Sidingshan formations (Feishui Group, central Anhui; Guilbaud et al., 2015; Sun et al., 2025a), sampled ~100 km southwest of Qingtongshan; and (2) from the stratigraphically overlying Shijia and Wangshan formations (Huaibei Group; Sun et al., 2025a), located ~20 km northwest of our section. Collectively, these datasets imply persistently ferruginous shallow-marine conditions during the early Neoproterozoic, establishing a regional framework for our research.

The sedimentary age of the studied intervals is constrained by three sets of radiometric ages (Fig. 1c). The youngest detrital zircon LA-ICP-MS U-Pb ages from the Xinxing and Niyuan formations are 1069 ± 27 Ma (Yang et al., 2012) and ~970 Ma (Sun et al., 2020), respectively. The oldest zircon SIMS U-Pb age from the mafic
intrusions within the Niyuan Formation is 940 ± 5 Ma (Zhao et al., 2020), while the SHRIMP U-Pb age of the mafic intrusions within the Jiudingshan Formation is 933 ± 14 Ma (Wang et al., 2012). Thus, based on these ages and the stratigraphic contact relationships, the sedimentary time span for the succession from the Jiayuan Formation to the Jiudingshan Formation can be approximately constrained to 1.0–0.95 Ga.

## 3. Materials and Methods

A total of 203 carbonate samples were collected from the Qingtongshan section (33°56′17.95″ N, 117°24′25.63″ E; Fig. 1d) in the Anhui Province. The samples were cut into chips, and only the fresh central parts were used for geochemical analyses. Eighteen representative samples that reflect the sedimentary characteristics of each formation were selected for mineralogical analysis. Microscopic features were observed on thin sections using a Zeiss Axio Scope A1 microscope. Ultrafine and micro-structures were investigated using a Zeiss Supra 55 field emission scanning electron microscope (FESEM) at the State Key Laboratory of Geomicrobiology and Environmental Changes, China University of Geosciences (Beijing). To obtain high-resolution images, the samples were coated with 10-nm-thick carbon for electric conduction before analysis. Topographic features were examined using a secondary electron imaging detector (SE2), and compositional differences were revealed with an AsB detector (backscattered electron, BSE, image). A Gatan ChromaCL2 cathodoluminescence (CL) detector connected to the FESEM was used to analyze the original textures.

The same 203 carbonate samples were further analyzed for iodine (I), major elements (Ca, Mg, Mn and Sr), and rare earth elements and yttrium (REE+Y) using ICP-MS/MS (Thermo iCAP™ TQ) in single quadrupole mode at the National Research Center for Geoanalysis in Beijing. For iodine analysis, approximately 24 mg of sample powder, washed with 18.25 MΩ·cm MQ water and dried, was weighed and placed into a 15 mL centrifuge tube. To completely dissolve the carbonate components, 4 mL of 3% HNO₃ was added to the centrifuge tube, followed by
centrifugation at 4000 rpm for 10 min. A 1 mL of the supernatant was transferred to a new 15 mL centrifuge tube, to which 1 mL of 3% tertiary amine (to prevent iodine volatilization; Lu et al., 2010) and 4 mL of MQ water were added for iodine analysis. For iodine analysis, 2 ppb Re and Rh solutions served as internal standards, and JDo-1 (certified I content = 0.58 ± 0.03 ppm; Xu et al., 2025) was used as the external standard. Uncertainty on the iodine analysis was less than 3.8% (Table S1). Then, 0.1 mL and 1 mL of the supernatant were taken and diluted to 6 mL for the analysis of major elements and REE+Y, respectively. Monitoring of the JDo-1 standard sample indicated that the test errors for Mg, Ca, Mn, and Sr were all less than 5% (Table S2). The uncertainty for ICP-MS/MS analysis of REE+Y was less than 10%.

Although using 3% HNO₃ to dissolve relatively pure carbonate does not significantly affect their REE+Y distribution patterns (Nothdurft et al., 2004), we also selected 30 samples at the intervals, using 2% acetic acid to dissolve 70% of the carbonate components in order to analyze REE+Y, thereby ensuring the reliability of the REE+Y results. The analytical results were normalized to the Post-Archean Australian Shale (PAAS) reference values (McLennan, 1989), with the subscript “SN” denoting shale-normalized concentrations. The Ce anomaly was calculated using the formula $Ce/Ce^{*}_{(SN)} = Ce_{SN}/[Pr^{2}_{SN}/Nd_{SN}]$ to avoid the effects of seawater La positive anomalies (Lawrence et al., 2006).

## 4. Results

The mineralogical characteristics of the carbonate samples from the studied interval are presented in Fig. 3. The samples are primarily composed of relatively pure carbonate minerals, with minimal siliciclastics and organic matter. The Zhaowei Formation and Member I of the Niyuan Formation are dominated by argillaceous limestone that is mainly composed of sub-rounded to rounded carbonate muds (Fig. 3a, e). Although recrystallization has caused some particles to increase in size, the average grain size is still less than 10 µm (Fig. 3b, f, c, g). CL images show that the samples are weakly luminescent (Fig. 3d, h). Member II of the Niyuan Formation contains argillaceous dolostone, with dolomite crystals having an average grain size
of less than 20 µm (Fig. 3i, j, k). CL images reveal that dolomite crystals exhibit dull luminescence (Fig. 3l). In the Jiudingshan Formation, argillaceous limestones are composed of carbonate muds that are sub-rounded to rounded in morphology (Fig. 3m), with an average crystal size of less than 10 µm (Fig. 3m, o). Notably, the Jiudingshan Formation exhibits well-developed molar tooth structures, with nearly spherical microspar calcite crystals (Fig. 3n), and an average crystal size of approximately 10 µm (Fig. 3n). CL images show that the samples are weakly luminescent (Fig. 3p).

The I/(Ca+Mg) value of the studied samples is generally lower than the Precambrian baseline of 0.5 µmol/mol (Fig. 4; Table S3), with two samples slightly exceeding this value (0.59 and 0.57 µmol/mol, respectively). The associated Mn/Sr ratio is relatively low (below 5 ppm/ppm; Fig. 4). In Member II of the Niyuan Formation, the Mg/Ca molar ratio is close to 1, whereas ratios in other strata approach 0.1 due to significantly lower Mg contents relative to Ca (Fig. 4). The I/(Ca+Mg) value shows a strong positive correlation with I ($r = 0.99, p < 0.05$; Fig. 5a), but no significant positive correlation is observed with Mn/Sr ($r = 0.3, p < 0.05$; Fig. 5b) or Mg/Ca ($r = 0.25, p < 0.05$; Fig. 5c).

To obtain data that are not significantly contaminated by siliciclastics and that accurately reflect the REE+Y distribution characteristics of seawater, the rare earth element data from nitric acid-dissolved samples (Table S4) were screened according to the criteria: $Th < 0.5$ ppm, $Y/Ho > 36$, $\SigmaREE < 12$ ppm, and $Pr/Yb_{(SN)} < 1$ (c.f., Ling et al., 2013). After screening (Fig. 5d; Table S5), the average Ce anomaly was $0.82 \pm 0.06$ ($n = 44$), with a minimum value of 0.70. Based on the overall trend in Ce anomaly variation, the studied section is divided into two intervals: Interval I, with an average Ce anomaly of $0.79 \pm 0.04$ ($n = 29$), and Interval II, with an average Ce anomaly of $0.88 \pm 0.04$ ($n = 15$). In these two intervals, $Ce/Ce_{(SN)}$ ($r = -0.49, p < 0.05$; $r = -0.12, p > 0.05$; Fig. 5e), $Pr/Yb_{(SN)}$ ($r = 0.47, p < 0.05$; $r = 0.02, p > 0.05$; Fig. 5f), and $Y/Ho$ ($r = -0.56, p < 0.05$; $r = -0.03, p > 0.05$; Fig. 5g) show no significant positive correlations with Th, a proxy of siliciclastic contamination.
261 Overall, both the Ce anomaly and the I/(Ca+Mg) values exhibit fluctuating characteristics.
262

