FESDIA (v1.0): exploring temporal variations of sediment biogeochemistry under the influence of flood events using numerical modelling

Episodic events of flood deposit in coastal environments are characterized by deposition of large quantities of sediment containing reactive organic matter within short periods of time. While steady-state modelling is common in sediment biogeochemical modelling, the inclusion of these events in current early diagenesis models has yet to be demonstrated. We adapted an existing model of early diagenetic processes to include the ability to mimic an immediate organic carbon deposition. The new model version (FESDIA) written in Fortran and R programming language was able to reproduce the basic trends from field sediment porewater data affected by the November 2008 flood event in the Rhône River prodelta. Simulation experiments on two end-member scenarios of sediment characteristics dictated by field observation (1–high thickness deposit, with low TOC (total organic carbon) and 2–low thickness, with high TOC), reveal contrasting evolutions of post-depositional profiles. A first-order approximation of the differences between subsequent profiles was used to characterize the timing of recovery (i.e. relaxation time) from this alteration. Our results indicate a longer relaxation time of approximately 4 months for SO42- and 5 months for DIC (dissolved inorganic carbon) in the first scenario, and less than 3 months for the second scenario which agreed with timescale observed in the field. A sensitivity analysis across a spectrum of these end-member cases for the organic carbon content (described as the enrichment factor α) and for sediment thickness indicates that the relaxation time for oxygen, sulfate, and DIC decreases with increasing organic enrichment for a sediment deposition that is less than 5 cm. However, for larger deposits (&gt;14 cm), the relaxation time for oxygen, sulfate, and DIC increases with α. This can be related to the depth-dependent availability of oxidant and the diffusion of species. This study emphasizes the significance of these sediment characteristics in determining the sediment's short-term response in the presence of an episodic event. Furthermore, the model described here provides a useful tool to better understand the magnitude and dynamics of flooding event on biogeochemical reactions on the seafloor.</p

Modélisation numérique des événements lors de la diagenèse précoce dans les écosystèmes côtiers : application aux dépôts de crue dans le prodelta du Rhône

