Comptes Rendus
Géoscience
Sciences de la Planète

Anastasia Chupryna, Jeremie Aubineau, Olabode Bankole, Ernest Chi Fru, Kurt Konhauser, Viktor Nesterovsky, Andreji Martyshyn, Yevheniia Soldatenko, Bruno Becker-Kerber, Pierre Sans-Jofre, Ibtissam Chraiki, Fatima Abdelfadel, Claude Fontaine, Claude Laforest, Alain Meunier and Abderrazak El Albani

Strong interplays between eustacy, nitrogen dynamics, and biodiversification
between the Ediacaran and Silurian interval

Volume 357 (2025), p. 201-223

Online since: 16 June 2025

https://doi.org/10.5802/crgeos.296

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

This article is licensed under the
CREATIVE COMMONS ATTRIBUTION 4.0 INTERNATIONAL LICENSE.

http://creativecommons.org/licenses/by/4.0/
Research article

Geochemistry, cosmochemistry

# Strong interplays between eustacy, nitrogen dynamics, and biodiversification between the Ediacaran and Silurian interval

Anastasia Chupryna<sup>a, b, c</sup>, Jeremie Aubineau<sup>a, c</sup>, Olabode Bankole<sup>a, d</sup>, Ernest Chi Fru<sup>a, d</sup>, Kurt Konhauser<sup>e, f</sup>, Viktor Nesterovsky<sup>b, f</sup>, Andreji Martyshyn<sup>b</sup>, Yevheniia Soldatenko<sup>f</sup>, Bruno Becker-Kerber<sup>g</sup>, Pierre Sans-Jofre<sup>h, g</sup>, Ibtissam Chraiki<sup>a, h</sup>, Fatima Abdelfadel<sup>i</sup>, Claude Fontaine<sup>a, i</sup>, Claude Laforest<sup>a, i</sup>, Alain Meunier<sup>a, i</sup> and Abderrazak El Albani<sup>*, a</sup>

**Abstract.** The Ediacaran to Silurian geological interval of the Podolia Basin, southwestern Ukraine, is associated with significant environmental change and biodiversification of morphologically complex biota. Using a multiproxy approach, including total Nitrogen (TN), nitrogen and carbon isotopes, total organic matter (TOC), and whole-rock geochemistry, we show that localised and transient nitrate scarcity may have persisted in otherwise oxygenated and nitrate-replete oceans during the period of extensive animal diversification. Bulk δ¹⁵N<sub>bulk</sub> data suggest stable partial denitrification between the middle Ediacaran and Silurian, indicating the existence of a large seawater nitrate reservoir in redox stratified waters that sustained the complex biota. Importantly, the switch to climate warming combined with the nitrate-rich waters in the early Paleozoic triggered a rapid phytoplankton and zooplankton bloom. This transition likely supported efficient nutrient flow to higher trophic levels, resulting in an increase in animal body size, biodiversification, and complexity during the Great Ordovician Biodiversification Event. Nonetheless, the upper Ediacaran and upper Silurian strata

<footnote><sup>a</sup> University of Poitiers, CNRS-IC2MP UMR, 7285 Poitiers, France</footnote>

<footnote><sup>b</sup> Institute of Geological Sciences of the NAS of Ukraine, Kyiv, Ukraine</footnote>

<footnote><sup>c</sup> Géosciences Environnement Toulouse, CNRS UMR 5563 (CNRS/UPS/IRD/CNES), Université de Toulouse, Observatoire Midi-Pyrénées, Toulouse, France</footnote>

<footnote><sup>d</sup> School of Earth and Ocean Sciences, Centre for Geobiology and Geochemistry, Cardiff University, Cardiff CF10 3AT, Wales, UK</footnote>

<footnote><sup>e</sup> Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada</footnote>

<footnote><sup>f</sup> Department of Geological Prospection, National Mining University of Ukraine, 49005, Dnipro, Ukraine</footnote>

<footnote><sup>g</sup> Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, USA</footnote>

<footnote><sup>h</sup> Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, Sorbonne Université, Paris, France</footnote>

<footnote><sup>i</sup> National Institute for Scientific and Technological Research in Water, City of Innovation Souss Massa, Ibn Zohr University, Agadir 80000, Morocco</footnote>

<footnote>E-mail: abder.albani@univ-poitiers.fr (A. El Albani)</footnote>

<footnote>* Corresponding author</footnote>
suggest a progressive near-quantitative denitrification with diazotrophic primary production as the
primary nitrogen source. The generated nitrate scarcity was likely transient with no significant
biological incidence since the early Paleozoic emergence and diversification of animals in the Podolia
Basin are well recorded. The shift to dominant lighter marine sedimentary δ¹⁵Nbulk composition
perhaps resulted from the expansion of anoxic conditions in open marine environments during the
Silurian sea-level rises and falls.

**Keywords.** Geochemistry, Ediacaran, Nitrogen, Ukraine.

Manuscript received 8 March 2025, revised 22 May 2025, accepted 23 May 2025.

**1. Introduction**

The Ediacaran (~635–539 Ma) to Silurian (~444–419 Ma) periods are biologically significant because this approximately 200 Myr time interval witnessed unparalleled animal diversification (R. Wood et al., 2020). The evolution of the Ediacaran biota between 575–539 Ma (Narbonne, 2005) is temporally linked to a period of climate change characterized by the end of the Marinoan Snowball Earth glaciation, which saw a substantial rise in atmospheric oxygen concentrations during the Neoproterozoic Oxygenation Event—NOE (Canfield, 2005; Sahoo, Planavsky, Kendall, et al., 2012), a dramatic rise in seawater levels (Sahoo, Planavsky, Kendall, et al., 2012; Reinhard, Planavsky, Gill, et al., 2017), and increased continental weathering (Frei et al., 2009; Bjerrum and Canfield, 2011). The Ediacaran biota, comprised largely of immobile soft-bodied fossils, declined or disappeared completely ~539 Ma, and were replaced by diverse morphologically complex multicellular algae and animals during the Ediacaran–Cambrian transition, culminating with the subsequent Cambrian Explosion of ancestral lineages of modern animals at ~541 to ~521 Ma (Narbonne, 2005; Butterfield, 2011). Enhanced oxygenic photosynthesis by marine phytoplankton promoted the diversification of zooplankton, fish and other larger animals during the Great Ordovician Biodiversification Event (GOBE) (Edwards et al., 2017). However, it remains unclear what caused the GOBE phytoplankton proliferation.

Existing evidence suggests the ensuing Silurian
Period experienced several events associated with
more expansive global oxygenation of seawater dur-
ing the so-called Paleozoic Oxygenation Event—POE
(Wallace et al., 2017; Krause et al., 2018), and corre-
lated with rising seawater levels, a wide distribution
of low continental landmass, a warm and stable cli-
mate, the diversification of fish and large-bodied an-
imals, an abundance of large-size shelly fossils, and
evidence for the appearance of the first terrestrial and

vascular plants (Dahl et al., 2010; Lenton et al., 2016; Krause et al., 2018). This evolutionary diversification has been widely related to increased oxygenation of the ocean–atmosphere system, especially during the Cambrian Explosion and beyond (Johnston, Poulton, Goldberg, et al., 2012; F. Bowyer et al., 2017; R. Wood et al., 2020). For instance, many Ediacaran rocks containing large and diverse biota were predominantly deposited beneath shallow oxygen-rich waters (F. Bowyer et al., 2017; F. T. Bowyer et al., 2020). Nonetheless, it is debated whether it was oxygen that triggered the rise of these animals or the other way round (Cole et al., 2020).

Pervasive anoxia, characterized by ferruginous
conditions and euxinic waters, persisted in some
productive Ediacaran deep-sea settings (Canfield,
Poulton, et al., 2008; Sahoo, Planavsky, Jiang, et
al., 2016), and well into the Cambrian and beyond
(Wallace et al., 2017; Krause et al., 2018; Li et al.,
2018; Sperling et al., 2021). These deoxygenated wa-
ters likely inhibited and delayed the global radiation
and biodiversification of aerobic life forms into the
available seawater realms. Although the accessibil-
ity of bio-limiting nutrients, such as nitrogen (N),
has been suggested as significant in regulating bi-
ological productivity, the metabolic activity across
the Neoproterozoic–Paleozoic interval and their ex-
act influence on the diversification of early marine
metazoans remain disputed (Kipp and Stüeken, 2017;
Reinhard, Planavsky, Gill, et al., 2017; Cole et al.,
2020). Moreover, the scarcity of near-continuous ma-
rine sedimentary sequences formed in a single basin
across this time interval makes the coherent testing
of these hypotheses unachievable.

Nitrogen isotopes have been successfully used
for reconstructing the paleoenvironment during the
Ediacaran–Cambrian transition and into the Paleo-
zoic (Melchin et al., 2013; Luo et al., 2016; Reinhard,
Planavsky, Gill, et al., 2017; Pehr et al., 2018; X. Wang
et al., 2018; C. Chang, Hu, et al., 2019; C. Chang,
Z. Wang, et al., 2022; Kipp, Algeo, et al., 2020; Y.
Liu et al., 2020; Yang et al., 2021). According to Algeo, Meyers, et al. (2014), climatic rather than redox fluctuations would have driven the long-term nitrogen dynamics in the Phanerozoic. However, the redox state of the Paleozoic oceans was likely not stable compared to those of the Mesozoic and Cenozoic eras, which would have disrupted the nitrogen cycle (Stüeken, Kipp, et al., 2016). Therefore, the influence of climate- and redox-driven variations on nitrogen cycling in the Paleozoic is yet to be determined. For example, the Ordovician–Silurian transition is thought to be globally marked by protracted nitrogen limitation resulting from quantitative denitrification (reduction of nitrate to dinitrogen) in successive icehouse and greenhouse intervals (Y. Liu et al., 2020; Yang et al., 2021; Melchin et al., 2013). Notably, the deep oceans in the Ordovician–Silurian transition may have been severely depleted in fixed nitrogen compounds, while the surface ocean remained oxygenated (Michel et al., 2013). Complex relationships among climate, eustasy, and redox changes have therefore been inferred to explain the nitrogen patterns during the first of the “Five Big” Phanerozoic extinction events (end-Ordovician).

Here we show dynamic interplays between eu-
stacy, nitrogen bioavailability, and the evolution
of complex life across the Late Neoproterozoic–
Paleozoic transition when the first ancestral forms
of modern animals first appeared. We report on
a rare but near-continuous Ediacaran to Silurian
marine sedimentary package in the Podolia Basin
in southwestern Ukraine (Figure 1). Biological and
lithological change across the sedimentary sequence
correlate well with sea-level fluctuations and the
sequence is biostratigraphically distinguished by
the exceptional preservation of representative Edi-
acaran to Silurian animal fossils (Figures 2 and 3).
The lower series contains a unique Ediacaran fossil
assemblage composed of *Podolimirus*, *Dickinsonia*,
*Kimberella*, *Aspidella*-type holdfasts “*Cyclomedusa
plana*”, *Shaanxithes*, *Intrites punctatus*, *Nemiana
simplex* (Ivantsov et al., 2015), *Hiemalora stellar-
sis*, *Astropolichnus sf. Hispanicus*, *Charniodiscus*,
*procerus*, *Finkoella ukrainica*, *Harlaniella podolica*,
*and Conomedusites lobatus*, (Figure 3a–c, Figure S1).
The Ediacaran–Cambrian transition is represented
by *Vendotaenia antiqua* (*Tyrasotaenia podolica*)
(Figure 3d) and *Treptichnus pedum* (Figure 3e),
while abundant Ordovician–Silurian macrofossils

include *crinoidea*, tetracoralla, corals, bryozoan,
brachiopods, gastropods, trilobites, *Eurypterus*, or-
thoceras, and trace fossils (Nikiforova et al., 1972)
(Figure 3f–j). Previous studies in the Podolia Basin
have largely focused on sedimentology and bios-
stratigraphy, with limited geochemical constraints
on nutrient cycling and redox conditions across the
Ediacaran–Silurian interval. Here, we present a new,
high-resolution multiproxy dataset—combining
bulk δ¹⁵N, carbon isotopes, TOC, and whole-rock
geochemistry—that enables a more detailed recon-
struction of nitrogen cycling dynamics and their
linkage to ocean redox evolution and biodiversifica-
tion events. This integrated geochemical approach
has not previously been applied in this basin across
this timespan, offering new insights into the nutrient
constraints that may have influenced early animal
ecosystems.