263 The acetic acid-digested (Table S6) and nitric acid-digested (Table S4) samples show consistent Ce anomaly trends, with average Ce anomalies of 1.01 ± 0.09 (n = 30) and 0.87 ± 0.06 (n = 203), respectively. For the acetic acid-digested samples, the Ce/Ce*(SN) (r = 0.11, p > 0.05), Pr/Yb(SN) (r = 0.26, p > 0.05), and Y/Ho (r = -0.64, p < 0.05) ratios show no significant correlation with Th (Table S6). A critical consideration in interpreting carbonate REE+Y data lies in recognizing that their ability to reflect seawater patterns may be predominantly governed by the inherent characteristics of the samples, particularly the potential contamination from clay minerals, rather than the digestion method (acetic acid versus nitric acid).
264
265
266
267
268
269
270
271
272 Previous studies demonstrated that nitric acid digestion can also reveal the original seawater rare earth signals (e.g., Nothdurft et al., 2004; Tostevin et al., 2016a).
273

274 **5. Discussion**

275 **5.1. Data Evaluation**

276 The I/(Ca+Mg) ratio in carbonate can be used to trace the redox state of
277 shallow marine environments (Hardisty et al., 2017; Lu et al., 2018), but it is prone to
278 diagenetic alteration (Hardisty et al., 2017; Hashim et al., 2022). Both burial
279 diagenesis and recrystallization processes result in a decrease in strontium (Sr)
280 contents (Banner & Hanson, 1990; Rimstidt et al., 1998), while meteoritic fluids tend
281 to increase manganese (Mn) contents (Veizer, 1983). The Mn/Sr ratio thus serves as a
282 proxy for diagenetic alteration, with values >10 ppm/ppm indicating strong
283 overprinting (Brand & Veizer, 1980; Higgins et al., 2018; Kaufman & Knoll, 1995). In
284 our samples, Mn/Sr ratios remain below 5 ppm/ppm (Fig. 4), and no significant
285 correlation exists between I/(Ca+Mg) and Mn/Sr (r = 0.3, p < 0.05; Fig. 5b),
286 suggesting minimal diagenetic overprinting. CL images show that most calcite and
287 dolomite are weakly luminescent (Fig. 3d, h, l, p), indicating low Mn contents
288 (slightly higher than 10–20 ppm; Mason & Mariano, 1990; Budd et al., 2000) and
suggesting minimal diagenetic alteration. Overall, the section is predominantly
argillaceous limestone, with dolomitization only occurring in Member II of the
Niyuan Formation (Figs. 3i–l, 4). The I/(Ca+Mg) value shows a weak correlation with
Mg/Ca ($r = 0.25, p < 0.05$; Fig. 5c), indicating that the I/(Ca+Mg) value is only
weakly affected by dolomitization. Therefore, the generally low I/(Ca+Mg) values in
the study samples are likely not related to diagenetic alteration or dolomitization but
may largely reflect the original seawater signal.

Rare earth elements and yttrium in carbonates can serve as another crucial
indicator for tracing shallow marine redox conditions. They exhibit strong resistance
to diagenetic alteration and dolomitization (e.g., Yang et al., 2024), but are susceptible
to the influence of siliciclastics (e.g., Ling et al., 2013; Olivier & Boyet, 2006).
Therefore, combining the carbonate I/(Ca+Mg) ratio with cerium anomaly
(Ce/Ce*(SN)) provides a more reliable construction of the seawater chemistry. The
samples in this study have low Th content, and no significant positive correlations are
observed between Ce/Ce*(SN), Pr/Yb(SN), Y/Ho, and Th (Fig. 5e, f, g), suggesting
minimal influence from siliciclastics. In addition, Ce/Ce*(SN) shows no significant
correlations with Mn/Sr ($r = -0.45, p < 0.05$; Fig. 5h) or Mg/Ca ($r = -0.03, p > 0.05$;
Fig. 5i), suggesting that diagenetic alteration or dolomitization had minimal effects on
Ce anomalies. Modern seawater has light-REE depleted distribution patterns (Byrne
& Sholkovitz, 1996; Sholkovitz & Elderfield, 1988), with a high Y/Ho ratio, positive
La anomalies, and weakly positive Gd anomalies (Byrne & Kim, 1990; Nagarajan et
al., 2011). These features are often used as key indicators of a seawater signal (e.g.,
Ling et al., 2013). In consideration of modern seawater characteristics, this study
selected samples based on the criteria Pr/Yb(SN) < 1, Th < 0.5 ppm, Y/Ho > 36, and
ΣREE < 12 ppm (cf. Ling et al., 2013), ensuring that the post-screened data accurately
reflect the rare earth element distribution in seawater at the time of deposition.

## 5.2. Reconstruction of Early Neoproterozoic Shallow Marine Redox Conditions

The pathway of shallow marine carbonate precipitation is strongly controlled
by seawater redox conditions (e.g., Fang et al., 2022; Riding et al., 2022; Sumner & Grotzinger, 1996). Fe²⁺ and Mn²⁺, strong inhibitors of carbonate precipitation, can significantly suppress calcite nucleation in the water column under anoxic, ferruginous conditions but allow aragonite to precipitate on the seafloor (Riding et al., 2022; Sumner & Grotzinger, 1996). In contrast, under suboxic to oxic conditions, as Fe²⁺ and Mn²⁺ are oxidized and removed, large amounts of carbonate mud (“whiting”) can form in the water column (Fang et al., 2022). In addition, the formation of molar tooth structures requires a water-column precipitated, viscous carbonate mud as a cohesive matrix, which is favorable for the fracturing of molar tooth structure cracks during the early diagenetic stage (Tang et al., 2023). Subsequent crack infilling is facilitated by microspar calcite precipitation, driven by the oxidation of porewater Fe(II) and Mn(II). Notably, Mn(II) oxidation occurs under suboxic conditions (≥0.5 µM O₂; Schippers et al., 2005). Collectively, these characteristics suggest that molar tooth structures formed in moderately oxidized shallow marine environments (Lyons et al., 2023; Tang et al., 2023). In this study, the carbonate components are primarily composed of water-column precipitated carbonate mud (Fig. 3), and molar tooth structures are abundant (Fig. 2f, l), suggesting that the shallow marine environment at the time was characterized by suboxic conditions, which could result in the oxidation of Fe(II) and Mn(II).

The I/(Ca+Mg) ratio of carbonates has been extensively used to reconstruct shallow marine redox states through geological periods (Hardisty et al., 2017; Lindskog et al., 2023; Lu et al., 2018; Shang et al., 2019). In modern seawater, iodine primarily exists in the forms of thermodynamically stable iodide (I⁻) and iodate (IO₃⁻) (Rue et al., 1997). From the oxidized surface seawater to anoxic oxygen minimum zones, IO₃⁻ can be fully reduced to I⁻, and the presence of IO₃⁻ in seawater requires a minimum oxygen concentration of 1–3 µM (Hardisty et al., 2017). Laboratory experiments on calcite synthesis show that only IO₃⁻ can enter the carbonate lattice with fixed partition coefficients, while I⁻ is excluded (Lu et al., 2010). Therefore, the I/(Ca+Mg) ratio of carbonates can track the IO₃⁻ concentration in seawater, thus
reflecting the seawater redox conditions (Lu et al., 2010), whereby higher I/(Ca+Mg) ratios indicate more oxic conditions, whereas lower ratios reflect reduced oxygenation. Statistical analysis of the I/(Ca+Mg) ratio in Precambrian carbonates reveals that during non-oxic intervals (as independently evidenced by other redox proxies; e.g., Ce/Ce\*(SN), δ⁹⁸Mo and δ⁵³Cr), over 95% of shallow marine carbonates exhibit I/(Ca+Mg) ratios lower than 0.5 µmol/mol (Lu et al., 2017; Shang et al., 2019). This value is regarded as the baseline under overall oxygen depleted conditions through the Precambrian ocean (Lu et al., 2017; Shang et al., 2019). I/(Ca+Mg) ratios consistently exceeding 0.5 µmol/mol is considered indicative of elevated oxygen concentrations (≥1–10 µM) in shallow marine environments (Shang et al., 2019), relative to the Precambrian background level. During the mid-Proterozoic, shallow marine oxygen concentrations were primarily controlled by levels of primary productivity (<1–10 µM; Reinhard et al., 2016). In this study, the I/(Ca+Mg) values of the vast majority (98%) of the samples are below 0.5 µmol/mol (Fig. 4). Based on this, it is speculated that the oxygen content in shallow marine environments during the early Neoproterozoic was likely lower than 10 µM, indicative of a suboxic environment (Uahengo et al., 2020). The I/(Ca+Mg) values in the terminal Mesoproterozoic Nanfen Formation (~1100–1000 Ma) of the NCC (~15 µmol/mol) approach modern levels (Tang et al., 2025), pointing to an oxygenation event in shallow marine environments. In contrast, the early Neoproterozoic strata in this study exhibit a significant decrease in the I/(Ca+Mg) ratio, suggesting a substantial decline in oxygen concentration (Fig. 6a), leading to overall suboxic shallow marine conditions during early Neoproterozoic.

