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\DOI{10.5802/crgeos.274}
\datereceived{2024-05-07}
\daterevised{2024-09-23}
\dateaccepted{2024-10-01}
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\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{review}

\title{Revisiting shallow glauconite factories: intertwined fates of
glauconite and iron}

\alttitle{Revisiter les usines de glauconite peu profondes : les
destins entrelac\'{e}s de la glauconite et du fer}

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

\keywords{\kwd{Boulonnais area}\kwd{Jurassic}\kwd{Cretaceous}\kwd{Chalk}\kwd{Cold seeps}\kwd{Greensands}}

\altkeywords{\kwd{Boulonnais}\kwd{Jurassique}\kwd{Cr\'{e}tac\'{e}}\kwd{Craie}\kwd{Suintements froids}\kwd{Sables verts}}

\thanks{INSU, as part of the Tellus Syster research program}

\begin{abstract}
Glauconite is an authigenic green mineral which has been studied
extensively because its conditions of formation make it possible to
evaluate certain parameters of the sedimentary environments where it is
found. The most widespread view is that glauconite forms very slowly
from a clay precursor, in environments where low sedimentation rates
allow prolonged contact over time between the authigenic mineral and
seawater. The latter is seen as the source of some of the chemical
elements necessary for the authigenic formation of this phyllosilicate,
in particular iron (Fe). Typically, for relatively shallow
environments, the distal portion of continental shelves is considered
the most suitable formation environment. This review article makes it
possible to broaden the spectrum of environments favorable to the
growth of glauconite, by identifying environments that experience very
shallow deposition depths and relatively high sedimentation rates.
These situations are well illustrated by the Jurassic and Cretaceous
deposits of the Boulonnais coast (Pas de Calais Strait, between the
English Channel and the North Sea). In particular, certain sedimentary
deposits show that glauconite could have grown during very early
diagenesis. It therefore appears that glauconite can form in relatively
varied environments, provided that two main conditions are met: the
presence of an abundance of reactive iron and weakly reducing redox
conditions. The emphasis of this paper is set on contrasted situations
where various sources of iron are involved in the presence of
glauconite. Last, abundant-pyrite accumulations imply large supplies of
iron for their formation and, as the same time, represent a
considerable iron sink with regard to the marine environment. 
\end{abstract}

\begin{altabstract}
La glauconite est un min\'{e}ral vert authig\`{e}ne qui a \'{e}t\'{e}
largement \'{e}tudi\'{e} car ses conditions de formation permettent
d'\'{e}valuer certains param\`{e}tres des environnements
s\'{e}dimentaires o\`{u} elle se trouve. Le point de vue le plus
r\'{e}pandu est que la glauconite se forme tr\`{e}s lentement \`{a}
partir d'un pr\'{e}curseur argileux, dans des environnements o\`{u} de
faibles taux de s\'{e}dimentation permettent un contact prolong\'{e}
dans le temps entre le min\'{e}ral authig\`{e}ne et l'eau de mer. Cette
derni\`{e}re est consid\'{e}r\'{e}e comme la source de certains des
\'{e}l\'{e}ments chimiques n\'{e}cessaires \`{a} la formation
authig\`{e}ne de ce phyllosilicate, en particulier le fer (Fe).
G\'{e}n\'{e}ralement, pour les environnements relativement peu
profonds, la partie distale des plateaux continentaux est
consid\'{e}r\'{e}e comme l'environnement de formation le plus
appropri\'{e}. Cet article de synth\`{e}se permet d'\'{e}largir le
spectre des milieux favorables \`{a} la croissance de la glauconite, en
identifiant les milieux qui connaissent des profondeurs de
d\'{e}p\^{o}t tr\`{e}s faibles et des vitesses de s\'{e}dimentation
relativement \'{e}lev\'{e}es. Ces situations sont bien illustr\'{e}es
par les gisements jurassiques et cr\'{e}tac\'{e}s de la c\^{o}te
boulonnaise (d\'{e}troit du Pas de Calais, entre la Manche et la mer du
Nord). En particulier, certains d\'{e}p\^{o}ts s\'{e}dimentaires
montrent que la glauconite a pu cro\^{i}tre au cours d'une
diagen\`{e}se tr\`{e}s pr\'{e}coce. Il appara\^{i}t donc que la
glauconite peut se former dans des milieux relativement vari\'{e}s,
\`{a} condition que deux conditions principales soient r\'{e}unies : la
pr\'{e}sence d'une abondance de fer r\'{e}actif et des conditions redox
faiblement r\'{e}ductrices. Dans cet article, l'accent est mis sur les
conditions contrast\'{e}es o\`{u} diverses sources de fer sont
impliqu\'{e}es dans la pr\'{e}sence de glauconite. Enfin, les
accumulations abondantes de pyrite impliquent de grandes quantit\'{e}s
de fer pour leur formation et, en m\^{e}me temps, repr\'{e}sentent un
puits de fer consid\'{e}rable par rapport \`{a} l'environnement marin.
\end{altabstract}