**2. Geological background**

The Podolia Basin, a part of the extensive Volyn-
Podolia-Moldavia Basin (VPMB), developed along
the passive northeastern margin of the East Euro-
pean Craton (EEC; also called Baltica) and the west-
ern flank of the Archean Ukrainian Shield (Figure 1)
(Shulga et al., 2007; Środoń et al., 2013; Poprawa et
al., 2018). Paleogeographic reconstructions and sed-
imentological studies show that the Ediacaran and
Paleozoic sedimentary successions consist of un-
metamorphosed mixed volcano-siliciclastic and car-
bonate continental shelf facies deposited during sea-
level fluctuations that at times flooded the continent
and led to an epi-continental sea (Poprawa et al.,
2018). Sedimentation on the Archean-Proterozoic
crystalline basement rocks along the proximal mar-
gin of the Ukrainian Shield started during tectonic
extension of the ECC in the Ediacaran and continued
to the Paleozoic (ibid.). Variations in lithofacies, fossil
diversity, and surface boundaries have been used to
establish the lithostratigraphy of the Ediacaran to Sil-
urian strata in the Podolia Basin (Figure 2) (Velikanov
et al., 1983; Shulga et al., 2007).

The development of the VPMB began with rift-
ing and rapid subsidence that led to the emplace-
ment of volcanoclastic sediments, the Volynska
Group (VG), which is predominantly composed of
mainly basaltic lavas and pyroclasts during the Edi-
acaran (Poprawa et al., 2018). Further extension and
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Location and geological map of the Podolia Basin. (A) Geological map showing the position of the studied outcrops in the Podolia Basin. Inset—location of the Podolia Basin in the southwestern part of the East European Craton and the western flank of the Archean Ukrainian Shield. (B) Geological cross-section showing the lithostratigraphic divisions across the studied area (modified after Soldatenko et al., 2019).</figcaption></figure>

subsidence resulted in the deposition of terrigenous materials comprising a dominantly coarse-grained conglomeratic sandstone in the lower part and alternating shale-siltstone in the upper section, reflecting an upward fining and deepening sequence

as sedimentation decreased during the Ediacaran (Poprawa et al., 2018). However, smaller scale cycles of upward coarsening and shallowing are also observed within the shale-siltstone-dominated upper part due to fluctuating sea level during deposition.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Lithostratigraphy of the Podolia Basin (modified after Ivantsov et al., 2015). PR—Precambrian; F—Formation.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Microphotographs of biota distributed across the Ediacaran to Silurian succession: (a) Colony of *Nemiana simplex* embedded in the Ediacaran sandstone. (b) *Eoporpita medusa*. (c) *Arborea arborea*. (d) *Vendotaenia* at the Ediacaran–Cambrian Transition. (e) Cambrian burrowing animals, *Treptichnus pedum*. (f) Ordovician *Orthoceras*. (g) Silurian *Eurypterus*. (h) Silurian coral embedded within mudstone matrix. (i) Complete shells of Silurian brachiopods. (j) Silurian echinoderms. Scale bars are 1 cm.</figcaption></figure>
The terrigenous Ediacaran sediments are further divided into the Mohyliv-Podilskyi and Kanilivska groups (Figures 1 and 2) (Velikanov et al., 1983; Ivantsov et al., 2015; Poprawa et al., 2018). The Mohyliv-Podilskyi Group consists of a thin conglomeratic sandstone bed that grades into alternating fine-grained sandstones interbedded with clayey sandstones, silty shales, thin bentonite beds, and phosphatic nodules towards the top. The overlying Kanilivska Group comprises fossiliferous shaly siltstones interbedded with thin beds of fine-grained sandstones, mudstones, and lenticular shaly limestone in the uppermost section. The minimum age of the Ediacaran strata in the Mohyliv-Podilskyi Group is constrained by U-Pb zircon dating of the interlayered tuff and thin bentonite beds to late Ediacaran, ~553 Ma (Grazhdankin et al., 2011) and 556.78 ± 0.1 Ma (Soldatenko et al., 2019).

Tectonic subsidence significantly diminished during the Cambrian and Ordovician when the basin was interpreted to be relatively stable and passive with a low sedimentation rate (Poprawa et al., 2018). The greenish glauconitic calcareous Cambrian sandstones unconformably overlie the Ediacaran strata with distinct levels of interlayered greenish mudstones rich in apatite (phosphatic horizons). These sediments contain acritarchs and macroalgae related to *Tyrasotenia podolica and Vendolenia antiqua*, bioglyphs, and skeletal fauna like *Sabelliditids* (Figure 3d,e) (Velikanov et al., 1983). The lower and middle Ordovician strata have been eroded in the basin (Shulga et al., 2007), and the locally preserved ~1.5 m thick upper Ordovician sediments consist of thin clayey calcareous fine-grained sandstones and shales that gradually shift to marly limestones at the top (Radkovets, 2015; Poprawa et al., 2018). These sediments contain macrofossils such as brachiopods, gastropods, trilobites, and Orthoceras (Figure 3f,g).

Again, sedimentation was truncated at the end of Ordovician due to an intense uplift and erosion of the southwestern margin of EEC during a global regression and glaciation (Radkovets, 2015). The Silurian flexural bending of the craton margin resulted in a rapid increase of subsiding conditions and subsequently deposition of clayey limestones and marlstones directly on the eroded Ordovician-early Silurian (Llandovery Series) sediments (Radkovets, 2015; Poprawa et al., 2018). Sedimentation of carbonate-rich sediments of various Silurian ages

(Wenlock, Ludlow and Pridoli series) became more pronounced in the shallow water shelf (lagoon and reef; <100 m depth) to open shelf environments during fluctuating transgression-regression cycles (Radkovets, 2015; Poprawa et al., 2018). These sediments preserve abundant and diverse macroscopic fossils such as crinoidea, tetracoralla, corals, bryozoan, and trace fossils (Figure 3h-j).

Samples were collected from an outcrop we designated as “Horaivka,” named after the nearby village located in the southern Ukrainian Shield (Figure 1). The section encompasses a range of lithologies spanning from the Ediacaran (Kalinivska Group, Studenytsia formation) to the Silurian. The transition between the Ediacaran and Cambrian intervals in the studied section is marked by an apparent greenish silty fine-grained sandstone at the base of the Cambrian unit. The Ordovician and Lower Silurian sediments are highly condensed and/or eroded in the studied section.

**3. Materials and methods**

**3.1. Sampling and sample preparation**

A total of 58 samples were collected from formations of different ages: 14 from the Silurian, 5 from the Ordovician, 6 from the Cambrian, and 33 from the Ediacaran. The samples were powdered using an agate mortar for subsequent geochemical analyses and mineralogical characterisation by X-ray diffraction (XRD).

**3.2. Petrography and mineralogy**

Polished thin sections were prepared for representative samples and examined under a polarized Nikon Eclipse E-600 optical microscope at the University of Poitiers. The thin sections were carbon-coated and studied using a JEOL scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) for detailed textural mineral relationships and point-chemical composition analysis in backscattered electron (BSE) mode. X-ray diffraction (XRD) analysis of bulk rock and clay fractions were performed with a Bruker D8 ADVANCE diffractometer using CuKα radiation operating at 40 mA and 40 kV. For bulk mineral composition, samples were gently powdered in an agate mortar and analyzed between 2 and 65° 2θ
angular range and step size of 0.025/s. The <2 µm, <1 µm, and <0.5 µm clay-size fractions were separated from the bulk materials after carbonate dissolution in acetic acid and removal of organic matter with hydrogen peroxide, by centrifugation. The extracted suspensions were saturated with Ca for uniform cation exchange and repeatedly rinsed with deionized water a minimum of three times and centrifuged to remove free ions (Moore and Reynolds Jr., 1997). The clay fractions were pipetted on glass slides and dried at room temperature. The oriented slides were analyzed in air-dried (AD) and after ethylene glycol (EG) treatment at a step size of 0.01/s from 2 to 35° 2θ angular range. The clay minerals were distinguished using their basal *00l* characteristic peaks (ibid.). Semi-quantification of the mineral components was determined using area ratios of the principal mineral peaks in decomposed bulk powder and EG treated XRD patterns (Hiller, 2003). The XRD patterns were decomposed using the FYTIK software and the mixed-layer clay minerals modelled with the NEWMOD software (Reynolds Jr., 1985).

### 3.3. Whole rock geochemistry

The bulk major and trace element concentrations of 58 selected samples (Tables S1–S3) were measured with inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS), respectively, according to Carignan et al. (2001). About 100 mg powdered samples were decomposed by alkali fusion and fused with 900 mg of ultra-pure lithium metaborate (LiBO₂) at 980 °C to form a glass. The glass was then digested in a mixture of 1M nitric acid, 0.5% hydrogen peroxide, and 10% glycerol for elemental analyses at the Service d'Analyse des Roches et des Minéraux (SARM; CRPG-CNRS), Nancy, France. The uncertainty for most major elements was better than 5%, 10% for Ca, >25% for P, lower than 5% for trace element concentrations >10 ppm, 15% for concentrations between 1 and 10 ppm, and 25% for elemental concentrations near the detection limits, when compared with international standards and analysis of replicate samples (ibid.).

The weathering intensity was determined using the chemical index of alteration (CIA = [Al₂O₃/ (Al₂O₃ + CaO* + Na₂O + K₂O)] × 100 in molar ratios), where CaO* is the amount in the silicate fraction,

to monitor the nature and weathering intensity of the detrital fluxes into the basin (Nesbitt and Young, 1984). Varying proportions of CaO-bearing phases in sedimentary rocks composed of carbonate and phosphate minerals falsify CIA values that are reliant on the fine-grained siliciclastic component of sediments for applicability. CaO is first corrected for that residing in apatite using P₂O₅ data (CaO' = CaO - 10/3 × P₂O₅). CaO' in silicates of our samples is assumed with CaO* = CaO' when the mole fraction of CaO' is ≤Na₂O and CaO* = Na₂O when the mole fraction of CaO' is ≥Na₂O (McLennan et al., 1993; Jian et al., 2013; Johnston, Poulton, Tosca, et al., 2013). Typically, highly weathered rocks in hot and humid climatic conditions have high CIA values (>80), moderately weathered rocks in wet and humid climate have values between 60 and 80, whereas lower CIA values (<60) indicate unweathered to weak chemical weathering in cold and dry climate (Nesbitt and Young, 1984; Fedo et al., 1995).

