Contrasting responses of the ocean’s oxygen minimum zones to artificial re-oxygenation

Studies assessing potential measures to counteract the marine deoxygenation attributed to anthropogenic activities have been conducted in a few coastal environments and at regional scale, but not yet on a global scale. One way toward global scale artificial oxygenation would be to use oxygen produced as a by-product from hydrogen-production through electrolysis. The low-carbon footprint renewable production of hydrogen from offshore wind energy offers such a possibility. Here, we assessed the potential of this artificial oxygenation method on a global scale using a coupled physical-biogeochemical numerical model. The anthropogenic oxygen source scenario assumes worldwide adoption of hydrogen, considering demographic changes and the feasibility of offshore wind turbine deployment. Following this scenario, artificial oxygenation had a negligible effect on the overall oxygen inventory (an increase of 0.07%) but showed a reduction in the overall volume of Oxygen Minimum Zones (OMZs) between 1.1% and 2.4%. Despite the decrease in the mean OMZ volume globally, OMZs display distinct and contrasting regional patterns notably due to the oxygen impacts on the nitrogen cycle. Artificial oxygenation can inhibit denitrification resulting in a net gain of nitrate that promotes locally and remotely increased biological productivity and consequent respiration. Increased respiration could ultimately lead to an oxygen loss at and beyond injection sites as in the Tropical Pacific and Indian Ocean and particularly expand the Bay of Bengal OMZ. In contrast, the tropical OMZ shrinkage in the Atlantic Ocean is attributed to oxygen enrichment induced by advective transport into the OMZ, while the absence of denitrification in this area precludes any biochemical feedback effect on oxygen levels. These results suggest that the impacts of artificial oxygenation on oxygen concentrations and ecosystems are highly non-linear. It can produce unexpected regional responses that can occur beyond the injection sites which make them difficult to forecast.publishedVersio

Modelling the impact of particulate iron from sedimentary origin on marine biogeochemical cycles

Il existe encore des incertitudes importantes concernant le cycle biogéochimique du fer, sa nature et la quantification de ses sources. Ce fer dissous (dFe) est considéré comme étant la forme la plus biodisponible ce qui a induit la sous-évaluation du rôle du fer particulaire (pFe) comme une source potentielle de dFe. Pourtant, la remise en suspension des sédiments libère davantage de pFe que de dFe. Dans ce contexte, ma thèse remet en question la vision traditionnelle du rôle du fer particulaire inorganique sédimentaire (pFeinorg) et propose la première modélisation de ce dernier comme source externe de dFe. Le modèle numérique PISCES a donc été adapté pour tenir compte d’un flux supplémentaire de fer en s’appuyant sur une climatologie de la dynamique à partir de la configuration NEMOPISCES globale à 2 degrés de résolution. Les simulations mettent en exergue la sensibilité de la biomasse phytoplanctonique à la forme de fer provenant des sédiments ; les limitations en macronutriments et celles en fer sont considérablement modifiées, ainsi que les gradients côte–large de chlorophylle. Le transport plus efficace du fer en tant que pFeinorg permet d’atteindre des régions éloignées de sa source. Son accumulation et sa dissolution dans les zones de convergences induisent via downwelling l’enrichissement de la surbsurface ; à ceci s’ajoute le processus de chute de la particule. Cependant, ces processus demeurent peu étudiés. Les tests de sensibilité ont montré que le gain (absence de chute) ou la perte (chute rapide) en fer dans l’océan, ou encore la prépondérance du pFe sur le dFe seraient modulés par le taux de dissolution. En revanche, la distribution de la chlorophylle est mieux représentée dans la mesure où les processus qui régissent la distribution du PFeinorg et du dFe qui en dérive sont, de concert, pris en compte. Une manière de mieux représenter les répercussions du fer sur les cycles biogéochimiques marins, serait de mieux contraindre les processus liés au PFeinorg.There are still substantial uncertainties in the iron biogeochemical cycle, including those related to the nature and magnitude of its external sources.Dissolved iron (dFe) is considered to be the most bioavailable form, which led to the underestimation of the role of particulate iron (pFe) as a potential source of dFe. Yet sediment resuspension releases more pFe than dFe. In this context, my thesis challenge the traditional view of the role of sedimentary inorganic particulate iron (pFeinorg) and proposes the first modeling of pFeinorg as a new external source of dFe. For this purpose, the PISCES numerical model has been adapted to take into account an additional iron flux based on a climatology of dynamics from the global NEMO-PISCES configuration at 2 degrees of resolution. Simulations highlight the sensitivity of phytoplankton biomass to the sedimentderived form of iron ; macronutrient limitations and iron limitations are considerably modified, as are coastal – open ocean chlorophyll gradients.The iron is more efficiently transported as a pFeinorg, allowing it to reach regions far from its source. Its accumulation and dissolution in the zones convergence zones would allow via downwelling to enrich the subsurface; in addition to this, the process of particle sinking. However, few studies have been conducted on these processes. Sensitivity tests have shown that the gain (no sinking velocity) or loss (relatively fast sinking velocity) of iron in the ocean or the preponderance of particulate iron over dissolved iron would be modulated by the dissolution rate. However the distribution of chlorophyll is better represented to the extent that the processes governing the distribution of pFeinorg and the dFe derived from it are jointly taken into account. One way to better represent the impact of iron on marine biogeochemical cycles would be to better constrain the processes associated with pFeinorg