\maketitle
{\vspace*{1.12pc}}

\twocolumngrid

\end{noXML}

\section{Introduction}

Glauconite is a green mineral belonging to the family of
phyllosilicates neoformed during diagenesis. Glauconite \textit{sensu
stricto} is very close to illite, from a mineralogical and chemical
point of view, with the characteristic presence of potassium (K$^{+}$)
and iron (both Fe$^{2+}$ and Fe$^{3+}$) ions \citep[][see the recent
synthesis by \citealp{RubioLopezPerez2024}]{Burst1958, OdinMatter1981,
StilleClauer1994, Velde2014, Huggett2021}.\ Glauconite is commonly
encountered in the form of green grains or pellets and such grains may
be referred to as glauconitic grains or glaucony, as suggested by 
\citet{OdinLetolle1980}.
In what follows, the mineral \textit{sensu stricto} will be termed by
the single word glauconite, the green grains or pellets will be
designated by the terms glauconitic grains, and facies rich in
glauconitic grains will be termed glaucony, regardless of their exact
mineralogical nature [glauconite, glauconitic micas, glauconitic
smectites, verdine, etc.; \citealp{OdinMatter1981}]. Since the
extensive work of Odin and co-workers, the presence of glauconitic
grains has been commonly considered to be a marker of the conditions
encountered on continental platforms sufficiently distal to
undergo a very low sedimentation rate \citep{OdinMatter1981,
GiresseWiewiora2001, Velde2014, LopezQuirosetal2019, Huggettetal2017,
Huggett2021}. Condensed sedimentation would be required for protracted
exchanges between seawater (source of dissolved~K) and the authigenic
mineral to exist \citep{McRae1972, OdinMatter1981, Amorosi1995,
Amorosi1997, Velde2014, Follmi2016, Giresse2022,
RubioLopezPerez2024}. This classic vision of glaucony began to be
complemented at least twenty years ago: it has been shown that
glauconitic grains can form in a variety of sedimentary environments
where sedimentation rates can even be relatively high
\citep{HuggettCuadros2010, ElAlbanietal2005, Banerjeeetal2012a,
Banerjeeetal2012b, Banerjeeetal2016a, Banerjeeetal2016b,
Bansaletal2020, Bansaletal2022, Bansaletal2023,
WilmsenBansal2021, Baldermannetal2013, Baldermannetal2022,
Tallobreetal2019, RoyChoudhuryetal2021a, RoyChoudhuryetal2021b,
RoyChoudhuryetal2023, RubioLopezPerez2024}.
However, it is still frequently taught that glauconite and glauconitic
grains are typical of condensed platform environments bathed under
mildly reducing conditions to explain the presence of reduced iron
(Fe$^{2+}$) in the crystal lattices of this mineral. This paper reports
on recent works showing that it is not a question of setting aside
``classical'' interpretations of the genesis of glauconite but of
illustrating the variety of contexts where it is encountered. In
particular, it is evidenced that glauconite can be present very early
on the proximal part of detrital platforms and that it can thus witness
the conditions at the redox interface (near the sediment-seawater
interface). Thus, what are the depositional requirements for such an
early formation? Beyond questioning the conditions of formation of
glauconite, a wider scale point may be addressed. The stratigraphic
record is punctuated with episodes of widespread accumulations of
glauconite; does it rely with peculiar events of the Earth's history?
Does it imply some constraints on iron availability? This paper aims to
provide some answers, notably stressing on the part played\break by iron.

{\vspace*{2pt}}

\section{The classic views} \label{sec2}

{\vspace*{2pt}}

Glauconite is an authigenic phyllosilicate, a member of the green clay
minerals family, which is found mainly in marine deposits and which is
quite common, both in carbonate sediments and in clastic ones. Its
presence is already observed in \mbox{sedimentary}
rocks of Precambrian age \citep{OdinMatter1981, Banerjeeetal2016a,
Huggett2021, Velde1992, RubioLopezPerez2024}. Its formula is
(K,Na)$_2$(Fe$^{3+}$,Fe$^{2+}$,Al,Mg)$_4$[Si$_6$(Si,Al)$_2$O$_{20}$](OH)$_4$.\ 
It~is commonly accepted that glauconite forms at the sediment-water
interface or a short distance below it. Indeed, the authigenic
formation of glauconite requires the capture of chemical elements
present in the water column, allowing a mineral rich in iron and
potassium to develop. These exchanges with seawater require that
relative proximity to sea water be respected. It is then logical to
consider that a low sedimentation rate is required for the growth of
glauconitic grains \citep{OdinMatter1981, Velde2014, Huggett2021,
Rafieietal2023}. Glauconite is also considered to develop from a
precursor, most commonly present in the form of an iron-bearing
smectite. This precursor evolves towards a glauconite through K$^{+}$
enrichment \citep{Charpentieretal2011, Gaudinetal2005}. According to
several authors \citep{OdinMatter1981, Baldermannetal2013,
LopezQuirosetal2019, RubioLopezPerez2024}, this incorporation could
occur over periods of up to $10^{3}$ years, or even $10^{6}$ years. The
so-called maturity of glauconite is therefore correlated with the
quantity of K$^{+}$ incorporated. Glauconite contains iron present with
both valences Fe(II) and Fe(III). This characteristic requires weakly
reducing conditions, making it possible to obtain reduced iron
simultaneously with oxidized iron. \citet{LopezQuirosetal2019} mention
values in the order of 0~mV, as regards the redox potential (Eh), and
7--8, as regards the seawater pH. \citet{Xiaetal2022} report a typical
Fe(III)/Fe(II) ratio of 9/1. Decaying organic matter and empty
foraminiferal tests or chambers supply the ideal environment for
glauconite genesis \citep{McRae1972}. More strongly reducing
conditions, where iron is entirely in the form of Fe(II), lead to the
formation of another authigenic mineral: pyrite \citep{Berner1981}.
Finally, glauconitic grains are used to date sedimentary deposits,
{via} the K--Ar or Rb--Sr methods \citep[e.g.,][and references
therein]{Vandenbergheetal2014, Rafieietal2023}. For a long time, the
prevailing opinion was that glauconite was mainly formed where
precursor clays are present and where the sedimentation rate is low,
that is to say, the edge of the continental shelf. In such an
environment, glauconitic grains could grow for periods of up to
$10^{6}$ years \citep{Velde1992}.

