Nicolas Tribovillard

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 Croï 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.

The littoral zone (including estuarine environments) is considered here as a regulator of the iron flow between the land and the marine environment, observed through the Jurassic sediments of Boulonnais (Northernmost France). Certain stages of deposition during the Kimmeridgian were accompanied by synsedimentary fault movements that caused sand injections, either in sands undergoing lithification (transforming into sandstone), or in claystone levels. The early cemented sandy material and that being hosted in a low-permeability environment (claystones) were as if frozen in their diagenetic evolution and protected against most subsequent transformations. We can see there early trapped and preserved iron-bearing authigenic minerals: pyrite, glauconite and magnetite. Glauconite can take on unusual forms: bryozoan colonies epigenized into glauconite, endolithic filaments mimicking microbial structures, a film enveloping a pyrite polyframboid. All these unusual forms, as well as the majority of glauconite grains, were not preserved in sands that were not cemented or protected early on, and the iron in these minerals was released into the surrounding environment during later stages of diagenesis. Generalizing from this example, the littoral environment can be an early iron trap, favoring the formation of iron-bearing minerals that would then tend to disappear during diagenesis, releasing iron into the marine environment. Furthermore, early cemented objects, where porosity/permeability was therefore rapidly occluded, are valuable insofar as they can record diagenetic stages that are subsequently no longer traced. This is the case here for iron-bearing minerals and this is an opportunity to question the highlighting of the role of microbial activity in the formation of glauconite.

Sedimentological evidence and detrital zircon provenance analysis show that the Amazonian Craton, Brazil, was subjected to large-scale subaerial erosion by c. 2.7 Ga. To assess weathering conditions attending the emergence of the Amazonian Craton, we used the chemical composition of terrigenous sediments deposited in the Carajás Basin, SE Amazonian Craton, between c. 2.7 and 2.6 Ga. Results from 41 samples collected in two drill cores along 150 and 250 m thick intervals show that emerged lands have been affected by strong chemical weathering conditions, similar in intensity to those occurring in the present-day inter-tropical belt. These intense chemical weathering conditions were probably due to the conjunction of warm temperatures (potentially as high as 22 ± 3°C), abundant precipitation (mean annual precipitation estimated at 1540 ± 150 mm) and high atmospheric CO ₂ content ( p CO ₂ ≥ 10 times the present atmospheric level) that enhanced the chemical alteration of labile minerals. These intense weathering conditions contrast with those inferred in other cratons, which suggests that regionally contrasted climates existed during the Neoarchean.

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.

Un nouveau forage réalisé sur le Campus de la Cité Scientifique de Villeneuve d’Ascq (Nord) a traversé 3,10 m de craie glauconieuse incluant plusieurs niveaux noduleux indurés ou hardgrounds. Cette unité lithologique, nommée ici Craie glauconieuse de Lezennes, est située aux confins de la limite Turonien-Coniacien. Une dizaine de niveaux repères à caractère lithologique (niveaux marneux et hardgrounds) identifiés dans l’intervalle étudié présentent une grande extension géographique à l’échelle régionale et constituent pour cette raison d’excellents outils de corrélation. La comparaison de plusieurs coupes échelonnées sur une distance d’une centaine de kilomètres depuis le Boulonnais au nord-ouest jusqu’à l’agglomération lilloise au sud-est, en passant par l’Artois, révèle le caractère ultra condensé de la Craie glauconieuse de Lezennes. Une analyse minéralogique complète l’étude. Enfin, au sein du forage, les variations de la surface piézométrique de la nappe de la craie du Turonien supérieur-Coniacien inférieur sont suivies et discutées en tenant compte de la sécheresse exceptionnelle qui a caractérisé l’année 2022.

This work examines the possibility of a rapid formation of glauconite in a relatively shallow platform environment (below fair-weather wave baseline). The materials studied here are uppermost Jurassic alternations of carbonate beds and marly interbeds, namely, the Assises de Croï Formation of the Boulonnais area (northernmost France). The carbonate beds yield field evidences of an early diagenetic origin and both beds and interbeds contain glauconite, questioning the duration of formation of the glauconite, relative to that of the diagenetic carbonate beds. Carbon and oxygen stable isotope composition of the carbonate beds confirm an early diagenetic growth. Contrasted grain-size distribution patterns of glauconite and quartz grain populations (isolated after acid digestion and magnetic separation) evidenced that glauconite formed after sediment deposition. Glauconite formation allegedly requires protracted episodes of ion capture from the water column, which is no longer possible when glauconite gets trapped within authigenic carbonates. Therefore, in-situ glauconite formation preceded carbonate authigenesis. Yet, the chemical composition of grains (Fe and K) typifies glauconite as highly evolved, meaning that its formation must have lasted over times, according to conventional views. Consequently, our results challenge these conventional views and confirm that glauconite can form in relatively shallow environments (which has been already brought to light previously) and it is concluded that early diagenetic glauconite can be markedly enriched in both K and Fe, which is an unprecedented result.