To further estimate the oxygen levels in seawater, we explored the REE+Y contents in selected carbonates. In seawater, Ce can exist in either Ce(III) or Ce(IV) forms. Under oxidizing conditions, Ce(IV) in seawater is adsorbed by Fe-Mn oxyhydroxides and removed from seawater (Byrne & Sholkovitz, 1996), resulting in a negative seawater Ce anomaly in contemporaneous carbonates (Byrne & Kim, 1990). When Fe-Mn oxyhydroxide particles pass through the Mn redox boundary, adsorbed
Ce(IV) is reduced to Ce(III), causing the disappearance of a negative Ce anomaly or even the appearance of a positive Ce anomaly in the seawater (Ling et al., 2013; Tostevin et al., 2016b). In the studied section, the average Ce anomaly value for Intervals I and II were $0.79 \pm 0.04$ ($n = 29$) and $0.88 \pm 0.04$ ($n = 15$), respectively (Fig. 4), indicating slight fluctuations in oxygen levels. The lowest negative Ce anomaly in the samples of this study is 0.70, which is significantly higher than modern seawater values (0.06; Byrne & Sholkovitz, 1996), and higher than values during the Neoproterozoic Oxygenation Event (i.e., 0.52, Cui et al., 2017), suggesting relatively low oxygen contents in seawater during this period. Compared to the pulsed oxygenation events of the Mesoproterozoic, as recorded for instance by the ~1.44 Ga Fengjiawan Formation in the southern margin of North China (Ce/Ce\*(SN) ~0.42; Yang et al., 2024), the Ce/Ce\*(SN) values are significantly higher. Compared to the Ce anomalies in mid-Proterozoic shallow seawater carbonates shown in Liu et al. (2021), Ce anomalies in our study are similar to the mid-Proterozoic background values (Fig. 6b), suggesting comparable oxygen levels (~1% PAL) to those of the mid-Proterozoic baseline (Liu et al., 2021). These findings further suggest that after a significant oxygenation event at the end of the Mesoproterozoic (Tang et al., 2025), there was a substantial fall in shallow marine oxygen levels during the early Neoproterozoic.

## 5.3. Early Neoproterozoic Evolutionary Stasis of Crown-Group Eukaryotes

The evolutionary stasis of crown-group eukaryotes during the early Neoproterozoic (ca. 1.0–0.8 Ga) can be attributed to a combination of direct physiological constraints and indirect biogeochemical feedbacks imposed by the low-oxygen marine environments reconstructed in this study. Aerobic respiration is a fundamental characteristic of eukaryotes, and oxygen concentration is a key factor influencing their evolution (Mills et al., 2014; Runnegar, 1991; Stolper et al., 2010). Laboratory studies on modern microbial eukaryotes indicate that oxygen concentrations below ~10.8 µM (~2.7% PAL) are insufficient to support their modern diversity (Mills et al., 2024). Although early crown-group eukaryotes may have had lower oxygen requirements, their diversification threshold undoubtedly exceeded that
of stem-group eukaryotes (>0.1% PAL; Runnegar, 1991). Our reconstruction of suboxic conditions (<10 µM O₂) in shallow seawaters (Section 5.2) indicates that early Neoproterozoic shallow marine oxygen levels fluctuated below this critical threshold. This would have directly limited their ability to expand into new ecological niches and compete with prokaryotes that thrive under suboxic conditions.
Consequently, low oxygen levels fundamentally restricted the ecological space available for crown-group eukaryotes, placing a constraint on their diversification.

Marine redox conditions exert a primary control on the bioavailability of essential nutrients such as nitrogen and phosphorus, thereby indirectly regulating biological evolution (Howarth, 1988; Luo et al., 2018; Planavsky et al., 2023). The low-oxygen and ferruginous conditions of the early Neoproterozoic ocean amplified this nutrient scarcity. Specifically, nitrogen limitation occurred via enhanced denitrification, as under the suboxic to anoxic conditions that prevailed in early Neoproterozoic shallow seas (this study; Zhang et al., 2022a), bioavailable nitrate (NO₃⁻) would have been readily reduced to gaseous nitrogen (N₂) through water-column denitrification (Stüeken et al., 2016). This process reduced nitrogen bioavailability, consistent with the generally low nitrogen isotope values of the early Neoproterozoic (-0.5‰ to +3.6‰; Figs. 4, 6c; Kang et al., 2023). In addition, phosphorus limitation occurred via efficient P removal and limited recycling under widespread ferruginous anoxia, which is indicated by iron speciation data from equivalent strata (Guilbaud et al., 2015; Sun et al., 2025a). This is because in ferruginous settings, phosphorus may be efficiently removed from the water column by adsorption onto ferric (oxyhydr) oxides (Bjerrum and Canfield, 2002) or green rust (Zegeye et al., 2012). This process removes phosphorus from the water column and buries it in sediments, drastically impeding its recycling back into the water column (Figs. 4, 6d; Guilbaud et al., 2020; Guilbaud, 2025). Therefore, the early Neoproterozoic's low-oxygen, nutrient-limited marine environment likely posed a key constraint on the diversification of crown-group eukaryotes (Fig. 6e).

In the early Neoproterozoic, this combination of direct physiological stress
and indirect nutrient scarcity effectively hindered the sustained evolution and diversification of early crown-group eukaryotes during the “Boring Billion”. It was not until the onset of the Neoproterozoic Oxygenation Event (Och & Shields-Zhou, 2012), which alleviated these constraints by increasing oxygen levels and perturbing nutrient cycles, that crown-group eukaryotes finally rose to ecological dominance (Brocks et al., 2017; Pang et al., 2024), ultimately paving the way for the subsequent radiation of metazoan life.

### 5.4. Global Implications of Early Neoproterozoic Shallow Marine Redox

Although this study focuses on the North China Craton, the reconstructed suboxic shallow marine conditions carry broad significance. This is supported by depositional evidence of an open basin connected to the global ocean (Hu et al., 2016), where sedimentary facies analysis indicates a shallow-water environment above wave base. Such a setting, likely being equilibrated with the atmosphere, suggests that the redox conditions likely reflect global atmospheric oxygen levels. This interpretation is further supported by independent proxies such as the increased Th/U ratio of arc igneous rocks, which signals pronounced global-scale deoxygenation in the early Neoproterozoic (Liu et al., 2019), and the Se/Co ratio in pyrite, which tracks a long-term decline in atmospheric $O_2$ to a minimum in the early Neoproterozoic (Large et al., 2018, 2022). In addition, the absence of a negative Ce anomaly in contemporaneous shallow marine carbonates from India and Russia (Liu et al., 2021) provides direct support for the prevalence of suboxic conditions in shallow seawater. Therefore, our results are not a local anomaly but reflect a widespread environmental condition during the “Boring Billion”, with profound implications for the delayed rise of complex life.

## 6. Conclusions

Through mineralogical and geochemical analyses including the $I/(Ca+Mg)$ ratios, and REE+Y patterns of the early Neoproterozoic carbonates (ca. 1.0–0.95 Ga) from the Xuhuai region on the southeastern margin of the NCC, the following
conclusions can be drawn:

(1) Water column precipitated carbonate mud is present in the studied intervals, and molar tooth structures occurred in both the Niyuan and Jiudingshan formations, indicating that the depositional environment was moderately oxidized (suboxic) relative to Fe(II) and Mn(II).

(2) $I/(Ca+Mg)$ values in the studied intervals are generally lower (98%) than the Precambrian baseline ($<0.5 \ \mu mol/mol$) and significant negative Ce anomalies are lacking ($0.82 \pm 0.06, n = 44$), further confirming that the shallow marine environment in the early Neoproterozoic ocean was in a low-oxygen (suboxic) state.

(3) During the early Neoproterozoic, direct oxygen limitation and indirect nutrient-poor conditions synergistically hindered crown-group eukaryote diversification, contributing to the 1.0–0.8 Ga evolutionary stasis.