\section{Classic views called into question} \label{sec3}

More recently, the spectrum of glauconitic grains formation conditions
has gradually expanded. Without denying the previous interpretations,
it is now possible to complement them, thanks to several works. An
extreme situation is the possible existence of glauconite on Mars
\citep{LosaAdamsetal2021}. \citet{HuggettCuadros2010} report onshore,
non-pellet glauconite formation. \citet{ElAlbanietal2005} observed
glauconite formed in extremely shallow environments (lagoonal setting).
\citet{WilmsenBansal2021} and \citet{Bansaletal2023} demonstrated the
formation of glauconite in shallow (nearshore) conditions during the
Cenomanian. At the other end of the depth spectrum,
\citet{Porrenga1967} already mentioned long ago the possibility that
glauconite could form at depths between 30 and 2000~m; see also
\citet{Duplayetal1989}. However, we must wait for much more recent work
for the formation of glauconite in deep marine environments to be
discussed \citep[e.g.,][to mention a few papers]{Baldermannetal2013,
Baldermannetal2022, Tallobreetal2019}. In other words, the formation of
glauconite is now identified in a wide range of sedimentary
environments. Furthermore, other recent work has called into question
the length of time required for glauconite and glauconitic grains to
form. \citet{MeunierElAlbani2007} discussed this aspect, showing that
it can sometimes be difficult to imagine that mature glauconitic grains
could have formed during times whose duration would have exceeded
100,000 years (or even more), taking into account the sedimentary
environments where glauconite is observed. The difficulty comes from
sedimentation rates which cannot reasonably have been close to zero for
so long. The authors reconciled the established ``dogma'' and their
observations by concluding that glauconite could have formed quickly
and that these episodes of rapid glauconitization could have been
repeated over long periods of time, between 100,000 years and $10^{6}$
years.\ Another way to smooth down the apparent paradox of the duration
of formation is to consider winnowing. Winnowing, also termed washing,
is the selective sorting, or removal, of fine particles by (wind or)
current action, leaving the coarser (denser) grains behind
\citep{BatesJackson1987}. In proximal settings, recurrent storms or the
action of marine currents could stir and sort the sediment, driving
away lighter \mbox{particles} and \mbox{concentrating} in situ denser
ones such as glauconitic grains. For instance, \citet{Giresse2022} and
\citet{Follmi2016} mention recent and ancient situations where
winnowing must be suspected.

More recently, \citet{WilmsenBansal2021} and \citet{Bansaletal2023}
demonstrated the early formation of glauconite in environments with a
high sedimentation rate. The duration of glauconite formation can be
nothing but short in such environments. \citet{Follmi2016} and
\citet{RubioLopezPerez2024} state that all the examples they reports on
in their respective synthesis indicate that condensation may play a
role in the formation and concentration of verdine and especially
glauconite, but not in all cases, in contrast to earlier views
\citep[e.g.,][]{VanHoutenPurucker1985}.

To conclude, the knowledge acquired recently shows that the formation
of glauconite is more ubiquitous than what previous work had defined:
from the shallowest shoreface to the deep marine setting, even
including (extra)terrestrial environments (soils, lagoons). In
addition, recent progress shows that glauconite can form in shorter
periods of time than was usually accepted for these authigenic
minerals.

\section{What conditions the abundance of{\hfill\break} glauconite?}
\label{sec4}

In light of the observations reported above, it is logical to ask the
following question: if the formation of glauconite and glauconitic
grains can be more rapid and more generalizable than previously
assumed, why is glauconite not more abundant in the stratigraphic
record? What could have been the limiting factors? In vitro, glauconite
was observed to form within a few months, in solutions containing Si,
Al, K and Fe at room temperature, provided iron was abundant enough
\citep{Harder1980}. This abiotic process requires specific redox
conditions: reducing enough to allow iron to be mobile and to reach
relatively high concentrations, and oxidizing enough to allow iron
hydroxides to form and react with dissolved elements, ending with the
formation of glauconite \citep{Harder1980}. This delicate balance could
be the whole crux of the story: shallow marine sediment most often
contain organic matter spanning a wide range of concentrations. The
organic-product decay usually induces the development of bacterially
mediated sulfate reduction reactions. The~HS$^{-}$~ions thus released
will favor pyrite precipitation and prevent glauconite formation.
Therefore, the formation of glauconite could be rather quick and easy
{in vitro}, but, {in vivo}, it is impeded by usual
environmental conditions: too little reactive iron, to much sulfide,
which can explain why glauconite is not as omnipresent as pyrite in the
geological\break record.

\looseness=-1
What is meant with the expression: ``reactive iron''? It must be
distinguished between ``reactive'' iron, which can be involved in
chemical reactions, and ``inert'' iron (firmly bound within crystal
lattices), which is poorly or not reactive during sediment early
diagenesis \citep[e.g.,][]{Canfieldetal1992, RaiswellCanfield2012,
Poulton2021, Vosteenetal2022}. Following the seminal work by
\citet{Canfield1989}, geochemists usually define reactive iron as iron
(oxyhydr)oxides that can be reductively dissolved by sodium dithionite,
or, more broadly, by HCl \citep[see][and references
therein]{Burdige2006}. In any case, reactive iron is considered to be
reactive toward sulfide ions. This definition is quite formal, and in
the present work, reactive iron will be considered to be the part of
the iron inventory capable of being involved in chemical reactions
and/or authigenic-mineral growth or formation. That is to say that
reactive iron is not locked inside mineral structures or lattices, but
may be desorbed, exchanged or released from, or out of, organic-mineral
complexes, clay-organic complexes, clay-mineral (surfaces). It can also
result from the bacterially mediated reduction of lowly soluble, iron
oxides that are released from emerged lands. They may come in a
soluble,{\break} reduced form, together with ground discharge of
oxygen-poor freshwater at the land-sea boundary \citep{Burdige2006,
RaiswellCanfield2012}.

