# Carbon, iron and sulfur records of lacustrine
paleo-environments during the middle Eocene in eastern
China

Xinping Liang, Sergei Katsev, Quanyou Liu, Gleb S. Pokrovski, Zhijun Jin

► To cite this version:

Xinping Liang, Sergei Katsev, Quanyou Liu, Gleb S. Pokrovski, Zhijun Jin. Carbon, iron and sulfur records of lacustrine paleo-environments during the middle Eocene in eastern China. Science of the Total Environment, 2024, 956, pp.177270. 10.1016/j.scitotenv.2024.177270. hal-04775135

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<table><tr><td>Manuscript Number:</td><td>STOTEN-D-24-35639R2</td></tr><tr><td>Article Type:</td><td>Research Paper</td></tr><tr><td>Keywords:</td><td>organic carbon burial; ferruginous conditions; volcanism; transgression; paralic lacustrine shale; middle Eocene</td></tr><tr><td>Corresponding Author:</td><td>Xinping Liang<br>Peking University<br>CHINA</td></tr><tr><td>First Author:</td><td>Xinping Liang</td></tr><tr><td>Order of Authors:</td><td>Xinping Liang<br>Sergei Katsev<br>Quanyou Liu<br>Gleb S. Pokrovski<br>Zhijun Jin</td></tr><tr><td>Abstract:</td><td>It is widely recognized that anoxic conditions facilitate the preservation of organic carbon in marine sediments. However, the specific geological factors that lead to the development of such conditions in paleo-lakes are less well understood. Owing to their smaller size, paleolakes could experience more frequent and stronger changes in geochemical conditions than oceans. Such changes, such as volcanism, hydrothermal fluids, or ocean transgressions, can also strongly affect the lacustrine organic carbon burial thereby complicating sediment diagenesis record. Here, we used total organic carbon content (TOC), organic carbon isotope (δ13Corg), iron speciation, and pyrite sulfur isotope (δ34Spy) data to establish relationships between organic carbon preservation and anoxic conditions in fine-grained sediments from the middle Eocene lacustrine depositional environments from the Shahejie Formation of the Jiyang Depression, Bohai Bay Basin, eastern China. The results reveal TOC between 1% and 10%, highly-reactive iron to total iron ratios greater than 0.38, and most TOC to total sulfur ratios exceeding 2. These data indicate that the organic-rich shales of the Shahejie Formation were formed as a result of high primary productivity during the warm and humid middle Eocene period, coupled with the efficient preservation of organic matter in anoxic bottom waters. Negative δ13Corg and δ34Spy excursions recorded in the Shahejie Formation indicate water column conditions to have been influenced by transient volcanic eruptions. Positive δ13Corg and negative δ34Spy excursions may have been caused by hydrothermal fluids input whereas δ34Spy values approaching 20‰ suggest frequent marine transgressions. In particular, despite potential inputs of S into the paleolake by volcanism, hydrothermal fluids, or marine transgressions, bacterial sulfate reduction efficiently depleted the sulfate pool to have created ferruginous geochemistry water conditions for the effective preservation of organic carbon in sediments. Our results establish a direct link between lacustrine shale geochemical signatures and geological phenomena that impact its sedimentation.</td></tr><tr><td>Suggested Reviewers:</td><td>Jun Shen, Dr.<br>Prof., China University of Geology (Wuhan)<br>shenjun@cug.edu.cn<br>Prof. Shen is a global environmentalist with a broad perspective, he published several papers in Nature Communications about volcanism<br><br>Mingyu Zhao, Dr.<br>Associate Prof., CAS Institute of Geology and Geophysics<br>mingyu.zhao@mail.iggcas.ac.cn<br>Dr. Zhao made a big progress in the sphere interaction and nutrient circulation, some of the papers are published in Nature Geoscience.</td></tr></table>
Zhengbing Wang, Dr.
Delft University of Technology
Zheng.Wang@wldelft.nl
Prof. Wang has made great progress in the ecological environment research of estuarine delta and near coastal zones.

Qiyuan Sun, Dr.
Fujian Normal University
minglei_2008@fjnu.edu.cn
Dr. Sun does research on the formation of organic matter in rivers and lakes for more than 10 years, and can give the comments from a morden perspective.

Meng Cheng, Dr.
Chengdu University of Technology
mengcheng@cdut.edu.cn
Dr. Cheng knows a lot about the carbon and sulfur cycles in oceans and lakes.

Jian Cao, Dr.
Nanjing University
jcao@nju.edu.cn

Xiting Liu, Dr.
Ocean University of China
liuxiting@ouc.edu.cn
former reviewer

Response to Reviewers:
Dear Editor Wei Ouyang,

We gratefully acknowledge the reviewer's positive evaluation of our work and agree with the comments of the second reviewer. We appreciate the editor's help in improving the manuscript, as well as the editorial support and suggestions. According to the proposed comments, we carefully revised the manuscript. The responses to the comments of the second reviewer are listed point-by-point in the attached file "Response to reviewers", the revisions are annotated red through the manuscript (in the version of track changes). The manuscript is co-authored by Sergei Katsev, Quanyou Liu, Gleb S. Pokrovski and Zhijun Jin. We kindly ask you to consider it for publication in "Science of the Total Environment".

In the revised manuscript, we used total organic carbon content, iron speciation, organic carbon isotope (δ¹³Corg), and pyrite sulfur isotope (δ³⁴Spy) data to establish relationships between organic carbon preservation and anoxic conditions in fine-grained sediments from the middle Eocene lacustrine depositional environments of the Jiyang Depression, Bohai Bay Basin, eastern China. Our results identified that despite potential inputs of S into the paleolake by volcanism, hydrothermal fluids, or transgression, bacterial sulfate reduction depleted the sulfate pool sufficiently to have created ferruginous geochemistry water conditions for the effectively preservation of organic carbon in sediments of Shahejie Formation. This is a direct link between lacustrine organic carbon burial and its controlling factors among multivariate geological events.

We believe that the revised manuscript will be of interest to the readership of researchers and professionals working on lacustrine environments. This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal. We have read and understood your journal's policies, and we believe that neither the manuscript nor the study violates any of these. There are no conflicts of interest to declare.

Thank you very much for your time and consideration. I look forward to hearing from you.

Sincerely,

Xinping Liang

Institute of Energy, Peking University, Beijing, 100871, China;

xinping.liang@pku.edu.cn
Carbon, iron and sulfur record of lacustrine paleo-environments
during the middle Eocene in eastern China

Xinping Liang<sup>a,c</sup>, Sergei Katsev<sup>b</sup>, Quanyou Liu<sup>a*</sup>, Gleb S. Pokrovski<sup>c</sup>,
Zhijun Jin<sup>a,d*</sup>

a. State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Institute of Energy, Peking University, Beijing 100871, China;

b. Large Lakes Observatory and Department of Physics, University of Minnesota
Duluth, 2205 E 5th St, Duluth, MN 55812, USA;

c. Géosciences Environnement Toulouse, Université Toulouse III – Paul Sabatier,
CNRS, IRD, CNES, OMP, 14 avenue Edouard Belin, F-31400 Toulouse, France;

d. Sinopec Petroleum Exploration and Production Research Institute, Beijing, 100083,
China

*Corresponding author: [liuqy@pku.edu.cn](mailto:liuqy@pku.edu.cn) or [qyouliu@sohu.com](mailto:qyouliu@sohu.com) (QY.L.) and
[jinzj1957@pku.edu.cn](mailto:jinzj1957@pku.edu.cn) (Z.J.J)
Dear Editor Wei Ouyang and dear reviewers,

We gratefully appreciate your help in improving the manuscript, as well as the editorial support and suggestions. Thank you for the positive evaluation of our work and we agree with the minor revisions. According to the proposed constructive comments, we carefully revised the manuscript and needed figures. The revisions are annotated red through the manuscript in the version of track changes. The manuscript is co-authored by Xinping Liang, Sergei Katsev, Quanyou Liu, Gleb S. Pokrovski and Zhijun Jin. We kindly ask you to consider it for publication in "*Science of the Total Environment*".

**1. Followed by the editor's suggestions, we submitted:**

1) Cover Letter;

2) revised manuscript with changes marked red;

3) revised manuscript with no changes marked;

4) Graphical Abstract;

5) Figures and Tables as separate files (Table 1 as separate file in Word format);

6) 3 points of Highlights (maximum 85 characters, including spaces, per bullet point);

7) response to Reviewers point-by-point;

and we REMOVED the original version of our submission.

**2. Here are the responses to the comments of Reviewer #2.**

**Comments:**

After reviewing the manuscript, I recommend minor revisions before acceptance.

The carbon isotopes of organic matter and the sulfur isotopes of pyrite are influenced by sedimentary environment evolution and sediment dynamics, which should be considered.

The iron speciation is affected by the sedimentary environment and redox conditions, with the contents of ferrous carbonate and pyrite competing with each other. I suggest the authors consider some modern sedimentary environment studies to enhance their discussion of the marine transgression in the study area. Relevant references for consideration include: Liu et al., 2023, SB; Zhang et al., 2024, 3P; Kong et al., 2024, MG; Zhang et al., CG, 2024.

**Response:**

We are grateful for this kind suggestion and the references. We agree with the reviewer's point that the influencing factors of carbon isotopes of organic matter ($\delta^{13}C_{org}$), the sulfur isotopes of pyrite ($\delta^{34}S_{py}$) and the iron speciation should be considered when these indicators are applied in the interpretation of paleoenvironments. As our samples are all fine-grained sediments in the semi-deep to deep lake, the TOC values vary between 1% and 10% (average 4%), and in the detailed core observations of the FY1 well, no strong storms, floods or fragments of higher plants were observed over a total length of 403.6 m, so we considered there was very few the terrestrial occasional effect, and the $\delta^{13}C_{org}$ and $\delta^{34}S_{py}$ could be used for most cases. Here we did not use the high C/S ratios to distinguish between marine and terrestrial sediments here because the sulfur content increasing can also be caused by volcanic or hydrothermal activities, neither does the pyrite content.

We agree with the reviewer's comment that the iron speciation is applied to interpret the sedimentary environment and redox conditions. Here as our magnetite content ($Fe_{mag}$) is low (< 0.13%), pointing to the absence of intense diagenetic transformation, so the iron speciation can be used to constrain the redox conditions of Shahejie Formation. As the oxidized Fe (Feox) is low, with
a maximum of 0.4%, so the contents of ferrous carbonate and pyrite compete with each other. This
conclusion is also consistent with Liu et al.'s (2023, SB) viewpoint (Fig1.A, B). The difference
between the samples of the two study areas is their total content, the total contents of ferrous
carbonate and pyrite is lower than ~2% and ~6% respectively (Fig1.C), indicating the ferruginous
to euxinic water conditions during Shahejie Formation.

As the TOC and δ¹³Corg values shows different sources of organic matter (Fig1.D) between ECMZ core of modern sedimentary environment (Kong et al., 2024, 3P) and FY-1 Well core of Shahejie Formation, we compared the values of highly reactive iron between 91 samples of ECMZ core and 66 samples of FY-1 Well core of Shahejie Formation, to try to enhance the discussion of the marine transgression during middle Eocene (Fig1.E, F). Here in core ECMZ, authigenic minerals such as framboid pyrite aggregates can document the seawater intrusion in East China Sea, and continuous and slight decrease in the contents of magnetite and haematite illustrates that the sea level continued to rise (Liu et al., 2023, SB). However, this could happen after volcanic and hydrothermal activities during Shahejie Formation in Jiyang Lake. For example, as volcanic activity brings large amounts of nutrients and trace metals to the surface water, the positive feedback can increase paleoproductivity, accelerate the reducing bottom water conditions, and formed the high content of authigenic minerals such as framboid pyrite.

Therefore, we combined δ¹³Corg to distinguish volcanism and transgressions because volcanism
can cause negative δ¹³Corg excursions, whereas marine transgressions usually happen in warmer
climates and cause positive δ¹³Corg excursions. Also, we propose that the precipitation of calcium
carbonate in organic-rich shale is the result of increased pH due to strong photosynthesis by algae
in the warm climate during marine transgression in the Jiyang calcareous lake. This may also be the
difference between shallow marine and terrestrial deposits. Our mineral analyses show that the
carbonate content in the study interval was generally high, with average calcite and dolomite
contents of 37% and 10%, respectively. For example, the content of Fecarb is higher than that of Fepy
from 3225 to 3200 m (Unit 5 in Fig.2), indicating that the sulfur isotope fractionation of pyrite in
the paralic lakes during the sulfate reduction process was constrained by changes in the global sea
level. The sea level increased in the warm and humid climate, brought large amounts of Ca²⁺ to form
carbonates as well as high paleo-productivity promoted the deposition of organic-rich layers
simultaneity. Thus formed the alternating carbonate and organic-rich shale layers, and after that the
organic matter was degraded in a limited manner under ferruginous conditions and was well
preserved. Saturated carbonate ions combined with iron and formed siderite, making its content
higher in lacustrine sediments than that in modern sedimentary environments (Fig1.E).

