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\DOI{10.5802/crgeos.325}
\datereceived{2025-11-28}
\daterevised{2026-01-15}
\dateaccepted{2026-01-15}
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\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\TopicFR{S\'edimentologie, stratigraphie, g\'eologie des bassins}
\TopicEN{Sedimentology, stratigraphy, basin geology}

\title{A magnetite-driven cryptic iron cycle}

\alttitle{Un cycle cryptique du fer r\'{e}v\'{e}l\'{e} par la magn\'{e}tite}

\author{\firstname{Nicolas} \lastname{Tribovillard}\CDRorcid{0000-0003-3493-5579}}
\address{Universit\'{e} de Lille, UMR 8187 LOG -- Laboratoire
d'Oc\'{e}anologie et de G\'{e}osciences, Univ. Lille, CNRS, Univ.
Littoral C\^{o}te d'Opale, IRD, 59000 Lille, France}
\email{nicolas.tribovillard@univ-lille.fr}

\keywords{\kwd{Boulonnais}
\kwd{Magnetite}
\kwd{Glauconite}
\kwd{Pyrite}
\kwd{Diagenesis}
\kwd{Authigenesis}}

\altkeywords{\kwd{Boulonnais}
\kwd{Magn\'{e}tite}
\kwd{Glauconite}
\kwd{Pyrite}
\kwd{Diagen\`{e}se}
\kwd{Authigen\`{e}se}}

\thanks{Programme Tellus Syster of the French Institut des Sciences
de l'Univers (INSU), Monique Gentric}

\begin{abstract}
Iron is essential for the proper functioning of the entire production
chain of marine biomass and of the ocean's food web. However, its
biogeochemical behavior often makes it a limiting factor in ocean
functioning. On a geological timescale, the initially reactive-iron
reservoir is most often stored as iron sulfides in the sedimentary
record. This study focuses on episodes occurring during the earliest
stages of diagenesis. It shows that there is a cycle of reversible
transformations of iron states before the situation becomes fixed by
the formation of iron sulfides, the most emblematic of which is pyrite.
The material studied here is an alternation of diagenetic limestone
beds and marly interbeds of Tithonian age, observed along the cliffs of
the Boulonnais region (Strait of Dover, France), and known as the
Assise de Cro\"{i} Formation. The early, authigenic carbonates of the
limestone beds trapped iron-bearing, authigenic minerals, notably
magnetite. This made visible the iron cycle, which can be described as
cryptic because it goes unnoticed if nothing reveals it. This
``fossilization'' of the early stages of iron distribution, through the
precipitation of diagenetic limestone, allows for a more refined
understanding of the carbon cycle, particularly in its shallow marine
compartment: indeed, the cryptic iron cycle actively participates in
the remineralization of fragile (labile) organic matter.
\vspace*{-4pt}
\end{abstract}

\begin{altabstract} 
Le fer est essentiel au bon fonctionnement de l'ensemble de la
cha\^{i}ne de production de la biomasse marine et du r\'{e}seau
trophique oc\'{e}anique. Cependant, son comportement biog\'{e}ochimique
en fait souvent un facteur limitant pour le fonctionnement des
oc\'{e}ans. \`{A} l'\'{e}chelle des temps g\'{e}ologiques, le
r\'{e}servoir de fer initialement r\'{e}actif est le plus souvent
stock\'{e} sous forme de sulfures de fer dans les archives
s\'{e}dimentaires. Cette \'{e}tude porte sur des \'{e}pisodes survenant
lors des premi\`{e}res \'{e}tapes de la diagen\`{e}se. Elle montre
l'existence d'un cycle de transformations r\'{e}versibles des \'{e}tats
du fer avant que la situation ne se stabilise par la formation de
sulfures de fer, dont le plus embl\'{e}matique est la pyrite. Le
mat\'{e}riau \'{e}tudi\'{e} ici est une alternance de bancs de calcaire
diag\'{e}n\'{e}tique et d'intercalations marneuses d'\^{a}ge tithonien,
observ\'{e}e le long des falaises du Boulonnais (Pas de Calais, France)
et connue sous le nom de Formation de l'Assise de Cro\"{i}. Les
carbonates authig\`{e}nes pr\'{e}coces des bancs de calcaire ont
pi\'{e}g\'{e} des min\'{e}raux authig\`{e}nes contenant du fer,
notamment la magn\'{e}tite. Ceci a rendu visible le cycle du fer, que
l'on peut qualifier de cryptique car il passe inaper\c{c}u en l'absence
de tout \'{e}l\'{e}ment r\'{e}v\'{e}lateur. Cette 
\og fossilisation \fg des premiers stades 
de la distribution du fer, par la pr\'{e}cipitation
de calcaire diag\'{e}n\'{e}tique, permet une compr\'{e}hension plus
fine du cycle du carbone, notamment dans son compartiment marin peu
profond : en effet, le cycle cryptique du fer participe activement
\`{a} la remin\'{e}ralisation de la mati\`{e}re organique fragile
(labile).
\end{altabstract}

%\input{CR-pagedemetas}

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\defcitealias{Mansy2007}{ibid.}