Petites particules de fer, un effet majeur sur l’océan ? Ce que la modélisation numérique à l’échelle globale nous apprend sur le rôle du fer particulaire lithogénique.

National audienc

Modélisation de l'impact du Fer particulaire d'origine sédimentaire sur les cycles biogéochimiques marins

There are still substantial uncertainties in the iron biogeochemical cycle, including those related to the nature and magnitude of its external sources.Dissolved iron (dFe) is considered to be the most bioavailable form, which led to the underestimation of the role of particulate iron (pFe) as a potential source of dFe. Yet sediment resuspension releases more pFe than dFe. In this context, my thesis challenge the traditional view of the role of sedimentary inorganic particulate iron (pFeinorg) and proposes the first modeling of pFeinorg as a new external source of dFe. For this purpose, the PISCES numerical model has been adapted to take into account an additional iron flux based on a climatology of dynamics from the global NEMO-PISCES configuration at 2 degrees of resolution. Simulations highlight the sensitivity of phytoplankton biomass to the sedimentderived form of iron ; macronutrient limitations and iron limitations are considerably modified, as are coastal – open ocean chlorophyll gradients.The iron is more efficiently transported as a pFeinorg, allowing it to reach regions far from its source. Its accumulation and dissolution in the zones convergence zones would allow via downwelling to enrich the subsurface; in addition to this, the process of particle sinking. However, few studies have been conducted on these processes. Sensitivity tests have shown that the gain (no sinking velocity) or loss (relatively fast sinking velocity) of iron in the ocean or the preponderance of particulate iron over dissolved iron would be modulated by the dissolution rate. However the distribution of chlorophyll is better represented to the extent that the processes governing the distribution of pFeinorg and the dFe derived from it are jointly taken into account. One way to better represent the impact of iron on marine biogeochemical cycles would be to better constrain the processes associated with pFeinorg.Il existe encore des incertitudes importantes concernant le cycle biogéochimique du fer, sa nature et la quantification de ses sources. Ce fer dissous (dFe) est considéré comme étant la forme la plus biodisponible ce qui a induit la sous-évaluation du rôle du fer particulaire (pFe) comme une source potentielle de dFe. Pourtant, la remise en suspension des sédiments libère davantage de pFe que de dFe. Dans ce contexte, ma thèse remet en question la vision traditionnelle du rôle du fer particulaire inorganique sédimentaire (pFeinorg) et propose la première modélisation de ce dernier comme source externe de dFe. Le modèle numérique PISCES a donc été adapté pour tenir compte d’un flux supplémentaire de fer en s’appuyant sur une climatologie de la dynamique à partir de la configuration NEMOPISCES globale à 2 degrés de résolution. Les simulations mettent en exergue la sensibilité de la biomasse phytoplanctonique à la forme de fer provenant des sédiments ; les limitations en macronutriments et celles en fer sont considérablement modifiées, ainsi que les gradients côte–large de chlorophylle. Le transport plus efficace du fer en tant que pFeinorg permet d’atteindre des régions éloignées de sa source. Son accumulation et sa dissolution dans les zones de convergences induisent via downwelling l’enrichissement de la surbsurface ; à ceci s’ajoute le processus de chute de la particule. Cependant, ces processus demeurent peu étudiés. Les tests de sensibilité ont montré que le gain (absence de chute) ou la perte (chute rapide) en fer dans l’océan, ou encore la prépondérance du pFe sur le dFe seraient modulés par le taux de dissolution. En revanche, la distribution de la chlorophylle est mieux représentée dans la mesure où les processus qui régissent la distribution du PFeinorg et du dFe qui en dérive sont, de concert, pris en compte. Une manière de mieux représenter les répercussions du fer sur les cycles biogéochimiques marins, serait de mieux contraindre les processus liés au PFeinorg

