Impact of the seismo-volcanic crisis offshore Mayotte on the Dziani Dzaha Lake

Since May 2018, an unexpected long and intense seismic crisis started offshore Mayotte (Indian Ocean, France). This ongoing seismic sequence is associated with the birth of a newly discovered submarine volcano 50 km east of Petite Terre. Here, we investigate the indirect impact of this crisis on the stability of an atypical ecosystem located in Mayotte, the Dziani Dzaha Lake. This lacustrine system presented physical, chemical and biogeochemical characteristics that were distinct from those classically observed in modern lakes or seawater, e.g. high salinity (up to 70 psu), lack of nitrate, sulfate content below 3 mM, and permanent water column anoxia below ca. 1.5 m depth (2012–2017 period). Based on three surveys conducted in 2020 and 2021, we documented an episode of water column oxygenation, a significant pH decrease and an impressive change in the carbon isotope signatures. These preliminary data suggest that the functioning of biogeochemical cycles in the Dziani Dzaha Lake is impacted by increased CO2 inputs and the changes in the lake mixing dynamics, which is an indirect consequence of the ongoing seismo-volcanic crisis

Enjeux des études de bibliothèques d'artistes et d'écrivains : le cas de la bibliothèque d'André Malraux au croisement des méthodologies

This work is a historiography dissertation that focuses on research issues in studies on the libraries of artists and writers. First, the thesis looks back at the university production of existing studies, putting them into perspective around different issues and different methodologies to lead to rethinking the corpus around the singularity of a library study. Secondly, the thesis explores the case of André Malraux's library under the prism of the historiographical gaze. The objective is to show how this library can be studied at the crossroads of methodologies, both by focusing on the possible influences of readings in the process of writing of Malraux, only by considering the way in which he himself gives us to see a certain idea that he has of the library, through two of his writings The Imaginary Museum and The Precarious Man and Literature.Ce travail est un mémoire d'historiographie qui s'intéresse aux enjeux de recherche dans les études sur les bibliothèques d'artistes et d'écrivains. Dans un premier temps, le mémoire revient sur la production universitaire d'études qui existent, en les mettant en perspective autour de différents enjeux et différentes méthodologies pour amener à repenser le corpus autour de la singularité d'une étude de bibliothèque. Dans un second temps, le mémoire explore le cas de la bibliothèque d'André Malraux sous le prisme du regard historiographique pour montrer comment cette bibliothèque peut être étudiée au croisement des méthodologies, tant en s'intéressant aux influences éventuelles des lectures dans le processus d’écriture de Malraux qu'en envisageant la manière dont lui-même nous donne à voir une certaine idée qu'il se fait de la bibliothèque, à travers deux de ses écrits Le Musée imaginaire et L’Homme précaire et la littérature

Dictionnaire de droit de la santé et de la biomédecine

Destiné aux juristes, ainsi qu'à l'ensemble des professionnels intervenant dans les secteurs sanitaires et sociaux, ce dictionnaire s'adresse plus largement à toutes les personnes sensibilisées aux problématiques sans cesse renouvelées des rapports du droit de la santé et de la biomédecine. Issu d'un travail collectif d'une quarantaine d'auteurs spécialisés dans les différentes branches du droit de la santé, cet ouvrage a pour objet non seulement la définition des notions fondamentales qui structurent la matière, mais aussi leur mise en perspective éthique et philosophique. Les définitions proposées ont ainsi pour but d'éclairer le lecteur dans sa prise de décision ou dans sa réflexion, que celle-ci soit médicale, éthique, déontologique ou juridique

Dictionnaire de droit de la santé et de la biomédecine

Destiné aux juristes, ainsi qu'à l'ensemble des professionnels intervenant dans les secteurs sanitaires et sociaux, ce dictionnaire s'adresse plus largement à toutes les personnes sensibilisées aux problématiques sans cesse renouvelées des rapports du droit de la santé et de la biomédecine. Issu d'un travail collectif d'une quarantaine d'auteurs spécialisés dans les différentes branches du droit de la santé, cet ouvrage a pour objet non seulement la définition des notions fondamentales qui structurent la matière, mais aussi leur mise en perspective éthique et philosophique. Les définitions proposées ont ainsi pour but d'éclairer le lecteur dans sa prise de décision ou dans sa réflexion, que celle-ci soit médicale, éthique, déontologique ou juridique