\section{Introduction}\label{sec1}
Iron is widely recognized for its crucial role in the functioning of
terrestrial and marine ecosystems \citep{Raiswell2012,Lin2025}.
However, in the marine environment, iron can only
influence productivity and the functioning of the food chain if it can
participate in (bio-)chemical reactions. This is what can be called
\textit{reactive iron}. Iron is not reactive if it is present, that is,
locked, within the crystal lattices of minerals, depending on the
mineral solubility \citep[e.g.,][]{Raiswell1998}. Located inside the
crystallographic network of a lowly soluble mineral such as pyrite,
iron cannot participate in the (bio-)chemical reaction. Iron, which is
\guillemotleft{}~trapped~\guillemotright{}~within the structure of minerals, can still be released
and become reactive if the crystal \mbox{lattices} that contain it are
disrupted during \mbox{diagenesis} or alteration. However, this transformation
can be quite time-consuming, ranging from a few days to a much longer
period, even a million years \citep[e.g.,][]{Canfield1999,Raiswell1998,Raiswell2012}. 
In marine environments, particularly
continental shelf environments, reactive iron inputs originate from the
emerged landmasses. These inputs are carried by rivers and winds 
\citep[e.g.,][and references therein]{Chester2012}. Fluvial (river-borne) iron
is in the form of organometallic and clay-humic complexes from which it
can easily be released. Aeolian (airborne) iron is primarily adsorbed
onto the surface of clay minerals and is carried away by the winds,
especially from desert regions worldwide
\citep{Chester2012,Vosteen2024}. In proximal \mbox{environments} (shoreface
and upper offshore), aeolian inputs are \mbox{often} masked by fluvial inputs,
the fluxes of which are generally much higher. Aeolian inputs should
not, however, be forgotten or neglected. Once introduced into the
marine environment, iron can present two redox states, Fe$^{2+}$ or
Fe$^{3+}$, and its solubility depends on the oxidation state. Several
factors contribute to immobilizing the iron present in the water column
and bringing it to the sediments: it can form poorly soluble oxides and
oxyhydroxides (and among them, magnetite); it can react with sulfide
ions, and form pyrite, directly or with intermediate phases; it can
also be incorporated into more complex authigenic minerals such as
phyllosilicates, and, in particular, glauconite. Our recent work
\citep{Tribovillard2025a} examined both glauconite
((K,Na)$_{2}$(Fe$^{3+}$,Fe$^{2+}$,Al,Mg)$_{4}$[Si$_{6}$(Si,Al)$_{2}$O$_{20}$](OH)$_{4}$)~and 
magnetite (Fe$_{3}$O$_{4}$). Both minerals contain Fe(II) and
Fe(III). Our result have shown that glauconite and magnetite can form
early and very rapidly in shallow environments where the abundance of
reactive iron was not a limiting factor and where redox conditions
allowed iron to be present in both its valences
\citep{Tribovillard2023a,Tribovillard2023b,Tribovillard2024}. We will
describe as \textit{weakly reducing} such conditions where iron is not
entirely present in its reduced form. Reciprocally, environments in
which reactive iron is exclusively present in its reduced form will be
designated as \textit{strongly reducing}. Our work has shown that
authigenic, iron bearing minerals formed in shoreface environments
(such as beach sands) can disappear during subsequent stages of
diagenesis if nothing protects them. Nevertheless, early carbonate
cementation can typically protect glauconite and magnetite from the
expected usual diagenetic evolution, namely, dissolution or
transformation into stable structures such as pyrite 
\citep{Tribovillard2025a,Tribovillard2025b}. In the present study, we
explore the authigenic trapping of iron in the form of magnetite, still
in continental shelf deposits, but in deeper environments (lower
offshore). We have identified Jurassic sedimentary deposits from the
Boulonnais (northernmost France) combining the presence of reactive
iron and weakly reducing redox conditions at the time of deposition. We
isolated magnetite from these rocks and observed it using electron
microscopy and Raman spectroscopy. Our goal was to provide \mbox{answers} to
the following questions: why is authigenic magnetite not more
frequently observed in sedimentary rocks, whereas glauconite is
relatively common? What is the role or place of authigenic magnetite in
the sedimentary iron cycle? What does the presence of magnetite mean
with regard to paleoenvironmental reconstruction?

\section{Material and methods}\label{sec2}

\subsection{Material}\label{sec21}

To study the distribution of authigenic or terrigenic (inherited)
magnetite, the geological formation called the Assise de Cro{\"{i}} was
chosen. This formation is visible on the cliff on both sides of the
town of Wimereux, wich t is to say to the north and to the south, and
the finest outcrop, from a visual point of view, is to the south of
Wimereux, near the small promontory called Pointe de la Cr\`{e}che
(Figure~\ref{fig1}). This formation, also called Argiles et Calcaires
de la Tour de Cro{\"{i}} \citep{Deconinck1996,Mansy2007,Deconinck2008}
is of Tithonian age (albani and glaucolithus ammonite zones) and
appears in the form of alternating limestone beds and marly interbeds 
(Figures~\ref{fig1} and~\ref{fig2}). Its total thickness is about 8 m;
the limestone beds are generally less than 20~cm thick and result from
the coalescence of limestone nodules. The marly interbeds are a bit
thicker than the beds (Figure~\ref{fig2}). The Assise de Cro{\"{i}} is
made up of sediments deposited under upper offshore conditions
\citep{Mansy2007}. {The Assise de Cro}{\"{i}} overlies the Argiles de
Wimereux Formation, a marly sequence also reflecting lower offshore
depositional conditions \citepalias{Mansy2007}. It is overlain by the
Gr\`{e}s de la Pointe aux Oies Formation, made up with sandstones beds
and corresponding to shoreface depositional conditions
\citep{Deconinck2000}.