Modélisation de l'impact du Fer particulaire d'origine sédimentaire sur les cycles biogéochimiques marins

There are still substantial uncertainties in the iron biogeochemical cycle, including those related to the nature and magnitude of its external sources.Dissolved iron (dFe) is considered to be the most bioavailable form, which led to the underestimation of the role of particulate iron (pFe) as a potential source of dFe. Yet sediment resuspension releases more pFe than dFe. In this context, my thesis challenge the traditional view of the role of sedimentary inorganic particulate iron (pFeinorg) and proposes the first modeling of pFeinorg as a new external source of dFe. For this purpose, the PISCES numerical model has been adapted to take into account an additional iron flux based on a climatology of dynamics from the global NEMO-PISCES configuration at 2 degrees of resolution. Simulations highlight the sensitivity of phytoplankton biomass to the sedimentderived form of iron ; macronutrient limitations and iron limitations are considerably modified, as are coastal – open ocean chlorophyll gradients.The iron is more efficiently transported as a pFeinorg, allowing it to reach regions far from its source. Its accumulation and dissolution in the zones convergence zones would allow via downwelling to enrich the subsurface; in addition to this, the process of particle sinking. However, few studies have been conducted on these processes. Sensitivity tests have shown that the gain (no sinking velocity) or loss (relatively fast sinking velocity) of iron in the ocean or the preponderance of particulate iron over dissolved iron would be modulated by the dissolution rate. However the distribution of chlorophyll is better represented to the extent that the processes governing the distribution of pFeinorg and the dFe derived from it are jointly taken into account. One way to better represent the impact of iron on marine biogeochemical cycles would be to better constrain the processes associated with pFeinorg.Il existe encore des incertitudes importantes concernant le cycle biogéochimique du fer, sa nature et la quantification de ses sources. Ce fer dissous (dFe) est considéré comme étant la forme la plus biodisponible ce qui a induit la sous-évaluation du rôle du fer particulaire (pFe) comme une source potentielle de dFe. Pourtant, la remise en suspension des sédiments libère davantage de pFe que de dFe. Dans ce contexte, ma thèse remet en question la vision traditionnelle du rôle du fer particulaire inorganique sédimentaire (pFeinorg) et propose la première modélisation de ce dernier comme source externe de dFe. Le modèle numérique PISCES a donc été adapté pour tenir compte d’un flux supplémentaire de fer en s’appuyant sur une climatologie de la dynamique à partir de la configuration NEMOPISCES globale à 2 degrés de résolution. Les simulations mettent en exergue la sensibilité de la biomasse phytoplanctonique à la forme de fer provenant des sédiments ; les limitations en macronutriments et celles en fer sont considérablement modifiées, ainsi que les gradients côte–large de chlorophylle. Le transport plus efficace du fer en tant que pFeinorg permet d’atteindre des régions éloignées de sa source. Son accumulation et sa dissolution dans les zones de convergences induisent via downwelling l’enrichissement de la surbsurface ; à ceci s’ajoute le processus de chute de la particule. Cependant, ces processus demeurent peu étudiés. Les tests de sensibilité ont montré que le gain (absence de chute) ou la perte (chute rapide) en fer dans l’océan, ou encore la prépondérance du pFe sur le dFe seraient modulés par le taux de dissolution. En revanche, la distribution de la chlorophylle est mieux représentée dans la mesure où les processus qui régissent la distribution du PFeinorg et du dFe qui en dérive sont, de concert, pris en compte. Une manière de mieux représenter les répercussions du fer sur les cycles biogéochimiques marins, serait de mieux contraindre les processus liés au PFeinorg

Sedimentary particulate iron : the missing micronutrients ?