To assess whether certain elements were enriched or depleted due to authigenic processes, we calculated enrichment factors (EFs) relative to the average composition of the upper continental crust (UCC), following the approach of Tribovillard, Algeo, Lyons, et al. (2006). The EF for an element X in siliciclastic sediments was determined using the formula:

$$X - \text{EF} = (X/\text{Al})_{\text{sample}}/(X/\text{Al})_{\text{UCC}}$$

(McLennan, 2001).

However, interpreting EF values in carbonate-dominated sediments is more complex due to their typically low aluminium content, which can lead to inflated EF values when compared to siliciclastic rocks. To address this issue, we also applied the concept of elemental excess (Krewer et al., 2024), calculated as:

$$\text{Element}_{\text{excess}} = \text{Element}_{\text{sample}} - (\text{Al}_{\text{sample}} \times [\text{Element}_{\text{UCC}}/\text{Al}_{\text{UCC}}])$$

$$\text{ElementEF}^* = (\text{Element}_{\text{excess}} + \text{Element}_{\text{UCC}})/ \text{Element}_{\text{UCC}}.$$

### 3.4. Carbon and nitrogen isotopes

Total organic carbon (TOC), total nitrogen (TN), and stable isotopes of organic carbon (δ¹³Cₗ₀) and bulk rock nitrogen (δ¹⁵Nₗ₀) were analyzed for 47 representative samples spanning the Ediacaran to Silurian (Table S4). About 300 mg of sample powders were
treated with 6 N HCl for ~1 h at 70 °C to remove car-
bonate minerals. The insoluble residues were cen-
trifuged and repeatedly rinsed with deionized wa-
ter until the supernatants achieved a neutral pH and
dried in a clean hood overnight. Small aliquots of
the de-carbonated powdered samples were weighed
and sealed in tin capsules for isotopic analyses with
a blank placed between each measurement. The
isotope data were measured by flash combustion
using a Thermo Scientific DELTA V Advantage iso-
tope ratio mass spectrometer (IRMS), operated un-
der a continuous helium flow mode, and coupled
with a Thermo Scientific Flash 2000 elemental ana-
lyzer at Laboratoire Géosciences Océan, University of
Brest, France. Certified standards USGS40 (δ¹³Corg =
-26.2‰, Corg = 40.82 wt% and δ¹⁵Nbulk = -4.5‰,
N = 9.52 wt%) and caffeine IAEA-600 (δ¹³Corg =
-27.77‰, and δ¹⁵Nbulk = 1‰) were used for the cal-
ibration. The obtained data were reported in the
delta (δ) notation in part per mil (‰) deviation rel-
ative to the Vienna Pee Dee Belemnite (VPDB) for
carbon (δ¹³Corg) and atmospheric N₂ for nitrogen
(δ¹⁵Nbulk). The TOC and TN concentrations were cal-
culated from peak areas of CO₂ and N₂, respectively.
We only report δ¹⁵Nbulk values of samples with TN
concentrations well above the detection limits.

4. Results

4.1. *Lithology and mineralogy*

Light microscopy and XRD mineralogical data
show that the Ediacaran to Silurian sediments are
composed mainly of quartz and calcite with trace
amounts of dolomite, ankerite, clay minerals, K-
feldspar, albite, with accessory apatite restricted to
the Cambrian samples (Figure 4). The bulk min-
eralogy of the siliciclastic Ediacaran and Cambrian
samples is dominated by 87–95% quartz, while the
carbonate minerals, composed of up to 90% cal-
cite, dominate the upper calcareous Ordovician
shales and Silurian limestones. The clay mineral and
feldspar abundances are higher in the Ediacaran than
the Cambrian siliciclastic samples, decreasing from
~10% to 0.5%. The greenish glauconitic sandstones,
common in some intervals of the Cambrian, contain
moderately sorted, sub-angular to angular quartz,
while the glauconitic siltstones and shales consist
mainly of fine-grained quartz in a clay-dominated
matrix (Figure 5a,b). Glauconite is identified by their

greenish to yellowish color, filling pores associated
with apatite in the lowermost “verdine facies”, while
that in the middle and upper levels are pale green to
yellowish-brown cement in association with carbon-
ate minerals or replacing granular grains. EDS anal-
ysis reveals that the grain-replacing and pore-filling
greenish glauconite minerals in the Cambrian sedi-
ments have heterogeneous octahedral compositions
of glauconite–berthierine/chamosite endmembers
of chlorite (Figure 5c,d). The Ordovician sandstones
are composed of fine to medium, sub-angular quartz
grains dispersed in calcite-dominated matrix.

XRD analysis of the separated <2 µm fractions provides the detailed mineralogy of the clay minerals (Figure S2). The clay mineral assemblage includes chlorite, illite, and illite-smectite (I/S) mixed layer minerals (MLMs) throughout the section, while kaolinite and chlorite-smectite (Ch/S) MLMs are absent in the Silurian deposits. Chamosite is identified in all samples by basal *00l* characteristic peaks at ~14.2, 7.1, 4.71, and 3.54 Å in AD and unaffected in EG state. Illite is defined in all the samples by the ~10, 5.0, and 3.3 Å peaks in AD samples, but remained relatively unchanged in the EG state. Kaolinite, present only in a few upper Ediacaran and Ordovician samples, is defined by the ~7.13 and 3.57 Å peaks. Decomposition of the broad and asymmetry peaks between 4 and 14° 2θ in EG patterns revealed Ch/S and I/S MLMs. I/S MLMs are present in all samples with a relatively decreasing abundance up-section; Ch/S MLMs have varying abundance and are absent in the Silurian samples. The bulk and clay mineral assemblages across the studied section show that mineral distribution and abundance are lithology dependent (Figure 2).

The most pronounced distinction through the
section is the authigenic glauconite and apatite min-
erals in the interlayered Cambrian verdine sedi-
ments. Authigenic green clay consists of a wide range
of chemical and mineralogical compositions, includ-
ing glauconite, berthierine, glauconite-smectite, and
berthierine-smectite (Odin and Matter, 1981; Me-
unier and El Albani, 2007; Velde, 2014). The pre-
cipitation of authigenic glauconite has the poten-
tial to trace the redox conditions and chemistry of
the depositional environments because these miner-
als typically form at shallow depths during diagene-
sis (Odin and Matter, 1981; Velde, 2014; Banerjee et
al., 2016; Bansal et al., 2018). Authigenic glauconite
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Mineral composition and relative abundances of the bulk and <2 µm clay-size fractions through the studied stratigraphic section. KF: K-feldspar; Pl: plagioclase; Q: quartz; Ca: calcite; Do: dolomite; Ank: ankerite; Apt: apatite; I/Sm: illite–smectite; Ch/Sm: chlorite/smectite; Kaol: kaolinite; Ch: chlorite. Legend of the lithology is presented in Figure 6.</figcaption></figure>

typically formed under pore water suboxic (mildly
reducing and low organic matter) conditions below
the seafloor in shallow water continental shelves,
where both Fe²⁺ and Fe³⁺ are simultaneously avail-
able. By contrast, berthierine, owing to the pre-
dominance of Fe²⁺, precipitates in suboxic to anoxic
pore waters (with high organic matter) near river
deltas in shallower waters relative to glauconite (Me-
unier and El Albani, 2007; Bansal et al., 2018; Al-
gabri et al., 2020). The occurrence of glauconite and
berthierine/chamosite minerals primarily in intersti-
tial pore spaces of the studied Cambrian samples
demonstrate their authigenic origin. The dissolu-
tion of detrital mineral grains (Figure 5c,d), availabil-
ity of cations (K, Fe, Mg, and Al) during diagenesis,

and change in conditions of formations (Meunier
and El Albani, 2007; Velde, 2014; Tang et al., 2017)
likely controls their heterogeneous chemical compo-
sitions. Interestingly, the presence of glauconite and
berthierine in the interbedded verdine facies of the
Podolia Basin during the Cambrian is consistent with
the global occurrence of glauconite at the Ediacaran-
Cambrian boundary from ~550 Myr onward (Balder-
mann et al., 2022).

4.2. *Whole rock elemental geochemistry*

The concentrations of major elements mainly reflect
lithofacies variation, with detrital tracers like SiO₂,
Al₂O₃, and TiO₂ being comparatively more abundant
<figure><img src="image_6.png" /><figcaption>**Figure 5.** (A) Photograph of the Ediacaran–Cambrian boundary. (B) Photomicrographs of pore and matrix filling amorphous glauconite minerals in Cambrian silty fine-grained sandstone. (C) Chemographic projection of the chemical composition of the glauconite minerals in the Cambrian sediments on the $M^+–4Si–R^{2+}$ ternary diagram after Meunier and Velde (2004). (D) Plot of $Fe/(Fe+Mg)$ versus $Al_{Tot}/Si$ of the glauconite minerals in the Cambrian verdine facies.</figcaption></figure>

in the siliciclastic sandstone and shale samples compared to the carbonates (Table S1). $Al_2O_3$ has a strong positive correlation with $SiO_2$ in the Silurian carbonate-rich rocks but a negative correlation in the siliciclastic-rich Ediacaran to Cambrian samples (Figure S3). The moderate to strong positive covariances of $TiO_2$, $K_2O$, $Fe_2O_3$, and MgO with $Al_2O_3$ (Figure S3b–e) in the siliciclastic and carbonate samples are also consistent with the prevalence of aluminosilicate minerals, including clays and feldspars, coupled with significant detrital flux to the basin. The higher Fe concentrations in the Cambrian rocks coincide with the enrichment of the authigenic Fe-rich glauconite/berthiérine. Consistent with XRD mineralogical analysis, CaO content is significantly higher in the carbonate-rich Silurian and Ordovician rocks

than the siliciclastic Ediacaran–Cambrian samples (Figure S3f). MnO and $P_2O_5$ contents are generally low through the section, except in a few interbedded Cambrian phosphate horizons with considerably higher concentrations of $P_2O_5$ (Table S1). The lack of an obvious relationship between CaO, MnO, and $P_2O_5$ with $Al_2O_3$ (Figure S3f–h), indicates an authigenic and/or a primary chemical weathering origin.

The concentrations of the incompatible high field strength elements (HFSE—Zr, Hf, Ta, Nb, and Th) and compatible transition trace elements (e.g., Sc and Cr) are significantly higher in the siliciclastic-dominated than the carbonate-rich rocks (Table S2). Excluding a few outliers, the strong positive covariances of Th, Sc, and Cr with Al (Figure S4a–c) show that they are dominantly hosted by the detrital
aluminosilicate minerals. The lack of a relationship between Zr and Al₂O₃, except in Silurian carbonates, hints that Zr is hosted in detrital zircon grains (Figure S4d). The concentrations of redox-sensitive trace elements (RSTE—e.g., Mo, U, V, Ni, Cu, and Zn) are generally low across the studied section (Table S2). The RSTEs and total REE (REE<sub>T</sub>) exhibit moderate positive correlations with Al except Mo (Figure S5), suggesting a strong detrital influence on their distributions in the sediments. Mo is constant in all the samples, except in the Silurian and Cambrian. The PAAS-normalized REE + Y (REY<sub>N</sub>) patterns in most samples are relatively flat (Figure S6), suggesting negligible fractionation and further support dominant terrigenous inputs (McLennan et al., 1993). A few samples, along the studied section, have dome-shaped patterns with depletion of LREE and HREE relative to middle REE (MREE: Sm-Dy), indicating preferential enrichment of MREE by authigenic phosphate minerals (Shields and Stille, 2001). The enrichment of the MREE in the glauconitic Cambrian sediments could have resulted from recrystallization during diagenetic alteration or uptake from sub-oxic to anoxic pore water during the formation of the glauconite assemblage (Meunier and El Albani, 2007).