This enhanced discussion of the marine transgression of the above explanation has been added
in the manuscript. Please see lines 447-464.

<figure><img src="image_2.png" /></figure>
<figure><img src="image_3.png" /><figcaption>Fig.1. Comparison of the highly reactive iron values, TOC and δ¹³Corg between samples of CEMZ core from modern sedimentary environment (Liu et al., 2023, SB; Zhang et al., 2024, 3P) and samples of FY1 Well from Jiyang Lake.</figcaption></figure>

Thank you for your time and consideration. I look forward to hearing from you.

Best wishes for the journal "Science of the Total Environment"!

Yours sincerely,

Xinping Liang on behalf of all authors

xinping.liang@pku.edu.cn

Institute of Energy, Peking University
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Carbon, iron and sulfur records of lacustrine paleo-environments during the
middle Eocene in eastern China

**Abstract**

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It is widely recognized that anoxic conditions facilitate the preservation of organic carbon in marine sediments. However, the specific geological factors that lead to the development of such conditions in paleo-lakes are less well understood. Owing to their smaller size, paleolakes could experience more frequent and stronger changes in geochemical conditions than oceans. Such changes, such as volcanism, hydrothermal fluids, or ocean transgressions, can also strongly affect the lacustrine organic carbon burial thereby complicating sediment diagenesis record. Here, we used total organic carbon content (TOC), organic carbon isotope ($\delta^{13}C_{org}$), iron speciation, and pyrite sulfur isotope ($\delta^{34}S_{py}$) data to establish relationships between organic carbon preservation and anoxic conditions in fine-grained sediments from the middle Eocene lacustrine depositional environments from the Shahejie Formation of the Jiyang Depression, Bohai Bay Basin, eastern China. The results reveal TOC between 1% and 10%, highly-reactive iron to total iron ratios greater than 0.38, and most TOC to total sulfur ratios exceeding 2. These data indicate that the organic-rich shales of the Shahejie Formation were formed as a result of high primary productivity during the warm and humid middle Eocene period, coupled with the efficient preservation of organic matter in anoxic bottom waters. Negative $\delta^{13}C_{org}$ and $\delta^{34}S_{py}$ excursions recorded in the Shahejie Formation indicate water column conditions to have been influenced by transient volcanic eruptions. Positive $\delta^{13}C_{org}$ and negative $\delta^{34}S_{py}$
excursions may have been caused by hydrothermal fluids input whereas δ³⁴Sₚy values
approaching 20‰ suggest frequent marine transgressions. In particular, despite
potential inputs of S into the paleolake by volcanism, hydrothermal fluids, or marine
transgressions, bacterial sulfate reduction efficiently depleted the sulfate pool to have
created ferruginous geochemistry water conditions for the effective preservation of
organic carbon in sediments. Our results establish a direct link between lacustrine shale
geochemical signatures and geological phenomena that impact its sedimentation.

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32 **Key words:** organic carbon burial; ferruginous conditions; volcanism; transgression; 33 paralic lacustrine shale; middle Eocene

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## 1. Introduction

36 Anoxic conditions facilitate burial and preservation of organic carbon by slowing
37 down microbial degradation, primarily in marine sediments (Shen Y. et al., 2003;
38 Planavsky et al., 2011; Clarkson et al., 2014, 2016; Guilbaud et al., 2015; Raiswell et
39 al., 2018), whilst factors that cause such conditions in lakes have not been sufficiently
40 investigated. Multiple geologic events, such as volcanism, subsurface hydrothermal
41 activity, and marine water incursion, can substantially affect sediment diagenesis and
42 the burial of organic matter in both marine and lacustrine basins. Volcanic ash layers,
43 for example, are prevalent in organic-rich deposits (organic carbon content greater than
44 2.0%) such as the Bazhenov Formation (western Siberia, Russia), the Eagle Ford
45 Formation (Gulf Coast Basin, USA), the Wufeng-Longmaxi Formation (Sichuan Basin,
46 China), the Fengcheng and Lucaogou Formations (Junggar Basin, China) (Ruebsam et
al., 2020; Liang et al., 2020; Liu et al., 2021, 2024). Volcanic activity may increase biological productivity in the photic zone by supplying nutrients and affect aquatic environments and their sedimentary records by providing fluxes of heat, dissolved carbon dioxide, and methane (Duggen et al., 2010; Langmann et al., 2010; Liu et al., 2019). Enhanced preservation of organic material in sediments can lead to the formation of organic-rich shales (Kim et al., 2015; LaRowe et al., 2020). Hydrothermal and volcanic inputs, recorded by high concentrations of metals and heterogeneous tuff in shales, are correlated with changes in paleo-productivity, anoxia, and organic matter burial (Magnall et al., 2016; He et al., 2020). However, the mechanisms by which geological events affect formation and preservation of organic matter in lacustrine environments are much less constrained than in marine environments.

Like the modern counterparts, ancient lakes were important sinks of organic carbon (Sklarew, 1979; Layton-Matthews et al., 2013; Swanner et al., 2020). Being much smaller than oceans, lakes are more sensitive to environmental changes and can record geologic events on shorter time scales. Paralic lakes, in particular, can be affected by volcanic activity, hydrothermal discharges, marine transgressions or other geological events (Malumián and Ramos, 1984; Duggen et al., 2010; Lee et al., 2018; Zeng et al., 2018; Zhang et al., 2018; Liu et al., 2024), so the controlling factors for the burial of lacustrine organic carbon are more complicated. For example, the Kabuno Bay (East Africa) is ferruginous, whereas Lake Kivu is sulfidic even though they both receive iron through hydrothermal sub-lacustrine springs (Bhattarai et al., 2012; Ross et al., 2014; Swanner et al., 2020). Studying variations in lacustrine conditions of sedimentation may help understand the analogous marine processes on their
70 correspondingly longer time scales.

71 In this regard, ancient lakes of the Jiyang Depression (Fig.1) in the Bohai Bay
72 Basin, eastern China offer an excellent natural environment to investigate in detail the
73 continental sedimentary record of the middle Eocene. A series of successive lacustrine
74 shale sediments 150-300 m thick were deposited between the upper sub-member of
75 Member 4 and the lower sub-member of Member 3 of the Shahejie Formation (Es4¹
76 and Es3³) (Chen et al., 2008; Liang C. et al., 2018; Shi et al., 2019). This depositional
77 period corresponds to a global increase in the atmospheric CO2 during the Middle
78 Eocene Climate Warming (MECO) (Fig.1, A; Lourens et al., 2005; Jovane et al., 2007;
79 Zachos et al., 2008; Bijl et al., 2010). Therefore, this study combines total organic
80 carbon contents (TOC) and total sulfur (TS) values of the Fanye-1 well (FY1, Fig. 1,
81 B), with the vertical variation in Fe speciation, organic carbon isotope (δ¹³Corg), and
82 pyrite sulfur isotope (δ³⁴Spy) data, to establish a direct link between organic carbon
83 preservation and anoxic conditions, and the factors that affect organic carbon burial in
84 lacustrine basins during middle Eocene.

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## 2. Geological Setting

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87 The Bohai Bay Basin is a large Cenozoic sedimentary basin, which was formed
88 at the intersection of three major tectonic regions that included the ancient Asian,
89 Tethys and Pacific Oceans (Fig. 1, B). The subduction of the West Pacific plate under
90 the North China Craton was the controlling factor in the tectonic evolution rate and
91 orientation of the Bohai Bay Basin (Chen et al., 2008). Between the Late Paleocene
92 and Eocene, subduction of the Pacific Plate decreased to its lowest rate of about 40

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mm/a at 43 Ma, and the orientation changed from north-north-west to north-west-west.

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The rate gradually increased to 60 mm/a from 43 to 32 Ma, making the entire Asia
continent an extended tectonic domain. This spreading reached its faulting limit at
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about 23 Ma in the Oligocene leading to the formation of the Late Mesozoic-Paleogene
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Bohai Bay Basin, which is divided into seven depressions and three uplifts (Chen et al.,
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2008).

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Among them, the Jiyang Depression is the largest sedimentation center of the
basin, spanning latitudes from 35 to 40°N. The depression is about 200 km long from
east to west and 120 km wide from north to south, with a total area of 26,000 km² (Song
et al., 2020). Cenozoic deposition during Es₄¹ and Es₃³ sub-members in the Bohai Bay
Basin coincided with an intensive fault depression, resulting in the formation of the
Jiyang Lake whose estimated ancient depth was 190-290 m (Fig. 1., C) with the
deepest area likely reaching 600 m (Chen et al., 2008). The lake has existed in the basin
for about 4 million years and was connected to the west Pacific paleo-ocean during
multiple periods, lasting for several thousand years each (Song et al., 2020). Between
these episodes, the lake basin likely remained closed (Chen et al., 2008).

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During Es₄¹ time, as the basin subsided, the lake experienced high salinities
affected by different geological processes. During the Es₃³ period of humid climate, as
the lake basin widened and subsidence increased, the lake connected to the ocean, thus
increasing accommodation space. Abundant semi-deep to deep waters (10-60 m depth)
provided favorable conditions for deposition of fine-grained sedimentary rocks such as
black shale. The rocks were predominantly argilitic with extensive layers of both
massive and lamellar textures and with veins of sparry limestone, limy mudstone, and

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116 dolomite (Chen et al., 2008; Liang C. et al., 2018; Song et al., 2020).

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118 **3. Materials and Methods**

119 Core analysis shows that the high-quality source rocks of Es₄¹ and Es₃³ (to ~1000 m) were widely distributed in semi-deep or deep lacustrine facies, intercalated with mm-to-cm think layers of volcanic ash in Jiyang Depression. Here a length of 403.6 m 120 of whole-core was taken from the FY1 well (Shi et al., 2019; Song et al., 2020). Black 121 and gray-black, layered, grainy, muddy shale dominates the Es₄¹ and Es₃³ lithologies. 122 A total of 270 core samples was obtained at a spacing of 1.0–1.5 m and these were 123 analyzed for TOC, TS, δ¹³Cₐrg and major element analyses. A subset of 70 samples at 124 an interval of 5–6 m was subjected to iron speciation and δ³⁴Sₚy analyses. 125 126

127 The concentrations of TOC and TS were determined using an Eltra infrared (IR) 128 C/S analyzer from the discrepancy between the total carbon (or sulfur) determined by 129 combustion and the total inorganic carbon (or sulfur) determined by acidification. 130 Ultrapure 6 N HCl was added in a silver cup to dissolve inorganic carbon and sulfur 131 (mostly carbonate and sulfide minerals) from weighed portions of powdered samples. 132 Analyses of δ¹³Cₐrg were performed using a Costech™ 4010 coupled with a Thermo 133 Finnigan MAT 253 via an open-split interface Conflo IV, with a reproducibility better 134 than ±0.1‰ as based on the international standard IAEA-600 (caffeine, δ¹³Cₐrg = - 135 27.77‰). 136

136 For major element concentrations (Fe, Al), 300 mg of powdered samples were 137 burnt at 600 °C for 12 h in a muffle furnace to remove volatiles compounds, followed 138 by a standard multi-acid (HF-HCl-HNO₃) digestion protocol designed for dissolution
in a Teflon bomb. An Agilent 7700 inductively coupled plasma mass spectrometer (ICP-MS) was used for the solution analyses, with an analytical precision of better than ± 5% (1σ) established using USGS standards (BHVO-2 and BCR-2).

Iron speciation and pyrite sulfur isotope analyses (in terms of δ³⁴Sₚy) were performed for a total of 70 samples at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan). Iron as pyrite (Feₚy) was extracted using the chromium reduction method and iron speciation was determined by the sequential extraction method (Poulton and Canfield, 2005). Sulfur in pyrite (Sₚy) was extracted as H₂S using a hot CrCl₂ distillation for 2 h, trapped as silver sulfide (Ag₂S), which was then quantified gravimetrically. The Feₚy content in each sample was then calculated assuming the ideal pyrite stoichiometry (FeS₂). Iron from carbonate minerals (Feₐcarb) was extracted by dissolution for 24 h in cold 10 wt% HCl. Then ferric Fe (oxyhydroxides, Feₐox) was liberated from the residue of each sample, treated for 2 h with a sodium dithionite solution (50 g/L) buffered to a pH of ~5 with acetic acid and sodium citrate. Finally, Fe bound in magnetite (Feₐmag) was extracted for 6 h using a 0.2 M ammonium oxalate and 0.17 M oxalic acid solution (pH ~3). After diluting the extracts by a factor of 100 in 2 wt% HNO₃ (suprapure), an Agilent 7500ce ICP-MS was used to analyze each sequential extract. To increase precision, samples underwent processing in conjunction with internal calibration standards and blanks, by analyzing in duplicate each 10th sample per batch. The analytical precision was better than 5% of the value.