### Acknowledgements

This research was supported by the National Natural Science Foundation of China (42372233 and 41930320), the “Deep-time Digital Earth” Science and Technology Leading Talents Team Funds for the Central Universities for the Frontiers Science Center for Deep-time Digital Earth, China University of Geosciences (Beijing) (2652023001). Thanks go to Chenming Wang, Yajie Liu, Lin Meng and Zhutong Ke for their kind helps in the field and laboratory work. We thank Hanqing Zhao for providing the section locality of the study area. RG acknowledges ANR grant ANR-21-CE49-0007-01. We are grateful to Eva Stüeken and the anonymous reviewer for their constructive comments, which have significantly improved the manuscript. We also thank Ben Bond-Lamberty and the Associate Editor for overseeing the review process.

### Conflict of Interest Statement

The authors have no conflicts of interest to disclose.
488 **Open Research**

489 Our data are archived at Hou et al. (2025).

490 **References**

491 Anderson, R. P., Mughal, S., & Wedlake, G. O. (2024). Proterozoic microfossils continue to provide new insights into the rise of complex eukaryotic life. *Royal Society Open Science*, *11*(7), 240154. https://doi.org/10.1098/rsos.240154

492
493
494 Banner, J. L., & Hanson, G. N. (1990). Calculation of simultaneous isotopic and trace element variations during water-rock interaction with applications to carbonate diagenesis. *Geochimica et Cosmochimica Acta*, *54*(11), 3123–3137.
497 https://doi.org/10.1016/0016-7037(90)90128-8

498 Bjerrum, C. J., & Canfield, D. E. (2002). Ocean productivity before about 1.9 Gyr ago limited by phosphorus adsorption onto iron oxides. *Nature*, *417*(6885), 159–162.
500 https://doi.org/10.1038/417159a

501 Brand, U., & Veizer, J. (1980). Chemical diagenesis of a multicomponent carbonate system; 1, Trace elements. *Journal of Sedimentary Research*, *50*(4), 1219–1236.
502
503 https://doi.org/10.1306/212F7BB7-2B24-11D7-8648000102C1865D

504 Brocks, J. J., Jarrett, A. J., Sirantoine, E., Hallmann, C., Hoshino, Y., & Liyanage, T. (2017). The rise of algae in Cryogenian oceans and the emergence of animals. *Nature*, *548*(7669), 578–581. https://doi.org/10.1038/nature23457

507 Brocks, J. J., Nettersheim, B. J., Adam, P., Schaeffer, P., Jarrett, A. J., Güneli, N., et al. (2023). Lost world of complex life and the late rise of the eukaryotic crown. *Nature*, *618*(7966), 767–773. https://doi.org/10.1038/s41586-023-06170-w

510 Budd, D. A., Hammes, U., & Ward, W. B. (2000). Cathodoluminescence in calcite cements: new insights on Pb and Zn sensitizing, Mn activation, and Fe quenching at low trace-element concentrations. *Journal of Sedimentary Research*, *70*(1), 217–226. https://doi.org/10.1306/2DC4090C-0E47-11D7-8643000102C1865D

515 Butterfield, N. J. (2000). *Bangiomorpha pubescens* n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic
517 radiation of eukaryotes. *Paleobiology*, 26(3), 386-404.

518 https://doi.org/10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2

519 Byrne, R. H., & Kim, K. H. (1990). Rare earth element scavenging in seawater.

520 *Geochimica et Cosmochimica Acta*, 54(10), 2645-2656.

521 https://doi.org/10.1016/0016-7037(90)90002-3

522 Byrne, R. H., & Sholkovitz, E. R. (1996). Marine chemistry and geochemistry of the

523 lanthanides. *Handbook on the physics and chemistry of rare earths*, 23, 497-593.

524 https://doi.org/10.1016/S0168-1273(96)23009-0

525 Chen, Z., Zhou, C., Meyer, M., Xiang, K., Schiffbauer, J. D., Yuan, X., & Xiao, S.

526 (2013). Trace fossil evidence for Ediacaran bilaterian animals with complex

527 behaviors. *Precambrian Research*, 224, 690-701.

528 https://doi.org/10.1016/j.precamres.2012.11.004

529 Clarkson, M. O., Sweere, T. C., Chiu, C. F., Hennekam, R., Bowyer, F., & Wood, R.

530 A. (2023). Environmental controls on very high δ²³⁸U values in reducing

531 sediments: Implications for Neoproterozoic seawater records. *Earth-Science

532 *Reviews*, 237, 104306. https://doi.org/10.1016/j.earscirev.2022.104306

533 Cohen, P. A., & Kodner, R. B. (2022). The earliest history of eukaryotic life:

534 Uncovering an evolutionary story through the integration of biological and

535 geological data. *Trends in Ecology & Evolution*, 37(3), 246-256.

536 https://doi.org/10.1016/j.tree.2021.11.005

537 Cohen, P. A., & Macdonald, F. A. (2015). The Proterozoic record of eukaryotes.

538 *Paleobiology*, 41(4), 610-632. https://doi.org/10.1017/pab.2015.25

539 Cole, D. B., Planavsky, N. J., Longley, M., Böning, P., Wilkes, D., Wang, X., et al.

540 (2020). Uranium isotope fractionation in non-sulfidic anoxic settings and the

541 global uranium isotope mass balance. *Global Biogeochemical Cycles*, 34(8),

542 e2020GB006649. https://doi.org/10.1029/2020GB006649

543 Cui, H., Kaufman, A. J., Xiao, S., Zhou, C., & Liu, X. M. (2017). Was the Ediacaran

544 Shuram Excursion a globally synchronized early diagenetic event? Insights from

545 methane-derived authigenic carbonates in the uppermost Doushantuo Formation,

546 *Chemical Geology*, 450, 59-80.
547 https://doi.org/10.1016/j.chemgeo.2016.12.010

548 Dou, L., Feng, L., & Jiang, Y. (2014). Sedimentary environments and genesis of the molar-tooth structures in the Neoproterozoic Jiayuan Formation in the Xuzhou region, Jiangsu. *Sedimentary Geology and Tethyan Geology*, 34(4), 40–47. (in Chinese). https://doi.org/10.3969/j.issn.1009-3850.2014.04.005

552 Fang, H., Tang, D., Shi, X., Zhou, L., Zhou, X., Wu, M., et al. (2022). Early Mesoproterozoic Ca-carbonate precipitates record fluctuations in shallow marine oxygenation. *Precambrian Research*, 373, 106630.
https://doi.org/10.1016/j.precamres.2022.106630

556 Erwin, D. H., Laflamme, M., Tweedt, S. M., Sperling, E. A., Pisani, D., & Peterson, K. J. (2011). The Cambrian conundrum: early divergence and later ecological success in the early history of animals. *Science*, 334(6059), 1091–1097.
https://doi.org/10.1126/science.1206375

560 Gibson, T. M., Shih, P. M., Cumming, V. M., Fischer, W. W., Crockford, P. W., Hodgskiss, M. S., et al. (2018). Precise age of *Bangiomorpha pubescens* dates the origin of eukaryotic photosynthesis. *Geology*, 46(2), 135–138.
https://doi.org/10.1130/G39829.1

564 Guilbaud, R., Andersen, M. B., Freeman, H. M., Perez, J. P. H., Uhlig, D., & Benning, L. G. (2025). Mackinawite partial oxidation to green rust produces a large, abiotic uranium isotope fractionation. *Communications Earth & Environment*, 6, 96. https://doi.org/10.1038/s43247-025-02088-5

568 Guilbaud, R., Poulton, S. W., Butterfield, N. J., Zhu, M., & Shields-Zhou, G. A. (2015). A global transition to ferruginous conditions in the early Neoproterozoic oceans. *Nature Geoscience*, 8(6), 466–470. https://doi.org/10.1038/ngeo2434

571 Guilbaud, R., Poulton, S. W., Thompson, J., Husband, K. F., Zhu, M., Zhou, Y., et al. (2020). Phosphorus-limited conditions in the early Neoproterozoic ocean maintained low levels of atmospheric oxygen. *Nature Geoscience*, 13(4), 296–301. https://doi.org/10.1038/s41561-020-0548-7

575 Guilbaud, R. (2025). Proterozoic evolution of the phosphorus cycle: Was it high or was it low? *Treatise on Geochemistry*, 5, 153–175. https://doi.org/10.1016/B978-
577 0-323-99762-1.00068-1

578 Gilleaudeau, G. J., Chen, X., Romaniello, S. J., Akam, S. A., Wittkop, C., Katsev, S.,
579 & Swanner, E. D. (2025). Uranium isotope systematics of a low-productivity
580 ferruginous ocean analog: Implications for the uranium isotope record of early
581 Earth. *Geochimica et Cosmochimica Acta.* 392, 195–206.

