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Climat : la fin des saisons ?

15/03/2023 19:00

L’hiver 2022/2023 sera-t-il le dernier ? De nombreux records de température ont été atteint en décembre 2022, après une vague de chaleur en automne. En 2021, le GIEC alertait sur des vagues de chaleur plus nombreuses dans le cadre d’un réchauffement à 1.5°. Quel effet sur les saisons ? A quoi s’attendre pour les années à venir ? Pourquoi ces phénomènes sont-ils inquiétants ?

34e Journées Scientifiques de l’Environnement (JSE)

14/03/2023 19:00

Rendez-vous pluridisciplinaire de découvertes et de débats autour de la recherche en environnement en Val-de-Marne, l’UPEC/OSU-EFLUVE et le Conseil départemental du Val-de-Marne organisent la 34e édition des Journées Scientifiques de l’Environnement (JSE) consacrée à « La sobriété dans tous ses états : définir, comprendre, agir ».

Pourquoi et comment initier une transition dans les laboratoires ?

15/02/2023 10:00

Pourquoi et comment initier une transition dans les laboratoires ? Le premier webinaire Labos 1point5 à destination des directions d’unité.

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Le climat en négociations : du GIEC aux COPs

26/01/2024 14:00

Nouvelle séance du séminaire « Changement Climatique : Sciences, Sociétés, Politique » co-organisé par le Centre Alexandre-Koyré (EHESS-CNRS) et l’ENS (CERES).

Le risque inondation est-il un risque urbain ?

26/01/2024 12:30

Cette conférence sera animée par Bruno Barroca professeur à l’Université Gustave Eiffel.

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Surface and subsurface evolution of mesoscale eddies under atmospheric forcings : case study in the Mediterranean sea

09/10/2023 14:00

Mesoscale eddies are ubiquitous turbulent structures in the oceans, in thermal wind balance with a signature in density. Anticyclones with negative vorticity are associated with a negative density anomaly translating in a sea surface height (SSH) elevation, and conversely for cyclones. Statistical studies really began with eddy automated detections based on gridded altimetry products. The first quantitative studies were done in a composite approach : many observations are collocated with eddy contours and gathered into a single mean eddy picture. This approach combined with remote-sensing and Argo profiling floats provided eddy average signature in sea surface temperature (SST), salinity, chlorophyll but also air-sea fluxes. Previous studies did not significantly investigate eddy temporal evolution, apart from trajectory statistics. Eddies interact with heat and momentum air-sea fluxes interact over both short and long timescale, but their evolution remains unknown. We then investigate the mesoscale evolution submitted to atmospheric interactions, in both surface and at depth. Mediterranean eddies provide an ideal case study with extensive in situ measurements and occurrence of long-lived anticyclones. In a first part, we define a Lagrangian method tracking eddies in altimetric data at 1/8°. Eddy observation are collocated with in situ vertical profiles to measure eddy subsurface physical properties, and an outside-eddy reference background is defined to retrieve the eddy-induced anomalies. In a second part, evolution of eddy SST anomalies reveals a strong seasonal signal. Anticyclonic cold and cyclonic warm surface signatures shift from very rare in winter to predominant in early summer. This seasonal oscillation also recovered tracking individual structures. Collocated vertical profiles reveals this summer shift to occur only in near-surface. Hence an eddy-modulated vertical mixing is hypothesized to drive this evolution, with increased mixing in anticyclones. Getting to the mixed layer depth (MLD) in a third part, anticyclones are observed to enhance winter mixed layer deepening (up to 350m anomaly) and significantly delay spring restratification (up to 2 months). Eddy MLD anomalies do not scale with relationship from previous composite studies, and are rather impacted by the subsurface density profile. In a fourth part, we assess the accuracy of eddy evolution in a high resolution numerical experiment with the CROCO model. Eddy seasonal variations in both SST and MLD are retrieved. Increased mixing in anticyclone is confirmed and found to be sensitive to grid resolution. Near-inertial waves triggered by high frequency winds propagate more into the anticyclonic negative relative vorticity and enhancing mixing, in a remarkable example of scales interaction. Last, remaining interactions are discussed, in particular the role of Ekman pumping, atmospheric retroactions and importance of salinity. This study highlights the rich evolution occurring in mesoscale eddies with atmospheric interactions, blurred in composite approach but observable using Lagrangian tracking, and not yet properly retrieved nor studied in global models.