\subsection{Iron as a limiting factor} \label{sec4.1}

\looseness=-1
Many authors have examined the origin of the \mbox{chemical} elements
necessary for the authigenic growth of glauconite: K$^{+}$, Fe$^{2+}$,
Fe$^{3+}$, Al$^{3+}$, Si$^{4+}$ \citep{OdinMatter1981, Berner1981,
MeunierElAlbani2007, RoyChoudhuryetal2021a, LopezQuirosetal2019}. These
authors consider that smectite constitutes the most common precursor,
providing silicon and aluminum. They also conclude that seawater is
unlikely to provide the necessary iron but
does provide potassium and that
detrital particles can provide Fe, K and Si, for shallow water deposits
\citep{SanchezNavasetal1998, LopezQuirosetal2019, RubioLopezPerez2024}.
These detrital minerals can be Fe(III)-oxides or Fe-kaolinite. The
elements K, Si, Fe, and Al have contrasting residence times and
concentrations in present-day seawater, as shown in Table~\ref{tab2}.
This table indicates that potassium is relatively abundant and stable
in the current ocean while iron is a limiting factor, with a residence
time counting in tens or hundreds of years according to the authors,
and its concentration in the water of sea is measured in picomol per kg
only. It is well understood that the precipitation of authigenic iron
minerals is conditioned by the availability of reactive iron, that is
to say, forms of minerals from which iron can be released over short
periods of time. Depending on the mineralogical nature of the support,
iron is released at very variable speeds \citep{Canfield1989,
Canfieldetal1992}. In shallow environments (shoreface to offshore),
reactive iron is mainly delivered from the emerged land, following the
leaching of continental masses \citep[][and references
therein]{Canfieldetal1992, RaiswellCanfield2012, Bansaletal2020,
RubioLopezPerez2024}.
Therefore, the authigenesis of glauconite in proximal environments is
directly linked to the amount of reactive iron from the emerged land
(the aspects of glauconite formed at great depths are not treated here
but it may be mentioned that \citet{Duplayetal1989} observed the
formation of glauconite at low temperature within ocean-flooring basalt
where halmyrolyse released solubilized iron). These iron inputs can be
transported by rivers and winds \citep{RaiswellCanfield2012} but in
proximal environments, fluvial inputs mask wind inputs which are less
quantitatively abundant. The main factors favoring iron inputs to
shallow marine environments are volcanic episodes or very hydrolyzing
climatic episodes \citep[hot and humid climates;][to mention a few
recent papers]{Shoenfeltetal2019, Bansaletal2020, 
Longmanetal2022, Vosteenetal2022, Schuncketal2023}.
Furthermore, the proximity of emerged
lands plays a crucial role for the presence of iron
\citep{Burdige2011}. Indeed, the iron resulting from leaching of the
emerged land is very quickly oxidized in seawater, which drastically
reduces its solubility (thence its short residence time). Consequently,
the quantity of reactive iron decreases rapidly as we move away from
the sources of input.

Concurrently, shallow-marine environments are highly sensitive to
sea-level variations that condition the dynamics of the sediment
distribution. Beyond the situations of the Boulonnais evoked below, and
despite the exceptions identified in the syntheses by
\citet{Follmi2016} and \citet{RubioLopezPerez2024}, the fact remains
that in such depositional settings, glauconite accumulations are most
frequently associated with episodes of condensation; in addition, a
number of such condensed accumulations are in turn associated with
sea-level rises, as stated by these authors.\ Therefore, land-derived
reactive-iron supply, plus transgression-induced episodes of condensed
sedimentation, are expected to favor the formation of glauconite-rich
sediments. However, conversely, not all episodes of transgression in
continental platform environments result in deposits rich in glauconite
\citep[and this did not escape the analysis of][]{Follmi2016}; these
are{\break} therefore often necessary but not sufficient conditions. The
hypothesis of a limiting factor which would be the availability of
reactive iron can therefore be formulated. Why iron rather than another
element also included in the chemical composition of glauconite?
Because iron is the element that is the most difficult to maintain in
solution or in a reactive state compared to the others (as recalled
above). In addition, the redox conditions cannot be reducing, otherwise
pyrite will form instead of glauconite. Consequently, it is suggested
here that reactive iron could be a key factor limiting the
genesis of glauconite because of its availability being so
delicate to maintain, as reminded by \citet{PoultonRaiswell2002}, and
\citet{Kendalletal2012}.

\citet{Baldermannetal2022} argue that large-scale, glauconitic grain
formation in shallow as well as deep marine settings is an important
iron sink that is currently underestimated. This point of view is very
interesting since iron is a bio-essential element; therefore, its
retention in authigenic minerals could impact or have impacted marine
productivity. Developing this point upstream of the reaction chain, we
could say that if glauconitic facies are quantitative iron traps, this
implies that, previously, reactive iron must have been present in
significant quantities in the marine environment. We will therefore
examine the contexts where iron inputs could have allowed or favored
the formation of glauconitic facies. These different cases will then be
compared with the deposits rich in glauconitic grains from the
Boulonnais~area.

\subsection{Iron delivery to shelfal environments} \label{sec4.2}

Regional-scale glauconite accumulations require regional-scale iron
supplies to seas. Such supplies to shelfal environments are attributed
to the volcanic activity or the weathering of emerged lands, as said
above. \citet{Follmi2016} listed the large accumulations of glauconite
through the Earth's history. As volcanoes cannot be systematically
invoked for each glauconite concentrations, it must be concluded that
continental weathering could have triggered such regional-scale
glauconite accumulations. Weathering can be stimulated by hydrolyzing
climate conditions or orogenies exposing fresh rocky material, or both.
In other words, with a bit of hindsight, glauconitic facies may be the
result of large-scale, paleoenvironmental and geodynamic (tectonic)
changes. Furthermore, it may also considered to be an important
archive, allowing geochemical proxies of long-term processes in the
oceans to be traced, e.g., changes and shifts in marine currents and
mixing of ocean waters \citep[see discussion about condensed
sedimentation in][]{Follmi2016}.