Negative Ce anomalies are generally lacking in the REE spider profiles, with a weak but persistent signal observed in the Silurian deposit. Nearly all the samples show suprachondritic Y/Ho ratios ranging from 25.0 to 28.7 with a mean of 26.8 ± 1.1 in the Ediacaran and Cambrian, before increasing to values ranging between from 26.4 and towards stronger seawater ratios of 35.2 that have a mean of 30.2 ± 2.4 in the Ordovician and Silurian rocks.

### 4.3. *Carbon and nitrogen isotopes*

The TOC and TN contents show similar stratigraphic trends with values ranging from 0.02 to 0.86 wt% and averaging 0.25 ± 0.21 wt% for TOC and 0.01 to 0.1 wt% with a mean of 0.04 ± 0.02 wt% for TN (Figure 6; Table S4). In the Ediacaran, δ¹³C<sub>org</sub> varies from -31.9‰ to -26.5‰ with a mean of -28.7 ± 1.2‰ and corresponding δ¹⁵N<sub>bulk</sub> values between +1.8 and +5.4‰, averaging +4.3 ± 1.0‰. The Cambrian δ¹³C<sub>org</sub> values show a narrower range of -28.9‰ to -27.6‰, with an average of -28.2 ± 0.5‰ similar to the Ediacaran. Likewise, the corresponding

Cambrian δ¹⁵N<sub>bulk</sub> values range from +3.7 to +5.2‰ with an average of +4.5 ± 0.7‰, remaining close to the Ediacaran δ¹⁵N<sub>bulk</sub> values. The Ordovician unit is characterized by δ¹³C<sub>org</sub> values similar to the Ediacaran and Cambrian periods, varying between -30.2‰ and -27.9‰ and a mean of -28.9 ± 0.8‰. The δ¹⁵N<sub>bulk</sub> values at this time span +0.8 to +5.1‰, with average values of +3.1 ± 1.8‰, being much lighter than the average Ediacaran and Cambrian δ¹⁵N<sub>bulk</sub> values. The Silurian samples display δ¹³C<sub>org</sub> values between -28.8‰ and -25.9‰ with an average of -27.8 ± 0.9‰. They contain the lowest δ¹⁵N<sub>bulk</sub> values of the section, ranging from +0.6 to +4.0‰ and averaging +2.8 ± 1.0‰). In light of these observations, δ¹³C<sub>org</sub> values do not significantly vary across the studied section, while the global δ¹⁵N<sub>bulk</sub> curve displays several trends.

We divide the studied section into four intervals I–IV based on the stratigraphic δ¹⁵N<sub>bulk</sub> variations (Figure 6). Interval I corresponds to lower Ediacaran strata, characterized by high and narrowly distributed δ¹⁵N<sub>(bulk)</sub> values and increasing total nitrogen (TN, wt%). Interval II includes upper Ediacaran samples, where δ¹⁵N<sub>(bulk)</sub> values decrease and TN (wt%) shifts to lower levels. The boundary between these two intervals corresponds to this shift in TN values. Interval III spans the Ediacaran–Cambrian transition to the lower Silurian and is marked by a return to high and narrowly ranged δ¹⁵N<sub>(bulk)</sub> values, similar to those in Interval I. Interval IV represents the upper Silurian and is characterized by a decreasing trend in δ¹⁵N<sub>bulk</sub> values (Figure 6).

## 5. Discussion

### 5.1. *Paleoenvironmental reconstructions*

#### 5.1.1. *Sediment provenance and paleoweathering processes*

The spatial distribution and assemblage of clay minerals in sedimentary rocks are routinely used as proxies for paleoclimatic, paleoenvironmental, and thermal historical events in Precambrian and Phanerozoic sedimentary rocks (e.g., Bristow et al., 2009). Clay minerals in fine-grained shale and mudstone siliciclastic sediments are major sinks for ions and can provide a physical and chemical record of
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Lithostratigraphic column associated with stratigraphic variations of total organic carbon (TOC), total nitrogen (TN), and carbon and nitrogen isotope composition across the Ediacaran to Silurian strata in the studied section, southwestern Ukraine. High-order (i.e., short-term) sea level changes are not shown, but well-documented in the Silurian Podolia Basin (Radkovets, 2015; Poprawa et al., 2018). Interval I: lower Ediacaran rocks, interval II: upper Ediacaran rocks, interval III: Ediacaran–Cambrian transition to lowermost Silurian rocks, interval IV: upper Silurian. Sediments of early Silurian in age were likely eroded in the Podolia Basin (see geological background). Brown circles, light green circles, green squares, red triangles, and blue triangles correspond to Ediacaran, Ediacaran–Cambrian transition, Cambrian, Ordovician, and Silurian samples.</figcaption></figure>

the weathering conditions that regulate the compo-
sition of the biosphere (Isson and Planavsky, 2018;
Rafiei and Kennedy, 2019). Importantly, the preser-
vation of low temperature chlorite/smectite and il-
lite/smectite mixed layer minerals, as well as glau-
conite/berthierine clay minerals, together with the
conspicuous absence of metamorphic minerals (Fig-
ures 4 and 5; Figure S2), imply that the Podolia

Basin sediments are well preserved and have only
undergone weak diagenetic modification (Meunier
and Velde, 2004; Bristow et al., 2009). Moreover, dif-
ferences in clay mineralogy between the siliciclas-
tic Ediacaran–Cambrian and the Ordovician–Silurian
carbonate rocks are consistent with changes in litho-
facies and prevailing environmental conditions. Fur-
ther, the moderate to strong co-variation of most of
the whole-rock major and trace elements composi-
tion with Al₂O₃ (Figures S3–S5) and flat REE patterns
(except for a few samples; Figure S6) indicate the de-
livery of these elements to the basin was mainly con-
trolled by detrital fluxes. Therefore, we interpret that
the mineralogy and bulk geochemistry as being con-
sistent with the dominance of lithological change.

The composition of parent rocks and the extent of
chemical weathering (i.e., their CIA) in the siliciclas-
tic rocks record significant influence on nutrient sup-
ply from the continents to the oceans, and thus influ-
ence on seawater primary productivity (Nesbitt and
Young, 1984; Cox et al., 2016; Stüeken, Kipp, et al.,
2016; Large et al., 2018). The high-field-strength el-
ements have been widely used in tracing sediment
provenance because they are; (i) more enriched in
felsic rocks relative to mafic rocks, (ii) largely inde-
pendent of redox conditions and biological evolu-
tion, (iii) relatively immobile, and (iv) resistant to al-
teration during sedimentary processes (Taylor and
McLennan, 1985; Bhatia and Crook, 1986; Condie,
1993; Large et al., 2018). We use the compositional
ratios of the incompatible thorium (Th), which tends
to be enriched in felsic rocks, with compatible scan-
dium (Sc) that is enriched in mafic rocks, to dis-
criminate the provenance of the sediments (Condie,
1993). Most of the samples plot between the gran-
ite and granodiorite endmembers on the La–Th–Sc
and the Th/Sc versus La/Sc compositional discrimi-
nant diagrams (Figure S7a,b), thus suggesting the de-
livery of detrital materials from a predominantly fel-
sic rock source (Bhatia and Crook, 1986; Large et al.,
2018). Moreover, the samples mostly fall away from
the A-CN axis and between the weathering lines of
granodiorite and granite on the A-CN-K diagram with
a mean of CIA values of 67.4 ± 5.7 in the studied sec-
tion (Figure S7c; Table S1) (Nesbitt and Young, 1984;
Fedo et al., 1995).

Regardless of stratigraphic position, our CIA
values suggest that parent rocks were moderately
weathered (CIA values between 60 and 80) in the
sediment source area, reflecting a temperate climate
(Algeo, Hong, et al., 2025). In contrast, variations in
lithofacies are more likely to reflect conditions as-
sociated with progressive sea-level rise and elevated
atmospheric CO₂ content during the deposition of
the carbonate-dominated Ordovician and Silurian
sediments (Maher and Chamberlain, 2014; S. Zhang
et al., 2021). This is consistent with the approximated

paleolatitude equatorial position of Ukraine during
the Ordovician to Silurian (Laverov et al., 2013), as
well as the generally higher CO₂ levels estimated for
that time. Interestingly, the Ordovician and Silurian
CIA values of the Podolia Basin do not significantly
vary compared to the Ediacaran and Cambrian. This
suggests that chemical weathering on land and/or
delivery of soluble elements to the depositional sites
was relatively stable in this weathering area.

5.1.2. *The paleoredox conditions*

The abundance of RSTEs in sediments has been
widely used to infer the paleoredox conditions of an-
cient seawaters due to their predictable response to
different redox conditions (Algeo and Maynard, 2004;
Tribovillard, Algeo, Lyons, et al., 2006; Algeo and Tri-
bovillard, 2009; Reinhard, Planavsky, Robbins, et al.,
2013). For instance, the redox sensitive Mo, U, Th,
Cr and V are typically soluble with little to no au-
thigenic enrichment in oxic sediments, but they be-
come immobilized and co-enriched in sediments un-
der ferruginous and euxinic anoxic conditions (Algeo
and Maynard, 2004; Tribovillard, Algeo, Lyons, et al.,
2006; Partin et al., 2013). The low concentrations of
Mo, U, and V across the sampled section, and their
positive correlations with Al (Figure S5), suggest var-
ious scenarios for basinal bioavailability of these el-
ements. This might be attributed to low weathering
rates of the provenance of lithology, incorporation in
detrital materials, the prevalence of predominantly
oxygenated depositional conditions (Tribovillard, Al-
geo, Lyons, et al., 2006), or a change in sedimentation
rates (J. Liu and Algeo, 2020). In addition, the Y/Ho
ratio commonly distinguishes marine from terrestrial
inputs, with sedimentary Y/Ho ratios >40 represen-
tative of a seawater signal and open marine settings,
33–40 near-shore settings, and a 25–27 terrestrial in-
put (Bau et al., 1997). Our Y/Ho ratios, averaging
around 26 in both the Ediacaran and Cambrian, and
increasing to 30.2 in the Ordovician and 30.21 in the
Silurian sediments, suggest a dominant input of de-
trital material consistent with deposition in a conti-
nental shelf environment. This observation further
reinforces the strong correlation between most of the
whole-rock elements and Al across the sequence, re-
gardless of lithological changes. Thus, the sea-level
changes in the Ediacaran and Cambrian were likely
not important to move the shoreline towards open
marine environment, which, in turn, allowed the
expression of near-uniform clastic Y/Ho ratios dur-
ing that time. In contrast, the increasing Y/Ho ratios
from the Ordovician and Silurian point to a gradual
increase in sea level (i.e., decrease of fresh water in-
put), (Bau et al., 1997; Halverson et al., 2011).

Redox-sensitive trace metals such as U, Mo, and
Cr are commonly employed as proxies to infer an-
cient redox conditions in sedimentary environments.
Their geochemical behavior is strongly influenced by
the redox state: they tend to remain dissolved un-
der oxic conditions but are more likely to be immobi-
lized and incorporated into sediments under anoxic
or suboxic settings. This characteristic makes them
valuable indicators of past oxygen levels in deposi-
tional environments (e.g., Brumsack, 2006; Algeo and
Maynard, 2004; Tribovillard, Algeo, Lyons, et al., 2006;
Tribovillard, Algeo, Baudin, et al., 2012; J. Zhang et al.,
2016).