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Location of the outcrop shown on
Figure~\ref{fig2}, with a schematic stratigraphic log of the topmost
part of the Jurassic deposits of the Boulonnais region. Maps from the
\textit{G\'{e}oportail} website of the French \textit{Institut national
de l'information g\'{e}ographique et foresti\`{e}re (IGN)}.}
\end{figure*}

\begin{figure*}
\includegraphics{fig02}
\vspace*{-1pt}
\caption{\label{fig2}(A)~The topmost part of the Jurassic deposits
exposed along the cliff between the Cap de la Cr\`{e}che and Wimereux
city (Figure~\ref{fig1}). (B)~Close up view of the alternation of
carbonate beds and marly interbeds of the Assise de Cro\"i Formation.}
\vspace*{-2pt}
\end{figure*}

Whether calcareous or marly, the Assise de Cro{\"{i}} deposits are
bioturbated and rich in bivalve bioclasts, most often \textit{Nanogyra}
and \textit{Trigonia}. They are also relatively rich in glauconite.
The work of \citet{Tribovillard2023b} showed that the nodular beds were
of early diagenetic origin and that the glauconite they contained was
authigenic and even earlier than the calcareous nodules. The
geochemical study by \citet{Tribovillard2025b} showed that the
enrichment of trace elements markers of redox conditions was modest and
that the depositional conditions were weakly reducing. The presence of
authigenic glauconite is consistent with these results, since this
mineral, which contains iron in its two oxidation states, can only form
under weakly reducing conditions. Moreover, the presence of this
mineral shows that reactive iron was not a limiting factor at the time
of deposition \citep{Tribovillard2024,Tribovillard2025a}. 
All these reasons explain why the Assise de Cro{\"{i}} was chosen to study
the magnetite content, whose authigenic formation conditions are
similar to those of glauconite.

The samples examined for the present work are the same as those studied
in \citet{Tribovillard2023b} and \citet{Tribovillard2025b}. The sample
set was collected from eight bed-interbed pairs in the lower part of
the Assise de Cro\"{i}, north of Wimereux, where the outcrop conditions
are propitious to sampling.

\subsection{Methods}\label{sec22}
The limestone or marl samples were crushed using a jaw crusher to
ensure that the maximum fragment size was only a few millimeters. The
analytical protocol is intended to concentrate iron-bearing minerals,
as well as other non-carbonate fractions (quartz, heavy minerals, clay
minerals,\,{\ldots}). \citet{Poulton2005} investigated the sequential
extractions that could be employed to analyze the distribution of iron
among various mineral phases. Notably, the authors emphasize that the
dissolution of carbonate while preserving magnetite necessitates the
use of acetic acid instead of hydrochloric acid. Here, we used dilute
acetic acid (10\%) to remove the carbonate fraction without damaging
the magnetite grains (as evidenced through microscope observation. The
samples were then rinsed and dried, and the magnetite grains were
isolated by magnetic separation. We used a Franz Isodynamic magnetic
separator applying the protocol described in 
\citet{Tribovillard2025a}. The particles were imaged using a scanning electron microscope
(SEM) equipped with an EDS-type analytical probe (FEI Quanta~200 SEM
equipped with a Quantax QX2 Roentec energy-dispersive X-ray
spectroscopy system). To determine or confirm the mineralogical
composition of magnetite and other accessory minerals, Raman
microspectroscopy was performed with a JY Horiba, Labram HR800UV,
equipped with an electronically cooled charge-coupled device (CCD)
detector, using 532\,nm laser excitation. Spectra was obtained using
$100{\times}$ magnification objective.

\section{Results}\label{sec3}
Magnetite grains are present in both facies of the Assise de Cro\"{i},
i.e., nodular limestone beds and marly interbeds. However, the
abundance of these grains is very low (a few grains out of 50--100~g of
rock) and could not be quantified. It should be reminded that~these two
facies are also rich in glauconite 
\unskip\break\citep{Tribovillard2023a,Tribovillard2023b}, 
it will be discussed below. 

\subsection{Authigenic carbonate beds}\label{sec31}


The nodular carbonate beds contain two types of magnetite grains: (1)
relatively small grains with euhedral morphology (Figure~\ref{fig3})
and, (2) to a lesser extent, larger, rounded grains showing signs of
wear (Figure~\ref{fig4}). The euhedral grains are made of nickel- and
chromium-rich magnetite and contain apatite inclusions
(Figure~\ref{fig3}). As regards the rounded grains, they do not contain
apatite inclusions, nor do they show enrichment in nickel or chromium,
but they are very clearly enriched in titanium. These are
titanomagnetite grains.