Évaluation de l'habitabilité de la surface de Mars en inventoriant la matière organique avec les expériences spatiales SAM (Mission MSL) et MOMA (Mission ExoMars)

27/09/2023 14:00

Français

Ma thèse de doctorat porte sur la détection de molécules organiques sur Mars. Je présente l’influence des sels sur la préservation et la détectabilité de ces molécules. En effet, des sels de chlorure ont été observés à plusieurs endroits sur Mars, mais les conséquences de leur présence dans des échantillons contenant de la matière organique restent à explorer.

J’ai mené des expériences en laboratoire pour reproduire les conditions dans lesquelles les échantillons martiens sont traités in situ par les suites instrumentales à bord de plusieurs sondes martiennes. J’ai travaillé sur un ensemble spécifique d’instruments présents dans la charge utile des sondes martiennes Viking, Curiosity et Rosalind Franklin afin d’analyser la composition moléculaire des échantillons de la surface de Mars : la chromatographie en phase gazeuse couplée à la spectrométrie de masse (GC-MS). J’ai d’abord testé l’influence des sels de chlorure et d’une matrice de phyllosilicate sur la préservation des composés organiques soumis aux rayonnements UV. Les résultats de cette étude ont montré que les sels de chlorure avaient le potentiel pour préserver les molécules organiques de la dégradation photocatalytique et pouvaient donc représenter d’excellents candidats pour l’analyse moléculaire des échantillons martiens.

Ces résultats ont conduit à explorer l’influence des sels de chlorure sur plusieurs composés organiques d’intérêt pour l’astrobiologie au cours d’expériences de pyrolyse-GC-MS. J’ai montré que lors de l’analyse, la présence de ces sels pouvait gêner la détection des composés organiques mais aussi former des précurseurs de composés chlorés, comme cela a déjà été détecté sur Mars.

Enfin, l’interaction entre les phases inorganiques et organiques dans l’environnement martien et lors des analyses in situ m’a amené à explorer la détectabilité d’un autre type de composés : les sels organiques aromatiques. Les sels organiques sont des molécules réfractaires, difficiles à détecter. J’ai utilisé les techniques de pyrolyse et de dérivatisation GC-MS utilisées par les instruments de vol pour comprendre le comportement et la signature de ces composés. Malgré leur nature non volatile, j’ai découvert qu’ils pouvaient être indirectement identifiés en combinant ces différentes techniques.

Dans l’ensemble, cette thèse de doctorat vise à faciliter l’interprétation des données in situ obtenues par les instruments embarqués à bord des anciennes, actuelles et futures missions martiennes, ainsi qu’à guider la recherche d’échantillons susceptibles de contenir des biosignatures.

 


 

English
Evaluation of the habitability of the surface of Mars by inventorying organic matter with the SAM (MSL Mission) and MOMA (ExoMars Mission) space experiments

My Ph.D. thesis focuses on the detection of organic molecules on Mars. I will present the influence of salts on the preservation and detectability of organic molecules at the Mars surface. Chloride salts have been observed at several locations on Mars, but the consequences of their presence in samples containing organic matter are yet to be explored. I conducted laboratory experiments to reproduce conditions in which Martian samples are processed in situ by the instrument suites onboard several Martian probes. I worked on a specific set of instruments present in the analytical chemistry payload of the Viking, Curiosity, and Rosalind Franklin surface probes to analyze in situ the molecular composition of Mars surface samples: Gas Chromatography-Mass Spectrometry (GC-MS).