The Mesozoic witnessed some episodes of marked accumulation and burial of organic matter (OM) in the Tethys Ocean and satellite basins, such as the Vocontian Basin (SE-France). These famous episodes, termed Oceanic Anoxic Events (OAEs), resulted from various factors, acting in complex synergies; the consensus about the key factors has not been reached yet. The Aptian–Albian Blue Marls Formation (Fm.) of the Vocontian Basin recorded the various substages of OAE1, plus additional organic-rich levels of regional extension. The semi-pelagic marlstones of the Blue Marls Fm. allow to carry out a detailed examination of the molecular fossils, to assess the respective weights of the factors involved in the OM storage process. In this work, we examined the lipid biomarkers of six organic-rich levels ranging from the Goguel Level to the Paquier Level in stratigraphic order. Biomarkers reputed to be characteristic of some OAEs are observed here: 2-methylhopanoids in the Goguel Level (OAE1a) and archaeal lipids in the Jacob, Kilian and Paquier Levels (OAE1b). This study shows that, in the Vocontian Basin, OM deposition resulted mostly from local factors and that each level has its own peculiarities; however, overarching connections with the Tethys Ocean were critical for the recording of global anoxic events.

Glauconite is an authigenic mineral reputed to form during long-lasting contact between a nucleus (a pre-existing phyllosilicate) and seawater. This protracted contact makes it possible to subtract the ions necessary for the construction of the neoformed phyllosilicate, here, glauconite (a mineral very close to an illite, rich in K and Fe). As a result, glauconite is often associated with sediments deposited in a transgressive context with a strong slowdown in the rate of sedimentation and a relatively large water layer thickness. This is the case of the Cenomanian chalk of Boulonnais (north of France). Being chemically and physically resistant, glauconite is a mineral that is often reworked, like quartz grains. This is frequently the case of the Jurassic deposits of the Boulonnais, where glauconite, almost ubiquitous, either in traces or in significant proportions of the sediments, presents a grain size sorting attesting to its transport and reworking. However, these Jurassic deposits are shallow (shoreface, upper offshore), which supports the idea that the “glauconite factory” was itself in the shallow areas of the Boulonnais. The only identified Jurassic facies of the Boulonnais where glauconite is both relatively abundant, large in size and unsorted (non reworked) are oyster reefs that formed at the outlet of cold seeps linked to a late-Jurassic synsedimentary tectonic (Kimmeridgian, Tithonian). Our work makes it possible to hypothesize that isolated oyster reefs were environments combining the redox conditions and in contact with seawater favoring the authigenic formation of glauconite. The weakly reducing conditions necessary for the formation of glauconite here are attested by the contents of metallic trace elements sensitive to redox conditions (vanadium, germanium, arsenic, in this case). Our work thus adds a new element to the understanding of the mechanisms of formation of glauconite in shallow environments.

The Bancs Jumeaux Formation appears as a succession of limestone beds and marly interbeds, of Late Jurassic age (Tithonian), cropping out in the cliffs of the Boulonnais (French shore of the English Channel). This formation is enriched in pyrite present in abundance in the forms of framboids, polyframboids and other concretions of a relatively large size for sedimentary pyrite (a few tens to hundreds of micrometers). Previous works have shown that (poly)framboids appeared during early authigenesis in reducing microenvironments within dysoxic sediments. The question is whether, like what is observed for bacteria causing carbonate mineralization of their outer wall, sulfate-reducing bacteria could have been trapped (locked up) in pyrite “sarcophagi”, induced by their own metabolism. In an attempt to provide answers, the large pyrites were extracted from the Bancs Jumeaux marls by physical separations and acid digests. Their analysis reveals the presence of organic matter in significant quantities. The typical morphology of certain polyframboids present in the studied samples suggests the early fossilization of bacterial colonies by pyrite. If pyrite appears to be a relatively significant trap for organic matter, then pyrite could represent an underevaluated component of the carbon cycle.

The Marnes Bleues Formation from the Vocontian Basin (Southeastern France) shows many dark-colored levels, some concomitant to oceanic anoxic events OAE1a (the so-called Goguel Level) and OAE1b (the Jacob, Kilian, and, above all, Paquier levels). These levels are usually referred to as being organic-rich; they are scattered through a thick, rather homogeneous, succession of marls, that is poor in organic matter (OM). Through a multi-parameter approach, the dark levels from the Aptian-Albian were characterized. Our results show that all dark levels are not OM-rich and that the dark levels exhibit variable characteristics, such as OM nature (marine vs. continental), sedimentation and accumulation rates, redox conditions, surface-water productivity and relative sea level, but they all show low to modest enrichments in OM. Furthermore, all the levels share in common the fact that they formed under conditions of normal to low productivity and oxic to suboxic conditions. Thus, our results strongly suggest that, in the absence of high productivity and anoxic bottom conditions, the other factors reputed to favor OM accumulation only led to sporadic and low enrichments in organic contents. It is as if such factors could only enhance OM accumulation but could not induce it alone. What was true for the Vocontian Basin may be extended to other settings, regardless of their time of deposition or location.