Sulfur isotope ratios (³⁴S/³²S) were determined using a Thermo Fisher Scientific Delta V Plus isotope-ratio mass spectrometer coupled with a flash EA. The results are
expressed as δ³⁴Sₚy following the standard delta notation as per mil deviations relative to the Vienna Cañon Diablo Troilite (Li et al., 2021). An analytical uncertainty of 0.2‰ (1σ) for pyrite δ³⁴S was determined from replicate analyses of the International Atomic Energy Agency (IAEA) standards: S1 (δ³⁴S = -0.3‰), IAEA S2 (δ³⁴S = +22.65‰), and IAEA S3 (δ³⁴S = -32.5‰).

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## 4. Results

The results of geochemical analyses of FY1 section are summarized in Table 1. The variation in chemical and isotopic composition of TOC, δ¹³Cₐrg, TS, δ³⁴Sₚy and Fe redox as a function of depth is presented in Fig. 2. The study interval was divided into 7 units according to the changes in δ¹³Cₐrg and δ³⁴Sₚy values.

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### 4.1. Organic carbon systematics

TOC values of the 270 samples vary from about 1 to 10 % with an average value close to 4%, and with maximum enrichment in Units 1 and 5 (Fig. 2), but without systematic differences between Es₃³ and Es₄¹. The δ¹³Cₐrg values for samples of Es₄¹ vary within a relatively narrow range of about -29‰ to -20‰ with an average of ~ -26‰, whereas those from Es₃³ vary within even narrower range of about -28‰ to -23‰ with a mean value -26‰ (Table 1). Samples with positive excursions of δ¹³Cₐrg values (> -24‰) are more frequent in Es₄¹, and are characterized by elevated TOC contents (> 4%). Samples showing more negative δ¹³Cₐrg (≤ -26‰) are also predominantly found in the Es₄¹ sub-member (Fig. 2A, B).

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### 4.2. Iron systematics
In the lacustrine samples of Es₄¹ and Es₃⁴, FeT values range between 1.4 and 8.0%, with an average close to 3% and are generally dominated by highly reactive iron (FeHR, including pyrite and carbonate iron, oxidized and magnetite iron) fraction (> 90% of FeT on average). Pyrite iron (Fepy) ranges from 0.1 to 4% with an average of ~1%, which is slightly less than or comparable to carbonate iron (Fecarb) content varying from 0.5 to 4 (average 1.5%). Oxidized iron (Feox) is low, with a maximum of 0.4%. The Fe/Al ratios are systematically higher than 0.44 (with a single sample exception of 0.18 at 3346.9 m). Magnetite content (Femag) is low (< 0.13%), pointing to the absence of digenesis; as a result, the iron speciation can be used to constrain the redox conditions. All FeHR/FeT values exceed 0.38, and almost all Fepy/FeHR ratios are lower than 0.6, which is mostly due to the relatively high Fecarb content.

Iron speciation is a widely used proxy for water column redox conditions, and can differentiate between oxic and anoxic (ferruginous or euxinic) environments (Poulton and Canfield, 2005, 2011). For anoxic conditions, Fepy/FeHR ratios are used to distinguish ferruginous (typically < 0.7) and euxinic (typically > 0.8) conditions; however, more recent work suggested values above 0.6 as evidence of euxinia (Poulton, 2021). Under anoxic conditions, either diagenetic transformation of FeHR minerals into Fe-rich clay, or the swift sedimentation of deposits lacking iron, can lower FeHR concentrations. Total Fe/Al ratios provide complementary information on water column redox conditions. Indeed, although oxidative weathering or digenesis of samples could obscure Fepy systematics, it would not significantly alter the Fe/Al ratios that are systematically lower than 0.44 in oxic marine sediments (Lyons and Severmann, 2006; Clarkson et al., 2014, 2016; Raiswell et al., 2018). Therefore, the
Fe/Al proxy in carbonate-rich sediments is relatively robust, provided $Fe_T$ > 0.5 wt% (Raiswell et al., 2018). For $Fe_T$ < 0.5 wt%, carbonate samples have a greater potential to be enriched in $Fe_{HR}$ due to processes other than water column processes that arise under anoxic conditions (e.g., post sedimentary diagenesis).

Overall, TOC values higher than 0.5%, with total iron > 0.5 wt%, $Fe_{HR}/Fe_T$ ratios > 0.38 and $Fe_{HR}/Fe_T$ ratios < 0.8 throughout the succession (Fig. 2, A, E, F) collectively demonstrate predominantly anoxic ferruginous during the depositional period, despite evidence of pyrite enrichment of considerable spatial and temporal variability that cannot exclude local intermittent euxinic conditions in the basin.

### 4.3. Sulfur systematics

Total sulfur (TS) concentrations vary from about 0.2 to 11%, with an average of ~1.4% across the two main intervals (Table 1, Fig. 2). Most samples (231) have TOC/TS weight ratios above 2, with only a few (36) having TOC/TS < 2 (Fig. 2A, C). According to Berner and Raiswell's (1984) analysis of modern sediments, TOC/TS values of < 2 would correspond to euxinic conditions, those between 2 and 3.6 represent "normal" marine conditions, those of 3.6 to 10 reflect freshwater and seawater transition zones, and those > 10 correspond to freshwater sediments. Therefore, most of our samples from the study interval would indicate freshwater or marine transitional conditions, with only rare instances of euxinia (Fig. 3).

Values of $\delta^{34}S_{py}$ exhibit large variations within the studied section (Fig. 2D), ranging from 9‰ to 36‰ with an average of ~ 20‰ with no significant differences between $Es_3^3$ and $Es_4^1$ (Table 1). As sulfur isotope values in marine sulfates were ~ 22 ‰
during middle Eocene (Payton et al., 1998) and elevated δ³⁴Sₚy values (> 20‰) are
consistent with the low levels of sulfate in the euxinic water column (Shen et al., 2003),
our large δ³⁴Sₚy variations (Fig. 2D) indicate that redox conditions fluctuated
considerably over time. However, despite the large environmental fluctuations in
Jiyang Lake during the middle Eocene, fine-grained sediments were formed during the
early stage of diagenesis in a closed system and were almost unaffected by late
digenesis, which was consistent with very limited influence of weathering and
metamorphism that could affect such sediments (Liang C. et al., 2018). This viewpoint
is supported by the fact that interbedded carbonates and mudstones have consistently
yielded similar redox interpretations across all instances of rock samples (Raiswell et
al., 2018). Generally, the unusually high dolomite content in fine-grained sediments
should indicate rather atypical lacustrine chemistry due to volcanism, hydrothermal
fluids, or transgressions that may affected variability in calcium carbonate saturation
leading to dolomite rather than calcite precipitation (Zhang et al., 2018; Liu et al., 2021).

# 5. Discussion

## 5.1. High primary productivity in middle Eocene warmhouse

Compared to the unmineralized fraction in deep sediments of modern large lakes
and marine sediments (Galazzo et al., 2013; Liu et al., 2019, 2021), our high TOC
values (average 4%) may indicate that the primary productivity of the lake over the
entire interval (Es<sub>4</sub><sup>1</sup> to Es<sub>3</sub><sup>3</sup>) was indeed higher than that during other MECO intervals
(1–3% TOC; Galazzo et al., 2013). Algae-rich sediments worldwide are well-known
to be a product of high primary productivity (Meyers, 1997). In Jiyang paleolakes,
algae-rich sedimentary layers, formed by alternating blooms of coccolithophytes and dinoflagellates, were a major feature of the Paleogene oil source rocks in the Bohai Bay Basin for more than 1 Ma (Xie et al., 2016; Song et al., 2020; Shi et al., 2021). These algal blooms are well recognized in the middle Eocene warming period of the Cenozoic (Bijl et al., 2010; Zachos et al., 2008; Pearson, 2010), which occurred 41.4–39.2 Ma ago (Shi et al., 2019). During this time, a warm climate and associated high productivity were favorable for oxygen consumption beneath the photic zone. The water column became anoxic, thereby leading to enhanced preservation and burial of organic matter. High productivity can be supported by systematically elevated concentrations of both TOC and barium (Zeng et al., 2018). TOC values above 6% are recorded here over 11 intervals across the depth range 3343.47 to 3102.67 m, with two intervals having TOC values as high as 8.4% and 10.4% at 3176.51 m and 3192.90 m, respectively (Fig. 2). The average Ba concentrations in $Es_4^1$ and $Es_3^3$ are 470 and 560 ppm, respectively. Over the entire section in the Jiyang Depression, Ba values in samples positively correlate with the corresponding TOC values (Supplementary Table 2). For comparison, in a similar high paleoproductivity Lake Malawi (East Africa), the TOC concentrations are ~ 6 % in the sediments from the anoxic deep parts of the lake (Li et al., 2018).

Meanwhile, high primary productivity can also be confirmed by organic carbon isotope values. Most samples show $δ^{13}C_{org}$ values between -29‰ and -25‰, indicating a source of lacustrine organic matter from exogenous terrestrial plants, authigenic phytoplankton photosynthesis, as well as possible allochthones (Meyers, 1997). Terrestrial C4 plants were very rare in the Jiyang Depression during the middle Eocene
as they are usually found in dry environments and have the δ¹³Corg values higher than -20‰. However, there might have been some C3 plants in the lake, because small fragments of higher plants (including leaves) can be windblown and floating debris could sink, although some of the compounds of terrestrial origin may have been subsequently degraded by diagenesis. In the detailed core observations of the FY1 well, over a total length of 403.6 m, no fragments of higher plants were observed. Generally, there is no direct relationship between the concentration of biomarkers and the source of organic matter because many biomarkers can be detected at very low concentrations.

There was probably some retene, oleanene, or β-carotene (Xu et al., 2020); however, the contribution of higher plants to the preserved organic matter is expected to be small.

For another, the relative content of the biomarkers may also change further with diagenesis. For example, Xu et al. (2020) also proposed an existing algae source of retene, oleanene, or β-carotene during organic-rich shale formation. To sum up, the contribution of organic matter of terrestrial origin from higher plants depends on the lake size and sedimentation process. In a very large paralic lake continental runoff input can be significant only due to strong storms, floods or other events from land. As there were no land events in the Jiyang Depression with a total area of 26,000 km², the detected compounds of higher plants do not mean that the terrestrial organic input is significant in this offshore lake.

Combined with the sedimentary characteristics of modern lakes, we propose that in the center of the paralic Jiyang lake like FY-1 Well, the organic matter may mainly come from lacustrine authigenesis, the input of terrigenous organic matter, such as plants, may only have occurred in brief periods. Of the studied 270 samples, only a few
(25 in Es₃³ and 27 in Es₄¹) showed δ¹³Cₐrg values above -25‰, with an average TOC of
5% (Fig.2), indicating that during this period organic matter may have been
occasionally derived from very limited amount of terrestrial plants. Thus, most of the
organic matter enrichment in the source rocks of the Shahejie Formation likely resulted
from native algal and bacterial production coupled with favorable preservation
conditions. Due to a globally significant increase of greenhouse gases in the MECO
atmosphere (Bijl et al., 2010), photosynthetic carbon fixation could lead to carbon
isotope ratios as low as -29‰ (Hodell and Schelske, 1998). Therefore, we propose that
the negative excursions of δ¹³Cₐrg corresponded to high primary productivity during
warm and humid climate caused by periods of elevated atmospheric pCO₂. Under these
elevated pCO₂, CO₂ was predominantly transported into lacustrine systems via
photosynthesis, and was reduced by biological processes into organic matter with
enrichment in the lighter isotope (¹²C), thereby yielding negative excursions of δ¹³C
values.

## 5.2. Ferruginous/euxinic water conditions

Lallier-Verges et al. (1993) quantified the degree of bacterial sulfate reduction (BSR) using the sulfate reduction index (SRI), which is defined as the ratio of primary organic carbon to residual organic carbon. Primary organic carbon encompasses both the organic carbon depleted via sulfate reduction, and the residual total organic carbon (TOC (wt.%) at present). Organic carbon loss (C<sub>loss</sub>) by degradation coupled with sulfate reduction may be assessed from the stoichiometry of the sulfate reduction equation proposed by Berner and Raiswell (1984):
$$323 \qquad SRI = (TOC + C_{loss})/TOC \qquad (1)$$

$$324 \qquad \text{where } C_{loss} = TS/1.33 \qquad (2)$$

Substituting (2) into (1) yields:

$$326 \qquad SRI = (TOC + TS/1.33)/TOC = 1 + 0.75 \times TS/TOC \qquad (3)$$

The SRI is regarded as the lowest degradation consumption index for total organic carbon. The higher the BSR activity, the higher the SRI value, and the greater the organic matter consumption. For SRI < 1.375, the BSR intensity can be limited by total sulfate and the amount of undegraded TOC may increase substantially. For SRI > 1.375, the BSR intensity increases, and the overall TOC value is relatively low (Liu et al., 2021). The relationship between the sulfate reduction strength and the organic carbon content (SRI/TOC) in the study interval shows that most of the SRI values in the ferruginous water column are less than 1.375 (Fig. 4 (D)), indicating a weak sulfate reduction, which may have been conductive to organic matter preservation and burial. In contrast, euxinic waters with strong BSR may result both in organic preservation and consumption. It can be seen in Fig. 3 and 4 (D) that in the euxinic zone, most samples with TOC < 2 % have SRI values > 1.375.