The enrichment factors of Mo, U, and Cr in the Podolia Basin show that samples from the Ediacaran to the Cambrian are generally not enriched in RSTEs, except for few samples from the Ediacaran-Cambrian transition. In contrast, samples from the Ordovician and the upper part of the Silurian show enrichment in Cr and U, while only the upper Silurian is enriched in Mo (Figure S8). This suggests that redox conditions across the section evolved significantly over time. During the Ediacaran to Cambrian interval, the environment was predominantly oxic. Conditions then shifted to anoxic during the Ordovician before returning to oxic in the lower Silurian (Algeo and Maynard, 2004; Tribovillard, Algeo, Lyons, et al., 2006; Tribovillard, Algeo, Baudin, et al., 2012; J. Zhang et al., 2016). The upper Silurian, however, appears to have been euxinic, as indicated by the marked enrichment Mo (Algeo and Maynard, 2004; Tribovillard, Algeo, Lyons, et al., 2006; Tribovillard, Algeo, Baudin, et al., 2012; J. Zhang et al., 2016).

5.1.3. *Preservation of primary C and N isotopic signals*

The molar C/N ratios in our Ukrainian samples av-
erage 6.9 ± 3.0 (Table S4), which is extremely close to
the molar C/N ratio of ~7 for the planktonic biomass
in modern oceans (Redfield, 1958). This suggests
an absence of extensive remineralization of organic
matter in the water column and sediments.

We then assessed the preservation of primary
$\delta^{13}\text{C}_{\text{org}}$ and $\delta^{15}\text{N}_{\text{bulk}}$ compositions in the sedi-

ments because diagenesis and metamorphism can
modify the primary signature (Stüeken, 2013; Ader,
Thomazo, et al., 2016; Stüeken, Kipp, et al., 2016).
Nitrogen in sedimentary rocks may occur as organic-
bound and inorganic clay-bound ammonium ($NH_4^+$)
species (Ader, Sansjofre, et al., 2014; Stüeken, Kipp,
et al., 2016; Zerkle, Poulton, et al., 2017). Degradation
of organic matter during early diagenesis may release
$NH_4^+$ into the pore water where it may undergo iso-
topic fractionation through redox-related processes
such as nitrification, denitrification, or diffusion,
potentially leading to $^{15}$N enrichment. $NH_4^+$ can
later be adsorbed by clay minerals and incorporated
into the interlayers of illite via substitution with $K^+$
cations during late diagenesis and metasomatism,
with minimal to no isotopic changes (Ader, Sansjofre,
et al., 2014; Stüeken, Kipp, et al., 2016; Aubineau et
al., 2021). However, $NH_4^+$ can be enriched in $^{15}$N,
which significantly affects the $\delta^{15}\text{N}_{\text{bulk}}$ values if
redox reactions are involved near the sediment-
water interface (Prokopenko et al., 2006). There is
no correlation between TN and $K_2\text{O}$ (Figure 7a) in
our Ukrainian rocks, indicating that N is derived
mainly from organic matter (Ader, Sansjofre, et al.,
2014; Ader, Thomazo, et al., 2016; Zerkle, Poulton,
et al., 2017). This view is supported by the posi-
tive correlation between TN and TOC in Ediacaran
and Cambrian samples (Figure 7b). The absence
of correlation between TN and TOC in the Ordovi-
cian and Silurian samples likely reflecting variable
mineral-bound nitrogen abundances. Overall, the
absence of a strong correlation between $\delta^{15}\text{N}_{\text{bulk}}$ and
K (Figure 7c) suggests that the $\delta^{15}\text{N}_{\text{bulk}}$ values are not
affected by the incorporation of N into illite during
illitization.

The preferential release of <sup>14</sup>N and <sup>12</sup>C during dia-
genetic and metamorphic thermal degradation of or-
ganic matter results in associated higher <sup>15</sup>N<sub>bulk</sub> and
<sup>13</sup>C<sub>org</sub> values (Cremonese, G. Shields-Zhou, et al.,
2013; Ader, Sansjofre, et al., 2014). In addition, ther-
mal degradation leads to the depletion of N relative
to C, which is reflected by an increase of and C/N mo-
lar ratios (Stüeken, Zaloumis, et al., 2017). This pro-
cess typically produces strong covariations between
<sup>15</sup>N<sub>bulk</sub> and C/N, <sup>15</sup>N<sub>bulk</sub> and <sup>13</sup>C<sub>org</sub>, and <sup>15</sup>N<sub>bulk</sub>
and TN. These covariations are not observed across
the studied Ukrainian section (Figure 7d–f), which
argues against diagenetic alteration of the primary
<sup>13</sup>C<sub>org</sub> and <sup>15</sup>N<sub>bulk</sub> signal.
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Preservation of primary isotopic signals. Cross plots showing the relationships between (A) TN and K₂O, (B) TN and TOC, (C) δ¹⁵Nbulk and K₂O, (D) δ¹⁵N and C/N, (E) δ¹⁵Nbulk and δ¹³C, (F) δ¹⁵Nbulk and TN in the studied section to check for possible alteration of primary isotopic compositions. Unfilled circles, green squares, red triangles, and black triangles correspond to Ediacaran, Cambrian, Ordovician, and Silurian samples, respectively.</figcaption></figure>
5.1.4. Evaluation of δ¹⁵N<sub>bulk</sub> patterns

The δ¹⁵N<sub>bulk</sub> has been extensively used to elu-
cidate connections between N cycling, nutrient
bioavailability, bio-evolution, and ocean redox con-
ditions in modern and ancient sedimentary basins
because N fractionates predictably during redox re-
actions (Canfield, Glazer, et al., 2010; Ader, Thomazo,
et al., 2016; Stüeken, Kipp, et al., 2016; Aubineau et
al., 2021). The exclusively positive δ¹⁵N<sub>bulk</sub> values,
mostly >+3‰ but <+5‰ for modern oxic sediments
(Tesdal et al., 2013), are broadly interpreted to imply
aerobic N cycling in a relatively stable nitrate reser-
voir (Bauersachs et al., 2009; Ader, Sansjofre, et al.,
2014). The positive δ¹⁵N<sub>bulk</sub> reflects partial denitri-
fication (i.e., reduction of NO₃⁻ and NO₂⁻) in oxygen
minimum zones or at the chemocline (oxic-anoxic
transition) in the water column, leaving behind a
residual ¹⁵N-enriched pool (Cremonese, G. Shields-
Zhou, et al., 2013; Cremonese, G. A. Shields-Zhou,
et al., 2014; Ader, Thomazo, et al., 2016; Stüeken,
Kipp, et al., 2016; Zerkle, Poulton, et al., 2017). Den-
itrification and anammox microbial processes re-
sult in large isotopic fractionation (~30‰, Stüeken,
Kipp, et al., 2016). Microbial denitrification, a pre-
dominantly anoxic process, generates a residual sed-
imentary ¹⁵N-enriched NO₃⁻ pool (δ¹⁵N<sub>bulk</sub> values
> 0‰) as most of the light ¹⁴N isotope is prefer-
entially returned to the atmosphere. Excess NO₃⁻
bioavailability is required to sustain denitrification
and retain a strong sedimentary isotopic signature.
On the other hand, N₂-fixation mediated by prokary-
otic cyanobacteria using Mo-bearing nitrogenase
enzyme (diazotrophs) is expected to shift sedimen-
tary δ¹⁵N<sub>bulk</sub> towards low or slightly negative isotopic
values (Bauersachs et al., 2009; X. Zhang et al., 2014).
This is because nitrogen isotope fractionation during
N₂ fixation is smaller (~2‰; Bauersachs et al., 2009;
Zerkle, Junium, et al., 2008). Bioavailable N is primar-
ily supplied to the ocean through N₂-fixation, where
atmospheric N₂ is converted to bioavailable NH₄⁺,
and returned to the atmosphere via denitrification
and anaerobic oxidation of NH₄⁺ (anammox) (Bauer-
sachs et al., 2009; Canfield, Glazer, et al., 2010; Cre-
monese, G. Shields-Zhou, et al., 2013; Cremonese,
G. A. Shields-Zhou, et al., 2014; Stüeken, 2013;
Stüeken, Kipp, et al., 2016). Furthermore, a well-
oxygenated surface environment of at least 11 µm
oxygen content (Fennel et al., 2005; Kipp, Stüeken,

et al., 2018) is a prerequisite for the oxidation of NH₄⁺
to NO₃⁻ by the aerobic nitrifying microorganisms.

The presence of profound eustatic changes in the Podolia sequence upward, shown by variations in lithology and increasing Y/Ho ratios, may have had an impact on the nitrogen dynamic from the Ediacaran to Silurian, which needs to be carefully considered. Intervals I and III, marked by mean positive δ¹⁵N<sub>bulk</sub> values of 4.6‰ (n = 18) and close to the ~+5‰ average value of modern oceans (Tesdal et al., 2013), are associated with sea-level rises. The transgressions during these intervals were likely not significant enough to cause major disruptions in biogeochemical nitrogen cycling. One possible explanation for the narrowing of N isotope composition is that water column partial denitrification and/or anammox at the redox transition zone produced a major loss of ¹⁴N delivered to the surface ocean. This also means that nitrogen fixation supported nitrification and denitrification, interpreted to mean that the water column must have been sufficiently oxidizing to promote quantitative conversion of NH₄⁺ to NO₃⁻. It was therefore likely that the latter dominated the dissolved inorganic nitrogen pool.

High bacterial abundance versus eukaryotic alga-
gae, together with low TOC contents and low hydro-
gen indices, revealed localized oligotrophic condi-
tions in the Podolia Basin during the middle to late
Ediacaran (Pehr et al., 2018). Nitrogen was not the
bio-limiting nutrient, as suggested by the positive
δ¹⁵N<sub>bulk</sub> values between +3.5 and +6.2‰, except one
sample of -2.8‰ (ibid.). The range of nitrogen iso-
tope compositions we report in the lowermost Edi-
acaran interval I strata (middle to late Ediacaran in
age, Soldatenko et al., 2019) is consistent with the
δ¹⁵N<sub>bulk</sub> range for other locations from epicontinen-
tal seas across the Podolia Basin (Pehr et al., 2018).
This strongly corroborates the idea that the micro-
bial ecosystem, dominated by prokaryotic N₂-fixing
microorganisms, could have formed a large seawater
nitrate pool. The proposed oligotrophic conditions
in the Podolia Basin could have been the result of low
nutrient fluxes between terrestrial and deep seawater
(ibid.). With persistent oligotrophic conditions, the
marine environments, localized in the Podolia Basin,
could have, however, favored the transient decrease
in δ¹⁵N<sub>bulk</sub> values to +1.8‰ in the upper Ediacaran
interval II. In nutrient-limited environments, pro-
ductivity becomes low, limiting the deposition of
organic matter, which is consistent with the lower
TOC and TN contents during interval II compared to
the underlying sediments (Figure 6). Therefore, ni-
trogen fixation rather than microbial denitrification
in the water column would have had, transiently, a
greater influence on δ¹⁵N<sub>bulk</sub> composition.