582 https://doi.org/10.1016/j.gca.2025.01.011

583 GSAP (Geological Survey of Anhui Province), 2006, *Geological Map of Lingbi: I-50-22*, scale 1:200,000, geological map (in Chinese).

585 Hardisty, D. S., Lu, Z., Bekker, A., Diamond, C. W., Gill, B. C., Jiang, G., et al. (2017). Perspectives on Proterozoic surface ocean redox from iodine contents in ancient and recent carbonate. *Earth and Planetary Science Letters*, 463, 159–170. https://doi.org/10.1016/j.epsl.2017.01.032

589 Hashim, M. S., Burke, J. E., Hardisty, D. S., & Kaczmarek, S. E. (2022). Iodine incorporation into dolomite: Experimental constraints and implications for the iodine redox proxy and Proterozoic Ocean. *Geochimica et Cosmochimica Acta*, 338, 365–381. https://doi.org/10.1016/j.gca.2022.10.027

593 Higgins, J. A., Blättler, C. L., Lundstrom, E. A., Santiago-Ramos, D. P., Akhtar, A. A.,
594 Ahm, A. C., et al. (2018). Mineralogy, early marine diagenesis, and the chemistry
595 of shallow-water carbonate sediments. *Geochimica et Cosmochimica Acta*, 220,
596 512–534. https://doi.org/10.1016/j.gca.2017.09.046

597 Hou, A., Shi, X., Guilbaud, R., Xie, B., Sun, L., Xu, L., et al. (2025). *Table S1-S6-Low oxygen in the early Neoproterozoic shallow seawater (Version 4)*. [Dataset].
598 Mendeley Data. https://doi.org/10.17632/rb7rndgy7w.4

600 Howarth, R. W. (1988). Nutrient limitation of net primary production in marine
601 ecosystems. *Annual review of ecology and systematics*, 19(1), 89–110.
602 https://doi.org/10.1146/annurev.es.19.110188.000513

603 Hu, J., Li, Z., Gong, W., Hu, G., & Dong, X. (2016). Meso-Neoproterozoic
604 stratigraphic and tectonic framework of the North China Craton. In: Zhai, M.,
605 Zhao, Y., Zhao, T. (Eds.), Main Tectonic Events and Metallogeny of the North
606 China Craton. Springer Singapore, Singapore (pp. 393–422).
607 https://doi.org/10.1007/978-981-10-1064-4_15

608 Kang, J., Gill, B., Reid, R., Zhang, F., & Xiao, S. (2023). Nitrate limitation in early

609 Neoproterozoic oceans delayed the ecological rise of eukaryotes. *Science*

610 *Advances*, 9(12), eade9647. https://doi.org/10.1126/sciadv.ade9647

611 Kaufman, A. J., & Knoll, A. H. (1995). Neoproterozoic variations in the C-isotopic

612 composition of seawater: stratigraphic and biogeochemical implications.

613 *Precambrian research*, 73(1–4), 27–49. https://doi.org/10.1016/0301-

614 9268(94)00070-8

615 Knoll, A. H., Javaux, E. J., Hewitt, D., & Cohen, P. (2006). Eukaryotic organisms in

616 Proterozoic oceans. *Philosophical Transactions of the Royal Society B:*

617 *Biological Sciences*, 361(1470), 1023–1038.

618 https://doi.org/10.1098/rstb.2006.1843

619 Large, R., Mukherjee, I., Danyushevsky, L., Gregory, D., Steadman, J., & Corkrey, R.

620 (2022). Sedimentary pyrite proxy for atmospheric oxygen: Evaluation of

621 strengths and limitations. *Earth-Science Reviews*, 227, 103941.

622 https://doi.org/10.1016/j.earscirev.2022.103941

623 Large, R. R., Mukherjee, I., Zhukova, I., Corkrey, R., Stepanov, A., & Danyushevsky,

624 L. V. (2018). Role of upper-most crustal composition in the evolution of the

625 Precambrian ocean-atmosphere system. *Earth and Planetary Science Letters*,

626 487, 44–53. https://doi.org/10.1016/j.epsl.2018.01.019

627 Lawrence, M. G., Greig, A., Collerson, K. D., & Kamber, B. S. (2006). Rare earth

628 element and yttrium variability in South East Queensland waterways. *Aquatic*

629 *Geochemistry*, 12, 39–72. https://doi.org/10.1007/s10498-005-4471-8

630 Li, D., Luo, G., Tang, Q., She, Z., & Xiao, S. (2023). New record of the green algal

631 fossil Proterocladus and coexisting microfossils from the Meso-Neoproterozoic

632 Diaoyutai Formation in southern Liaoning, North China. *Precambrian Research*,

633 393, 107104. https://doi.org/10.1016/j.precamres.2023.107104

634 Lindskog, A., Young, S. A., Bowman, C. N., Kozik, N. P., Newby, S. M., Eriksson, M.

635 E., et al. (2023). Oxygenation of the Baltoscandian shelf linked to Ordovician

636 biodiversification. *Nature Geoscience*, 16(11), 1047–1053.
637 https://doi.org/10.1038/s41561-023-01287-z

638 Ling, H. F., Chen, X., Li, D. A., Wang, D., Shields-Zhou, G. A., & Zhu, M. (2013).

639 Cerium anomaly variations in Ediacaran-earliest Cambrian carbonates from the
640 Yangtze Gorges area, South China: implications for oxygenation of coeval
641 shallow seawater. *Precambrian Research*, 225, 110–127.

642 https://doi.org/10.1016/j.precamres.2011.10.011

643 Liu, H., Zartman, R. E., Ireland, T. R., & Sun, W. D. (2019). Global atmospheric
644 oxygen variations recorded by Th/U systematics of igneous rocks. *Proceedings
645 of the National Academy of Sciences*, 116(38), 18854–18859.

646 https://doi.org/10.1073/pnas.1902833116

647 Liu, P., Xiao, S., Yin, C., Chen, S., Zhou, C., & Li, M. (2014). Ediacaran
648 acanthomorphic acritarchs and other microfossils from chert nodules of the upper
649 Doushantuo Formation in the Yangtze Gorges area, South China. *Journal of
650 Paleontology*, 88(72), 1–139. https://doi.org/10.1666/13-009

651 Liu, X. M., Kah, L. C., Knoll, A. H., Cui, H., Wang, C., Bekker, A., & Hazen, R. M. (2021). A persistently low level of atmospheric oxygen in Earth's middle age. *Nature communications*, 12(1), 351. https://doi.org/10.1038/s41467-020-20484-7

654 Loron, C. C., François, C., Rainbird, R. H., Turner, E. C., Borensztajn, S., & Javaux,
655 E. J. (2019). Early fungi from the Proterozoic era in Arctic Canada. *Nature*,
656 570(7760), 232–235. https://doi.org/10.1038/s41586-019-1217-0

657 Lu, L., Li, Z., Ye, C., Kang, P., Zhang, X. (2013). Characterization of Tempestite
658 Sediments of the Neoproterozoic Zhaowei Formation in Xuzhou Area, Jiangsu
659 Province. *Geological Journal of China Universities*, 19(3), 520–528. (in
660 Chinese). https://doi.org/10.16108/j.issn1006-7493.2013.03.014

661 Lu, W., Ridgwell, A., Thomas, E., Hardisty, D. S., Luo, G., Algeo, T. J., et al. (2018). Late inception of a resiliently oxygenated upper ocean. *Science*, 361(6398), 174–177. https://doi.org/10.1126/science.aar5372

664 Lu, W., Wörndle, S., Halverson, G. P., Zhou, X., Bekker, A., Rainbird, R. H., et al. (2017). Iodine proxy evidence for increased ocean oxygenation during the Bitter Springs Anomaly. *Geochemical Perspectives Letters*, 5, 53–57.
667 https://doi.org/10.7185/geochemlet.1746

668 Lu, Z., Jenkyns, H. C., Rickaby, R. E. (2010). Iodine to calcium ratios in marine carbonate as a paleo-redox proxy during oceanic anoxic events. *Geology*, **38**(12), 1107–1110. https://doi.org/10.1130/G31145.1

671 Luo, G., Junium, C. K., Izon, G., Ono, S., Beukes, N. J., Algeo, T. J., et al. (2018). Nitrogen fixation sustained productivity in the wake of the Palaeoproterozoic Great Oxygenation Event. *Nature Communications*, **9**(1), 978. https://doi.org/10.1038/s41467-018-03361-2