Several mechanisms may have created ferruginous conditions at the bottom of the Jiyang lake. A combination of volcanism, hydrothermal fluids, and transgression may have brought reactive iron and maintained anoxic conditions, while the primary productivity was high. For example, volcanism could have increased nutrient influxes (Liu et al., 2019, 2024), and productivity, and stimulated oxygen consumption below the photic zone. Meanwhile, the isotopically “light” sulfur (δ³⁴Sₚ₃ of 0 ‰) from a volcanic source was preferentially utilized by BSR, resulting in low δ³⁴Sₚ₃ values.
However, intermittent sulfidization events in a ferruginous Jiyang Lake may not have led to elevated Fe<sub>py</sub>/Fe<sub>HR</sub> ratios compared to typical marine values of 0.7, because such an increase may be obscured by the high carbonate (Fe<sub>carb</sub>) content largely dominating over Fe<sub>py</sub> (Fig. 2, Unit 2, 3, 5). For all the samples, there are only two Fe<sub>py</sub>/Fe<sub>HR</sub> values above 0.7, with the corresponding δ<sup>34</sup>S<sub>py</sub> values of 24–25‰) (Fig. 4 A, C). The two δ<sup>34</sup>S<sub>py</sub> values, higher than those in the lower adjacent interval (~13 ‰), indicate that the basin became relatively closed and euxinic after a period of volcanism or hydrothermal activity. The positive δ<sup>34</sup>S<sub>py</sub> values (> 20‰) are higher than the sulfate <sup>34</sup>S from seawater during middle Eocene, indicating the depletion of the light sulfur isotopes in sulfate under strong BSR. Meanwhile, the TOC values for these two samples are 3.4 and 2.8% at 3332.39 and 3226.24 m, respectively (Fig. 2), which are less than the average value (4.0%) of the study interval, indicating that BSR consumed the primary organic matter and converted sulfate to H<sub>2</sub>S, and promoted pyrite formation. A rough positive covariation between the Fe<sub>py</sub>/Fe<sub>HR</sub> ratios and TOC also supports the BSR occurrence (Fig. 4 A, B). However, the high TOC/TS ratios (> 2.0) and low sulfate reduction index (SRI) (< 1.375) suggest that it was sulfate availability, rather than organic matter supply, that constrained BSR in deep waters (Fig. 4 (D)).

For Jiyang Lake, there may have been occasional euxinic conditions as the TOC/TS values are less than 2 (in 36 samples from 270). Euxinia in the photic zone (from biomarker data) and deeper water (based on framboid size distributions) has been identified in Silurian, Devonian, Permian and Eocene oceans (Racka et al., 2010; Marynowski et al., 2012; Liu et al., 2019; Xu et al., 2020; Percival et al., 2022). Euxinia is prevalent in anoxic marine environments where sulfate is abundant. Modern
meromictic lakes develop euxinia when sulfate concentrations exceed 100 µM, while below that threshold their monimolimnia (the deep stagnant layers) tend to be ferruginous. Xu et al. (2020) demonstrated that there were different euxinic water conditions during the deposition of the Shahejie Formation, evidenced by variations in the aryl isoprenoid ratio, relative amount of isorenieratane, long-chain n-alkane carbon isotopic composition and other molecular indicators, which indicated different intensity of bacterial sulfate reaction (BSR). In Jiyang Lake, potential sources for extra sulfate could have been episodic (for 5–10 ka) marine transgressions (Unit 2 and Unit 5), volcanism (Unit 1, 2, 3 of Es₄¹, and Unit 5 of Es₃³) or hydrothermal fluids (Unit 4 of Es₃³) (Fig. 2). Outside these episodes, sulfate would become depleted by BSR, Feₚy/FeₘR values were mostly < 0.6 and FeₘR/Feₜ values > 0.38 (Fig. 4 A), which also suggests that the deep-water column throughout the middle Eocene was predominantly ferruginous. Meanwhile, there might also be very occasional oxic episodes identified by the presence of bioturbation in anoxic brackish water (Song et al., 2020). Although Fe shuttling could potentially create local enrichments of Feₐcarb (Fig. 4 B)) or Feₐox, resulting in high FeₘR/Feₜ and low Feₚy/FeₘR ratios, the deep lake was probably anoxic and ferruginous during most of the depositional period. The reduced Fe transported from the anoxic water column could have led to Feₐcarb enrichments in bioturbated sediments. As anoxic ferruginous conditions expanded and contracted over time, reactive iron enrichment could form in even deeper settings.

In summary, the FeₘR values between 1.2% and 6.6% generally show the same evolution trend as the total of Feₚy and Feₐcarb, while Feₐox and FeₘR have little correlation with the FeₘR content trend. Thus, organic matter enrichment was primarily created by
both enhanced preservation under ferruginous water conditions and high paleo-
productivity with limited dilution of the shale by terrigenous clastic materials (Xie et
al., 2016; Song et al., 2020).

**5.3. Impacts of geological events on organic carbon burial**

Our core analyses demonstrate that volcanism, hydrothermal activity, and marine
transgressions affected the study area. Only few investigations of carbon and pyrite
sulfur isotopic variations focused on lacustrine sedimentary intervals during the MECO.
Our study uses different covariations between δ³⁴Spy and δ¹³Corg values (Fig. 2B, D) to
establish a relationship between geological events, biological productivity, and water
redox conditions. It has been shown that a negative excursion of 2 to 6‰ in δ¹³Corg of
both carbonate and organic carbon is linked to global volcanism (Lee et al., 2018; Liu
et al., 2019; Longman et al., 2019; Shen J. et al., 2019b; Li Y. et al., 2021). Indeed, in
the study intervals, volcanism effects are evident in nine prominent negative excursions
in δ¹³Corg (Fig. 2B), which are correlated with tuff interlayers (Fig. 5). The pyroclastic
interlayers in the shales are predominantly vitreous and crystalline debris of andesite
(Fig. 5 (C, D)), in which microfractures were developed and ostracod fragments were
deposited. Such negative excursions in δ¹³Corg values are typically associated with
increases in primary productivity, which can be brought about by the increased delivery
of nutrients following the volcanic events.

The same layers with highly negative δ¹³Corg are also associated with negative
shifts in δ³⁴Sₚy (as low as 8.9‰; Fig. 2D). The water column in the deep lake was
usually closed when there were no geological events. In this case, the SO₄²⁻ reservoir
is limited and could not be replenished. With the continuing exhaustion of the sulfate
reservoir (enriched in ³⁴S) by sulfur reduction through dissimilation, ³⁴Sₚy gradually increased and approached that of the initial sulfate. In contrast to open marine sediments, ³⁴Sₚy values in an almost infinite sea water sulfate reservoir, are usually below 0‰ (Li et al., 2021). Volcanic and hydrothermal activity generate sulfur with δ³⁴S of 0 to 5‰ (Huston, 1999), which is substantially below the values of water surface-sourced sulfate (~20‰ during study period). If sub-lacustrine eruptions of a volcano or associated geothermal fluids carried H₂³²S-enriched sulfide sulfur into a closed lake, its reaction with highly reactive iron could form isotopically equivalent Fe³²S₂-enriched pyrite within a quite short period of time (i.e. several years). This process caused the decrease in δ³⁴Sₚy values along with the sulfate of the light sulfur isotope was consumed by BSR. As volcanic H₂³²S was quickly consumed by iron and the BSR began to reduce sulfate in a closed system, the resulting pyrite had lighter δ³⁴S values. Higher δ³⁴Sₚy values then formed again at low sulfate concentrations at the end of sulfate reduction. If volcanic activities or deep fluids carried large amounts of oxidized volatiles, such as SO₂, BSR would have been enhanced and more ³²S consumed. As a result, ³⁴Sₚy first decreased and then increased with an increased amount of sulfate consumption. Indeed, this trend could be identified especially in Units 1 and 3 of Es₄¹ (Fig. 2D). Because the bottom waters of the paleolake seemed to have remained ferruginous, the iron input (as Fe (II); Isley and Abbott, 1999; Kump and Seyfried, 2005) during these volcanic or hydrothermal activities must have exceeded the inputs of sulfur into the lake.

The presence of marine ostracods (Fig 5 (A)), elevated δ³⁴Sₚy values (~20 ‰) and high carbonate content (Feₚcarb to ~5%) collectively indicate marine transgressions,
which is consistent with the interpretation of the strata using the method of astronomical cycles (Ma et al., 2023). During middle Eocene warming, marine transgressions increased the lake water depth and supplied <sup>34</sup>S-enriched sulfate, causing the δ<sup>34</sup>S<sub>py</sub> value of pyrite to approach that of seawater. The δ<sup>34</sup>S<sub>py</sub> values vary between 9 and 36 ‰, being both lower and higher than those of the marine sulfates identified during the MECO (20.9 ± 0.5‰; Longinelli, 1989; Kampschulte et al., 2001).

Marine transgressions may have occurred roughly every 1,000 years over a period of 5,000 years (Fig. 2, Units 2 and 5). As BSR generates isotopically light sulfide, the heavier δ<sup>34</sup>S<sub>py</sub> values can be attributed to the consumption of sulfate sulfur after staged transgression when the lake system closed again. Mineral analyses show that the total content of Fe<sub>mag</sub> and Fe<sub>ox</sub> is low, with a maximum of 0.37%, so the contents of ferrous carbonate and pyrite compete with each other. This is consistent with most sediments in modern paralic environment (Liu et al., 2023; Zhang et al., 2024). Only the difference between the samples of the two study areas is the contents of ferrous carbonate, indicating the different water conditions during their sediment deposition.

In Shahejie Formation, the carbonate content in the study interval was generally high, with average calcite and dolomite contents of 37% and 10%, respectively. For example, the content of Fe<sub>carb</sub> is higher than that of Fe<sub>py</sub> from 3225 to 3200 m in Unit 5 (Fig. 2), indicating that the sulfur isotope fractionation of pyrite in the paralic lakes during the sulfate reduction process was constrained by changes in the global sea level. The sea level increased in the warm and humid climate, and transgression brought large amounts of Ca<sup>2+</sup> to form carbonates. Meanwhile, high paleo-productivity also promoted the deposition of organic-rich layers. Therefore, the precipitation of calcium
carbonate and organic matter layers is the result of increased pH due to strong
photosynthesis by algae in the warm climate during marine transgressions in the Jiyang
calcareous lake. After that, the organic matter was degraded in a limited manner under
ferruginous conditions and was well preserved.

The delicate equilibrium between ferruginous and euxinic chemical conditions
could be influenced by the comparable rates of Fe<sub>HR</sub> and sulfate input fluxes. On a
global scale, continental sources stand out as the primary contributors to the potential
Fe<sub>HR</sub> flux. At the regional scale, geological events, such as volcanic activity or ocean
transgression, could also have liberated some reactive iron and sulfate into the lake,
providing a conceivable mechanism for the transportation of ferrous iron and resulting
in a disproportionate increase in the Fe<sub>HR</sub> flux relative to that of sulfate. The interplay
of all these processes have generated dominantly ferruginous environments during the
middle Eocene in the Jiyang Depression.

On the basis of the aforementioned research, we propose that there was an
extensive burial of lacustrine organic carbon in Eastern China during the warm and
humid climate of the middle Eocene, which thereby created a positive feedback loop
in the Earth's climate system. Frequent volcanic activity during the early stages of Es<sub>4</sub><sup>1</sup>
might have brought aerosols to increase the albedo in the short-term cold climate
(Fig.6B, maybe to 10 years). Most importantly, it released abundant CO<sub>2</sub> into the
atmosphere and brought reactive iron and nutrients to create optimal environments for
the growth and expansion of algae and plankton in the later stages (Fig.6A, to 1 million
years) of the middle Eocene. As temperatures increased, the biological productivity in
Jiyang Lake increased, leading to enhanced production of total organic carbon in
ferruginous water conditions. This positive feedback mechanism may have contributed to the sustained warmth of the middle Eocene climate. Based on this analysis, we highlight a combined effect on sedimentation of external events such as input of volcanic ash, intermittent hydrothermal fluids and marine transgressions during middle Eocene continental warming of western Pacific. These events could not only supply nutrients and catalytic elements to promote biological productivity and cause oxygen consumption, but they also enhanced the reactive iron input to form the dynamic anoxic ferruginous conditions that were conducive for the preservation of organic carbon.