Marine N₂-fixation is an energy-intensive enzy-
matic process, which requires 16 ATP molecules per
N₂ molecule. Diazotrophic cyanobacteria, however,
compete poorly with eukaryotic algae except under
oligotrophic conditions that were dominant dur-
ing the Ediacaran intervals I and II. Therefore, the
rapid return of a nitrate reservoir strong enough to
leave a positive isotopic fingerprint at the Ediacaran-
Cambrian transition (lower part of the interval III)
could have first resulted from rapid replenishment
of the nitrate reservoir by N₂-fixing cyanobacteria
initiated in the upper Ediacaran interval II. This ni-
trate buildup would have been followed by changes
of ecological conditions at the interval II–III tran-
sition, where denitrifying organisms outcompeted
diazotrophic cyanobacteria. Intriguingly, the per-
sistence of this relatively stable NO₃⁻ pool coincides
with the rapid biodiversification and oxygenation
events that produced the Ediacaran and Cambrian
biota (Figure 8).

Although the presence of eustatic variations be-
tween intervals I and III, the δ¹³Corg values vary
moderately around a mean of about -28.5% and are
within the range reported in the Podolia Basin (Fig-
ure 6; Pehr et al., 2018). Given that these rocks have
not experienced thermal alteration, this suggests
that the carbon fixation pathways that produced
δ¹³C fractionation in organic compounds did not
experience radical compositional change through
this time period. Moreover, high Corg fractionation
linked to small-cell size, high surface/volume ratio,
and slow growth rates under oligotrophic conditions,
may have significantly lowered the δ¹³Corg values
(Tolosa et al., 2008). Therefore, we propose that the
δ¹³Corg values between intervals I and III have largely
originated from a similar microbial C source, which
would suggest that the chemocline—the locus of
multiple microbial processes—was deeper than the
depositional settings.

The persistent aerobic N cycling in the Ordovi-
cian and lowermost Silurian rocks (upper part of
the interval III, see geological background for ex-
planation of sedimentary gaps) could be intimately

linked to the GOBE and POE (Figures 3f–j, 6). These
observations may suggest that the availability of a
large seawater NO₃⁻ pool was possibly linked to the
evolution of the metazoans and the formation of a
complex aquatic food chain that promoted the in-
crease in complexity and animal size in the early
Paleozoic (Figure 8). The progressive decreasing
δ¹⁵Nbulk trend upward preserved in back-reef la-
goonal to reef environments associated with multi-
ple high-order transgressive-regressive cycles with
no definite long-term trends in the upper Silurian
rocks (interval IV) (Figure 6) (Verniers et al., 2008;
Radkovets, 2015), represents a shift towards the
dominance of diazotrophic primary productivity
over denitrification. Eustatic fluctuations have been
invoked in older depositional settings to explain the
increasing expression of quantitative denitrification
(M. Chang et al., 2019; Aubineau et al., 2021). Fur-
thermore, the δ¹⁵Nbulk pattern is not accompanied
by a comparable decrease in δ¹³Corg values, which
suggests that the C-fixing microbial ecosystem re-
mains similar and unperturbed above the redox
transition zone despite successive transgressive-
regressive cycles.

The depth of the chemocline controls the amount
of nitrate available for recharging the surface ocean
(Ader, Sansjofre, et al., 2014). The shallower the
chemocline is, the smaller the size of the nitrate
pool. In the Podolia Basin, the Silurian interval IV
was deposited on shallow water shelves within an
eustatic cyclicity (Radkovets, 2015; Poprawa et al.,
2018), which would have allowed the introduc-
tion of anoxic to euxinic deep waters into the shal-
low platform. Consistent with the latter proposi-
tion, accumulation of organic-rich sediments in
the open shelf (>100 m) of Baltica and other paleo-
continents was recorded, resulting from the ex-
pansion of anoxic/euxinic environments in the Sil-
urian (Verniers et al., 2008; Melchin et al., 2013; Rad-
kovets, 2015). The spread of anoxia in the Silurian
was attributed to several interwoven factors (e.g.,
sea-level rise, global warming, and reduced rates
of overturning circulation, among others; Melchin
et al., 2013). As explained above, it is reasonable
to assume that the shallow chemocline in the wa-
ter column of the Podolia Basin (<100 m) associ-
ated with repetitive transgression-regression cycles,
would have favored near-quantitative denitrifica-
tion and the expression of N₂ fixation in the studied
<figure><img src="image_9.png" /><figcaption>**Figure 8.** (A) Artist's representation illustrating the near-continuous Ediacaran to Silurian shallow seawater margin in southwestern Ukraine (from left to the right), reconstructed from the fossil assemblage preserved across the section (see Figure 1). White circle shows the emergence of abundant plankton biomass during the Silurian (copyright A. *El Albani*). (B) Dominant features of the Ediacaran–Silurian nitrogen cycle. The size of the arrows is proportional to the changes that accompanied the magnitude of the nitrogen and oxygen biogeochemical cycles and their influence on life forms in the seawater column. The nitrate-replete conditions operated in lower Ediacaran rocks, and Ediacaran–Cambrian transition to the lowermost Silurian rocks (intervals I and III), while the nitrate-poor conditions operated in the upper Ediacaran and upper Silurian rocks (intervals II and IV). The rapid return of nitrate-rich conditions was likely promoted by a high rate of nitrogen fixation in the late Ediacaran. The nitrate drawdown during the Silurian likely resulted from repetitive sea level rises and falls.</figcaption></figure>

depositional environments (Figure 8), lowering the
$\delta^{15}$N$_{bulk}$ toward 0‰. Deciphering whether this ni-
trate depletion was globally extended requires fur-
ther investigations. Alternatively, the low $\delta^{15}$N$_{bulk}$
signature could have resulted from a completely oxic
ocean with all denitrification restricted to sediments
(Ader, Sansjofre, et al., 2014; Ader, Thomazo, et al.,
2016). However, this is inconsistent with the pres-
ence of anoxic waters in the open shelf of Baltica
during the Silurian (Verniers et al., 2008; Melchin
et al., 2013; Radkovets, 2015). Thus, our inferences
highlight that the early Paleozoic life may have suf-

fered from transient and localized nitrate scarcity in
the Podolia Basin. Ocean redox conditions remained
regionally heterogeneous during the Silurian, like the
Neoproterozoic–Cambrian transition.

**6. Conclusion**

Our study presents the first comprehensive nitro-
gen isotope dataset across the Ediacaran–Silurian in-
terval in the Podolia Basin, integrated with carbon
and oxygen isotopes, TOC, and whole-rock geochem-
istry. This novel multiproxy approach reveals a direct
cause-effect relationship between seawater nitrate bioavailability and biological evolution. We propose that the increase in nutrient supply to seawater at the end of the Cryogenian Neoproterozoic Snowball Earth glaciations (Frei et al., 2009; Bjerrum and Canfield, 2011; Butterfield, 2011) accelerated remarkable Ediacaran–Cambrian diazotrophic nitrogen fixation, which, in turn, promoted the expansion of bioavailable N, including nitrate. Although geological hiasuses in the Podolia sedimentary sequence were observed in the early Paleozoic, the $NO_3^-$-rich waters lasted until the Silurian, leading to the remarkable complexification of life. The consequential occurrence of a long-term nitrate reservoir likely led to the rise of phytoplankton blooms, providing a rich food source that enabled the emergence of a new assemblage of efficient zooplankton that promoted the growth of larger size and diverse Silurian animals. The subsequent switch to progressive quantitative denitrification indicates that the Silurian seas of Baltica may have not been fully oxygenated. This proposition is best explained by the decreasing Upper Silurian $\delta^{15}N_{bulk}$ trend pointing to the consumption of the available seawater-rich $NO_3^-$ reservoir by intense eustatic fluctuations and the warm climate-invigorated growth of phytoplankton blooms in the photic zone.

## Declaration of interests

The authors do not work for, advise, own shares in, or receive funds from any organisation that could benefit from this article, and have declared no affiliations other than their research organisations.

## Acknowledgements

La Region Nouvelle-Aquitaine, French Embassy in Kiev. Universities of Schevchenko and National Mining University of Ukraine, French Ministry for Foreign Affairs and Centre National de la Recherche Scientifique (CNRS) are acknowledged for collaboration and support. Permit was provided by the University of Schevchenko. For assistance, we acknowledge C. Lebailly.

## Supplementary data

Supporting information for this article is available on the journal's website under https://doi.org/10.5802/crgeos.296 or from the author.

## References

Ader, M., P. Sansjofre, G. P. Halverson, V. Busigny, R. I. F. Trindade, M. Kunzmann and A. C. R. Nogueira, "Ocean redox structure across the Late Neoproterozoic oxygenation event: A nitrogen isotope perspective", *Earth Planet. Sci. Lett.* **396** (2014), pp. 1–13.

Ader, M., C. Thomazo, P. Sansjofre, V. Busigny, D. Papineau, R. Laffont, P. Cartigny and G. P. Halverson, "Interpretation of the nitrogen isotopic composition of Precambrian sedimentary rocks: Assumptions and perspectives", *Chem. Geol.* **429** (2016), pp. 93–110.

Algabri, M., Z. She, L. Jiao, et al., "Apatite-glucony association in the Ediacaran Doushantuo Formation, South China and implications for marine redox conditions", *Precambrian Res.* **347** (2020), article no. 105842.

Algeo, T. J., H. Hong and C. Wang, "The chemical index of alteration (CIA) and interpretation of ACNK diagrams", *Chem. Geol.* **671** (2025), article no. 122474.

Algeo, T. J. and J. B. Maynard, "Trace-element behavior and redox facies in core shales of Upper Pennsylvanian Kansas-type cyclothems", *Chem. Geol.* **206** (2004), pp. 289–318.

Algeo, T. J., P. A. Meyers, R. S. Robinson, H. Rowe and G. Q. Jiang, "Icehouse–greenhouse variations in marine denitrification", *Biogeosciences* **11** (2014), pp. 1273–1295.

Algeo, T. J. and N. Tribovillard, "Environmental analysis of paleo-oceanographic systems based on molybdenum–uranium co-variation", *Chem. Geol.* **268** (2009), pp. 211–225.

Aubineau, J., A. El Albani, E. Chi Fru, M. A. Kipp, J. Ngwal'ghoubou Ikouanga and A. Bekker, "Benthic redox conditions and nutrient dynamics in the ca. 2.1 Ga Franceville sub-basin", *Precambrian Res.* **360** (2021), article no. 106234.

Baldermann, A., S. Banerjee, G. Czuppon, et al., "Impact of green clay authigenesis on element sequestration in marine settings", *Nat. Commun.* **13** (2022), article no. 1527.

Banerjee, S., U. Bansal, K. Pande and S. S. Meena, "Compositional variability of glauconites within the Upper Cretaceous Karai Shale Formation, Cauvery Basin, India: Implications for evaluation of stratigraphic condensation", *Sedim. Geol.* **331** (2016), pp. 12–29.

Bansal, U., S. Banerjee, D. K. Ruidas and K. Pande, "Origin and geochemical characterization of the glauconites in the Upper Cretaceous Lameta Formation, Narmada Basin, central India", *J. Palaeogeogr.* **7** (2018), pp. 99–116.

Bau, M., P. Möller and P. Dulski, "Ytrium and lanthanides in eastern Mediterranean seawater and their fractionation during redox-cycling", *Mar. Chem.* **56** (1997), pp. 123–131.

Bauersachs, T., S. Schouten, J. Compaoré, U. Wollenzien, L. J. Stal and J. S. S. Damsteé, "Nitrogen isotopic fractionation associated with growth on dinitrogen gas and nitrate by cyanobacteria", *Limnol. Oceanogr.* **54** (2009), pp. 1403–1411.