\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3}Small dimensioned, euhedral grains of authigenic
magnetite. (A)~Picture after photonic microscope observation, (B,~C) 
scanning electron microscope (SEM) images.}
\end{figure}

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Large dimensioned, rounded grains of terrigenic
magnetite. (A)~Picture after photonic microscope observation; 
(B--D)~SEM images of magnetic grains, commonly yielding the typical lattice
structures observed with magnetite from igneous rocks (D).}
\end{figure}

\subsection{Marlstone interbeds}\label{sec32}
The marly interbeds contain only titanomagnetite grains, which also
show signs of wear. No euhedral magnetite was observed in the
interbeds.

To summarize, worn titanomagnetite grains are present both in the beds
and interbeds, but in addition, the beds also contain small euhedral
magnetite grains showing apatite inclusions and often enriched in
nickel and chromium.

\section{Interpretations}\label{sec4}

\subsection{Two types of magnetite grains}\label{sec41}
Our observations allow two types of grains to be identified. The
largest ones are made up of titanomagnetite and show traces of wear.
Titanomagnetite is common in igneous (and metamorphic) rocks 
\citep[strictly, it is a homogeneous cubic solid solution of 
ilmenite and magnetite;
e.g.,][]{Moskowitz2015}. Consequently, we will consider that, in
the present case, the titanomagnetite grains are of terrigenous origin,
consistent with their round-shaped morphologies. 

Reciprocally, the small euhedral grains will be considered to be
authigenic. They are too large to be interpreted as deriving directly
from bacterial activity (magneto-tactic bacteria): bacterially secreted
biogenic grains are reported to be sub-micrometric in scale
\citep[e.g.,][]{Lin2020,Yamazaki2020,Mathon2024}. However, bio-induced
magnetite resulting from the activity of dissimilatory iron-reducing
bacteria may be micrometric or pluri-micrometer large \citep{Lin2024}.
This~size~range is compatible with what is observed in the present
work. Therefore, terrigenic magnetite is present everywhere in the
Assise de Cro\"{i}, whereas authigenic magnetite is present only in the
carbonate nodular beds.

\subsection{Reconstruction of the authi-diagenetic sequence}\label{sec42}
The deposits of the Assise de Cro\"{i}, whatever their facies---either
diagenetic carbonate beds or marly interbeds, contain small amounts of
terrigenic magnetite, closer to titanomagnetite than to magnetite
\textit{stricto sensu}. This terrigenic (titano)magnetite accompanies
quartz grains that are also terrigenic \citep{Tribovillard2024}, and
it was not completely dissolved during later stages of diagenesis. What
does this tell us about paleoenvironmental conditions?

Magnetite and titanomagnetite are minerals relatively sensitive to
reductive dissolution \citep{Nowaczyk2011}. This is to be ascribed
to the redox variations of the iron oxidation state. According to the
classical reaction succession of early diagenesis associated with
organic matter remineralization (Figure~\ref{fig5}), iron reduction
(from Fe$^{3+}$ to Fe$^{2+}$) starts after the sequential progression
from aerobic respiration, to nitrate reduction (a.k.a. denitrification)
and to manganese reduction 
\citep{Jorgensen2006,Jorgensen2021,Burdige2006,Adhikari2017}. Below the
iron redox boundary, where iron reduction begins, magnetic mineral
dissolution may occur \citep{Canfield1987,Yamazaki2020}, as the
following equation may illustrate it:
{\begin{equation}\label{eq1}
\mathrm{Fe}_{3}\mathrm{O}_{4} + \mathrm{H}_{2} + 
3 \mathrm{H}_{2}\mathrm{S} \rightarrow 3 \mathrm{FeS} + 
4 \mathrm{H}_{2}\mathrm{O}
\end{equation}}\unskip
With on-going reduction, sediments undergo sulfidic diagenesis and iron
sulfide minerals such as greigite and pyrite may grow within the
sediments at the expense of magnetite
\citep{Rowan2009,Roberts2015,Runge2023}, provided that reactive iron
is present in appropriate amounts. Chemical and biological reduction of
magnetite by dissolved sulfide has been extensively studied. For
instance, \textit{Shewanella putrefaciens} is capable of rapid
dissolution and reduction of magnetite via enzymatic processes at the
in vitro scale \citep{Kostka1995}. The bacterially mediated
reduction of \mbox{magnetite} (using organic matter as an electron 
\mbox{supplier})
may be illustrated with the following equation:
{\begin{equation}\label{eq2}
2 \mathrm{Fe}_{3}\mathrm{O}_{4} + 
\mathrm{CH}_{2}\mathrm{O}+ 11 \mathrm{H}^{+} \rightarrow 6 
\mathrm{Fe}^{2+} + \mathrm{HCO}_{3}^{-} + 6 \mathrm{H}_{2}\mathrm{O}
\end{equation}}\unskip
In addition, within this diagenetic sequence, biogenic magnetite is
considered to be lost earlier than terrigenic magnetite because of its
finer grain size \citep{Karlin1990}. Accessorily, weathering can also
affect magnetite, favoring the formation of maghemite
(Fe$_{2}$O$_{3}$); \citep{Xu1997,Sundman2017,Zhang2021}. 