492

## 6. Conclusions

The widespread terrestrial sedimentary and geochemical records of the middle Eocene in the Eastern China have been established using novel TOC, TS, iron speciation, $\delta^{13}C_{org}$ and $\delta^{34}S_{py}$ data. These data, including the highly-reactive iron to total iron ratios ($Fe_{HR}/Fe_{T} > 0.38$), along with a wide range of total organic carbon content (TOC, from 1 to 10%) exceeding that of total sulfur (TOC/TS > 2), and low sulfate reduction indexes (SRI < 1.375), collectively suggest widespread anoxic and ferruginous conditions that would have been favorable for burial of lacustrine organic carbon during the middle Eocene in the paralic lacustrine environments of the Jiyang Depression. The temporal stability of lacustrine ferruginous water conditions may have been due to the warm and humid climate during the middle Eocene, with high biological productivity and large oxygen consumption in the water column, leading to efficient burial of organic carbon in deep sediments.

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Both $\delta^{13}C_{org}$ and $\delta^{34}S_{py}$ values reveal transient geological events such as volcanism,
hydrothermal fluids, and transgressions. Negative $\delta^{13}C_{org}$ and $\delta^{34}S_{py}$ excursions along the sedimentary column point to transient volcanic events, whereas positive $\delta^{13}C_{org}$ along with negative $\delta^{34}S_{py}$ excursions indicate injection of associated hydrothermal fluids; in contrast, elevated $\delta^{34}S_{py}$ (to ~20‰) point to frequent marine transgressions. Despite potential inputs of sulfur into the paleolake as a result of these geological events, bacterial sulfate reduction efficiently consumed the sulfate pool, thereby creating ferruginous water conditions favorable for the efficient preservation of organic carbon.

## Acknowledgments

We thank Yunqing Hao for assistance with microscopic slice analyses and for discussions, Prof. Chao Li and Prof. Yan'an Shen for help with the geochemical analyses, Prof. Bing Shen and Dr. Huiyuan Xu for discussions. This work was supported by the National Natural Science Foundation of China (Grants 42172151, 41811530094, and 41625009), the China Postdoctoral Science Foundation (Grant 2021M690204), and the National Key Research and Development Program (Grant 2023YFF0806200). We thank the two reviewers and the editorial support for the constructive comments, which have greatly improved the quality of this paper.

## Research Data

All of the processed data discussed **has been uploaded in the supplementary tables.**

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Table and Figure captions

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736

Table 1. Summary data of total organic carbon (TOC), organic carbon isotope
composition (δ¹³Corg), total sulfur (TS), pyrite sulfur isotope (δ³⁴Spy), Fe/Al ratio, and
iron speciation of samples in the FY1 Well (data range, arithmetic average and number
of samples, see Supplementary Table 1 for data points).

737

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Figure 1. (A) Map of sites with existing MECO records on a paleogeographic
reconstruction for the middle Eocene at 40 Ma (modified from Ploeg et al., 2023). (B)

742

743

Simplified geological map of Bohai Bay Basin in western Pacific, showing the core
location of FY-1 Well in the Jiyang Depression in eastern China (modified from Liang
et al., 2016).

744

745

Stratigraphic column of the study interval in the middle Eocene. The
Es₄¹ and Es₃³ sediments are deposited during the rifting stage over 38-50 Ma ago

746
(modified from Shi et al., 2019).

748

Figure 2. Iron speciation, δ¹³Corg, TOC, and δ³⁴Spy values as a function of depth from the middle Eocene (Es₄¹⁻Es₃³) black shales in the Jiyang Depression (geological timescale from Shi et al., 2019). (A) total organic carbon (TOC), (B) δ¹³Corg, (C) total sulfur content (TS), (D) pyrite sulfur isotope (δ³⁴Spy), (E) ratio of highly reactive Fe to total Fe (FeHR/FeT), (F) ratio of pyrite iron to highly reactive iron (Fepy/FeHR), (G) content of pyrite iron, (H) content of carbonate iron. Note that FeHR = Fepy+Feox+Fe mag +Fe carb, the thresholds for oxic versus anoxic (0.22–0.38) and ferruginous versus euxinic conditions (0.6–0.8) are from Poulton et al. (2021); the units of the study interval are based on lithological and geochemical differences, as discussed in the text.

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Figure 3. Plots of TOC vs TS, showing that the salinity of the most samples was similar to the salinity of seawater, corresponding to ferruginous water conditions.

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Figure 4. Plots of (A) FeHR/FeT vs Fepy/FeHR; (B) Fecarb vs Fepy; (C) Fepy/FeHR vs δ³⁴Spy; (D) TOC vs SRI.

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Figure 5. Microscopic features of volcanic ash in the Middle Eocene shale of the Jiyang Depression. (A) ostracod fragments in shales; (B) interbedded tuffs and shale; (C) interbedded tuffs and shale with micro fractures; (D) pyroclastic interlayer in shales.

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769

Figure 6. Sedimentary model of the paralic lake basin in the Jiyang Depression

770
771 developed in this study. (A) High primary productivity during middle Eocene long-

772 time warmth. (B) Impacts of different geological events on organic carbon burial.

773 During a short period of cooling caused by volcanism, despite potential external sulfur

774 inputs, bacterial sulfate reduction sufficiently depleted the sulfate pool to have created

775 ferruginous conditions.

776
Highlights

* The primary productivity was high during the Shahejie Formation in middle Eocene.

* Ferruginous conditions were predominant during lacustrine OC preservation.

* Multivariate geological events enhanced OC burial in paralic paleoenvironments.
1

Carbon, iron and sulfur records of lacustrine paleo-environments during the

2

middle Eocene in eastern China

3

4

**Abstract**

5

It is widely recognized that anoxic conditions facilitate the preservation of organic

6

carbon in marine sediments. However, the specific geological factors that lead to the

7

development of such conditions in paleo-lakes are less well understood. Owing to their

8

smaller size, paleolakes could experience more frequent and stronger changes in

9

geochemical conditions than oceans. Such changes, such as volcanism, hydrothermal

10

fluids, or ocean transgressions, can also strongly affect the lacustrine organic carbon

11

burial thereby complicating sediment diagenesis record. Here, we used total organic

12

carbon content (TOC), organic carbon isotope (δ¹³Corg), iron speciation, and pyrite

13

sulfur isotope (δ³⁴Spy) data to establish relationships between organic carbon

14

preservation and anoxic conditions in fine-grained sediments from the middle Eocene

15

lacustrine depositional environments from the Shahejie Formation of the Jiyang

16

Depression, Bohai Bay Basin, eastern China. The results reveal TOC between 1% and

17

10%, highly-reactive iron to total iron ratios greater than 0.38, and most TOC to total

18

sulfur ratios exceeding 2. These data indicate that the organic-rich shales of the

19

Shahejie Formation were formed as a result of high primary productivity during the

20

warm and humid middle Eocene period, coupled with the efficient preservation of

21

organic matter in anoxic bottom waters. Negative δ¹³Corg and δ³⁴Spy excursions

22

recorded in the Shahejie Formation indicate water column conditions to have been

23

influenced by transient volcanic eruptions. Positive δ¹³Corg and negative δ³⁴Spy
excursions may have been caused by hydrothermal fluids input whereas δ³⁴Sₚy values
approaching 20‰ suggest frequent marine transgressions. In particular, despite
potential inputs of S into the paleolake by volcanism, hydrothermal fluids, or marine
transgressions, bacterial sulfate reduction efficiently depleted the sulfate pool to have
created ferruginous geochemistry water conditions for the effective preservation of
organic carbon in sediments. Our results establish a direct link between lacustrine shale
geochemical signatures and geological phenomena that impact its sedimentation.

31

32 **Key words:** organic carbon burial; ferruginous conditions; volcanism; transgression;

33 paralic lacustrine shale; middle Eocene

34

1. Introduction

36 Anoxic conditions facilitate burial and preservation of organic carbon by slowing
37 down microbial degradation, primarily in marine sediments (Shen Y. et al., 2003;
38 Planavsky et al., 2011; Clarkson et al., 2014, 2016; Guilbaud et al., 2015; Raiswell et
39 al., 2018), whilst factors that cause such conditions in lakes have not been sufficiently
40 investigated. Multiple geologic events, such as volcanism, subsurface hydrothermal
41 activity, and marine water incursion, can substantially affect sediment diagenesis and
42 the burial of organic matter in both marine and lacustrine basins. Volcanic ash layers,
43 for example, are prevalent in organic-rich deposits (organic carbon content greater than
44 2.0%) such as the Bazhenov Formation (western Siberia, Russia), the Eagle Ford
45 Formation (Gulf Coast Basin, USA), the Wufeng-Longmaxi Formation (Sichuan Basin,
46 China), the Fengcheng and Lucaogou Formations (Junggar Basin, China) (Ruebsam et
al., 2020; Liang et al., 2020; Liu et al., 2021, 2024). Volcanic activity may increase biological productivity in the photic zone by supplying nutrients and affect aquatic environments and their sedimentary records by providing fluxes of heat, dissolved carbon dioxide, and methane (Duggen et al., 2010; Langmann et al., 2010; Liu et al., 2019). Enhanced preservation of organic material in sediments can lead to the formation of organic-rich shales (Kim et al., 2015; LaRowe et al., 2020). Hydrothermal and volcanic inputs, recorded by high concentrations of metals and heterogeneous tuff in shales, are correlated with changes in paleo-productivity, anoxia, and organic matter burial (Magnall et al., 2016; He et al., 2020). However, the mechanisms by which geological events affect formation and preservation of organic matter in lacustrine environments are much less constrained than in marine environments.

Like the modern counterparts, ancient lakes were important sinks of organic carbon (Sklarew, 1979; Layton-Matthews et al., 2013; Swanner et al., 2020). Being much smaller than oceans, lakes are more sensitive to environmental changes and can record geologic events on shorter time scales. Paralic lakes, in particular, can be affected by volcanic activity, hydrothermal discharges, marine transgressions or other geological events (Malumián and Ramos, 1984; Duggen et al., 2010; Lee et al., 2018; Zeng et al., 2018; Zhang et al., 2018; Liu et al., 2024), so the controlling factors for the burial of lacustrine organic carbon are more complicated. For example, the Kabuno Bay (East Africa) is ferruginous, whereas Lake Kivu is sulfidic even though they both receive iron through hydrothermal sub-lacustrine springs (Bhattarai et al., 2012; Ross et al., 2014; Swanner et al., 2020). Studying variations in lacustrine conditions of sedimentation may help understand the analogous marine processes on their
70 correspondingly longer time scales.

71 In this regard, ancient lakes of the Jiyang Depression (Fig.1) in the Bohai Bay
72 Basin, eastern China offer an excellent natural environment to investigate in detail the
73 continental sedimentary record of the middle Eocene. A series of successive lacustrine
74 shale sediments 150-300 m thick were deposited between the upper sub-member of
75 Member 4 and the lower sub-member of Member 3 of the Shahejie Formation (Es4¹
76 and Es3³) (Chen et al., 2008; Liang C. et al., 2018; Shi et al., 2019). This depositional
77 period corresponds to a global increase in the atmospheric CO2 during the Middle
78 Eocene Climate Warming (MECO) (Fig.1, A; Lourens et al., 2005; Jovane et al., 2007;
79 Zachos et al., 2008; Bijl et al., 2010). Therefore, this study combines total organic
80 carbon contents (TOC) and total sulfur (TS) values of the Fanye-1 well (FY1, Fig. 1,
81 B), with the vertical variation in Fe speciation, organic carbon isotope (δ¹³Corg), and
82 pyrite sulfur isotope (δ³⁴Spy) data, to establish a direct link between organic carbon
83 preservation and anoxic conditions, and the factors that affect organic carbon burial in
84 lacustrine basins during middle Eocene.

85

## 2. Geological Setting

86

87 The Bohai Bay Basin is a large Cenozoic sedimentary basin, which was formed
88 at the intersection of three major tectonic regions that included the ancient Asian,
89 Tethys and Pacific Oceans (Fig. 1, B). The subduction of the West Pacific plate under
90 the North China Craton was the controlling factor in the tectonic evolution rate and
91 orientation of the Bohai Bay Basin (Chen et al., 2008). Between the Late Paleocene
92 and Eocene, subduction of the Pacific Plate decreased to its lowest rate of about 40

89

91

92
93

mm/a at 43 Ma, and the orientation changed from north-north-west to north-west-west.