At a local scale, an additional source of reactive iron can also be
mentioned: cold seep fluids, often rich in methane (CH$_{4}$) or other
forms of organic matter \citep[e.g.,][]{Lemaitreetal2014, Hongetal2020,
Taetal2024}. Incidentally, beyond shelfal environments, (hot)
hydrothermal fluids can also release iron in oceanic settings. In
addition to dissolved iron, cold seep fluids are also reported to
release dissolved potassium \citep{Oluetal1996, Suess2014,
Wangetal2017, Zhangetal2022}. That is to say that cold seep sites are
places where the ``ingredients'' of glauconite are available and where
redox conditions may not be reducing. For the latter point, see
discussion in the recent paper by \citet{Haaseetal2024} and references
therein. To sum up, cold seep sites are places where conditions may be
necessary if not sufficient for glauconite to form.

Lastly, some works suggest that anoxic sediments contribute some
release of iron to the sediment-water interface and, possibly, to the
bottom waters \citep[conditions termed ferruginous;
e.g.,][]{LyonsSevermann2006}.\ Under reducing conditions, iron would be
solubilized in the Fe$^{2+}$ state and could \mbox{migrate} upward
through the sediment and even escape from it {via} diffusion and
pore water expulsion \citep[e.g.,][]{FortinLangley2005,
LyonsSevermann2006, Burdige2011, Pasquieretal2022}. Nevertheless, in
marine platform environments with normal organic productivity, the
sediment is usually the place for an intense bacterial activity,
dominated by sulfate-reduction reactions. These reactions follow two
ways: organoclastic sulfate reduction and sulfate-dependent anoxic
oxidation of methane \citep[e.g.,][]{Jorgensen2019, Jorgensenetal2019,
Huangetal2022}. Either way leads to the transformation of dissolved
sulfate into soluble sulfide. Reduced iron has so high an affinity
toward sulfide ions that iron is more likely to be fixed inside the
sediment in the form of iron sulfides (such as pyrite) than to reach
and rise above the sediment-water interface. In other words, sediments
rich in organic matter, stimulating bacterially mediated decay of
organic products and favoring sulfate reduction reactions are {a
priori} not suitable for glauconite growth. Nevertheless, the
remineralization of moderate to low amounts of marine organic matter,
or the presence of terrestrial, recalcitrant, organic matter, resisting
extensive decay, could lead to limited dissolved-O$_{2}$ consumption
and create redox conditions prone to glauconite \mbox{formation}.
\looseness=-1

\section{The case of the Boulonnais} \label{sec5}

In France, the Upper Jurassic and Cretaceous geological formations in
Boulonnais (French coastline of the Strait of Pas-de-Calais, in the
English Channel) show levels sometimes rich in glauconitic grains
\citep{Mansyetal2007}.\ Geological formations containing such levels are
shown in Figure~\ref{fig1} and listed in Table~\ref{tab1}.\ Their
detailed studies can be found in the corresponding papers, indicated in
this table. The Upper Jurassic formations (Kimmeridgian and Tithonian)
were deposited on a shallow continental platform (shoreface-lower
offshore) which terminated the London-Brabant Basin to the east
\citep{Mansyetal2003}. The stratigraphy of the Late jurassic-Cretaceous
sequence can be summarized as follows. The Jurassic deposits consist
essentially of a succession of sandstone formations and marl
formations.\ Among them, some contain levels rich in glauconitic
grains.\ The transition between the \mbox{Jurassic} and the Cretaceous
is accompanied by a prolonged \mbox{emergence} of the region
(Purbeckian facies then depositional gap). Marine sedimentation will
only resume in the Aptian--Albian with terrigenic deposits (glauconitic
sandstones) topped by Upper Cretaceous Chalk \citep{Mansyetal2007}. The
Cenomanian chalk, deposited during the great transgression of the Upper
Cretaceous, is marked at its base by a very dark gray limestone level,
this color being due to a large abundance of glauconite (around 44\% of
the weight of the rock). This level is sometimes called Tourtia in the
literature \citep{AmedroRobaszynski1999}. This basal level is overcome
by much finer glauconite-rich levels, the presence of which is
associated with periods of sea level rise, finding their place in a
pattern of sequential stratigraphy \citep{AmedroRobaszynski1999,
AmorosiCentineo2000}.

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Location of the sections sampled alongshore the
Boulonnais, Strait of Pas de Calais, English Channel. The numbers on
the coastline refer to the simplified stratigraphic column. Map after
the SHOM website (Service Hydrographique \& Oc\'{e}anographique de la
Marine; \url{https://www.shom.fr/}).}
\end{figure*}

\begin{table*}%tab1
\fontsize{8.9}{10.9}\selectfont
\tabcolsep=2.3pt
\caption{\label{tab1}Short description of the levels sampled in the
stratigraphic succession of the Boulonnais{\vspace*{-2pt}}}
\begin{tabular}{cccccc}
\thead
Age && \parbox[t]{2cm}{\centering Geological formations} 
& \parbox[t]{1cm}{\centering\# in
\mbox{Figure}~\ref{fig1}}\vspace*{2pt} & \parbox[t]{3.3cm}{\centering
Levels rich in glauconite in Boulonnais} & \parbox[t]{3.3cm}{\centering
Location (see Figure~\ref{fig1})}\\
\endthead
\morerows{1}{\raisebox{-2.5pc}{Cretaceous}} && Cenomanian Chalk & 1 & \parbox[t]{4cm}{\raggedright
Glauconious chalk at the lowermost part of the formation, beginning
with the so-called Tourtia bed, rich in glauconite}\vspace*{3pt} &
\parbox[t]{3.8cm}{\raggedright Beach between Strouanne and Cap Blanc
Nez}\\
&& \parbox[t]{2.5cm}{\centering Aptian--Albian sandstones} & 2 &
\parbox[t]{4cm}{\raggedright Dark-colored sands and sandstones visible
at low tide}\vspace*{3pt} & Strouanne Beach\\
\xxbotline
\morerows{4}{\raisebox{-7.5pc}{Jurassic}} & \morerows{2}{\raisebox{-5pc}{Tithonian}} & Assises de Cro\"{i} & 3 &
\parbox[t]{4cm}{\raggedright Alternating carbonate levels and marly
interbeds} & \parbox[t]{3.8cm}{\raggedright Sampled at Pointe aux Oies
(North Wimereux), pictured at Rochers du Fort Cro\"{i} (South
Wimereux)}\vspace*{3pt}\\
&& Bancs Jumeaux & 4 & \parbox[t]{4cm}{\raggedright P1 \& P2:
phosphate- and clastic-rich carbonate levels, at the base and top of
the formation}\vspace*{3pt} & \parbox[t]{3.8cm}{\raggedright South to
Pointe aux Oies (place called Pointe de la Rochette)}\\
&& Argiles de la Cr\`eche & 5 & \parbox[t]{4cm}{\raggedright Silty
marls at the very base of the formation} & \parbox[t]{3.8cm}{\raggedright
Rochers du Fort Cro\"{i} (South Wimereux)}\vspace*{10pt}\\
& \morerows{1}{\raisebox{-2pc}{Kimmeridgian}} & Argiles de Ch\^atillon & 6 &
\parbox[t]{4cm}{\raggedright Coquina beds at the
Kimmeridgian-Tithonian boundary}\vspace*{3pt} &
\parbox[t]{3.8cm}{\raggedright North of Audresselles (Cran du
Noirda)}\\
&& Argiles de Ch\^atillon & 6 & \parbox[t]{4cm}{\raggedright Oyster
patch reefs at the base of the formation (the Boundary Bed,
Kimmerdigian)}\vspace*{2pt} & \parbox[t]{3.8cm}{\raggedright North of
Audresselles (Cran du Noirda)}
\botline
\end{tabular}
\end{table*}