\begin{figure*}
\includegraphics{fig05}
\caption{\label{fig5}This figure summarizes the main stages of
remineralization of organic matter in marine sediments. This process
involves aerobic and anaerobic respiration, fermentation, and finally,
methanogenesis (left-hand side of the figure). In particular, under
anaerobic conditions, the reduction of sulfate, iron oxyhydroxides, and
manganese oxides can be replenished with electron acceptors by the
reactions illustrated on the right-hand side of the figure. The former
process consumes organic matter, but the latter does not, although
these reaction (not involving organic-matter consumption) do prolong
this anaerobic respiration stage by regenerating electron acceptors.
These repeated cycles of iron reduction and oxidation allow magnetite
to form and disappear in successive loops, a phenomenon referred to
here as the cryptic iron cycle. This cycle also allows the formation of
glauconite, which does not disappear as easily because this mineral is
less labile than magnetite under reducing conditions. This figure is
developed and enriched from that of \citet{Schulz2000}.}
\end{figure*}
In the situation examined in this paper, the presence of terrigenic
magnetites indicates that the sediments of the Assise de Cro\"{i} did
not undergo sustained or intense sulfidic diagenesis, as also evidenced
by the low enrichment of the rocks in trace metal elements that are
sensitive to reductive conditions 
\citep{Tribovillard2023a,Tribovillard2023b,Tribovillard2025b}. 
The sediment was therefore not subjected to strongly reducing conditions
during the early diagenesis; otherwise any form of magnetite would have
disappeared.

In addition, the carbonate beds have preserved intact authigenic
magnetite minerals, which are even more fragile than the inherited
magnetite. However, these minerals are absent from the marly interbeds.
This suggests that the conditions for preserving labile magnetite were
not gathered in the marly interbeds, or that magnetite could not have
formed there during early diagenesis. Since authigenic glauconite,
another iron-bearing mineral, is present in the interbeds, it can be
deduced that reactive iron was present in the pore waters, and that the
conditions were favorable for the formation of authigenic minerals
mixing Fe$^{2+}$ and Fe$^{3+}$. Therefore, it may be deduced that
authigenic magnetite could also have formed within the marly interbeds,
but that it did not survive the diagenetical conditions. If labile
magnetite is still present in the nodular beds, it is because the very
early diagenetic growth and cementation of the nodules sealed
micro-environments hosting both authigenic and inherited magnetite. The
partitioning or closure of the internal environment of the nodules no
longer allowing the circulation of fluids, the magnetite was protected
against any further evolution, that is to say, from any degradation or
dissolution. The early cementation of the limestone nodules effectively
\guillemotleft{}~fossilized~\guillemotright{}~the geological conditions at the time of deposition,
or shortly thereafter.

\section{Discussion}\label{sec5}
Our interpretations are that the Assise de Cro\"{i} sediments hosted
authigenic magnetite growth during the early stages of diagenesis. This
magnetite then disappeared during later stages of diagenesis, likely
through reductive dissolution during the sulfidic phase, induced by the
decomposition of organic matter 
\citep[organoclastic sulfate reduction;][]{Jorgensen2019}. 
However, this magnetite was preserved in certain horizons where the
initial marly deposits were cemented early into carbonate beds made up
with coalescent nodules. This indicates that the geochemical conditions
were right for the growth of iron-rich minerals to take place. These
interpretations are based on the usual conditions leading to the
dissolution of magnetite, as well as factors that prevented such
destruction, contrary to all expectations. However, it can be
instructive to question the conditions of magnetite formation.

\subsection{What are the conditions for the authigenic growth of magnetite?}\label{sec51}

As mentioned above, the growth of iron-bearing minerals such as
magnetite and glauconite requires weakly reducing conditions, as well
as the presence of reactive iron. The literature provides several
scenarios for magnetite \citep{Hu2025}: (1) Iron may be released by the
dissolution of pre-existing pyrite or titanopyrite, whose alteration
during deposition could provide the required iron 
\citep{Katz1998,Nowaczyk2011,Badesab2023}. (2) \citet{Lin2024} observed
micrometer-sized euhedral magnetites in sediments impacted by cold
methane seeps, with iron released by dissimilatory iron-reducing
bacteria. (3) \citet{Karlin1987} identified magnetite as a byproduct of
metabolic reactions conducted by microorganisms using iron redox
transitions as an energy source. All these scenarios rule out a simpler
hypothesis regarding the origin of iron: that reactive iron could be
brought to the marine environment in the form of metal-organic or
metal-clay complexes, without necessarily resorting to a pre-existing
source of iron in the marine environment that needs to be remobilized.
In addition, \citet{Halevy2017} and \citet{Koeksoy2019} reported that
green rust, a mixed-valence hydroxide that contains carbonate ions in
the interlayer, could be a precursor to magnetite. According to these
authors, carbonate green rust can form near the iron redoxcline and
commonly decomposes into magnetite and/or siderite.\looseness=-1