675 Lyons, T. W., Tu, C., & Hancock, L. (2023). Giving some tooth to Precambrian carbonates and the tales they tell about ancient oceans. *Journal of Geophysical Research: Biogeosciences*, **128**(4), e2023JG007491. https://doi.org/10.1029/2023JG007491

679 Mason, R. A., & Mariano, A. N. (1990). Cathodoluminescence activation in manganese-bearing and rare earth-bearing synthetic calcites. *Chemical Geology*, **88**(1–2), 191–206. https://doi.org/10.1016/0009-2541(90)90113-L

682 McLennan, S.M. (1989). Rare earth elements in sedimentary rocks: influence of provenance and sedimentary processes. *Geochemistry and Mineralogy of Rare Earth Elements*. *Review in Mineralogy*, **21**(1), 169–200. https://doi.org/10.1515/9781501509032-010

686 Meng, Y., Li, Z., Shen, Y., Yang, T., Jing, Y., Wen, Z., et al. (2023). Sedimentary environment evolution and event deposition of the Neoproterozoic Niyuan Formation of Zhaowei section in Xuzhou, Jiangsu Province, China. *Journal of Palaeogeography*, **25**(2), 308–322. (in Chinese). https://doi.org/10.7605/gdlxb.2023.02.034

691 Mills, D. B., Simister, R. L., Sehein, T. R., Hallam, S. J., Sperling, E. A., & Crowe, S. A. (2024). Constraining the oxygen requirements for modern microbial eukaryote diversity. *Proceedings of the National Academy of Sciences*, **121**(2), e2303754120. https://doi.org/10.1073/pnas.2303754120

695 Mills, D. B., Ward, L. M., Jones, C., Sweeten, B., Forth, M., Treusch, A. H., & Canfield, D. E. (2014). Oxygen requirements of the earliest animals. *Proceedings*
697 of the National Academy of Sciences, 111(11), 4168-4172.

698 https://doi.org/10.1073/pnas.1400547111

699 Nagarajan, R., Madhavaraju, J., Armstrong-Altrin, J. S., & Nagendra, R. (2011).

700 Geochemistry of neoproterozoic limestones of the Shahabad formation, Bhima

701 basin, Karnataka, southern India. *Geosciences Journal*, 15, 9-25.

702 https://doi.org/10.1007/s12303-011-0005-0

703 Narbonne, G. M. (2005). The Ediacara biota: Neoproterozoic origin of animals and

704 their ecosystems. *Annu. Rev. Earth Planet. Sci.*, 33(1), 421-442.

705 https://doi.org/10.1146/annurev.earth.33.092203.122519

706 Nothdurft, L. D., Webb, G. E., & Kamber, B. S. (2004). Rare earth element

707 geochemistry of Late Devonian reefal carbonates, Canning Basin, Western

708 Australia: confirmation of a seawater REE proxy in ancient limestones.

709 *Geochimica et Cosmochimica Acta*, 68(2), 263-283.

710 https://doi.org/10.1016/S0016-7037(03)00422-8

711 Och, L. M., & Shields-Zhou, G. A. (2012). The Neoproterozoic oxygenation event:

712 Environmental perturbations and biogeochemical cycling. *Earth-Science

713 *Reviews*, 110(1-4), 26-57. https://doi.org/10.1016/j.earscirev.2011.09.004

714 Olivier, N., & Boyet, M. (2006). Rare earth and trace elements of microbialites in

715 Upper Jurassic coral-and sponge-microbialite reefs. *Chemical Geology*, 230(1-2), 105-123. https://doi.org/10.1016/j.chemgeo.2005.12.002

716

717 Pang, K., Tang, Q., Li, G., Wan, B., Ouyang, Q., Wang, X. (2024). Origin and Early

718 Evolution of Multicellular Eukaryotic Algae in the Proterozoic Eon. *China Basic

719 *Science*, 26(4), 1-16. (in Chinese). https://doi.org/10.3969/j.issn.1009-2412.2024.04.001

720

721 Planavsky, N. J., Asael, D., Rooney, A. D., Robbins, L. J., Gill, B. C., Dehler, C. M.,

722 et al. (2023). A sedimentary record of the evolution of the global marine

723 phosphorus cycle. *Geobiology*, 21(2), 168-174.

724 https://doi.org/10.1111/gbi.12536

725 Planavsky, N. J., Reinhard, C. T., Wang, X., Thomson, D., McGoldrick, P., &

726 Rainbird, R. H. (2014). Low Mid-Proterozoic atmospheric oxygen levels and the
727 delayed rise of animals. *Science*, 346(6209), 635–638.

728 https://doi.org/10.1126/science.1258410

729 Reinhard, C. T., Planavsky, N. J., Olson, S. L., Lyons, T. W., & Erwin, D. H. (2016).

730 Earth's oxygen cycle and the evolution of animal life. *Proceedings of the National Academy of Sciences*, 113(32), 8933–8938.

732 https://doi.org/10.1073/pnas.1521544113

733 Riding, R., Liang, L., & Fralick, P. (2022). Oxygen-induced chemocline precipitation between Archean Fe-rich and Fe-poor carbonate seas. *Precambrian Research*, 383, 106902. https://doi.org/10.1016/j.precamres.2022.106902

736 Rimstidt, J. D., Balog, A., & Webb, J. (1998). Distribution of trace elements between carbonate minerals and aqueous solutions. *Geochimica et Cosmochimica Acta*, 62(11), 1851–1863. https://doi.org/10.1016/S0016-7037(98)00125-2

739 Rue, E. L., Smith, G. J., Cutter, G. A., & Bruland, K. W. (1997). The response of trace element redox couples to suboxic conditions in the water column. *Deep Sea Research Part I: Oceanographic Research Papers*, 44(1), 113–134.
742 https://doi.org/10.1016/S0967-0637(96)00088-X

743 Runnegar, B. (1991). Precambrian oxygen levels estimated from the biochemistry and physiology of early eukaryotes. *Palaeogeography, Palaeoclimatology, Palaeoecology*, 97(1–2), 97–111. https://doi.org/10.1016/0031-0182(91)90186-U

746 Schippers, A., Neretin, L. N., Lavik, G., Leipe, T., & Pollehne, F. (2005). Manganese (II) oxidation driven by lateral oxygen intrusions in the western Black Sea. *Geochimica et Cosmochimica Acta*, 69(9), 2241–2252.
749 https://doi.org/10.1016/j.gca.2004.10.016

750 Shang, M., Tang, D., Shi, X., Zhou, L., Zhou, X., Song, H., & Jiang, G. (2019). A pulse of oxygen increase in the early Mesoproterozoic ocean at ca. 1.57–1.56 Ga. *Earth and Planetary Science Letters*, 527, 115797.
753 https://doi.org/10.1016/j.epsl.2019.115797

754 Sholkovitz, E. R., & Elderfield, H. (1988). Cycling of dissolved rare earth elements in Chesapeake Bay. *Global Biogeochemical Cycles*, 2(2), 157c176.
756 https://doi.org/10.1029/GB002i002p00157
757 Stolper, D. A., Revsbech, N. P., & Canfield, D. E. (2010). Aerobic growth at nanomolar oxygen concentrations. *Proceedings of the National Academy of Sciences*, 107(44), 18755-18760. https://doi.org/10.1073/pnas.1013435107

760 Stüeken, E. E., Kipp, M. A., Koehler, M. C., & Buick, R. (2016). The evolution of Earth's biogeochemical nitrogen cycle. *Earth-Science Reviews*, 160, 220-239.
https://doi.org/10.1016/j.earscirev.2016.07.007

763 Stüeken, E. E., Pellerin, A., Thomazo, C., Johnson, B. W., Duncanson, S., & Schoepfer, S. D. (2024). Marine biogeochemical nitrogen cycling through Earth's history. *Nature Reviews Earth & Environment*, 5(10), 732-747.
https://doi.org/10.1038/s43017-024-00591-5

767 Sumner, D. Y., & Grotzinger, J. P. (1996). Were kinetics of Archean calcium carbonate precipitation related to oxygen concentration? *Geology*, 24(2), 119-122.
https://doi.org/10.1130/0091-7613(1996)024%3C0119:WKOACC%3E2.3.CO;2

770 Sun, F., Peng, P., Zhou, X., Magalhaes, A. J. C., Guadagnin, F., Zhou, X., et al. (2020). Provenance analysis of the late Mesoproterozoic to Neoproterozoic Xuhuai Basin in the southeast North China Craton: Implications for paleogeographic reconstruction. *Precambrian Research*, 337, 105554.
https://doi.org/10.1016/j.precamres.2019.105554

775 Sun, L., Shi, X., Zhou, X., Zhou, L., Huang, K., Xu, L., et al. (2025b). Dynamic redox conditions in Mesoproterozoic shallow seawater: Constraints from carbonate fabrics and geochemistry. *Geological Society of America Bulletin*, 137(1-2), 190-202. https://doi.org/10.1130/B37544.1

779 Sun, Y., Wang, W., Lang, X., Guan, C., Ouyang, Q., Pang, K., et al. (2025a). A shallow-water oxygen minimum zone in an oligotrophic Tonian basin. *Nature Communications*, 16(1), 725.