94

The rate gradually increased to 60 mm/a from 43 to 32 Ma, making the entire Asia
continent an extended tectonic domain. This spreading reached its faulting limit at
95
96 about 23 Ma in the Oligocene leading to the formation of the Late Mesozoic-Paleogene
97 Bohai Bay Basin, which is divided into seven depressions and three uplifts (Chen et al.,
98 2008).

98

99

Among them, the Jiyang Depression is the largest sedimentation center of the
basin, spanning latitudes from 35 to 40°N. The depression is about 200 km long from
east to west and 120 km wide from north to south, with a total area of 26,000 km² (Song
et al., 2020). Cenozoic deposition during Es₄¹ and Es₃³ sub-members in the Bohai Bay
Basin coincided with an intensive fault depression, resulting in the formation of the
Jiyang Lake whose estimated ancient depth was 190-290 m (Fig. 1., C) with the
deepest area likely reaching 600 m (Chen et al., 2008). The lake has existed in the basin
for about 4 million years and was connected to the west Pacific paleo-ocean during
multiple periods, lasting for several thousand years each (Song et al., 2020). Between
these episodes, the lake basin likely remained closed (Chen et al., 2008).

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103

104

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108

109

During Es₄¹ time, as the basin subsided, the lake experienced high salinities
affected by different geological processes. During the Es₃³ period of humid climate, as
the lake basin widened and subsidence increased, the lake connected to the ocean, thus
increasing accommodation space. Abundant semi-deep to deep waters (10-60 m depth)
provided favorable conditions for deposition of fine-grained sedimentary rocks such as
black shale. The rocks were predominantly argilitic with extensive layers of both
massive and lamellar textures and with veins of sparry limestone, limy mudstone, and

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116 dolomite (Chen et al., 2008; Liang C. et al., 2018; Song et al., 2020).

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118 **3. Materials and Methods**

119 Core analysis shows that the high-quality source rocks of Es₄¹ and Es₃³ (to ~1000 m) were widely distributed in semi-deep or deep lacustrine facies, intercalated with mm-to-cm think layers of volcanic ash in Jiyang Depression. Here a length of 403.6 m 120 of whole-core was taken from the FY1 well (Shi et al., 2019; Song et al., 2020). Black 121 and gray-black, layered, grainy, muddy shale dominates the Es₄¹ and Es₃³ lithologies. 122 A total of 270 core samples was obtained at a spacing of 1.0–1.5 m and these were 123 analyzed for TOC, TS, δ¹³Cₐrg and major element analyses. A subset of 70 samples at 124 an interval of 5–6 m was subjected to iron speciation and δ³⁴Sₚy analyses. 125 126

127 The concentrations of TOC and TS were determined using an Eltra infrared (IR) 128 C/S analyzer from the discrepancy between the total carbon (or sulfur) determined by 129 combustion and the total inorganic carbon (or sulfur) determined by acidification. 130 Ultrapure 6 N HCl was added in a silver cup to dissolve inorganic carbon and sulfur 131 (mostly carbonate and sulfide minerals) from weighed portions of powdered samples. 132 Analyses of δ¹³Cₐrg were performed using a Costech™ 4010 coupled with a Thermo 133 Finnigan MAT 253 via an open-split interface Conflo IV, with a reproducibility better 134 than ±0.1‰ as based on the international standard IAEA-600 (caffeine, δ¹³Cₐrg = - 135 27.77‰). 136

136 For major element concentrations (Fe, Al), 300 mg of powdered samples were 137 burnt at 600 °C for 12 h in a muffle furnace to remove volatiles compounds, followed 138 by a standard multi-acid (HF-HCl-HNO₃) digestion protocol designed for dissolution
in a Teflon bomb. An Agilent 7700 inductively coupled plasma mass spectrometer (ICP-MS) was used for the solution analyses, with an analytical precision of better than ± 5% (1σ) established using USGS standards (BHVO-2 and BCR-2).

Iron speciation and pyrite sulfur isotope analyses (in terms of δ³⁴Sₚy) were performed for a total of 70 samples at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan). Iron as pyrite (Feₚy) was extracted using the chromium reduction method and iron speciation was determined by the sequential extraction method (Poulton and Canfield, 2005). Sulfur in pyrite (Sₚy) was extracted as H₂S using a hot CrCl₂ distillation for 2 h, trapped as silver sulfide (Ag₂S), which was then quantified gravimetrically. The Feₚy content in each sample was then calculated assuming the ideal pyrite stoichiometry (FeS₂). Iron from carbonate minerals (Feₐcarb) was extracted by dissolution for 24 h in cold 10 wt% HCl. Then ferric Fe (oxyhydroxides, Feₐox) was liberated from the residue of each sample, treated for 2 h with a sodium dithionite solution (50 g/L) buffered to a pH of ~5 with acetic acid and sodium citrate. Finally, Fe bound in magnetite (Feₐmag) was extracted for 6 h using a 0.2 M ammonium oxalate and 0.17 M oxalic acid solution (pH ~3). After diluting the extracts by a factor of 100 in 2 wt% HNO₃ (suprapure), an Agilent 7500ce ICP-MS was used to analyze each sequential extract. To increase precision, samples underwent processing in conjunction with internal calibration standards and blanks, by analyzing in duplicate each 10th sample per batch. The analytical precision was better than 5% of the value.

Sulfur isotope ratios (³⁴S/³²S) were determined using a Thermo Fisher Scientific Delta V Plus isotope-ratio mass spectrometer coupled with a flash EA. The results are
expressed as $\delta^{34}$S$_{py}$ following the standard delta notation as per mil deviations relative to the Vienna Cañon Diablo Troilite (Li et al., 2021). An analytical uncertainty of 0.2‰ (1σ) for pyrite $\delta^{34}$S was determined from replicate analyses of the International Atomic Energy Agency (IAEA) standards: S1 ($\delta^{34}$S = -0.3‰), IAEA S2 ($\delta^{34}$S = +22.65‰), and IAEA S3 ($\delta^{34}$S = -32.5‰).

## 4. Results

The results of geochemical analyses of FY1 section are summarized in Table 1. The variation in chemical and isotopic composition of TOC, $\delta^{13}$C$_{org}$, TS, $\delta^{34}$S$_{py}$ and Fe redox as a function of depth is presented in Fig. 2. The study interval was divided into 7 units according to the changes in $\delta^{13}$C$_{org}$ and $\delta^{34}$S$_{py}$ values.

### 4.1. Organic carbon systematics

TOC values of the 270 samples vary from about 1 to 10 % with an average value close to 4%, and with maximum enrichment in Units 1 and 5 (Fig. 2), but without systematic differences between $Es_3^3$ and $Es_4^1$. The $\delta^{13}$C$_{org}$ values for samples of $Es_4^1$ vary within a relatively narrow range of about -29‰ to -20‰ with an average of ~ -26‰, whereas those from $Es_3^3$ vary within even narrower range of about -28‰ to -23‰ with a mean value -26‰ (Table 1). Samples with positive excursions of $\delta^{13}$C$_{org}$ values (> -24‰) are more frequent in $Es_4^1$, and are characterized by elevated TOC contents (> 4%). Samples showing more negative $\delta^{13}$C$_{org}$ (≤ -26‰) are also predominantly found in the $Es_4^1$ sub-member (Fig. 2A, B).

### 4.2. Iron systematics
In the lacustrine samples of Es₄¹ and Es₃⁴, Fe_T values range between 1.4 and 8.0%, with an average close to 3% and are generally dominated by highly reactive iron (Fe_HR, including pyrite and carbonate iron, oxidized and magnetite iron) fraction (> 90% of Fe_T on average). Pyrite iron (Fe/py) ranges from 0.1 to 4% with an average of ~1%, which is slightly less than or comparable to carbonate iron (Fe_carb) content varying from 0.5 to 4 (average 1.5%). Oxidized iron (Fe_ox) is low, with a maximum of 0.4%. The Fe/Al ratios are systematically higher than 0.44 (with a single sample exception of 0.18 at 3346.9 m). Magnetite content (Fe_mag) is low (< 0.13%), pointing to the absence of digenesis; as a result, the iron speciation can be used to constrain the redox conditions. All Fe_HR/Fe_T values exceed 0.38, and almost all Fe py/Fe_HR ratios are lower than 0.6, which is mostly due to the relatively high Fe_carb content.

Iron speciation is a widely used proxy for water column redox conditions, and can differentiate between oxic and anoxic (ferruginous or euxinic) environments (Poulton and Canfield, 2005, 2011). For anoxic conditions, Fe py/Fe HR ratios are used to distinguish ferruginous (typically < 0.7) and euxinic (typically > 0.8) conditions; however, more recent work suggested values above 0.6 as evidence of euxinia (Poulton, 2021). Under anoxic conditions, either diagenetic transformation of Fe HR minerals into Fe-rich clay, or the swift sedimentation of deposits lacking iron, can lower Fe HR concentrations. Total Fe/Al ratios provide complementary information on water column redox conditions. Indeed, although oxidative weathering or digenesis of samples could obscure Fe py systematics, it would not significantly alter the Fe/Al ratios that are systematically lower than 0.44 in oxic marine sediments (Lyons and Severmann, 2006; Clarkson et al., 2014, 2016; Raiswell et al., 2018). Therefore, the
Fe/Al proxy in carbonate-rich sediments is relatively robust, provided $Fe_T$ > 0.5 wt% (Raiswell et al., 2018). For $Fe_T$ < 0.5 wt%, carbonate samples have a greater potential to be enriched in $Fe_{HR}$ due to processes other than water column processes that arise under anoxic conditions (e.g., post sedimentary diagenesis).

Overall, TOC values higher than 0.5%, with total iron > 0.5 wt%, $Fe_{HR}/Fe_T$ ratios > 0.38 and $Fe_{HR}/Fe_T$ ratios < 0.8 throughout the succession (Fig. 2, A, E, F) collectively demonstrate predominantly anoxic ferruginous during the depositional period, despite evidence of pyrite enrichment of considerable spatial and temporal variability that cannot exclude local intermittent euxinic conditions in the basin.

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### 4.3. Sulfur systematics

Total sulfur (TS) concentrations vary from about 0.2 to 11%, with an average of ~1.4% across the two main intervals (Table 1, Fig. 2). Most samples (231) have TOC/TS weight ratios above 2, with only a few (36) having TOC/TS < 2 (Fig. 2A, C). According to Berner and Raiswell's (1984) analysis of modern sediments, TOC/TS values of < 2 would correspond to euxinic conditions, those between 2 and 3.6 represent "normal" marine conditions, those of 3.6 to 10 reflect freshwater and seawater transition zones, and those > 10 correspond to freshwater sediments. Therefore, most of our samples from the study interval would indicate freshwater or marine transitional conditions, with only rare instances of euxinia (Fig. 3).

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Values of $\delta^{34}S_{py}$ exhibit large variations within the studied section (Fig. 2D), ranging from 9‰ to 36‰ with an average of ~ 20‰ with no significant differences between $Es_3^3$ and $Es_4^1$ (Table 1). As sulfur isotope values in marine sulfates were ~ 22 ‰
during middle Eocene (Payton et al., 1998) and elevated δ³⁴Sₚy values (> 20‰) are
consistent with the low levels of sulfate in the euxinic water column (Shen et al., 2003),
our large δ³⁴Sₚy variations (Fig. 2D) indicate that redox conditions fluctuated
considerably over time. However, despite the large environmental fluctuations in
Jiyang Lake during the middle Eocene, fine-grained sediments were formed during the
early stage of diagenesis in a closed system and were almost unaffected by late
digenesis, which was consistent with very limited influence of weathering and
metamorphism that could affect such sediments (Liang C. et al., 2018). This viewpoint
is supported by the fact that interbedded carbonates and mudstones have consistently
yielded similar redox interpretations across all instances of rock samples (Raiswell et
al., 2018). Generally, the unusually high dolomite content in fine-grained sediments
should indicate rather atypical lacustrine chemistry due to volcanism, hydrothermal
fluids, or transgressions that may affected variability in calcium carbonate saturation
leading to dolomite rather than calcite precipitation (Zhang et al., 2018; Liu et al., 2021).