We have no satisfactory means of rejecting or retaining this or that
hypothesis, and in this case, it does not matter much. What matters is
that authigenic magnetite could have formed. It formed before being
embedded in carbonate nodules that have been proven to be very early,
because very rapid confinement would have prevented any circulation of
mineralizing fluids essential for authigenesis. Therefore, magnetite,
like glauconite, already existed when the calcareous nodules formed.
This means that at the time of sediment deposition, a certain amount of
reactive iron was present in the sedimentary deposits and could have
fueled the authigenic growth of magnetite and glauconite. The work of
\citet{Tribovillard2025a} reached the same conclusion, but for much
more proximal sandy deposits (shoreface sands of the Gr\`{e}s de
Ch\^{a}tillon Formation). This study showed that early-cemented sand
injectites contained magnetite grains that had formed during the
earliest diagenesis. In the present study, the deposits took place on a
significantly deeper platform, with of course drastically contrasting
depositional conditions. It is concluded that the phenomenon of iron
diagenesis goes beyond the coastline alone and can be observed more
widely on the continental shelf. This discovery broadens the spectrum
of all phenomena involving iron during very early \mbox{diagenesis} in the
continental shelf domain.

\subsection{The diagenetical transition zone where magnetite can
form}\label{sec52}

When iron is trapped in the form of pyrite, or more broadly iron
sulfides, it is relatively tightly retained in a chemically stable
medium, which has a low probability of being oxidized later during
diagenesis. Indeed, the classic diagenetic sequence, as described in
the works of J\o{}rgensen and co-workers 
\citep{Jorgensen2006,Jorgensen2021,Jorgensen2009,Jorgensen2019} or that
of \citet{Burdige2006,Canfield2006}, leads inexorably from an environment where
aerobic \mbox{respiration} dominates to one where reducing conditions prevail
and remain unchanged. Re-oxidation of the sedimentary pile can only be
caused by phenomena affecting the entire thickness of the water column,
simultaneously with the first few decimeters or meters of sediment.
These phenomena can be sudden, such as large-scale turbidites, or
large-scale, but slow, phenomena such as those associated with
sea-level variations. Consequently, the classic sequence of diagenetic
phenomena leads to the sequestration of iron in sulfides and authigenic
phyllosilicates. What is stressed on here is that this virtually
inexorable phenomenon can be significantly slowed by redox loops
involving labile minerals, such as magnetite. The same way water can
turn into ice and ice melts at 0~\textdegree C, magnetite can form or
be dissolved at the redox boundary where iron has its tipping point
between Fe$^{2+}$ and Fe$^{3+}$. In the diagenetical zone above the
horizon where sulfate reduction takes place, iron is involved in
bacterially mediated reactions of iron oxyhydroxides reduction with
organic matter being an electron supplier, as illustrated with
Equations~(\ref{eq3}) and~(\ref{eq4}).\looseness=-1
{\begin{eqnarray}
&&\hspace*{-7pt}
2 \mathrm{Fe}_{2}\mathrm{O}_{3} + \mathrm{CH}_{2}\mathrm{O} + 3 
\mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{HCO}_{3}^- + 4 
\mathrm{Fe}^{2+} + 7 \mathrm{OH}^- \label{eq3}
\quad 
\Seqnsplit
&&\hspace*{-7pt}
4 \mathrm{FeOOH} + \mathrm{CH}_{2}\mathrm{O} + \mathrm{H}_{2}
\mathrm{O} \rightarrow \mathrm{HCO}_{3}^- + 4 \mathrm{Fe}^{2+} 
+ 7 \mathrm{OH}^-
\quad
\label{eq4}
\end{eqnarray}}\unskip

However, iron thus reduced by iron-reducing bacteria (also termed
ferri-reducing bacteria) may fuel oxidation reactions mediated by
iron-oxidizing bacteria (so-called ferro-oxidizing bacteria) and get
back to the Fe(III) state \citep[e.g.,][]{Burdige2006,Miot2016,Laufer2016}. 
In addition, H$_{2}$S released by sulfate reducers may also reduce
iron as illustrated by the following equations, because iron is also
involved in chemical reactions operating without organic matter acting
as an electron supplier:
{\begin{eqnarray}
&& 
2 \mathrm{FeOOH} + \mathrm{HS}^- + 5 \mathrm{H}^{+} 
\rightarrow 2 \mathrm{Fe}^{2+} + 4 \mathrm{H}_{2}\mathrm{O} + 
\mathrm{S}^{0}\label{eq5}
\quad 
\Seqnsplit
&& 
2 \mathrm{Fe(OH)}_{3} + \mathrm{HS}^- + 5 \mathrm{H}^{+} 
\rightarrow 2 \mathrm{Fe}^{2+} + 6 \mathrm{H}_{2}\mathrm{O} 
+ \mathrm{S}^{0} \label{eq6}
\quad 
\Seqnsplit
&&
\mathrm{Fe}_{2}\mathrm{O}_{3} + \mathrm{HS}^- + 5 \mathrm{H}^{+} 
\rightarrow 2 \mathrm{Fe}^{2+} + 3 \mathrm{H}_{2}\mathrm{O} + 
\mathrm{S}^{0} \label{eq7}
\quad
\end{eqnarray}}\unskip