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 an important case-study site for quantifying the impact of these extreme events on early diagenetic process 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 term 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-enrichment sediment was deposited (up to a factor of 7). My research demonstrated that these differences were due to the nature of organic carbon delivered to the proximal delta of the Rhone 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 sequence of flood deposition with the conclusion that the 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 the 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 DIC, SO_4^{2-} and CH_4 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) flux out of the sediment.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.L'objectif principal de ce travail est d'étudier la réponse biogéochimique des fonds marins côtiers soumis à des dépôts massifs épisodiques de sédiments. Le Rhône et ses marges côtières constituent un cas d'étude important pour quantifier l'impact des dépôts de crue sur les processus diagénétiques dans les sédiments superficiels, car juqu'à 80% des apports de sédiments sont réalisés lors d'événements courts et intenses de crues. Ces événements extrêmes sont rares et imprévisibles, et il est donc difficile d'évaluer par l'observation directe leur impact sur les processus biogéochimiques des sédiments. Afin d'étudier la réponse à court et moyen terme de la biogéochimie des sédiments dans ces conditions de changement brutal, un modèle numérique de diagenèse précoce a été spécifiquement développé au cours de cette thèse. En utilisant les données publiées de deux inondations contrastées en 2008, le modèle a montré sa capacité à simuler les changements induits par l'apport de sédiments sur les profils d'eau interstitielle pour différents solutés. Le modèle suggère que ces inondations pourraient produire des réponses biogéochimiques différentes, dont l'ampleur est déterminée par les caractéristiques du dépôt sédimentaire. Nous avons constaté que les taux de minéralisation du carbone organique totaux ont été multipliés par deux au cours de la crue du printemps 2008 par rapport aux conditions antérieures, et qu'ils ont encore augmenté à l'automne lorsqu'un sédiment enrichi en carbone très labile a été déposé (acrroissement d'un facteur 7). Mes recherches ont démontré que ces différences étaient dues à la nature du carbone organique du delta proximal du Rhône ainsi qu'à l'épaisseur du dépôt. Ces caractéristiques intrinsèques pourraient également être responsables du temps de relaxation des divers solutés de l'eau interstitielle (par exemple l'oxygène, le carbone inorganique dissous, le sulfate) autour de quelques mois. La thèse explore également le concept d'effet mémoire d'une séquence temporellement connectée de plusieurs dépôts de crue. L'occurrence multiple de ces événements peut déclencher une superposition temporelle entre les crues qui a un effet substantiel sur les processus opérant en profondeur (tels que la méthanogénèse et la réduction des sulfates) mais négligeable pour les processus oxiques et suboxiques superficiels. Cela a des conséquences importantes dans les scénarios futurs d'augmentation de la fréquence de ces événements extrêmes.La récente série temporelle de composition de l'eau interstitielle obtenue au cours des campagnes hivernales de 2021-22 étudie l'évolution temporelle du sédiment après un dépôt de sédiments estimé à 25 cm. Une modification remarquable des profils de DIC, SO_4^{2-} and CH_4 a été observée, qui se distingue de la situation antérieure à la crue. Les simulations du modèle décrivent de manière adéquate l'ensemble des données et montrent que ces événements hivernaux peuvent entraîner une augmentation de 75 % de la minéralisation totale du carbone, augmentant ainsi la production de DIC à plus long terme dans les sédiments. Cette inondation hivernale entraîne également un découplage des deux voies de réduction du sulfate - la réduction organoclastique du sulfate et l'oxydation anaérobie du méthane - et est associée à un enfoncement dans le sédiment de la zone de transition entre le sulfate et le méthane. Ceci pourrait renforcer l'efficacité du piège de méthane (un gaz à effet de serre crucial dans le contexte du changement climatique) dans le sédiment. Dans l'ensemble, cette thèse fournit pour la première fois une synthèse issue de l'exploration numérique du rôle d'un événement épisodique tel qu'un dépôt de crue massif sur la dynamique spatio-temporelle des processus biogéochimiques dans les sédiments

Modélisation numérique des événements lors de la diagenèse précoce dans les écosystèmes côtiers : application aux dépôts de crue dans le prodelta du Rhône