782 Tang, D., Fang, H., Shi, X., Liang, L., Zhou, L., Xie, B., et al. (2023). Mesoproterozoic molar tooth structure related to increased marine oxygenation. *Journal of Geophysical Research: Biogeosciences*, 128(1), e2022JG007077.
https://doi.org/10.1029/2022JG007077

786 Tang, Q., Pang, K., Yuan, X., Wan, B., & Xiao, S. (2015). Organic-walled
787 microfossils from the Tonian Gouhou Formation, Huaibei region, North China

788 Craton, and their biostratigraphic implications. *Precambrian Research*, 266,

789 296–318. https://doi.org/10.1016/j.precamres.2015.05.025

790 Tang, Q., Pang, K., Yuan, X., & Xiao, S. (2020). A one-billion-year-old multicellular

791 chlorophyte. *Nature Ecology & Evolution*, 4(4), 543–549.

792 https://doi.org/10.1038/s41559-020-1122-9

793 Tang, Q., Zheng, W., Zhang, S., Fan, J., Riedman, L. A., Hou, X., et al. (2024).

794 Quantifying the global biodiversity of Proterozoic eukaryotes. *Science*,

795 386(6728), eadm9137. https://doi.org/10.1126/science.adm9137

796 Tang, D., Zhou, H., Jiang, G., Shi, X., Li, X., Wang, L., et al. (2025). Terminal

797 Mesoproterozoic (1.1–1.0 Ga) shallow ocean oxygenation and the rise of crown-

798 group eukaryotes. *Geological Society of America Bulletin*. 137, 3452–3462.

799 https://doi.org/10.1130/B37943.1

800 Tesdal, J. E., Galbraith, E. D., & Kienast, M. (2013). Nitrogen isotopes in bulk marine

801 sediment: linking seafloor observations with subseafloor records.

802 *Biogeosciences*, 10(1), 101–118. https://doi.org/10.5194/bg-10-101-2013

803 Tostevin, R., Shields, G. A., Tarbuck, G. M., He, T., Clarkson, M. O., & Wood, R. A.

804 (2016a). Effective use of cerium anomalies as a redox proxy in carbonate-

805 dominated marine settings. *Chemical Geology*, 438, 146–162.

806 https://doi.org/10.1016/j.chemgeo.2016.06.027

807 Tostevin, R., Wood, R. A., Shields, G. A., Poulton, S. W., Guilbaud, R., Bowyer, F., et

808 al. (2016b). Low-oxygen waters limited habitable space for early animals. *Nature*

809 *Communications*, 7(1), 12818.

810 Uahengo, C. I., Shi, X., Jiang, G., & Vatuva, A. (2020). Transient shallow-ocean

811 oxidation associated with the late Ediacaran Nama skeletal fauna: evidence from

812 iodine contents of the Lower Nama Group, southern Namibia. *Precambrian*

813 *Research*, 343, 105732. https://doi.org/10.1016/j.precamres.2020.105732

814 Veizer, J. (1983). Trace elements and isotopes in sedimentary carbonates. *Reviews in*

815 *mineralogy*, 11, 265–300. https://doi.org/10.1515/9781501508134-012

816 Wan, B., Tang, Q., Pang, K., Wang, X., Bao, Z., Meng, F., et al. (2019). Repositioning
817 the Great Unconformity at the southeastern margin of the North China Craton.

818 *Precambrian Research*, 324, 1–17.

819 https://doi.org/10.1016/j.precamres.2019.01.014

820 Wang, Q., Yang, D., & Xu, W. (2012). Neoproterozoic basic magmatism in the southeast margin of North China Craton: Evidence from whole-rock

821 geochemistry, U–Pb and Hf isotopic study of zircons from diabase swarms in the
822 Xuzhou-Huaibei area of China. *Science China Earth Sciences*, 55, 1461–1479.

823 https://doi.org/10.1007/s11430-011-4237-7

825 Xiao, S., Shen, B., Tang, Q., Kaufman, A. J., Yuan, X., Li, J., & Qian, M. (2014).
826 Biostratigraphic and chemostratigraphic constraints on the age of early

827 Neoproterozoic carbonate successions in North China. *Precambrian Research*,
828 246, 208–225. https://doi.org/10.1016/j.precamres.2014.03.004

829 Xu, L., Shi, X., Poulton, S. W., Sun, L., Zhou, X., Zhou, L., & Tang, D. (2025).
830 Shallow seawater oxygenation at ca. 1.44 Ga: A reflection of local seafloor

831 oxygen oases or extensive water-column oxygenation?. *Geological Society of America Bulletin*. https://doi.org/10.1130/B38041.1

833 Yang, D. B., Xu, W. L., Xu, Y. G., Wang, Q. H., Pei, F. P., & Wang, F. (2012). U–Pb
834 ages and Hf isotope data from detrital zircons in the Neoproterozoic sandstones
835 of northern Jiangsu and southern Liaoning Provinces, China: Implications for the
836 Late Precambrian evolution of the southeastern North China Craton.

837 *Precambrian Research*, 216, 162–176.

838 https://doi.org/10.1016/j.precamres.2012.07.002

839 Yang, H., Shi, X., Xu, L., Sun, L., Xie, B., Zhou, L., et al. (2024). 1.44 Ga
840 Oxygenation Event: Evidence From the Southern Margin of North China.

841 *Journal of Geophysical Research: Biogeosciences*, 129(4), e2023JG007787.
842 https://doi.org/10.1029/2023JG007787

843 Zang, W., & Walter, M. R. (1992). Late Proterozoic and Early Cambrian microfossils
844 and biostratigraphy, northern Anhui and Jiangsu, central-eastern China.

845 *Precambrian Research*, 57(3–4), 243–323. https://doi.org/10.1016/0301-9268
846 (92) 90004-8

847 Zegeye, A., Bonneville, S., Benning, L. G., Sturm, A., Fowle, D. A., Jones, C., et al. (2012). Green rust formation controls nutrient availability in a ferruginous water column. *Geology*, *40*(7), 599-602. https://doi.org/10.1130/G32959.1

848
849
850 Zhang, F., Stockey, R. G., Xiao, S., Shen, S. Z., Dahl, T. W., Wei, G. Y., et al. (2022a).
Uranium isotope evidence for extensive shallow water anoxia in the early Tonian
oceans. *Earth and Planetary Science Letters*, 583, 117437.
https://doi.org/10.1016/j.epsl.2022.117437

854 Zhang, K., Zhu, X., Wood, R. A., Shi, Y., Gao, Z., & Poulton, S. W. (2018).
855 Oxygenation of the Mesoproterozoic ocean and the evolution of complex
eukaryotes. *Nature Geoscience*, 11(5), 345-350. https://doi.org/10.1038/s41561-018-0111-y

858 Zhang, S., Cao, R., Lan, Z., Li, Z., Zhao, Z., Wan, B., et al. (2022b). SIMS Pb-Pb
dating of phosphates in the Proterozoic strata of SE North China Craton:
Constraints on eukaryote evolution. *Precambrian Research*, 371, 106562.
https://doi.org/10.1016/j.precamres.2022.106562

862 Zhao, H., Zhang, S., Ding, J., Chang, L., Ren, Q., Li, H., et al. (2020). New
863 geochronologic and paleomagnetic results from early Neoproterozoic mafic sills
and late Mesoproterozoic to early Neoproterozoic successions in the eastern
864 North China Craton, and implications for the reconstruction of Rodinia. *Bulletin*,
132(3-4), 739-766. https://doi.org/10.1130/B35198.1