Reciprocally, iron is also oxidized in the presence of nitrate ions:
{\begin{equation}\label{eq8}
 10 \mathrm{Fe}^{2+} + 12 \mathrm{H}^{+} + 2 \mathrm{NO}_{3}^- \rightarrow 10 
 \mathrm{Fe}^{3+} + \mathrm{N}_{2} + 6 \mathrm{H}_{2}\mathrm{O}
\end{equation}}\unskip

Therefore, this diagenetic zone, where reactive species such as
nitrate, manganese, or iron are present and interact the ones with the
others, is the place where magnetite may form because this mineral may
result from iron reduction reactions as well as iron oxidation ones
\citep{Karlin1987,Brown1995,GibbsEggar1999,Chaudhuri2001}. Lastly,
\citet{Usman2012} showed that when ferric minerals, such as
ferrihydrite, are aged within a Fe$^{2+}$ containing media (such as
ferruginous pore waters), they readily transform into green rust, which
can be a precursor to magnetite, as mentioned above. However, when the
sediment gradually passes into the sulfate-reducing zone 
\citep[organoclastic sulfate reduction or sulfate-dependent anoxic
oxidation of methane;][]{Jorgensen2019,Jorgensen2021} as a consequence
of continuous particle settling, magnetite will definitively disappear
because of iron reduction \citep{Canfield1987,Canfield2010}.

\vspace*{-1pt}
\subsection{Magnetite at the expense of sedimentary organic matter}\label{sec53}
\vspace*{-1pt}

Magnetite can form or disappear in the diagenetic zone where conditions
easily switch between the ``world'' of oxidation and that of reduction.
Consequently, iron can be trapped or released numerous times before the
situation becomes permanently locked in highly reducing conditions. In
other words, within this zone, iron is reversibly trapped. It remains
reactive, even when transiently trapped in mineral phases because it
can be released. Thus, in the sediments of shallow marine environments,
Fe remains usable by living organisms and accessible to the chain of
reactions that determines the remineralization or conversely,
preservation of organic matter as shown in Figure~\ref{fig5}. 
Since these loops
keep iron in a reactive state for a relatively long time, it can be
remobilized by the action of bottom dwellers, and it can be involved,
on the one hand, in chemical reactions mobilizing organic matter as an
electron supplier in redox reactions, or on the other hand, in
oxidation/reduction reactions not involving organic molecules
\citep[][Figure~\ref{fig5}]{Adhikari2017,Arellano2019}.\looseness=-1

\vspace*{-1pt}
\subsection{Iron, an agent playing a role in the increased degradation of organic matter}\label{sec54}
\vspace*{-1pt}

First, iron is known to play a negative, indirect but effective, role
in the preservation of organic matter. In a few words, organic matter
may be sulfurized (vulcanized) in the presence of sulfide ions, as has
been known for decades now
\citep[][to mention a few recent
papers]{Vandenbroucke2007,Amrani2014,Quijada2016,Raven2016,CohenSadon2025,Ma2025}. 
Nevertheless, sulfide ions have a high
chemical affinity for reduced iron
\citep{Rickard2012,Rickard2021,Rickard2024,CohenSadon2025,Ma2025}, and
when Fe$^{2+}$ is available in pore waters or even at the sea bottom,
sulfide ions are monopolized by reactive iron. If iron is present in
abundance, the entire pool of sulfide ions may be caught by iron
sulfide formation ; therefore, no sulfurization of organic matter can
occur, which facilitates the remineralization of organic products
\citep{Tribovillard2015}. Second, before reaching the point where iron
is entirely transformed into iron sulfide, Fe can participate in the
redox loops described above and fuel reactions that destroy organic
matter through iron-mediated anaerobic respiration. Consequently,
environments where reactive iron is abundant are unfavorable for the
preservation of fragile organic matter. Such milieus are typically
shallow environments located near the mouths of large rivers, where
iron may be supplied in large amounts
\citep{Burdige2011,Bianchi2011,Bianchi2016}. There, organic matter
accumulated in the sediments is mainly (if not exclusively) present in
the form of refractory organic products (land-derived woody fragments),
because all the labile organic matter would have got lost, as the
result of the iron-mediated remineralization reactions, among other
processes remineralizing organic \mbox{matter}.

\subsection{Magnetite and redox/iron-sensitive trace metals}\label{sec55}
Following the events occurring with reactive iron, all these diagenetic
transformations of iron-bearing phases discussed here can affect trace
metal elements, whose behavior is related to that of iron, whether in
the form of sulfides or oxides and hydroxides 
\citep[molybdenum, arsenic, copper, nickel, chromium, antimony, etc.;
e.g.,][]{Banerjee2016,Baldermann2022,Chanda2025}. To explore this
aspect, sufficient quantities of authigenic magnetite are required to
allow for chemical analysis. So far, we have not been able to achieve
this, but it represents a challenge for our ongoing work. The point is
to examine possible enrichments in redox-sensitive trace metals within
authigenic magnetite grains that offer a double opportunity to capture
them: the presence of iron and of (mildly) reducing conditions.