L'objectif principal de ce travail est d'étudier la réponse biogéochimique des fonds marins côtiers soumis à des dépôts massifs épisodiques de sédiments. Le Rhône et ses marges côtières constituent un cas d'étude important pour quantifier l'impact des dépôts de crue sur les processus diagénétiques dans les sédiments superficiels, car juqu'à 80% des apports de sédiments sont réalisés lors d'événements courts et intenses de crues. Ces événements extrêmes sont rares et imprévisibles, et il est donc difficile d'évaluer par l'observation directe leur impact sur les processus biogéochimiques des sédiments. Afin d'étudier la réponse à court et moyen terme de la biogéochimie des sédiments dans ces conditions de changement brutal, un modèle numérique de diagenèse précoce a été spécifiquement développé au cours de cette thèse. En utilisant les données publiées de deux inondations contrastées en 2008, le modèle a montré sa capacité à simuler les changements induits par l'apport de sédiments sur les profils d'eau interstitielle pour différents solutés. Le modèle suggère que ces inondations pourraient produire des réponses biogéochimiques différentes, dont l'ampleur est déterminée par les caractéristiques du dépôt sédimentaire. Nous avons constaté que les taux de minéralisation du carbone organique totaux ont été multipliés par deux au cours de la crue du printemps 2008 par rapport aux conditions antérieures, et qu'ils ont encore augmenté à l'automne lorsqu'un sédiment enrichi en carbone très labile a été déposé (acrroissement d'un facteur 7). Mes recherches ont démontré que ces différences étaient dues à la nature du carbone organique du delta proximal du Rhône ainsi qu'à l'épaisseur du dépôt. Ces caractéristiques intrinsèques pourraient également être responsables du temps de relaxation des divers solutés de l'eau interstitielle (par exemple l'oxygène, le carbone inorganique dissous, le sulfate) autour de quelques mois. La thèse explore également le concept d'effet mémoire d'une séquence temporellement connectée de plusieurs dépôts de crue. L'occurrence multiple de ces événements peut déclencher une superposition temporelle entre les crues qui a un effet substantiel sur les processus opérant en profondeur (tels que la méthanogénèse et la réduction des sulfates) mais négligeable pour les processus oxiques et suboxiques superficiels. Cela a des conséquences importantes dans les scénarios futurs d'augmentation de la fréquence de ces événements extrêmes.La récente série temporelle de composition de l'eau interstitielle obtenue au cours des campagnes hivernales de 2021-22 étudie l'évolution temporelle du sédiment après un dépôt de sédiments estimé à 25 cm. Une modification remarquable des profils de DIC, SO_4^{2-} and CH_4 a été observée, qui se distingue de la situation antérieure à la crue. Les simulations du modèle décrivent de manière adéquate l'ensemble des données et montrent que ces événements hivernaux peuvent entraîner une augmentation de 75 % de la minéralisation totale du carbone, augmentant ainsi la production de DIC à plus long terme dans les sédiments. Cette inondation hivernale entraîne également un découplage des deux voies de réduction du sulfate - la réduction organoclastique du sulfate et l'oxydation anaérobie du méthane - et est associée à un enfoncement dans le sédiment de la zone de transition entre le sulfate et le méthane. Ceci pourrait renforcer l'efficacité du piège de méthane (un gaz à effet de serre crucial dans le contexte du changement climatique) dans le sédiment. Dans l'ensemble, cette thèse fournit pour la première fois une synthèse issue de l'exploration numérique du rôle d'un événement épisodique tel qu'un dépôt de crue massif sur la dynamique spatio-temporelle des processus biogéochimiques dans les sédiments.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 an important case-study site for quantifying the impact of these extreme events on early diagenetic process 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 term 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-enrichment sediment was deposited (up to a factor of 7). My research demonstrated that these differences were due to the nature of organic carbon delivered to the proximal delta of the Rhone 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 sequence of flood deposition with the conclusion that the 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 the 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 DIC, SO_4^{2-} and CH_4 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) flux out of the sediment.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

Modélisation numérique des événements lors de la diagenèse précoce dans les écosystèmes côtiers : application aux dépôts de crue dans le prodelta du Rhône