Impact of the seismo-volcanic crisis offshore Mayotte on the Dziani Dzaha Lake

International audienceSince May 2018, an unexpected long and intense seismic crisis started offshore Mayotte (Indian Ocean, France). This ongoing seismic sequence is associated with the birth of a newly discovered submarine volcano 50 km east of Petite Terre. Here, we investigate the indirect impact of this crisis on the stability of an atypical ecosystem located in Mayotte, the Dziani Dzaha Lake. This lacustrine system presented physical, chemical and biogeochemical characteristics that were distinct from those classically observed in modern lakes or seawater, e.g. high salinity (up to 70 psu), lack of nitrate, sulfate content below 3 mM, and permanent water column anoxia below ca. 1.5 m depth (2012-2017 period). Based on three surveys conducted in 2020 and 2021, we documented an episode of water column oxygenation, a significant pH decrease and an impressive change in the carbon isotope signatures. These preliminary data suggest that the functioning of biogeochemical cycles in the Dziani Dzaha Lake is impacted by increased CO 2 inputs and the changes in the lake mixing dynamics, which is an indirect consequence of the ongoing seismo-volcanic crisis

L'énergie de cohérence du noyau 29si

We studied the behaviour of the 28Si(n, α)25Mg reaction cross-section for incident neutrons from 14.37 MeV to 14.57 MeV. The neutrons were obtained from the T(d, n)4He reaction ; the deuterons were accelerated by means of a Van de Graaff accelerator, they were selected by an analyzing magnet and their energy was measured by using nuclear magnetic resonance. The energy spread of the neutrons received by the detector was less than 22 keV. We have calculated the correlation functions relating, on the one hand, to the best defined levels, and on the other to the sum of several levels. From our data, we obtain a coherence energy Γ lower than the one already found [Réf. [6], [7], [8], [9]] for the same compound nucleus, at the same excitation energy, but with a larger incident energy spread.Nous avons étudié le comportement de la section efficace de la réaction 28Si(n, α)25Mg pour des neutrons d'énergie comprise entre 14,37 MeV et 14,57 MeV. Un accélérateur Van de Graaff nous a fourni les neutrons de la réaction T(d, n)4He. Les deutérons étaient triés par un analyseur magnétique et leur énergie mesurée à l'aide d'un gaussmètre à résonance protonique. La dispersion en énergie des neutrons reçus par le détecteur était inférieure à 22 keV. Nous avons calculé les fonctions de corrélation relatives aux pics les mieux définis d'une part, à la somme de certains d'entre eux d'autre part. L'énergie de cohérence Γ que nous tirons de nos résultats se révèle être inférieure à celle qui a déjà été déterminée [6], [7], [8], [9] pour le même noyau composé, pour une énergie d'excitation sensiblement égale, mais avec des énergies incidentes moins bien définies

The Dziani Dzaha Lake: A long‐awaited modern analogue for superheavy pyrites

International audienceSedimentary records of superheavy pyrites in Phanerozoic and Proterozoic successions (i.e., extremely positive delta S-34(pyrite) values together with higher delta S-34(pyrite) than coeval delta S-34(CAS)) are mostly interpreted as resulting either from secondary postdepositional processes or from multiple redox reactions between sulfate and sulfide in stratified sulfate-poor environments. We report here the first observation of strongly positive delta S-34 values for both dissolved sulfate and sulfide (average delta S-34(diss.sulfate) value of 34.6 parts per thousand and delta S-34(diss.sulfide) values of 36.7 parts per thousand) compared to the present-day seawater delta S-34(diss).(sulfate) (similar to 21 parts per thousand), with a negative apparent fractionation between sulfate and sulfide (Delta S-34(diss.sulfate-diss.sulfide) similar to -2.1 +/- 1.4 parts per thousand), in the sulfate-poor (m) modern thalassohaline lacustrine system Dziani Dzaha (Mayotte, Indian Ocean). Overall, surface sediments faithfully record the water column isotopic signatures including a mainly negative Delta S-34(sed.sulfate-sed.sulfide) (-4.98 +/- 4.5 parts per thousand), corresponding to the definition of superheavy pyrite documented in the rock record. We propose that in the Dziani Dzaha this superheavy pyrite signature results from a two-stage evolution of the sulfur biogeochemical cycle. In a first stage, the sulfur cycle would have been dominated by sulfate from initially sulfate-rich marine waters. Overtime, Raleigh distillation by microbial sulfate reduction coupled with sulfide burial in the sediment would have progressively enriched in S-34 the water column residual sulfate. In a second still active stage, quantitative sulfate reduction not only occurs below the halocline during stratified periods but also in the whole water column during fully anoxic episodes. Sulfates are then regenerated by partial oxidation of sulfides as the oxic-anoxic interface moves downward. These results demonstrate that the atypical superheavy pyrite isotope signature does not necessarily require postdepositional or secondary oxidative processes and can result from primary processes in restricted sulfate-poor and highly productive environments analogous to the Dziani Dzaha