867 **Figure captions**

868 **Figure 1.** Geological setting. (a) Tectonic division of the NCC (modified after Zhang 869 et al., 2022b). (b) Paleogeography of the Huaibei Group sedimentation period on the 870 southeastern margin of the NCC (modified after Hu et al., 2016). (c) Stratigraphic 871 sequence of the Huaibei Group in the Xuhuai area (modified after Sun et al., 2020).
872 U-Pb ages are from (1) Yang et al. (2012); (2) Sun et al. (2020); (3) Zhao et al. 873 (2020); (4) Wang et al. (2012). (d) Geological map of the study area (modified from 874 GSAP, 2006).
**Figure 2.** Stratigraphic column and sedimentary features of the Qingtongshan section, Suzhou, Anhui. (a) Jiayuan Formation: sandy limestone. (b) Member I of the Zhaowei Formation: gray thick-bedded argillaceous dolomite-bearing limestone containing storm debris (arrow). (c) Member I of the Zhaowei Formation: columnar stromatolites (the dashed line marks a single stromatolitic lamina). (d) Member II of the Zhaowei Formation: medium-thin-bedded grey argillaceous dolomite-bearing limestone interbedded with calcareous shale. (e) Member I of the Niyuan Formation: medium-bedded argillaceous limestone and argillaceous dolomite-bearing limestone (water column precipitated carbonate mud). (f) Member I of the Niyuan Formation: argillaceous limestone containing molar tooth structure (arrows). (g) Member I of the Niyuan Formation: columnar stromatolites (the dashed line marks a single stromatolitic lamina). (h) Member I of the Niyuan Formation: oolitic limestone (arrow). (i) Member I of the Niyuan Formation: flat pebble conglomerate (arrow). (j) Member II of the Niyuan Formation: medium-bedded laminated carbonate. (k) Member II of the Niyuan Formation: argillaceous dolostone interbedded with thin layers of calcareous shale. (l) Jiudingshan Formation: storm debris (lower box) and limestone containing molar tooth structures (upper box).

**Figure 3.** Mineralogical characteristics of the Neoproterozoic carbonates from the Qingtongshan section. (a and b) Photomicrographs with low and high magnification, showing water-column precipitated carbonate mud in the Zhaowei argillaceous dolomite-bearing limestone with weak recrystallization and few siliciclastic contamination (stratigraphic height = 176 m). (c) BSE image of the Zhaowei argillaceous dolomite-bearing limestone, showing nearly round calcite (Cal) crystals and rhombohedral dolomite (Dol) crystals. (d) CL image of the same area as panel c, showing weakly luminescent calcite (Cal) and dolomite (Dol). (e and f) Photomicrographs with low and high magnification, showing water-column precipitated carbonate mud in Niyuan argillaceous limestone with weak recrystallization and few siliciclastic contamination (stratigraphic height = 237.7 m). (g) BSE image of Niyuan argillaceous limestone, showing nearly round calcite (Cal)
crystals and rhombohedral dolomite (Dol) crystals. (h) CL image of the same area as panel g, showing weakly luminescent calcite (Cal) and dolomite (Dol). (i and j) Photomicrographs with low and high magnification, showing argillaceous dolostone from the Niyuan Formation with weak recrystallization and few siliciclastic contamination (stratigraphic height = 556 m). (k) BSE image of Niyuan argillaceous dolostone, showing rhombohedral dolomite (Dol) crystals. (l) CL image of the same area as panel k, showing dull luminescent dolomite (Dol). (m) Photomicrographs with low magnification, showing molar tooth structures in Jiudingshan argillaceous limestone with ptygmatically folded ribbon-like morphology (stratigraphic height = 650.5 m). (n) Photomicrographs with high magnification, showing equigranular, microspar calcite crystals. (o) BSE image of Jiudingshan argillaceous limestone, showing nearly round calcite (Cal) crystals. (p) CL image of the same area as panel o, showing weakly luminescent calcite (Cal) and dolomite (Dol).

**Figure 4.** Geochemical characteristics of the Jiayuan, Zhaowei, Niyuan, and Jiudingshan formations from the Qingtongshan section. The horizontal blue dashed line represents the boundary between the two intervals divided according to Ce/Ce\*(SN). The early Neoproterozoic deep-sea ferruginous conditions (Guilbaud et al., 2015) and low phosphorus recycling (Guilbaud et al., 2020) are illustrated alongside the stratigraphic column. In the I/(Ca+Mg) profile, the red vertical dashed line represents the Precambrian baseline (0.5 µmol/mol; c.f., Lu et al., 2017). The δ¹⁵Nₐₗ represents the data from the Zhaowei and Liangtang sections in the Anhui Province (Kang et al., 2023) are compared here, with the corresponding data for the respective formations. The yellow vertical dashed line represents the average δ¹⁵N value of sediments in modern oceans (5‰; Tesdal et al., 2013).

**Figure 5.** Cross plots of geochemical data from the Qingtongshan section. (a–c) Cross plots of I/(Ca+Mg) with I, Mn/Sr, and Mg/Ca. (d) REE+Y distribution diagram for post-screened samples (selection criteria: Th < 0.5 ppm, Y/Ho > 36, ΣREE < 12 ppm, Pr/Yb(SN) < 1; c.f. Ling et al., 2013). The green line represents Interval I data, and the orange line represents Interval II data. (e–g) Cross plots between Ce anomaly,
933 Pr/Yb<sub>(SN)</sub>, Y/Ho, and Th for post-screened samples from Intervals I and II. (h)

934 Ce/Ce*(SN) vs. Mn/Sr. (i) Ce/Ce*(SN) vs. Mg/Ca.

935 **Figure 6.** Secular variations of marine chemical conditions and the evolution of crown-group eukaryotes. (a) I/(Ca+Mg) through time (modified from Sun et al., 2025b; Tang et al., 2025). (b) Cerium anomaly of shallow seawater carbonates through time (modified from Liu et al., 2021). (c) Sedimentary δ<sup>15</sup>N through time (modified from Stüeken et al., 2024). (d) Changes in phosphorus concentrations in siliciclastic marine sediments through time (modified from Planavsky et al., 2023). (e)

941 Early eukaryotic evolutionary history based on molecular clocks and fossil records (modified from Brocks et al., 2023; Planavsky et al., 2014), with an evolutionary stasis during ca. 1.0–0.8 Ga. (1) Butterfield (2000), Gibson et al. (2018), (2) Li et al. (2023), Tang et al. (2020), (3) Loron et al. (2019), (4) Planavsky et al. (2014), (5) Erwin et al. (2011).

946

947 **Supporting information**

948 **Table S1.** Iodine concentration of the dolostone reference material JDo-1 measured by ICP-MS/MS.

949 **Table S2.** Major elements of the dolostone reference material JDo-1 measured by ICP-MS/MS.

950 **Table S3.** Geochemical data of the carbonates from the Jiayuan, Zhaowei, Niyuan and Jiudingshan formations of North China.

952 **Table S4.** Pre-screened nitric acid-digested REE data of carbonates from the Jiayuan, Zhaowei, Niyuan and Jiudingshan formations of North China.

954 **Table S5.** Post-screened nitric acid-digested REE data of carbonates from the Zhaowei, Niyuan and Jiudingshan formations of North China.

956 **Table S6.** Acetic acid-digested REE data of carbonates from the Zhaowei, Niyuan and Jiudingshan formations of North China.
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

Argillaceous dolostone

<figure><img src="image_9.png" /></figure>

Argillaceous calcareous dolostone

<figure><img src="image_10.png" /></figure>

Argillaceous limestone

<figure><img src="image_11.png" /></figure>

Argillaceous dolomite-bearing limestone

<figure><img src="image_12.png" /></figure>

Silty/sandy limestone

<figure><img src="image_13.png" /></figure>

Shale/mudstone

<figure><img src="image_14.png" /></figure>

Mafic sill

<figure><img src="image_15.png" /></figure>

Molar tooth structure

<figure><img src="image_16.png" /></figure>

Stromatolite
<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>
<figure><img src="image_33.png" /></figure>

Global deep water ferruginous

<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>

<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /></figure>

<figure><img src="image_38.png" /></figure>

<figure><img src="image_39.png" /></figure>

<figure><img src="image_40.png" /></figure>
<figure><img src="image_41.png" /></figure>

<figure><img src="image_42.png" /></figure>

<figure><img src="image_43.png" /></figure>

<figure><img src="image_44.png" /></figure>

<figure><img src="image_45.png" /></figure>

<figure><img src="image_46.png" /></figure>

<figure><img src="image_47.png" /></figure>

<figure><img src="image_48.png" /></figure>

<figure><img src="image_49.png" /></figure>