# 5. Discussion

## 5.1. High primary productivity in middle Eocene warmhouse

Compared to the unmineralized fraction in deep sediments of modern large lakes
and marine sediments (Galazzo et al., 2013; Liu et al., 2019, 2021), our high TOC
values (average 4%) may indicate that the primary productivity of the lake over the
entire interval (Es<sub>4</sub><sup>1</sup> to Es<sub>3</sub><sup>3</sup>) was indeed higher than that during other MECO intervals
(1–3% TOC; Galazzo et al., 2013). Algae-rich sediments worldwide are well-known
to be a product of high primary productivity (Meyers, 1997). In Jiyang paleolakes,
algae-rich sedimentary layers, formed by alternating blooms of coccolithophytes and dinoflagellates, were a major feature of the Paleogene oil source rocks in the Bohai Bay Basin for more than 1 Ma (Xie et al., 2016; Song et al., 2020; Shi et al., 2021). These algal blooms are well recognized in the middle Eocene warming period of the Cenozoic (Bijl et al., 2010; Zachos et al., 2008; Pearson, 2010), which occurred 41.4–39.2 Ma ago (Shi et al., 2019). During this time, a warm climate and associated high productivity were favorable for oxygen consumption beneath the photic zone. The water column became anoxic, thereby leading to enhanced preservation and burial of organic matter. High productivity can be supported by systematically elevated concentrations of both TOC and barium (Zeng et al., 2018). TOC values above 6% are recorded here over 11 intervals across the depth range 3343.47 to 3102.67 m, with two intervals having TOC values as high as 8.4% and 10.4% at 3176.51 m and 3192.90 m, respectively (Fig.2). The average Ba concentrations in $Es_4^1$ and $Es_3^3$ are 470 and 560 ppm, respectively. Over the entire section in the Jiyang Depression, Ba values in samples positively correlate with the corresponding TOC values (Supplementary Table 2). For comparison, in a similar high paleoproductivity Lake Malawi (East Africa), the TOC concentrations are ~ 6 % in the sediments from the anoxic deep parts of the lake (Li et al., 2018).

Meanwhile, high primary productivity can also be confirmed by organic carbon isotope values. Most samples show $δ^{13}C_{org}$ values between -29‰ and -25‰, indicating a source of lacustrine organic matter from exogenous terrestrial plants, authigenic phytoplankton photosynthesis, as well as possible allochthones (Meyers, 1997). Terrestrial C4 plants were very rare in the Jiyang Depression during the middle Eocene
as they are usually found in dry environments and have the δ¹³Corg values higher than -20‰. However, there might have been some C3 plants in the lake, because small fragments of higher plants (including leaves) can be windblown and floating debris could sink, although some of the compounds of terrestrial origin may have been subsequently degraded by diagenesis. In the detailed core observations of the FY1 well, over a total length of 403.6 m, no fragments of higher plants were observed. Generally, there is no direct relationship between the concentration of biomarkers and the source of organic matter because many biomarkers can be detected at very low concentrations.

There was probably some retene, oleanene, or β-carotene (Xu et al., 2020); however, the contribution of higher plants to the preserved organic matter is expected to be small.

For another, the relative content of the biomarkers may also change further with diagenesis. For example, Xu et al. (2020) also proposed an existing algae source of retene, oleanene, or β-carotene during organic-rich shale formation. To sum up, the contribution of organic matter of terrestrial origin from higher plants depends on the lake size and sedimentation process. In a very large paralic lake continental runoff input can be significant only due to strong storms, floods or other events from land. As there were no land events in the Jiyang Depression with a total area of 26,000 km², the detected compounds of higher plants do not mean that the terrestrial organic input is significant in this offshore lake.

Combined with the sedimentary characteristics of modern lakes, we propose that in the center of the paralic Jiyang lake like FY-1 Well, the organic matter may mainly come from lacustrine authigenesis, the input of terrigenous organic matter, such as plants, may only have occurred in brief periods. Of the studied 270 samples, only a few
(25 in Es₃³ and 27 in Es₄¹) showed δ¹³Cₐrg values above -25‰, with an average TOC of
5% (Fig.2), indicating that during this period organic matter may have been
occasionally derived from very limited amount of terrestrial plants. Thus, most of the
organic matter enrichment in the source rocks of the Shahejie Formation likely resulted
from native algal and bacterial production coupled with favorable preservation
conditions. Due to a globally significant increase of greenhouse gases in the MECO
atmosphere (Bijl et al., 2010), photosynthetic carbon fixation could lead to carbon
isotope ratios as low as -29‰ (Hodell and Schelske, 1998). Therefore, we propose that
the negative excursions of δ¹³Cₐrg corresponded to high primary productivity during
warm and humid climate caused by periods of elevated atmospheric pCO₂. Under these
elevated pCO₂, CO₂ was predominantly transported into lacustrine systems via
photosynthesis, and was reduced by biological processes into organic matter with
enrichment in the lighter isotope (¹²C), thereby yielding negative excursions of δ¹³C
values.

**5.2. Ferruginous/euxinic water conditions**

Lallier-Verges et al. (1993) quantified the degree of bacterial sulfate reduction (BSR) using the sulfate reduction index (SRI), which is defined as the ratio of primary organic carbon to residual organic carbon. Primary organic carbon encompasses both the organic carbon depleted via sulfate reduction, and the residual total organic carbon (TOC (wt.%) at present). Organic carbon loss (C<sub>loss</sub>) by degradation coupled with sulfate reduction may be assessed from the stoichiometry of the sulfate reduction equation proposed by Berner and Raiswell (1984):
$$323 \quad SRI = (TOC + C_{loss})/TOC \quad (1)$$

$$324 \quad \text{where } C_{loss} = TS/1.33 \quad (2)$$

Substituting (2) into (1) yields:

$$326 \quad SRI = (TOC + TS/1.33)/TOC = 1 + 0.75 \times TS/TOC \quad (3)$$

The SRI is regarded as the lowest degradation consumption index for total organic carbon. The higher the BSR activity, the higher the SRI value, and the greater the organic matter consumption. For SRI < 1.375, the BSR intensity can be limited by total sulfate and the amount of undegraded TOC may increase substantially. For SRI > 1.375, the BSR intensity increases, and the overall TOC value is relatively low (Liu et al., 2021). The relationship between the sulfate reduction strength and the organic carbon content (SRI/TOC) in the study interval shows that most of the SRI values in the ferruginous water column are less than 1.375 (Fig. 4 (D)), indicating a weak sulfate reduction, which may have been conductive to organic matter preservation and burial. In contrast, euxinic waters with strong BSR may result both in organic preservation and consumption. It can be seen in Fig. 3 and 4 (D) that in the euxinic zone, most samples with TOC < 2 % have SRI values > 1.375.

Several mechanisms may have created ferruginous conditions at the bottom of the Jiyang lake. A combination of volcanism, hydrothermal fluids, and transgression may have brought reactive iron and maintained anoxic conditions, while the primary productivity was high. For example, volcanism could have increased nutrient influxes (Liu et al., 2019, 2024), and productivity, and stimulated oxygen consumption below the photic zone. Meanwhile, the isotopically “light” sulfur ($\delta^{34}S_{py}$ of 0 ‰) from a volcanic source was preferentially utilized by BSR, resulting in low $\delta^{34}S_{py}$ values.
However, intermittent sulfidization events in a ferruginous Jiyang Lake may not have led to elevated Fe<sub>py</sub>/Fe<sub>HR</sub> ratios compared to typical marine values of 0.7, because such an increase may be obscured by the high carbonate (Fe<sub>carb</sub>) content largely dominating over Fe<sub>py</sub> (Fig. 2, Unit 2, 3, 5). For all the samples, there are only two Fe<sub>py</sub>/Fe<sub>HR</sub> values above 0.7, with the corresponding δ<sup>34</sup>S<sub>py</sub> values of 24–25‰) (Fig. 4 A, C). The two δ<sup>34</sup>S<sub>py</sub> values, higher than those in the lower adjacent interval (~13 ‰), indicate that the basin became relatively closed and euxinic after a period of volcanism or hydrothermal activity. The positive δ<sup>34</sup>S<sub>py</sub> values (> 20‰) are higher than the sulfate <sup>34</sup>S from seawater during middle Eocene, indicating the depletion of the light sulfur isotopes in sulfate under strong BSR. Meanwhile, the TOC values for these two samples are 3.4 and 2.8% at 3332.39 and 3226.24 m, respectively (Fig. 2), which are less than the average value (4.0%) of the study interval, indicating that BSR consumed the primary organic matter and converted sulfate to H<sub>2</sub>S, and promoted pyrite formation. A rough positive covariation between the Fe<sub>py</sub>/Fe<sub>HR</sub> ratios and TOC also supports the BSR occurrence (Fig. 4 A, B). However, the high TOC/TS ratios (> 2.0) and low sulfate reduction index (SRI) (< 1.375) suggest that it was sulfate availability, rather than organic matter supply, that constrained BSR in deep waters (Fig. 4 (D)).

For Jiyang Lake, there may have been occasional euxinic conditions as the TOC/TS values are less than 2 (in 36 samples from 270). Euxinia in the photic zone (from biomarker data) and deeper water (based on framboid size distributions) has been identified in Silurian, Devonian, Permian and Eocene oceans (Racka et al., 2010; Marynowski et al., 2012; Liu et al., 2019; Xu et al., 2020; Percival et al., 2022). Euxinia is prevalent in anoxic marine environments where sulfate is abundant. Modern
meromictic lakes develop euxinia when sulfate concentrations exceed 100 µM, while below that threshold their monimolimnia (the deep stagnant layers) tend to be ferruginous. Xu et al. (2020) demonstrated that there were different euxinic water conditions during the deposition of the Shahejie Formation, evidenced by variations in the aryl isoprenoid ratio, relative amount of isorenieratane, long-chain n-alkane carbon isotopic composition and other molecular indicators, which indicated different intensity of bacterial sulfate reaction (BSR). In Jiyang Lake, potential sources for extra sulfate could have been episodic (for 5–10 ka) marine transgressions (Unit 2 and Unit 5), volcanism (Unit 1, 2, 3 of Es₄¹, and Unit 5 of Es₃³) or hydrothermal fluids (Unit 4 of Es₃³) (Fig. 2). Outside these episodes, sulfate would become depleted by BSR, Feₚy/FeₘR values were mostly < 0.6 and FeₘR/Feₜ values > 0.38 (Fig. 4 A), which also suggests that the deep-water column throughout the middle Eocene was predominantly ferruginous. Meanwhile, there might also be very occasional oxic episodes identified by the presence of bioturbation in anoxic brackish water (Song et al., 2020). Although Fe shuttling could potentially create local enrichments of Feₐₚ₄ (Fig. 4 B) or Feₐₓ, resulting in high FeₘR/Feₜ and low Feₚy/FeₘR ratios, the deep lake was probably anoxic and ferruginous during most of the depositional period. The reduced Fe transported from the anoxic water column could have led to Feₐₚ₄ enrichments in bioturbated sediments. As anoxic ferruginous conditions expanded and contracted over time, reactive iron enrichment could form in even deeper settings.

In summary, the FeₘR values between 1.2% and 6.6% generally show the same evolution trend as the total of Feₚy and Feₐₚ₄, while Feₐₓ and FeₘR have little correlation with the FeₘR content trend. Thus, organic matter enrichment was primarily created by
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both enhanced preservation under ferruginous water conditions and high paleo-
productivity with limited dilution of the shale by terrigenous clastic materials (Xie et
al., 2016; Song et al., 2020).

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**5.3. Impacts of geological events on organic carbon burial**

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Our core analyses demonstrate that volcanism, hydrothermal activity, and marine
transgressions affected the study area. Only few investigations of carbon and pyrite
sulfur isotopic variations focused on lacustrine sedimentary intervals during the MECO.
Our study uses different covariations between δ³⁴Sₚy and δ¹³Cₐrg values (Fig. 2B, D) to
establish a relationship between geological events, biological productivity, and water
redox conditions. It has been shown that a negative excursion of 2 to 6‰ in δ¹³Cₐrg of
both carbonate and organic carbon is linked to global volcanism (Lee et al., 2018; Liu
et al., 2019; Longman et al., 2019; Shen J. et al., 2019b; Li Y. et al., 2021). Indeed, in
the study intervals, volcanism effects are evident in nine prominent negative excursions
in δ¹³Cₐrg (Fig. 2B), which are correlated with tuff interlayers (Fig. 5). The pyroclastic
interlayers in the shales are predominantly vitreous and crystalline debris of andesite
(Fig. 5 (C, D)), in which microfractures were developed and ostracod fragments were
deposited. Such negative excursions in δ¹³Cₐrg values are typically associated with
increases in primary productivity, which can be brought about by the increased delivery
of nutrients following the volcanic events.