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 an important case-study site for quantifying the impact of these extreme events on early diagenetic process 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 term 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-enrichment sediment was deposited (up to a factor of 7). My research demonstrated that these differences were due to the nature of organic carbon delivered to the proximal delta of the Rhone 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 sequence of flood deposition with the conclusion that the 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 the 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 DIC, SO_4^{2-} and CH_4 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) flux out of the sediment.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.L'objectif principal de ce travail est d'étudier la réponse biogéochimique des fonds marins côtiers soumis à des dépôts massifs épisodiques de sédiments. Le Rhône et ses marges côtières constituent un cas d'étude important pour quantifier l'impact des dépôts de crue sur les processus diagénétiques dans les sédiments superficiels, car juqu'à 80% des apports de sédiments sont réalisés lors d'événements courts et intenses de crues. Ces événements extrêmes sont rares et imprévisibles, et il est donc difficile d'évaluer par l'observation directe leur impact sur les processus biogéochimiques des sédiments. Afin d'étudier la réponse à court et moyen terme de la biogéochimie des sédiments dans ces conditions de changement brutal, un modèle numérique de diagenèse précoce a été spécifiquement développé au cours de cette thèse. En utilisant les données publiées de deux inondations contrastées en 2008, le modèle a montré sa capacité à simuler les changements induits par l'apport de sédiments sur les profils d'eau interstitielle pour différents solutés. Le modèle suggère que ces inondations pourraient produire des réponses biogéochimiques différentes, dont l'ampleur est déterminée par les caractéristiques du dépôt sédimentaire. Nous avons constaté que les taux de minéralisation du carbone organique totaux ont été multipliés par deux au cours de la crue du printemps 2008 par rapport aux conditions antérieures, et qu'ils ont encore augmenté à l'automne lorsqu'un sédiment enrichi en carbone très labile a été déposé (acrroissement d'un facteur 7). Mes recherches ont démontré que ces différences étaient dues à la nature du carbone organique du delta proximal du Rhône ainsi qu'à l'épaisseur du dépôt. Ces caractéristiques intrinsèques pourraient également être responsables du temps de relaxation des divers solutés de l'eau interstitielle (par exemple l'oxygène, le carbone inorganique dissous, le sulfate) autour de quelques mois. La thèse explore également le concept d'effet mémoire d'une séquence temporellement connectée de plusieurs dépôts de crue. L'occurrence multiple de ces événements peut déclencher une superposition temporelle entre les crues qui a un effet substantiel sur les processus opérant en profondeur (tels que la méthanogénèse et la réduction des sulfates) mais négligeable pour les processus oxiques et suboxiques superficiels. Cela a des conséquences importantes dans les scénarios futurs d'augmentation de la fréquence de ces événements extrêmes.La récente série temporelle de composition de l'eau interstitielle obtenue au cours des campagnes hivernales de 2021-22 étudie l'évolution temporelle du sédiment après un dépôt de sédiments estimé à 25 cm. Une modification remarquable des profils de DIC, SO_4^{2-} and CH_4 a été observée, qui se distingue de la situation antérieure à la crue. Les simulations du modèle décrivent de manière adéquate l'ensemble des données et montrent que ces événements hivernaux peuvent entraîner une augmentation de 75 % de la minéralisation totale du carbone, augmentant ainsi la production de DIC à plus long terme dans les sédiments. Cette inondation hivernale entraîne également un découplage des deux voies de réduction du sulfate - la réduction organoclastique du sulfate et l'oxydation anaérobie du méthane - et est associée à un enfoncement dans le sédiment de la zone de transition entre le sulfate et le méthane. Ceci pourrait renforcer l'efficacité du piège de méthane (un gaz à effet de serre crucial dans le contexte du changement climatique) dans le sédiment. Dans l'ensemble, cette thèse fournit pour la première fois une synthèse issue de l'exploration numérique du rôle d'un événement épisodique tel qu'un dépôt de crue massif sur la dynamique spatio-temporelle des processus biogéochimiques dans les sédiments

Biogeochemical response of New Caledonia lagoon sediments to tropical events: a numerical modeling approach

International audienceNew Caledonia main island is made of ultramafic rocks largely covered by metal-rich laterites. Mining activity enhances erosion and detrital deposit to the lagoon coastline. This anthropogenic process is itself enhanced by tropical cyclones. This numerical work explores long term early diagenesis processes, especially the formation of authigenic minerals such iron sulfides and "green clays" in shallow sediment. As well, the short term biogeochemical response is investigated regarding different scenarios: a massive sediment deposit due to river flooding and sediment resuspension events. Modeling outputs are compared to results obtained in 2016 and 2018 from campaigns at sea in the Northwest of New Caledonia

Biogeochemical response of New Caledonia lagoon sediments to tropical events: a numerical modeling approach

International audienceNew Caledonia main island is made of ultramafic rocks largely covered by metal-rich laterites. Mining activity enhances erosion and detrital deposit to the lagoon coastline. This anthropogenic process is itself enhanced by tropical cyclones. This numerical work explores long term early diagenesis processes, especially the formation of authigenic minerals such iron sulfides and "green clays" in shallow sediment. As well, the short term biogeochemical response is investigated regarding different scenarios: a massive sediment deposit due to river flooding and sediment resuspension events. Modeling outputs are compared to results obtained in 2016 and 2018 from campaigns at sea in the Northwest of New Caledonia

Biogeochemical response of New Caledonia lagoon sediments to tropical events: a numerical modeling approach