I first tested the influence of chloride salts and a phyllosilicate matrix on preserving organic compounds subjected to UV radiation reaching the Martian surface. The results of this study showed that chloride salts had the potential to preserve organic molecules from photocatalytic degradation and could represent, therefore, excellent candidates for sample analysis. These results led to exploring the influence of chloride salts on several organic compounds of interest for astrobiology during pyrolysis-GC-MS experiments. I showed that during molecular analysis, the presence of these salts could hinder the detection of organic compounds but could also form precursors of chlorinated molecules, as already detected on Mars.

Finally, the interaction between inorganic and organic phases in the Martian environment and during in situ analyses led me to explore the detectability of another type of compound: aromatic organic salts. Organic salts are refractory molecules, challenging to detect. I used pyrolysis and derivatization GC-MS techniques as used in the flight instruments to understand the behavior and signature of these compounds.

Despite their non-volatile nature, I found that they could be indirectly identified by combining these different techniques. Overall, this Ph.D. thesis aims to help interpreting in situ data as performed by the instruments onboard former, current, and future Martian missions, as well as guide the search for samples that could contain preserved biosignatures.

Event-driven numerical modelling of early diagenesis in coastal ecosystems: application to flood deposits in Rhône River prodelta

13/10/2023 14:00

The main purpose of this work is to study the biogeochemical response of coastal seafloor subject to episodic massive sediment deposition from floods events. The Rhône River and its connected coastal margins serve as a case-study site for quantifying the impact of these extreme events on early diagenetic processes because it receives significant inputs of sediment (estimated to be up to 80%) during short and intense events. These extreme events are rare and unpredictable, thus the assessment of their impact on sediment biogeochemical processes is difficult.

In order to study the short and intermediate terms response of the sediment biogeochemistry under these abruptly changing conditions, an event-driven numerical model of early diagenesis was specifically developed during this thesis. Using published data of two contrasting floods in year 2008, the model showed reliable capability to simulate the changes induced by the sediment input on the porewater profiles for various solutes. The model suggests that these floods could produce differing biogeochemical response, the extent of which is determined by the underlying characteristics of the flood layer deposit. We found a two-fold increase in overall mineralization rates during the 2008 spring flood event from pre-flood conditions in the spring, which increased further in the fall when a very labile carbon enriched sediment was deposited. My research demonstrated that these differences were due to the nature of organic carbon delivered to  Rhone delta as well as the scale (thickness) of deposition.

These intrinsic characteristics might also be responsible for constraining the relaxation timescale of the various porewater solutes (e.g oxygen, dissolved inorganic carbon, sulfate) to a few months as observed in the field. Furthermore, this research also demonstrated that the strong internal cycling and the role of secondary redox processes such as pyrite precipitation (which were enhanced during these flood events) might be responsible for the maintenance of non-sulfidic condition observed in Rhône prodelta sediment. The thesis also briefly explores the concept of ”memory effect” of temporally connected flood depositions. Multiple occurrence of these events can also trigger temporal interaction between floods which has a substantial effect on the processes operating in the deep (such as methanogenesis and sulfate reduction) but negligible for superficial oxic and suboxic processes. This has significant ramification in future scenarios of increasing frequency of these extreme events.

More recent time series of porewater composition obtained during winter campaigns in 2021-22 investigates the temporal evolution of the porewater following an estimated 25 cm of sediment deposition. A remarkable modification of the dissolved inorganic carbon, sulfate and dissolved methane profiles were observed which was distinguishable from the pre-flood situation. Model simulations describes adequately the dataset and showed that these winter events can result to as much as 75% increase in total carbon mineralization, thus enhancing longer-term DIC production in the sediment. This winter flood also leads to a decoupling of the two pathways for sulfate reduction – organoclastic sulfate reduction and anaerobic oxidation of methane and is associated to vertical displacement of the sulfate-methane transition zone. This observation has important implications since further deepening of the AOM maximum zone due to flood deposition could enhance the effective trapping of methane (a ”green house” gas crucial in the context of climate change).

Overall, the numerical exploration in this thesis provides for the first time, a synthesis of the role of episodic event such as the massive flood deposition on spatio-temporal dynamics of the biogeochemical processes in the sediment.

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