\begin{table*}%tab2
\tabcolsep=10pt
\caption{\label{tab2}Concentrations in seawater and residence times of
silicium, aluminum, iron and potassium{\vspace*{-3pt}}}
\begin{tabular}{ccccc}
\thead
& Fe & Al & Si & K\\
\endthead
Atomic number & 26 & 13 & 14 & 19\\
Atomic weight & 55.847 & 26.98154 & 28.0855 & 39.0983\\
Average concentration in ocean & 540~pmol/kg & 1.11~nmol/kg & 100~$\upmu$mol/kg & 10.2~mmol/kg\\
Residence time & 50--500 yrs & 200 yrs & 20,000 yrs & 12,000,000 yrs
\botline
\end{tabular}
\tabnote{Data from \citet{Bruland1983} and \citet{Byrneetal1988}
collected on the website of the Monterey Bay Aquarium Research
Institute (MBARI):
\url{https://www.mbari.org/know-your-ocean/periodic-table-of-elements-in-the-ocean/summary-table/}.}
{\vspace*{-.3pc}}
\end{table*}

\subsection{Cretaceous deposits} \label{sec5.1}

The Mesozoic geological formations of the Boulonnais illustrate the
complexity or, at least, the diversity of the conditions of formation
of glauconite. The largest-scale phenomenon is probably the presence of
green sands (also written greensands) of Aptian--Albian age. They may
contain glauconite but also other types of green minerals often
referred to with the words verdine and glaucony \citep{Follmi2016,
RubioLopezPerez2024}. Such green sands and sandstones are reported over
a wide array of distribution through the Cretaceous world but,
regarding the geographical location of the present study, some major,
regional-scale, accumulations of green sands may be mentioned in
England, Germany, France and the North Sea \citep[e.g.,][]{Owen1969,
Triat1983, RobaszynskiAmedro1986, Hartley1995, Lehmann2013}. This
remarkable period of deposition green sands is accounted for by the
conjunction of two phenomena: (1)~the resuming marine sedimentation,
related to an incipient eustatic sea-level rise, and (2)~a climate
favoring continental weathering, therefore, iron release to the seas
\citep{Schoutenetal2003, DumitrescuBrassell2006, Galeetal2008,
Mutterloseetal2010, Mutterloseetal2014, Corentinetal2020,
Deconincketal2021, Bloketal2022, Caillaudetal2022, Jiaetal2022}. This
period of time witnessed the development of epicontinental seas. The
relatively shallow depth of deposition must have favored the
oxygenation of water masses. In addition, the coarse-grained sand
deposits prevented or, at least, hampered the development of anoxic
conditions within the sediment. Thus, the redox conditions were
probably not reducing, therefore, preventing pyrite accumulations to
take place, in spite of the abundance of reactive iron. Instead,
glauconite could develop widely \mbox{becoming} the distinctive mark of
the sand and sandstone of mid-Cretaceous in shelfal environments. 

Later on, the eustatic sea-level rise kept on during the Cenomanian and
the worldwide development of epicontinental seas witnessed the
deposition of the chalk facies over large surfaces
\citep{HancockKauffman1979, Haq2014, Anetal2017, LeCallonnecetal2021}.
In France (and England), the base of the Cenomanian chalk is rich in
glauconite. The very base (called Tourtia) is made of more than 40~wt\%
of glauconite \citep{Tribovillardetal2021} and is overlain by thin,
recurrent, glauconite-rich seams, each of them being related to
small-scale fluctuations of the sea-level \citep{RobaszynskiAmedro1986,
AmorosiCentineo2000}. The chalk is characterized by the overwhelming
accumulation of coccoliths. The proliferation of small-dimension
phytoplankton (e.g., coccolithophorids {versus} diatoms) is the
sign of oligotrophic marine conditions \citep{Chester2000}. In other
words, the epicontinental seas of that period of time must have known
low-production conditions \citep{MutterloseBottini2013,
LeCallonnecetal2021}. Again, the sediments, lacking abundant
sedimentary organic matter, developed redox conditions compatible with
glauconite formation, but not with pyrite \citep[except for nodules of
later diagenetic origin;][]{Rickard2012}.