The Plenus Cold Event Record in the Abyssal DSDP Site 367 (Cape Verde, Central Atlantic): Environmental Perturbations and Impacts on the Nitrogen Cycle

International audienceThe Oceanic Anoxic Event 2, at the Cenomanian-Turonian boundary (∼93.9 Ma), was an episode of widespread burial of organic matter in marine sediments, underlined by a positive carbon-isotope (δ 13 C) excursion observed worldwide. Within this episode of O 2 -depleted conditions, a short interval of cooling, termed as the Plenus Cold Event, has been recorded in many sites and sections in the northern hemisphere (Tethyan domain, Western Interior Seaway, proto-North Atlantic Ocean). But, its record and its impact on the biogeochemical cycles of carbon and nitrogen in the southern part of Central Atlantic Ocean has not been explored yet. Here, we present a detailed geochemical study of the Deep Sea Drilling Project site 367 (Cape Verde) based on a compilation of previous and new data of carbon and nitrogen isotope signals as well as trace element concentrations. The aim of this study is to better constrain the evolution of oxygenation in the water column and the associated changes in nitrogen cycle before and during the Oceanic Anoxic Event 2 in order to understand the paleoceanographic and environmental consequences of the Plenus Cold Event at one of the deepest site of the Central Atlantic Ocean. Our new dataset improves the resolution of the δ 13 C curve for this site, and we propose a new chemo-stratigraphic frame of the carbon excursion allowing for a better identification of the short-term negative carbon isotope excursion associated to the Plenus Cold Event. The detailed evolution of redox-sensitive proxies (Mo, U, V, Fe, Cu, Ni enrichments and C org /P total ) and isotopic signals (δ 13 C org and δ 15 N total ) evidence that this deep site was impacted by this cooling event. While anoxic conditions prevailed in bottom waters before and during the onset of the Oceanic Anoxic Event 2 characterized by euxinic NH 4 + -rich water column, this cooling event was accompanied by reoxygenation of the water column, which had affected the behavior of the redox-sensitive elements and caused changes in nitrogen biogeochemical cycling

Nitrogen Isotope Discrepancy Between Primary Producers and Sediments in an Anoxic and Alkaline Lake

Nitrogen isotope compositions (δ15N) in sedimentary rocks are extensively used to investigate the biogeochemical nitrogen cycle through geological times. This use relies on the observation that, in modern continental platforms and anoxic basins, surface sediments faithfully record the δ15N of primary producers, assuming that it was similar in the past. Over Earth’s history, however, surface environments experienced profound changes, including the transition of ammonium-dominated to nitrate-dominated waters and the transition from exclusively microbial ecosystems to ecosystems including multicellularity, which make modern environments significantly different compared to earlier ones, potentially invalidating the fundamental assumption that surface sediments faithfully record the δ15N of primary producers. In order to improve our understanding of the nitrogen isotopic information contained in the early Earth’s sedimentary rock record, we investigate here the nitrogen isotope systematics in a microbial, nitrate free and ammonium-rich modern system, the Dziani Dzaha Lake. In this modern system, the δ15N of the reduced dissolved inorganic nitrogen (i.e., NH4+ and NH3) in the water column is close to ∼7‰ . δ15N of suspended particulate matter (SPM) show a similar average value in surface waters (i.e., where SPM is massively composed of active primary producers), but increases up to 14‰ in the deeper part of the water column during periods when it is enriched in dissolved reduced species (i.e., CH4, H2S/HS− and NH4+/NH3). Surface sediments δ15N, with values comprised between 10 and 14 ‰, seem to preferentially record these positive isotopic signatures, rather than those of active primary producers. We propose here that the observed isotopic pattern is mainly linked to the assimilation of ammonium strongly enriched in 15N by isotope exchange with ammonia under basic conditions. Although ammonium assimilation seems here to be responsible for a significant isotopic enrichment due to the basic conditions, in neutral anoxic environments inhabited by similar microbial ecosystems, this process may also significantly impact the δ15N of primary producers towards more negative values. This would have strong implications for our interpretation of the Precambrian sedimentary record as this finding challenges one the fundamental hypotheses underlying the use of sedimentary δ15N in paleo-oceanographic reconstructions, i.e. that surface sediments faithfully record the δ15N of active primary producers in the photic zone