Bhatia, M. R. and K. A. W. Crook, "Trace element characteristics of graywackes and tectonic setting discrimination of sedimentary basins", *Contr. Mineral. Petrol.* **92** (1986), pp. 181–193.

Bjerrum, C. J. and D. E. Canfield, "Towards a quantitative understanding of the late Neoproterozoic carbon cycle", *Proc. Natl. Acad. Sci. USA* **108** (2011), pp. 5542–5547.
Bowyer, F., R. A. Wood and S. W. Poulton, "Controls on the evolution of Ediacaran metazoan ecosystems: A redox perspective", *Geobiology* **15** (2017), pp. 516–551.

Bowyer, F. T., A. J. Shore, R. A. Wood, et al., "Regional nutrient decrease drove redox stabilisation and metazoan diversification in the late Ediacaran Nama Group, Namibia", *Sci. Rep.* **10** (2020), article no. 2240.

Bristow, T. F., M. J. Kennedy, A. Derkowski, M. L. Droser, G. Jiang and R. A. Creaser, "Mineralogical constraints on the paleoenvironments of the Ediacaran Doushantuo Formation", *Proc. Natl. Acad. Sci. USA* **106** (2009), pp. 13190–13195.

Brumsack, H. J., "The trace metal content of recent organic carbon-rich sediments: Implications for Cretaceous black shale formation", *Palaeogeogr. Palaeoclimatol. Palaeoecol.* **232** (2006), no. 2–4, pp. 344–361.

Butterfield, N. J., "Animals and the invention of the Phanerozoic Earth system", *Trends Ecol. Evol.* **26** (2011), pp. 81–87.

Canfield, D. E., "The early history of atmospheric oxygen: Homage to Robert M. Garrels", *Annu. Rev. Earth Planet. Sci.* **33** (2005), pp. 1–36.

Canfield, D. E., A. N. Glazer and P. G. Falkowski, "The evolution and future of Earth's nitrogen cycle", *Science* **330** (2010), pp. 192–196.

Canfield, D. E., S. W. Poulton, A. H. Knoll, G. M. Narbonne, G. Ross, T. Goldberg and H. Strauss, "Ferruginous conditions dominated Later Neoproterozoic deep-water chemistry", *Science* **321** (2008), pp. 949–952.

Carignan, J., P. Hild, G. Mevelle, J. Morel and D. Yeghicheyan, "Routine analyses of trace elements in geological samples using flow injection and low pressure on-line liquid chromatography coupled to ICP-MS: A study of geochemical reference materials BR, DR-N, UB-N, AN-G and GH", *Geostand. Newsl.* **25** (2001), pp. 187–198.

Chang, C., W. Hu, X. Wang, K. J. Huang, A. Yang and X. Zhang, "Nitrogen isotope evidence for an oligotrophic shallow ocean during the Cambrian Stage 4", *Geochim. Cosmochim. Acta* **257** (2019), pp. 49–67.

Chang, C., Z. Wang, K. J. Huang, H. Yun and X. Zhang, "Nitrogen cycling during the peak Cambrian explosion", *Geochim. Cosmochim. Acta* **336** (2022), pp. 50–61.

Chang, M., Y. Wang, Y. Pan, K. Zhang, L. Lyu, M. Wang and T. Zhu, "Nitrogen removal from wastewater via simultaneous nitrification and denitrification using a biological folded non-aerated filter", *Bioresour. Technol.* **289** (2019), article no. 121696.

Cole, D. B., D. B. Mills, D. H. Erwin, E. A. Sperling, S. M. Porter, C. T. Reinhard and N. J. Planavsky, "On the co-evolution of surface oxygen levels and animals", *Geobiology* **18** (2020), pp. 260–281.

Condie, K. C., "Chemical composition and evolution of the upper continental crust: Contrasting results from surface samples and shales", *Chem. Geol.* **104** (1993), pp. 1–37.

Cox, G. M., A. Jarrett, D. Edwards, P. W. Crockford, G. P. Halverson, A. S. Collins, A. Poirier and Z.-X. Li, "Basin redox and primary productivity within the Mesoproterozoic Roper Seaway", *Chem. Geol.* **440** (2016), pp. 101–114.

Cremonese, L., G. Shields-Zhou, U. Struck, H.-F. Ling, L. Och, X. Chen and D. Li, "Marine biogeochemical cycling during the early Cambrian constrained by a nitrogen and organic carbon isotope study of the Xiaotan section, South China", *Precambrian Res.* **225** (2013), pp. 148–165.

Cremonese, L., G. A. Shields-Zhou, U. Struck, H.-F. Ling and L. M. Och, "Nitrogen and organic carbon isotope stratigraphy of the Yangtze Platform during the Ediacaran–Cambrian transition in South China", *Palaeogeogr. Palaeoclimatol. Palaeoecol.* **398** (2014), pp. 165–186.

Dahl, T. W., E. U. Hammarlund, A. D. Anbar, et al., "Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish", *Proc. Natl. Acad. Sci. USA* **107** (2010), pp. 17911–17915.

Edwards, C. T., M. R. Saltzman, D. L. Royer and D. A. Fike, "Oxygenation as a driver of the great ordovician biodiversification event", *Nat. Geosci.* **10** (2017), no. 12, pp. 925–929.

Fedo, C. M., H. W. Nesbitt and G. M. Young, "Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols, with implications for paleoweathering conditions and provenance", *Geology* **23** (1995), pp. 921–924.

Fennel, K., M. Follows and P. G. Falkowski, "The co-evolution of the nitrogen, carbon and oxygen cycles in the Proterozoic ocean", *Am. J. Sci.* **305** (2005), pp. 526–545.

Frei, R., C. Gaucher, S. W. Poulton and D. E. Canfield, "Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes", *Nature* **461** (2009), pp. 250–253.

Grazhdankin, D. V., V. V. Marusin, J. Meert, M. T. Krupenin and A. V. Maslov, "Kotlin regional stage in the South Urals", *Dokl. Earth Sci.* **440** (2011), p. 1222.

Halverson, G. P., F. Poitrasson, P. F. Hoffman, A. Nédélec, J.-M. Montel and J. Kirby, "Fe isotope and trace element geochemistry of the Neoproterozoic syn-glacial Rapitan iron formation", *Earth Planet. Sci. Lett.* **309** (2011), pp. 100–112.

Hiller, S., "Quantitative analysis of Clay and other minerals in sandstones by X-ray powder diffraction (XRPD)", *International Association of Sedimentologists Special Publication* **34** (2003), pp. 213–251.

Isson, T. T. and N. J. Planavsky, "Reverse weathering as a long-term stabilizer of marine pH and planetary climate", *Nature* **560** (2018), pp. 471–475.

Ivantsov, A. Y., V. Grytsenko, V. Paliy, et al., *Upper Vendian Macrofossils of Eastern Europe. Middle Dniester Area and Volhynia, PIN RASS: Moscow, 2015.*

Jian, X., P. Guan, W. Zhang and F. Feng, "Geochemistry of Mesozoic and Cenozoic sediments in the northern Qaidam basin, northeastern Tibetan Plateau: Implications for provenance and weathering", *Chem. Geol.* **360–361** (2013), pp. 74–88.

Johnston, D. T., S. W. Poulton, T. Goldberg, V. N. Sergeev, V. Podkovyrov, N. G. Vorob'eva, A. Bekker and A. H. Knoll, "Late Ediacaran redox stability and metazoan evolution", *Earth Planet. Sci. Lett.* **335–336** (2012), pp. 25–35.

Johnston, D. T., S. W. Poulton, N. J. Tosca, T. O'Brien, G. P. Halverson, D. P. Schrag and F. A. Macdonald, "Searching for an oxygenation event in the fossiliferous Ediacaran of northwestern Canada", *Chem. Geol.* **362** (2013), pp. 273–286.

Kipp, M. A., T. J. Algeo, E. E. Stüeken and R. Buick, "Basinal hydrographic and redox controls on selenium enrichment and isotopic composition in Paleozoic black shales", *Geochim. Cosmochim. Acta* **287** (2020), pp. 229–250.

Kipp, M. A. and E. E. Stüeken, "Biomass recycling and Earth's early phosphorus cycle", *Sci. Adv.* **3** (2017), article no. eaa04795.

Kipp, M. A., E. E. Stüeken, M. Yun, A. Bekker and R. Buick, "Pervasive aerobic nitrogen cycling in the surface ocean across
the Paleoproterozoic Era", *Earth Planet. Sci. Lett.* **500** (2018), pp. 117–126.

Krause, A. J., B. J. W. Mills, S. Zhang, N. J. Planavsky, T. M. Lenton and S. W. Poulton, "Stepwise oxygenation of the Paleozoic atmosphere", *Nat. Commun.* **9** (2018), article no. 4081.

Krewer, C., S. W. Poulton, R. J. Newton, C. März, B. J. W. Mills and T. Wagner, "Controls on the termination of cretaceous oceanic anoxic event 2 in the Tarfaya Basin, Morocco", *Am. J. Sci.* **324** (2024), article no. 11.

Large, R. R., I. Mukherjee, I. Zhukova, R. Corkrey, A. Stepanov and L. V. Danyushevsky, "Role of upper-most crustal composition in the evolution of the Precambrian ocean–atmosphere system", *Earth Planet. Sci. Lett.* **487** (2018), pp. 44–53.

Laverov, N. P., L. I. Lobkovsky, M. V. Kononov, N. L. Dobretsov, V. A. Vernikovsky, S. D. Sokolov and E. V. Shipilov, "A geodynamic model of the evolution of the Arctic basin and adjacent territories in the Mesozoic and Cenozoic and the outer limit of the Russian Continental Shelf", *Geotectonics* **47** (2013), pp. 1–30.

Lenton, T. M., T. W. Dahl, S. J. Daines, B. J. W. Mills, K. Ozaki, M. R. Saltzman and P. Porada, "Earliest land plants created modern levels of atmospheric oxygen", *Proc. Natl. Acad. Sci. USA* **113** (2016), pp. 9704–9709.

Li, Z.-Q., L.-C. Zhang, C.-J. Xue, et al., "Earth's youngest banded iron formation implies ferruginous conditions in the Early Cambrian ocean", *Sci. Rep.* **8** (2018), article no. 9970.

Liu, J. and T. J. Algeo, "Beyond redox: Control of trace-metal enrichment in anoxic marine facies by watermass chemistry and sedimentation rate", *Geochim. Cosmochim. Acta* **287** (2020), pp. 296–317.

Liu, Y., J. M. Magnall, S. A. Gleeson, F. Bowyer, S. W. Poulton and J. Zhang, "Spatio-temporal evolution of ocean redox and nitrogen cycling in the early Cambrian Yangtze ocean", *Chem. Geol.* **554** (2020), article no. 119803.

Luo, G., T. J. Algeo, R. Zhan, D. Yan, J. Huang, J. Liu and S. Xie, "Perturbation of the marine nitrogen cycle during the Late Ordovician glaciation and mass extinction", *Palaeogeogr. Palaeoclimatol. Palaeoecol.* **448** (2016), pp. 339–348.

Maher, K. and C. P. Chamberlain, "Hydrologic regulation of chemical weathering and the geologic carbon cycle", *Science (New York, NY)* **343** (2014), pp. 1502–1504.

McLennan, S. M., "Relationships between the trace element composition of sedimentary rocks and upper continental crust", *Geochem. Geophys. Geosyst.* **2** (2001), no. 4, article no. 1021.