\subsection{Jurassic deposits: condensed sedimentation} \label{sec5.2}

Contrary to the Cretaceous deposits, the Late Jurassic ones are rich in
clay minerals. Such land-derived minerals are considered to be good
carrier phases for adsorbed reactive iron; in addition Fe-rich clays
(e.g., smectites) may release iron into the pore waters through
diagenesis \citep{Canfield1989, Kendalletal2012}. Furthermore, the
Jurassic deposits accumulated on a shallow ramp close to the eastern
end of the Weald-Boulonnais Basin \citep{Mansyetal2003}, that is, close
to the sources of land-derived reactive iron
\citep[e.g.,][]{Jilbertetal2018, Herzogetal2020}. In other words, iron
may have not been the limiting factor with regard to glauconite or
pyrite formation. The growth of authigenic minerals was thus directed
towards glauconite or pyrite, depending on the redox conditions at the
sediment-water interface or a short distance below. This alternative is
illustrated by what is observed with the Bancs Jumeaux Fm. (Tithonian).
The P1 and P2 levels, corresponding to winnowed, probably erosional,
surfaces are rich in glauconite, whereas the marlstones in-between are
rich in abundant, large-size, pyrite framboids and polyframboids
\citep{Tribovillardetal2008, Tribovillardetal2023b,
Tribovillardetal2023a}. The condensed sedimentation of the P1 \& P2
levels \citep{DeconinckBaudin2008} favored the formation of glauconite
whereas the fine-grained, clay-rich, sediments of the marlstones
favored the development of micro-niches or micro-environments where the
conditions were reducing, therefore favoring the formation of
large-dimensioned (poly-) framboids of pyrite
\citep{Tribovillardetal2008}. \looseness=-1

Finally, the Assises de Cro\"{i} Fm., with glauconite-rich,
alternating, carbonate beds and marly interbeds, is also associated
with condensed sedimentation \citep{DeconinckBaudin2008}
but with no occurrences of seep sites being identified. This rather
thin formation (about ten meters) is covering several ammonite zones
\citep[Albani, Glaucolithus and Okusensis pro
parte;][]{TownsonWimbledon1979}. In this case, the presence of iron
must have been owing to the proximity of the shore and associated
fluvial inputs \citep{Mansyetal2007}, and the early diagenetic growth
of glauconite must have been owing to the condensed sedimentation:
well-sorted populations of quartz grains suggest a hydrodynamic
activity such as winnowing
\citep[][\mbox{submitted}]{Tribovillardetal2023b,Tribovillardetal2023a}.
\looseness=-1

{\vspace*{-2pt}}

\subsection{Jurassic deposits: iron supply from seep sites}
\label{sec5.3}

{\vspace*{-2pt}}

{\advance\baselineskip by -.2pt

The transitional level separating the Gr\`{e}s de Ch\^{a}tillon Fm. and
the overlying Argiles de Ch\^{a}tillon Fm. (Kimmeridgian) nested
numerous oyster patch reefs. The reefs developed upon cold-fluid seeps
associated with synsedimentary fault movements 
\citep{Hatemetal2014,Hatemetal2016}. To account for the abundant authigenic
glauconite associated with the oyster patch reef, a possible link
between iron, oyster reefs and glauconite was evoked by
\citet{Tribovillardetal2023b,Tribovillardetal2023a} but with no
mention of iron being possibly supplied with seeping fluids close to
the vents on the sea floor \citep{Lemaitreetal2014, Hongetal2020,
Zhangetal2022}. Thus, here too, a potential source of iron, namely, the
seeping fluids, is associated to the presence of glauconite. Regarding
cold seep sites of about the same age, the well-known
``pseudo-bioherms'' of Beauvoisin \citep[southeastern
France;][]{Gaillardetal1992, Peckmannetal1999, Tribovillardetal2013,
Gayetal2019, Gayetal2020} also yield authigenic glauconite, as observed
by \citet{Gaillard1983}. Regarding much younger objects,
\citet{Hanetal2004} observed abundant glauconite grains associated with
seep-site carbonates offshore Costa Rica but they did not evoke a
genetical link. The same is true with \citet{Himmleretal2015} with seep
carbonates of the Arabian Sea. \citet{Zhangetal2022} examined the role
of the iron released at seep sites during the Last Glacial Maximum in
the South China Sea. They explained how the iron was reductively
dissolved and trapped in the form of pyrite through bacterially
mediated reactions of sulfate-dependent anoxic oxidation of methane.
Meanwhile, part of the iron supply was consumed by glauconite growth in
dead-foraminifer chambers.\looseness=-1

}

Thus, examining the influence of reactive iron supply upon glauconite
formation leads us to suggest that cold seep sites could be places
favorable to the formation of this authigenic mineral. Apart from
supplying reactive iron, such sites are not known to experience
particularly low sedimentation rates nor to endure winnowing currents.
Consequently, it may be postulated that glauconite growth must be
relatively rapid in such settings. To test the hypothesis formulated
here, glauconite must be looked for, close to the seep vents as well as
at some distance, to observe a possible gradient in abundance. Modern,
underwater, sites are not convenient for trying to observe such mineral
gradients {in situ} and fossil, on-land, seepage zones are
easier to investigate. To the best of our knowledge, such a gradient
has not been reported in the literature, perhaps because no one tried
hitherto to observe it.

{\vspace*{-2pt}}

\subsection{Partial conclusion} \label{sec5.4}

{\vspace*{-2pt}}

The Upper Jurassic and Cretaceous deposits of the Boulonnais correspond
well to the range of situations where reactive iron is brought into the
marine environment from emerged lands (rivers and groundwater
discharge) or the seabed (seepage). Large-scale climatic and tectonic
(transgressions) events affecting the Tethyan realm during the
mid-Cretaceous times led to wide-scale accumulations of greensand-type
deposits (Aptian--Albian sands and base of the Cenomanian chalk
formation, in the Boulonnais). Regional/local-scale factors, such as
the hydrographic network, the proximity of the coastline, the
occurrence of marine currents, led to medium-scale accumulations of
glauconite-rich sediments (e.g., the Bancs Jumeaux and Assises de
Cro\"{i} stratigraphic formations). Finally, localized sources of
iron-rich seeping fluids such as vents or pockmarks correspond to
places where glauconite is observed (the transition level between the
Gr\`{e}s de Ch\^{a}tillon and Argiles de Ch\^{a}tillon formations, that
yields numerous oyster patch reefs). 