\vspace*{-2pt}

\section{The cryptic iron cycle}\label{sec6}

\vspace*{-2pt}

{\advance\baselineskip by -.2pt

The same way a cryptic sulfur cycle has been conceptualized by
\citet{Canfield2010}, a cryptic iron cycle may be put forward here. As
explained by \citet{Callbeck2018} and \citet{GrijalvaRodriguez2025}, the
cryptic S cycle refers to the simultaneous activity of bacterially
mediated, sulfate-reduction and sulfide-oxidation pathways in, for
instance, a marine particle aggregate or any other closely defined
space. Sulfide produced locally by sulfate-reducing bacteria is
immediately reoxidized to elemental sulfur or sulfate ions by
sulfide-oxidizing bacteria, and dissolved sulfide remains at
submicromolar concentrations. Here, iron is captured in magnetite but
the loose capture is most of the time only transient, even if it may be
repeated through cycles of capture/release of iron in
precipitating/dissolving magnetite. Magnetite can form and dissolve,
and form again and dissolve again as long as the hosting sediment is
maintained in mildly reducing conditions, that is, in the ``oscillatory
zone'' of diagenesis where iron is cyclically oxidized/reduced. This
cyclic, possibly protracted, redox transition can be conducted by
bacterially mediated reactions or abiotic ones. Nevertheless, once
diagenetic conditions turn to reducing, beyond all return, then
magnetite may be quasi-permanently transformed into pyrite 
\citep[see][for an exception to the rule]{Brothers1996}. Consequently, the
magnetite-driven cryptic iron cycle leaves no trace in the sediments,
unless an early-diagenetic cementation occurs (as is the case in the
present work), allowing the authigenic magnetite to be preserved and
recorded. A consequence of the cryptic iron cycle is to stimulate
iron-depending bacterial activity at the expense of labile organic
matter in marine sediments. The longer the cryptic cycle, the larger
the amount of remineralized, labile, organic matter, and thus, the
higher the proportion of recalcitrant (also termed refractory) organic
matter in the organic content of the sediments.

}

\vspace*{-2pt}

\section{Conclusions}\label{sec7}

\vspace*{-2pt}

The examination of magnetite distribution in the Cro\"{i} Formation,
revealing two contrasting facies, provides new information on early
diagenesis. The sediments of this geological formation, regardless of
their facies, contain detrital magnetite, indicating that the redox
conditions throughout early diagenesis were slightly reducing and did
not allow for the complete dissolution of detrital magnetite (which is
more resistant to reducing dissolution than authigenic magnetite).
Authigenic magnetite is found only in the early diagenetic limestone
beds that protected the tiny euhedral magnetite grains from reducing
dissolution. The early precipitation of a kind of carbonate matrix thus
encapsulated and fixed the results of the initial stages of diagenesis,
during which magnetite could precipitate and dissolve repeatedly,
following a dynamic and cyclic redistribution of iron. The subsequent
stages of diagenesis, through sulfate reduction reactions, resulted in
the long-term capture of iron as iron sulfide, erasing all traces of
the iron's ``versatile life'' during its early diagenesis. This turbulent
early life, which we refer to here as the cryptic iron cycle, allowed
for the remineralization of reactive organic matter by bacteria whose
metabolism relies on the reduction or oxidation of iron, thus providing
energy to the microorganisms.\looseness=1

The cryptic iron cycle is predictably enhanced when large quantities of
reactive iron are introduced into the marine environment: typically off
the mouths of major rivers, on the continental shelf. In these
environments, the sediment grain size is generally relatively coarse
(clay-poor sediments), and the (relatively) shallow depth promotes
sediment oxygenation through atmospheric O$_{2}$ diffusion on the one
hand, and through seawater turbulence on the other hand. Consequently,
shallow marine environments are not conducive to the development of
reducing conditions at the base of the water column. Thus, the
significant iron input, combined with slightly reducing conditions,
stimulates the cryptic iron cycle, resulting in substantial
remineralization of labile organic matter. As a result, the sediments
accumulated in these environments primarily reflect the preservation of
recalcitrant or refractory organic products.\looseness=-1

Disclosure of interests. The author does not work for, consult, own
shares in, or receive funding from any company or organization that
would benefit from this article, and has disclosed no relevant
affiliations beyond his academic appointment.


\section*{Acknowledgments}
This work has been performed using the Plateform CARMIN \textendash{}
Ulille infrastructure and technical support. Our thanks go to the
Programme Tellus Syster of the French Institut des Sciences de
l'Univers (INSU) for funding our work. We thank Monique Gentric for the
financial/administrative management, as well as Sandra Ventalon and
Marion Delattre for the technical support of this project, and lastly,
the LOG laboratory and the Department of Earth Sciences of the
University of Lille for their support. Lastly, I would like to express
my sincere gratitude to the anonymous reviewers of the manuscript for
their assistance.\looseness=-1

\section*{Declaration of interests}

The author does not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and has
declared no affiliations other than their research organization.

\section*{Supplementary materials}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crgeos-325-suppl.pdf}}

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