McLennan, S. M., S. Hemming, D. K. McDaniel and G. N. Hanson, "Geochemical approaches to sedimentation, provenance, and tectonics", in *Processes Controlling the Composition of Clastic Sediments* (Johnsson, M. J. and A. Basu, eds.), Geological Society of America, 1993.

Melchin, M. J., C. E. Mitchell, C. Holmden and P. Šorch, "Environmental changes in the Late Ordovician–early Silurian: Review and new insights from black shales and nitrogen isotopes", *GSA Bull.* **125** (2013), pp. 1635–1670.

Meunier, A. and A. El Albani, "The glauconite–Fe-illite–Fe-smectite problem: a critical review", *Terra Nova* **19** (2007), pp. 95–104.

Meunier, A. and B. Velde, "The mineralogy of illite—what is illite?", in *Illite: Origins, Evolution and Metamorphism* (Meunier, A. and B. Velde, eds.), Springer: Berlin, Heidelberg, 2004, pp. 3–62.

Michel, J., E. H. Owens, S. Zengel, et al., "Extent and degree of shoreline oiling: Deepwater Horizon oil spill, Gulf of Mexico, USA", *PLoS One* **8** (2013), no. 6, article no. e65087.

Moore, D. M. and R. C. Reynolds Jr., *X-ray Diffraction and the Identification and Analysis of Clay Minerals*, 2nd edition, Oxford University Press: Oxford, 1997.

Narbonne, G. M., "The Ediacara biota: Neoproterozoic origin of animals and their ecosystems", *Annu. Rev. Earth Planet. Sci.* **33** (2005), pp. 421–442.

Nesbitt, H. W. and G. M. Young, "Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations", *Geochim. Cosmochim. Acta* **48** (1984), pp. 1523–1534.

Nikiforova, O. I., N. N. Priedtchiensky, A. F. Abushik, M. M. Ignatovich, T. L. Modzalevskaya, A. Y. Berger, L. S. Novoselova and Y. K. Burkov, *Silurian and Lower Devonian Key Section of Podolia*, Ministry of Geology of the USSR, Transactions, 5, Ministry of Geology of the USSR, 1972, pp. 1–258.

Odin, G. S. and A. Matter, "De glauconiarum origine", *Sedimentology* **28** (1981), pp. 611–641.

Partin, C. A., A. Bekker, N. J. Planavsky, et al., "Large-scale fluctuations in Precambrian atmospheric and oceanic oxygen levels from the record of U in shales", *Earth Planet. Sci. Lett.* **369–370** (2013), pp. 284–293.

Pehr, K., G. D. Love, A. Kuznetsov, V. Podkovyrov, C. K. Junium, L. Shumlyanskyy, T. Sokur and A. Bekker, "Ediacara biota flourished in oligotrophic and bacterially dominated marine environments across Baltica", *Nat. Commun.* **9** (2018), article no. 1807.

Poprawa, P., N. Radkovets and J. Rauball, "Ediacaran–Paleozoic subsidence history of the Volyn-Podillya-Moldavia Basin (W and SW Ukraine, Moldova, NE Romania)", *Geol. Q.* **62** (2018), no. 3, pp. 459–486.

Prokopenko, M. G., D. E. Hammond and L. D. Stott, "Lack of isotopic fractionation of d15N of organic matter during long-term diagenesis in marine sediments, ODP Leg 202, Sites 1234 and 1235", in *Proceedings of the Ocean Drilling Program, Scientific Results* (Tiedemann, R., A. C. Mix, C. Richter and W. F. Ruddiman, eds.), Ocean Drilling Program: College Station, TX, 2006, pp. 1–22. Sediment and pore water N and d15N concentrations of ODP Holes 202-1234A and 202-1235A (supplement).

Radkovets, N., "The Silurian of southwestern margin of the East European Platform (Ukraine, Moldova and Romania): lithofacies and palaeoenvironments", *Geol. Q.* **59** (2015), pp. 105–118.

Rafiei, M. and M. Kennedy, "Weathering in a world without terrestrial life recorded in the Mesoproterozoic Velkerri Formation", *Nat. Commun.* **10** (2019), article no. 3448.

Redfield, A. C., "The biological control of chemical factors in the environment", *Am. Sci.* **46** (1958), 230A, 205–221.

Reinhard, C. T., N. J. Planavsky, B. C. Gill, et al., "Evolution of the global phosphorus cycle", *Nature* **541** (2017), pp. 386–389.

Reinhard, C. T., N. J. Planavsky, L. J. Robbins, et al., "Proterozoic ocean redox and biogeochemical stasis", *Proc. Natl. Acad. Sci. USA* **110** (2013), pp. 5357–5362.

Reynolds Jr., R. C., *NEWMOD, a computer program for the calculation of one-dimensional diffraction patterns of mixed-layered clays*, 1985. RC Reynolds, 8.
Sahoo, S. K., N. J. Planavsky, G. Jiang, et al., "Oceanic oxygenation events in the anoxic Ediacaran ocean", *Geobiology* **14** (2016), pp. 457–468.

Sahoo, S. K., N. J. Planavsky, B. Kendall, et al., "Ocean oxygenation in the wake of the Marinoan glaciation", *Nature* **489** (2012), pp. 546–549.

Shields, G. and P. Stille, "Diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies: an isotopic and REE study of Cambrian phosphorites", *Chem. Geol.* **175** (2001), pp. 29–48.

Shulga, V. F., A. Zdanovski, L. B. Zaytseva and A. V. Ivanova, *Correlation of the Carboniferous coal-bearing formation of the Lviv-Volyn and Lublin basins*, Varta: Kiev, 2007, p. 427.

Soldatenko, Y., A. El Albani, M. Ruzina, C. Fontaine, V. Nesterovsky, J.-L. Paquette, A. Meunier and M. Ovtcharova, "Precise U-Pb age constrains on the Ediacaran biota in Podolia, East European Platform, Ukraine", *Sci. Rep.* **9** (2019), article no. 1675.

Sperling, E. A., M. J. Melchin, T. Fraser, et al., "A long-term record of early to mid-Paleozoic marine redox change", *Sci. Adv.* **7** (2021), article no. eabf4382.

Środon, J., M. Paszkowski, D. Drygant, A. Anczkiewicz and M. Banaś, "Thermal history of Lower Paleozoic Rocks on the Peri-Tornquist Margin of the East European Craton (Podolia, Ukraine) inferred from combined XRD, K-Ar, and AFT data", *Clays Clay Miner.* **61** (2013), pp. 107–132.

Stüeken, E. E., "A test of the nitrogen-limitation hypothesis for retarded eukaryote radiation: Nitrogen isotopes across a Mesoproterozoic basinal profile", *Geochim. Cosmochim. Acta* **120** (2013), pp. 121–139.

Stüeken, E. E., M. A. Kipp, M. C. Koehler and R. Buick, "The evolution of Earth's biogeochemical nitrogen cycle", *Earth-Sci. Rev.* **160** (2016), pp. 220–239.

Stüeken, E. E., J. Zaloumis, J. Meixnerová and R. Buick, "Differential metamorphic effects on nitrogen isotopes in kerogen extracts and bulk rocks", *Geochim. Cosmochim. Acta* **217** (2017), pp. 80–94.

Tang, D., X. Shi, G. Jiang, X. Zhou and Q. Shi, "Ferruginous seawater facilitates the transformation of glauconite to chamosite: An example from the Mesoproterozoic Xiamaling Formation of North China", *Am. Mineral.* **102** (2017), pp. 2317–2332.

Taylor, S. R. and S. M. McLennan, *The Continental Crust: Its Evolution and Composition*, Blackwell: London, 1985.

Tesdal, J.-E., E. D. Galbraith and M. Kienast, "Nitrogen isotopes in bulk marine sediment: linking seafloor observations with subseafloor records", *Biogeosciences* **10** (2013), pp. 101–118.

Tolosa, I., J.-C. Miquel, B. Gasser, P. Raimbault, C. Goyet and H. Claustre, "Distribution of lipid biomarkers and carbon isotope

fractionation in contrasting trophic environments of the South East Pacific", *Biogeosciences* **5** (2008), pp. 949–968.

Tribovillard, N., T. J. Algeo, F. Baudin and A. Riboulleau, "Analysis of marine environmental conditions based onmolybdenum-uranium covariation—Applications to Mesozoic paleoceanography", *Chem. Geol.* **324–325** (2012), pp. 46–58.

Tribovillard, N., T. J. Algeo, T. Lyons and A. Riboulleau, "Trace metals as paleoredox and paleoproductivity proxies: An update", *Chem. Geol.* **232** (2006), pp. 12–32.

Velde, B., "9.12 - Green clay minerals", in *Treatise on Geochemistry*, 2nd edition (Holland, H. D. and K. K. Turekian, eds.), Elsevier: Oxford, 2014, pp. 351–364.

Velikanov, V. A., E. A. Asseeva and M. A. Fedonkin, *Ukranian Vendi* (Ryabenko, V. A., ed.), Naukova Dumka, 1983, p. 164.

Verniers, J., J. Maletz, J. Kříž, Ž. Žigait, F. Paris, H. P. Schönlaub and R. Wrona, "Silurian", in *Geology of the Central Europe*, The Geological Society: London, 2008, pp. 249–302.

Wallace, M. W., A. vS. Hood, A. Shuster, A. Greig, N. J. Planavsky and C. P. Reed, "Oxygenation history of the Neoproterozoic to early Phanerozoic and the rise of land plants", *Earth Planet. Sci. Lett.* **466** (2017), pp. 12–19.

Wang, X., G. Jiang, X. Shi, Y. Peng and D. C. Morales, "Nitrogen isotope constraints on the early Ediacaran ocean redox structure", *Geochim. Cosmochim. Acta* **240** (2018), pp. 220–235.

Wood, R., P. C. J. Donoghue, T. M. Lenton, A. G. Liu and S. W. Poulton, "The origin and rise of complex life: progress requires interdisciplinary integration and hypothesis testing", *Interface Focus* **10** (2020), article no. 20200024.

Yang, S., W. Hu, X. Wang and J. Fan, "Nitrogen isotope evidence for a redox-stratified ocean and eustasy-driven environmental evolution during the Ordovician–Silurian transition", *Glob. Planet. Change* **207** (2021), article no. 103682.

Zerkle, A. L., C. K. Junium, D. E. Canfield and C. H. House, "Production of ¹⁵N-depleted biomass during cyanobacterial N₂-fixation at high Fe concentrations", *J. Geophys. Res. BiogeoSCI* **113** (2008), no. G3, article no. G03014.

Zerkle, A. L., S. W. Poulton, R. J. Newton, C. Mettam, M. W. Claire, A. Bekker and C. K. Junium, "Onset of the aerobic nitrogen cycle during the Great Oxidation Event", *Nature* **542** (2017), pp. 465–467.

Zhang, J., T. Fan, T. J. Algeo, Y. Li and J. Zhang, "Paleo-marine environments of the Early Cambrian Yangtze Platform", *Palaeogeogr. Palaeoclimatol. Palaeoecol.* **443** (2016), pp. 66–79.

Zhang, S., X. Bai, C. Zhao, et al., "Global CO₂ consumption by silicate rock chemical weathering: its past and future", *Earth's Future* **9** (2021), article no. e2020EF001938.

Zhang, X., D. M. Sigman, F. M. M. Morel and A. M. L. Kraepiel, "Nitrogen isotope fractionation by alternative nitrogenases and past ocean anoxia", *Proc. Natl. Acad. Sci. USA* **111** (2014), pp. 4782–4787.