{\vspace*{-2pt}}

\section{Conclusion: a model using glauconite{\hfill\break} as a tool
for paleoenvironmental{\hfill\break} reconstructions} \label{sec6}

{\vspace*{-2pt}}

This examination of the various conditions in which glauconite could
form in relatively shallow, shelfal, environments illustrates how much
this authigenic mineral is impacted by the environmental context:
glauconite is a multi-scale sensor. Since it appears that glauconite
can form relatively easily, why is it not more abundant or more
frequent in the stratigraphic record of sedimentary basins? The answer
to this (somewhat provocative) question can be reduced to two essential
factors: redox conditions and the availability of reactive iron,
implying that the role played by the duration of the process can be
minimized. Furthermore, significant accumulations of glauconite require
massive iron inputs. From these statements, an integrated model of the
use of glauconite as a tool for paleo-environmental reconstructions can
be proposed (Figure~\ref{fig2}). As a preliminary remark, let us recall
that all the work presented here is based on the observed presence of
glauconite \textit{sensu stricto} and it cannot be claimed that the
conclusions put forward here are necessarily valid for sedimentary
deposits containing other types of green minerals (verdine,
berthierine, glauconitic smectites, etc.). To be used as a
paleoenvironmental tool, glauconite grains must not be reworked, and
they must have formed at the sediment-water interface or the closest
possible to it. Regarding the Boulonnais, the two points are discussed
in the Appendices~\ref{app1} and~\ref{app2}, herein below. To conclude,
glauconite can be envisioned as a multi-dimension sensor for
paleoenvironmental reconstructions. 

\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig02}
\caption{\label{fig2}Glauconite as a tool for deciphering
paleoenvironmental conditions.}
\end{figure*}

\section*{Declaration of interests}

The author does not work for, consult, own or receive funding from any
organization that would benefit from this article, and have declared no
affiliations other than their sponsoring research organizations.

\section*{Funding}

This work was supported by INSU, as part of the Tellus Syster research
program.

\section*{Acknowledgements}

Thanks to Monique Gentric (LOG) for administration and the Earth
Science Department of the Faculty of Sciences \& Technologies of the
University of Lille for its support. Thanks to the editorial and
technical team of the C.~R.~Geoscience journal.
{\vspace*{-.3pc}}

\back{}

\appendix{}

\gdef\appendixlabel{Appendix A}

\section{How to know whether glauconite is autochthonous or reworked?}
\label{app1}

The reworking can be demonstrated in two complementary ways: the
morphology of the grains observed with a scanning electron microscope
(SEM) and particle size analysis \citep{Tribovillardetal2021}. The
Boulonnais formations studied here almost all present glauconite and
quartz (except certain chalk levels from which quartz is absent). These
two \mbox{minerals} are extracted and separated into pure phases after
decarbonation with HCl and magnetic separation using a Frantz
isodynamic apparatus. Each of the two phases is then analyzed with a
Malvern Mastersizer device to obtain the particle size distribution
\citep[details in][]{Tribovillardetal2023b,Tribovillardetal2023a}.
Quartz is always terrigenic while glauconite can be ``terrigenic''
(that is, reworked) or autochthonous (synsedimentary, non reworked).
Additionally, quartz and glauconite have comparable densities.
Therefore, the grain size distribution curves are similar when both
minerals are terrigenic, with a unimodal distribution exhibiting good
sorting. On the other hand, if the authigenic glauconites are
autochthonous, the respective curves of quartz and glauconite are
contrasting and the glauconite curves may be plurimodal and the sorting
is significantly worse. Thus, in the case of Boulonnais, it was
possible to show that the Jurassic levels characterized by the presence
of oyster reefs associated with cold seeps of hydrocarbons
\citep{Hatemetal2014, Hatemetal2016} were kinds of glauconite
factories. This glauconite is observed in the oyster patch reefs
themselves as well as in the tempestites they nourished
\citep{Tribovillardetal2023b}. Similarly, the glauconites of the
Assises de Cro\"{i} Fm. have been proved to be autochthonous
\citep{Tribovillardetal2023b,Tribovillardetal2023a}.

{\vspace*{-.3pc}}

\gdef\appendixlabel{Appendix B}

\section{} \label{app2}

With the help of isotopic ages, numerous papers showed that glauconite
growth may be quite long, even at the geological scale, as exposed
above. However, this rule is not absolute.\ In the Boulonnais, the
Assises de Cro\"{i} Fm.\ yields an alternation of limestone beds and
marly interbeds.\ Both \mbox{facies} contains glauconite that was
identified as autochthonous and synsedimentary
\citep{Tribovillardetal2023b}. The nodular, rather contorted, limestone
beds were formed during diagenesis, based on stable isotope
composition. As they have been bioturbated, it was inferred that the
beds were formed during early diagenesis, while the sediment was still
rather soft and hosting dwelling in-fauna. The early-formed carbonate
objects contain glauconite that was evidenced to pre-date the
precipitation of the carbonates nodules and beds
\citep{Tribovillardetal2023b}. Therefore, glauconite grew during the
earliest stages of diagenesis. This is a relative chronology and, as
everybody knows, the word early does not necessarily implies short
time, especially if winnowing currents affected the sea bottom
\citep[discussion in][]{Giresse2022}. Whatever, being formed during
earliest diagenesis, glauconite was a witness of what was occurring on
the sediment-water interface or just\break below it.

\bibliographystyle{crgeos}
\bibliography{crgeos20240374}
\refinput{crgeos20240374-reference.tex}

\end{document}