International audienc

How abiotic sedimentary particulate iron may impact the dissolved iron distribution and may change our understanding on global ocean biogeochemical cycles ?

International audienc

Contrasting responses of the ocean’s oxygen minimum zones to artificial re-oxygenation

Studies assessing potential measures to counteract the marine deoxygenation attributed to anthropogenic activities have been conducted in a few coastal environments and at regional scale, but not yet on a global scale. One way toward global scale artificial oxygenation would be to use oxygen produced as a by-product from hydrogen-production through electrolysis. The low-carbon footprint renewable production of hydrogen from offshore wind energy offers such a possibility. Here, we assessed the potential of this artificial oxygenation method on a global scale using a coupled physical-biogeochemical numerical model. The anthropogenic oxygen source scenario assumes worldwide adoption of hydrogen, considering demographic changes and the feasibility of offshore wind turbine deployment. Following this scenario, artificial oxygenation had a negligible effect on the overall oxygen inventory (an increase of 0.07%) but showed a reduction in the overall volume of Oxygen Minimum Zones (OMZs) between 1.1% and 2.4%. Despite the decrease in the mean OMZ volume globally, OMZs display distinct and contrasting regional patterns notably due to the oxygen impacts on the nitrogen cycle. Artificial oxygenation can inhibit denitrification resulting in a net gain of nitrate that promotes locally and remotely increased biological productivity and consequent respiration. Increased respiration could ultimately lead to an oxygen loss at and beyond injection sites as in the Tropical Pacific and Indian Ocean and particularly expand the Bay of Bengal OMZ. In contrast, the tropical OMZ shrinkage in the Atlantic Ocean is attributed to oxygen enrichment induced by advective transport into the OMZ, while the absence of denitrification in this area precludes any biochemical feedback effect on oxygen levels. These results suggest that the impacts of artificial oxygenation on oxygen concentrations and ecosystems are highly non-linear. It can produce unexpected regional responses that can occur beyond the injection sites which make them difficult to forecast

Propriétés turbulentes de la couche limite de surface à partir des données OCARINA

National audienc

Air-Sea Turbulent Fluxes From a Wave-Following Platform During Six Experiments at Sea

Turbulent fluxes at the air‐sea interface are estimated with data collected in 2011 to 2017 with a low‐profile platform during six experiments in four regions. The observations were carried out with moderate winds (2–10 m/s) and averaged wave heights of 1.5 m. Most of the time, there was a swell, with an averaged wave age (the ratio between wave phase speed and wind speed) being equal to 2.8 ± 1.6. Three flux calculation methods are used, namely, the eddy covariance (EC), the inertial dissipation (ID), and the bulk methods. For the EC method, a spectral technique is proposed to correct wind data from platform motion. A mean bias affecting the friction velocity (u*) is then evaluated. The comparison between EC u* and ID u* estimates suggests that a constant imbalance term (ϕimb) equal to 0.4 is required in the ID method, possibly due to wave influence on our data. Overall, the confidence in the calculated u* estimates is found to be on the order of 10%. The values of the drag coefficient (CD) are in good agreement with the parameterizations of Smith (1988, https://doi.org/10.1029/JC093iC12p15467) in medium‐range winds and of Edson et al. (2013, https://doi.org/10.1175/JPO‐D‐12‐0173.1) in light winds. According to our data, the inverse wave age varies linearly with wind speed, as in Edson et al. (2013, https://doi.org/10.1175/JPO‐D‐12‐0173.1), but our estimates of the Charnock coefficient do not increase with wind speed, which is possibly related to sampling swell‐dominated seas. We find that the Stanton number is independent from wind speed