The contrasting origins of glauconite in the shallow marine environment highlight this mineral as a marker of paleoenvironmental conditions

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

A novel approach to volcano surveillance using gas geochemistry

The aim of this paper is to test a simple damage model of a cohesive granular medium to study the relationship between the damage and velocity of elastic waves. Our numerical experiments of edometric compression show that the mi- croscopic deformation quickly becomes very heterogeneous, while our simulations of elastic waves propagation show that a small amount of damage induces a dra- matic decrease in the elastic velocity. This shows that cohesive discrete media are very sensitive to strain field heterogeneity, and that the wave velocities in these media can measure subtle transient deformation processes, such as earthquake initiation phases

Understanding the Distributions of Benthic Foraminifera in the Adriatic Sea with Gradient Forest and Structural Equation Models

Abstract: In the last three decades, benthic foraminiferal ecology has been intensively investigated to improve the potential application of these marine organisms as proxies of the effects of climate change and other global change phenomena. It is still challenging to define the most important factors affecting foraminiferal communities and derived faunistic parameters. In this study, we examined the abiotic-biotic relationships of foraminiferal communities in the central-southern area of the Adriatic Sea using modern machine learning techniques. We combined gradient forest (Gf) and structural equation modeling (SEM) to test hypotheses about determinants of benthic foraminiferal assemblages. These approaches helped determine the relative effect of sizes of different environmental variables responsible for shaping living foraminiferal distributions. Four major faunal turnovers (at 13–28 m, 29–58 m, 59–215 m, and >215 m) were identified along a large bathymetric gradient (13–703 m water depth) that reflected the classical bathymetric distribution of benthic communities. Sand and organic matter (OM) contents were identified as the most relevant factors influencing the distribution of foraminifera either along the entire depth gradient or at selected bathymetric ranges. The SEM supported causal hypotheses that focused the factors that shaped assemblages at each bathymetric range, and the most notable causal relationships were direct effects of depth and indirect effects of the Gf-identified environmental parameters (i.e., sand, pollution load Index–PLI, organic matter–OM and total nitrogen–N) on foraminifera infauna and diversity. These results are relevant to understanding the basic ecology and conservation of foraminiferal communitie

Syndepositional glauconite as a paleoenvironmental proxy - the lower Cenomanian Chalk of Cap Blanc Nez (N-France)

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A sedimentological oxymoron: highly evolved glauconite of earliest diagenetic origin.

International audienceThis 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

LES ORIGINES CONTRASTÉES DE LA GLAUCONITE EN MILIEU MARIN PEU PROFOND METTENT EN VALEUR CE MINÉRAL EN TANT QUE MARQUEUR DES CONDITIONS PALÉOENVIRONNEMENTALES

International audienceLa glauconite est un minéral authigénique réputé se former à l’occasion de contact prolongé entre un nucléus (un phyllosilicate pré-existant) et l’eau de mer. Ce contact prolongé permet de soustraire les ions nécessaires pour la construction du phyllosilicate néoformé, la glauconite (minéral très proche d’une illite, riche en K et en Fe). De ce fait, la glauconite est souvent associée à des sédiments déposés en contexte transgressif avec fort ralentissement du taux de sédimentation et une épaisseur de tranche d’eau relativement importante. C’est le cas de la craie cénomanienne du Boulonnais (N de la France). Minéral résistant chimiquement et physiquement, la glauconite peut aussi être un minéral souvent remanié, à l’instar des grains de quartz. C’est fréquemment le cas des dépôts jurassiques du Boulonnais, où la glauconite, quasi-omniprésente que ce soit à l’état de traces ou de proportions notables du sédiment, présentent un tri granulométrique attestant son transport et son remaniement. Pourtant ces dépôts jurassiques sont peu profonds (shore face, offshore supérieur), cela accrédite l’idée que "l’usine à glauconite" ait été elle-même dans les zones peu profondes du Boulonnais. Les seuls faciès identifiés du Jurassique boulonnais où la glauconite est à la fois relativement abondante, de grande taille et non triée (non remaniée) sont des récifs à huitres qui se sont formés au droit de suintements froids liés à une tectonique synsédimentaire fini-jurassique (Kimméridgien, Tithonien). Nos travaux permettent de formuler l’hypothèse selon laquelle les récifs à huitres isolés aient été des environnements réunissant les conditions de d’ambiance redox et de contact avec l’eau de mer favorisant la formation authigénique de glauconite. Les conditions faiblement réductrices nécessaires à la formation de glauconite (présence simultanée de Fe2+ et Fe3+ dans le même réseau cristallin) sont ici attestées par les teneurs en éléments traces métalliques sensibles aux conditions redox (vanadium, germanium, arsenic, en l’occurrence)

Radiogenic isotopic and clay mineralogical signatures of terrigenous particles as water-mass tracers: New insights into South Atlantic deep circulation during the last termination

International audienceThe past evolution of the Southern Ocean, one of the major components of the climatic system, is still a matter of debate. This study provides new insights into the deep Southern Ocean circulation based on the radiogenic isotopes and clay mineralogical signature of the terrigenous fractions transported by the main deep water masses to sediments recovered in core MD07-3076Q from the central South Atlantic. This approach successfully permits: (1) provenance identification of the various grain-size fractions (clay, cohesive silt and sortable silt); (2) assignment of each grain-size fraction to a specific water-mass; (3) reconstruction of past changes in the main deep water-mass pathways. These data document the evolution of deep-water masses in the South Atlantic Ocean during the last deglaciation. The Antarctic Bottom Water (AABW) speed and northward extension were maximum at the end of the Last Glacial Maximum (LGM), associated with strong bottom water production in the Weddell Sea, together with a vigorous Lower Circumpolar Deep Water (LCDW). In contrast the North Atlantic Deep Water (NADW) circulation was weaker than today. The onset of the deglaciation (from 17.5 ka to 15 ka,~Heinrich Stadial 1, HS 1) was marked by weakening and southerly retreat of the AABW and by an increase of mixing between AABW and LCDW. The speed of the AABW remained at its lowest during the Bølling Allerød (B/ A) and the Younger Dryas (YD), and the LCDW slowed and retreated to the south, while the NADW progressively migrated southward, deepened, and strengthened between the beginning of the Bølling Allerød and the Holocene (from~15 to 10 ka)