International audienceNew Caledonia main island is made of ultramafic rocks largely covered by metal-rich laterites. Mining activity enhances erosion and detrital deposit to the lagoon coastline. This anthropogenic process is itself enhanced by tropical cyclones. This numerical work explores long term early diagenesis processes, especially the formation of authigenic minerals such iron sulfides and "green clays" in shallow sediment. As well, the short term biogeochemical response is investigated regarding different scenarios: a massive sediment deposit due to river flooding and sediment resuspension events. Modeling outputs are compared to results obtained in 2016 and 2018 from campaigns at sea in the Northwest of New Caledonia

Biogeochemical response of New Caledonia lagoon sediments to tropical events: a numerical modeling approach

International audienceNew Caledonia main island is made of ultramafic rocks largely covered by metal-rich laterites. Mining activity enhances erosion and detrital deposit to the lagoon coastline. This anthropogenic process is itself enhanced by tropical cyclones. This numerical work explores long term early diagenesis processes, especially the formation of authigenic minerals such iron sulfides and "green clays" in shallow sediment. As well, the short term biogeochemical response is investigated regarding different scenarios: a massive sediment deposit due to river flooding and sediment resuspension events. Modeling outputs are compared to results obtained in 2016 and 2018 from campaigns at sea in the Northwest of New Caledonia

Biogeochemical response of New Caledonia lagoon sediments to tropical events: a numerical modeling approach

International audienceNew Caledonia main island is made of ultramafic rocks largely covered by metal-rich laterites. Mining activity enhances erosion and detrital deposit to the lagoon coastline. This anthropogenic process is itself enhanced by tropical cyclones. This numerical work explores long term early diagenesis processes, especially the formation of authigenic minerals such iron sulfides and "green clays" in shallow sediment. As well, the short term biogeochemical response is investigated regarding different scenarios: a massive sediment deposit due to river flooding and sediment resuspension events. Modeling outputs are compared to results obtained in 2016 and 2018 from campaigns at sea in the Northwest of New Caledonia

Characterization of the benthic biogeochemical dynamics after flood events in the Rhône River prodelta: A data-model approach

At the land-sea interface, the benthic carbon cycle is strongly influenced by the export of terrigenous particulate material across the river-ocean continuum. Episodic flood events delivering massive sedimentary materials can occur, but their short-term impact on carbon cycling is poorly understood. In this paper, we use a coupled data-model approach to estimate the temporal variations of sediment-water fluxes, biogeochemical pathways and their reaction rates during these abrupt phenomena. We studied one episodic depositional event in the vicinity of the Rhône River mouth (NW Mediterranean Sea) during the fall-winter of 2021–2022. The distribution of dissolved inorganic carbon (DIC), sulfate (SO42−) and methane (CH4) were measured in sediment porewater collected every 2 weeks before and after the deposition of a 25 cm sediment layer during the main winter flood event. Significant changes in the distribution of DIC, SO42− and CH4, concentrations were observed in the sediment porewaters. The use of an early diagenetic model (FESDIA) to calculate biogeochemical reaction rates and fluxes revealed that this type of flooding event can increase the total organic carbon mineralization rate in the sediment by 75 % a few days after deposition, essentially by increasing the sulfate reduction contribution to total mineralization relative to non-flood depositional period. It predicts a short-term decrease of the DIC flux out of the sediment from 100 to 55 mmol m−2 d−1 after the deposition of the new sediment layer with a longer-term increase by 4 %, therefore implying an initial internal storage of DIC in the newly deposited layer and a slow release over relaxation of the system. Furthermore, examination of the stoichiometric ratios of DIC and SO42− as well as model output over this five-months window shows a decoupling between the two modes of sulfate reduction following the deposition – organoclastic sulfate reduction (OSR) intensified in the newly deposited layer below the sediment surface, whereas anaerobic oxidation of methane (AOM) intensified at depth below the former buried surface. This depth-wise bifurcation of both pathways of sulfate reduction in the sediment column is clearly related to the deepening of the sulfate-methane transition zone (SMTZ) by 25 cm after the flood deposition. Our findings highlight the significance of short-term transient biogeochemical processes at the seafloor and provide new insights on the benthic carbon cycle in the coastal ocean