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The same layers with highly negative δ¹³Corg are also associated with negative
shifts in δ³⁴Sₚy (as low as 8.9‰; Fig. 2D). The water column in the deep lake was
usually closed when there were no geological events. In this case, the SO₄²⁻ reservoir
is limited and could not be replenished. With the continuing exhaustion of the sulfate

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reservoir (enriched in ³⁴S) by sulfur reduction through dissimilation, ³⁴Sₚy gradually increased and approached that of the initial sulfate. In contrast to open marine sediments, ³⁴Sₚy values in an almost infinite sea water sulfate reservoir, are usually below 0‰ (Li et al., 2021). Volcanic and hydrothermal activity generate sulfur with δ³⁴S of 0 to 5‰ (Huston, 1999), which is substantially below the values of water surface-sourced sulfate (~20‰ during study period). If sub-lacustrine eruptions of a volcano or associated geothermal fluids carried H₂³²S-enriched sulfide sulfur into a closed lake, its reaction with highly reactive iron could form isotopically equivalent Fe³²S₂-enriched pyrite within a quite short period of time (i.e. several years). This process caused the decrease in δ³⁴Sₚy values along with the sulfate of the light sulfur isotope was consumed by BSR. As volcanic H₂³²S was quickly consumed by iron and the BSR began to reduce sulfate in a closed system, the resulting pyrite had lighter δ³⁴S values. Higher δ³⁴Sₚy values then formed again at low sulfate concentrations at the end of sulfate reduction. If volcanic activities or deep fluids carried large amounts of oxidized volatiles, such as SO₂, BSR would have been enhanced and more ³²S consumed. As a result, ³⁴Sₚy first decreased and then increased with an increased amount of sulfate consumption. Indeed, this trend could be identified especially in Units 1 and 3 of Es₄¹ (Fig. 2D). Because the bottom waters of the paleolake seemed to have remained ferruginous, the iron input (as Fe (II); Isley and Abbott, 1999; Kump and Seyfried, 2005) during these volcanic or hydrothermal activities must have exceeded the inputs of sulfur into the lake.

The presence of marine ostracods (Fig 5 (A)), elevated δ³⁴Sₚy values (~20 ‰) and high carbonate content (Feₐ₄b to ~5%) collectively indicate marine transgressions,
which is consistent with the interpretation of the strata using the method of
astronomical cycles (Ma et al., 2023). During middle Eocene warming, marine
transgressions increased the lake water depth and supplied <sup>34</sup>S-enriched sulfate,
causing the δ<sup>34</sup>S<sub>py</sub> value of pyrite to approach that of seawater. The δ<sup>34</sup>S<sub>py</sub> values vary
between 9 and 36 ‰, being both lower and higher than those of the marine sulfates
identified during the MECO (20.9 ± 0.5‰; Longinelli, 1989; Kampschulte et al., 2001).
Marine transgressions may have occurred roughly every 1,000 years over a period of
5,000 years (Fig. 2, Units 2 and 5). As BSR generates isotopically light sulfide, the
heavier δ<sup>34</sup>S<sub>py</sub> values can be attributed to the consumption of sulfate sulfur after staged
transgression when the lake system closed again. Mineral analyses show that the total
content of Fe<sub>mag</sub> and Fe<sub>ox</sub> is low, with a maximum of 0.37%, so the contents of ferrous
carbonate and pyrite compete with each other. This is consistent with most sediments
in modern paralic environment (Liu et al., 2023; Zhang et al., 2024). Only the
difference between the samples of the two study areas is the contents of ferrous
carbonate, indicating the different water conditions during their sediment deposition.
In Shahejie Formation, the carbonate content in the study interval was generally high,
with average calcite and dolomite contents of 37% and 10%, respectively. For example,
the content of Fe<sub>carb</sub> is higher than that of Fe<sub>py</sub> from 3225 to 3200 m in Unit 5 (Fig.2),
indicating that the sulfur isotope fractionation of pyrite in the paralic lakes during the
sulfate reduction process was constrained by changes in the global sea level. The sea
level increased in the warm and humid climate, and transgression brought large
amounts of Ca<sup>2+</sup> to form carbonates. Meanwhile, high paleo-productivity also
promoted the deposition of organic-rich layers. Therefore, the precipitation of calcium
carbonate and organic matter layers is the result of increased pH due to strong photosynthesis by algae in the warm climate during marine transgressions in the Jiyang calcareous lake. After that, the organic matter was degraded in a limited manner under ferruginous conditions and was well preserved.

The delicate equilibrium between ferruginous and euxinic chemical conditions could be influenced by the comparable rates of Fe<sub>HR</sub> and sulfate input fluxes. On a global scale, continental sources stand out as the primary contributors to the potential Fe<sub>HR</sub> flux. At the regional scale, geological events, such as volcanic activity or ocean transgression, could also have liberated some reactive iron and sulfate into the lake, providing a conceivable mechanism for the transportation of ferrous iron and resulting in a disproportionate increase in the Fe<sub>HR</sub> flux relative to that of sulfate. The interplay of all these processes have generated dominantly ferruginous environments during the middle Eocene in the Jiyang Depression.

On the basis of the aforementioned research, we propose that there was an extensive burial of lacustrine organic carbon in Eastern China during the warm and humid climate of the middle Eocene, which thereby created a positive feedback loop in the Earth's climate system. Frequent volcanic activity during the early stages of Es<sub>4</sub><sup>1</sup> might have brought aerosols to increase the albedo in the short-term cold climate (Fig.6B, maybe to 10 years). Most importantly, it released abundant CO<sub>2</sub> into the atmosphere and brought reactive iron and nutrients to create optimal environments for the growth and expansion of algae and plankton in the later stages (Fig.6A, to 1 million years) of the middle Eocene. As temperatures increased, the biological productivity in Jiyang Lake increased, leading to enhanced production of total organic carbon in
ferruginous water conditions. This positive feedback mechanism may have contributed to the sustained warmth of the middle Eocene climate. Based on this analysis, we highlight a combined effect on sedimentation of external events such as input of volcanic ash, intermittent hydrothermal fluids and marine transgressions during middle Eocene continental warming of western Pacific. These events could not only supply nutrients and catalytic elements to promote biological productivity and cause oxygen consumption, but they also enhanced the reactive iron input to form the dynamic anoxic ferruginous conditions that were conducive for the preservation of organic carbon.

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## 6. Conclusions

The widespread terrestrial sedimentary and geochemical records of the middle Eocene in the Eastern China have been established using novel TOC, TS, iron speciation, $\delta^{13}C_{org}$ and $\delta^{34}S_{py}$ data. These data, including the highly-reactive iron to total iron ratios ($Fe_{HR}/Fe_{T} > 0.38$), along with a wide range of total organic carbon content (TOC, from 1 to 10 %) exceeding that of total sulfur (TOC/TS > 2), and low sulfate reduction indexes (SRI < 1.375), collectively suggest widespread anoxic and ferruginous conditions that would have been favorable for burial of lacustrine organic carbon during the middle Eocene in the paralic lacustrine environments of the Jiyang Depression. The temporal stability of lacustrine ferruginous water conditions may have been due to the warm and humid climate during the middle Eocene, with high biological productivity and large oxygen consumption in the water column, leading to efficient burial of organic carbon in deep sediments.

494

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497

498

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500

501

502

503

504

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506

Both $\delta^{13}C_{org}$ and $\delta^{34}S_{py}$ values reveal transient geological events such as volcanism,
hydrothermal fluids, and transgressions. Negative $\delta^{13}C_{org}$ and $\delta^{34}S_{py}$ excursions along the sedimentary column point to transient volcanic events, whereas positive $\delta^{13}C_{org}$ along with negative $\delta^{34}S_{py}$ excursions indicate injection of associated hydrothermal fluids; in contrast, elevated $\delta^{34}S_{py}$ (to ~20‰) point to frequent marine transgressions. Despite potential inputs of sulfur into the paleolake as a result of these geological events, bacterial sulfate reduction efficiently consumed the sulfate pool, thereby creating ferruginous water conditions favorable for the efficient preservation of organic carbon.

## Acknowledgments

We thank Yunqing Hao for assistance with microscopic slice analyses and for discussions, Prof. Chao Li and Prof. Yan'an Shen for help with the geochemical analyses, Prof. Bing Shen and Dr. Huiyuan Xu for discussions. This work was supported by the National Natural Science Foundation of China (Grants 42172151, 41811530094, and 41625009), the China Postdoctoral Science Foundation (Grant 2021M690204), and the National Key Research and Development Program (Grant 2023YFF0806200). We thank the two reviewers and the editorial support for the constructive comments, which have greatly improved the quality of this paper.

## Research Data

All of the processed data discussed has been uploaded in the supplementary tables.

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Table and Figure captions

736
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738
739

Table 1. Summary data of total organic carbon (TOC), organic carbon isotope composition (δ¹³Corg), total sulfur (TS), pyrite sulfur isotope (δ³⁴Spy), Fe/Al ratio, and iron speciation of samples in the FY1 Well (data range, arithmetic average and number of samples, see Supplementary Table 1 for data points).

740

741
742
743
744
745
746

Figure 1. (A) Map of sites with existing MECO records on a paleogeographic reconstruction for the middle Eocene at 40 Ma (modified from Ploeg et al., 2023). (B) Simplified geological map of Bohai Bay Basin in western Pacific, showing the core location of FY-1 Well in the Jiyang Depression in eastern China (modified from Liang et al., 2016). (C) Stratigraphic column of the study interval in the middle Eocene. The Es₄¹ and Es₃³ sediments are deposited during the rifting stage over 38-50 Ma ago
(modified from Shi et al., 2019).

748

Figure 2. Iron speciation, δ¹³Corg, TOC, and δ³⁴Spy values as a function of depth from the middle Eocene (Es₄¹⁻Es₃³) black shales in the Jiyang Depression (geological timescale from Shi et al., 2019). (A) total organic carbon (TOC), (B) δ¹³Corg, (C) total sulfur content (TS), (D) pyrite sulfur isotope (δ³⁴Spy), (E) ratio of highly reactive Fe to total Fe (FeHR/FeT), (F) ratio of pyrite iron to highly reactive iron (Fepy/FeHR), (G) content of pyrite iron, (H) content of carbonate iron. Note that FeHR = Fepy+Feox+Fe mag +Fe carb, the thresholds for oxic versus anoxic (0.22–0.38) and ferruginous versus euxinic conditions (0.6–0.8) are from Poulton et al. (2021); the units of the study interval are based on lithological and geochemical differences, as discussed in the text.

749

750

751

752

753

754

755

756

757

758

759

Figure 3. Plots of TOC vs TS, showing that the salinity of the most samples was similar to the salinity of seawater, corresponding to ferruginous water conditions.

760

761

762

Figure 4. Plots of (A) FeHR/FeT vs Fepy/FeHR; (B) Fecarb vs Fepy; (C) Fepy/FeHR vs δ³⁴Spy; (D) TOC vs SRI.

763

764

765

Figure 5. Microscopic features of volcanic ash in the Middle Eocene shale of the Jiyang Depression. (A) ostracod fragments in shales; (B) interbedded tuffs and shale; (C) interbedded tuffs and shale with micro fractures; (D) pyroclastic interlayer in shales.

766

767

768

769

Figure 6. Sedimentary model of the paralic lake basin in the Jiyang Depression

770
771 developed in this study. (A) High primary productivity during middle Eocene long-

772 time warmth. (B) Impacts of different geological events on organic carbon burial.

773 During a short period of cooling caused by volcanism, despite potential external sulfur

774 inputs, bacterial sulfate reduction sufficiently depleted the sulfate pool to have created

775 ferruginous conditions.

776
<figure><img src="image_4.png" /></figure>

<table><thead><tr><th colspan="2">Paleogene</th><th>Neogene</th><th>Period</th><th>Stage</th><th>Stage</th><th>Member</th><th>Thicknes<br>(m)</th><th>Lithology</th></tr></thead><tbody><tr><td rowspan="11">Eocene</td><td rowspan="11">Shale<br>(m)</td><td rowspan="11">Oligocene</td><td rowspan="11">Denby<br>(m)</td><td rowspan="11">Ganesh<br>(m)</td><td rowspan="11">Majestic<br>(m)</td><td>Nm</td><td rowspan="11">100-450</td><td rowspan="11"></td></tr><tr><td>Nm</td></tr><tr><td>Nm</td></tr><tr><td>Nm</td></tr><tr><td>Ed<sub>1</sub></td></tr><tr><td>Ed<sub>2</sub></td></tr><tr><td>Ed<sub>3</sub></td></tr><tr><td>Ed<sub>4</sub></td></tr><tr><td>Ed<sub>5</sub></td></tr><tr><td>0-450</td></tr><tr><td>0-350</td></tr><tr><td>700-1200</td><td rowspan="2">Study<br>Marine</td><td rowspan="2"></td></tr><tr><td>0-1500</td></tr><tr><td>0-100</td><td></td><td></td></tr><tr><td>0-30</td><td></td><td></td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_6.png" /></figure>

Brackish-salt water,
ferruginous conditions
("normal marine")
<figure><img src="image_7.png" /></figure>

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

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

<figure><img src="image_10.png" /></figure>
<figure><img src="image_11.png" /></figure>

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

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

<figure><img src="image_14.png" /></figure>
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>
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**Supplementary Material**

Supplementary Table 1+2.xlsx
**Declaration of interests**

[x] The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[ ] The author is an Editorial Board Member/Editor-in-Chief/Associate Editor/Guest Editor for *Journal name* and was not involved in the editorial review or the decision to publish this article.

[ ] The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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