Reduced CaCO3 Flux to the Seafloor and Weaker Bottom Current Speeds Curtail Benthic CaCO3 Dissolution Over the 21st Century

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153324/1/gbc_2019_sulpisetal_reducedCaCO3flux_21stcentury.pdfDescription of gbc_2019_sulpisetal_reducedCaCO3flux_21stcentury.pdf : Main articl

RADIv1: a non-steady-state early diagenetic model for ocean sediments in Julia and MATLAB/GNU Octave

We introduce a time-dependent, one-dimensional model of early diagenesis that we term RADI, an acronym accounting for the main processes included in the model: chemical reactions, advection, molecular and bio-diffusion, and bio-irrigation. RADI is targeted for study of deep-sea sediments, in particular those containing calcium carbonates (CaCO3). RADI combines CaCO3 dissolution driven by organic matter degradation with a diffusive boundary layer and integrates state-of-the-art parameterizations of CaCO3 dissolution kinetics in seawater, thus serving as a link between mechanistic surface reaction modeling and global-scale biogeochemical models. RADI also includes CaCO3 precipitation, providing a continuum between CaCO3 dissolution and precipitation. RADI integrates components rather than individual chemical species for accessibility and is straightforward to compare against measurements. RADI is the first diagenetic model implemented in Julia, a high-performance programming language that is free and open source, and it is also available in MATLAB/GNU Octave. Here, we first describe the scientific background behind RADI and its implementations. Following this, we evaluate its performance in three selected locations and explore other potential applications, such as the influence of tides and seasonality on early diagenesis in the deep ocean. RADI is a powerful tool to study the time-transient and steady-state response of the sedimentary system to environmental perturbation, such as deep-sea mining, deoxygenation, or acidification events

Aragonite dissolution protects calcite at the seafloor.

peer reviewedIn the open ocean, calcium carbonates are mainly found in two mineral forms. Calcite, the least soluble, is widespread at the seafloor, while aragonite, the more soluble, is rarely preserved in marine sediments. Despite its greater solubility, research has shown that aragonite, whose contribution to global pelagic calcification could be at par with that of calcite, is able to reach the deep-ocean. If large quantities of aragonite settle and dissolve at the seafloor, this represents a large source of alkalinity that buffers the deep ocean and favours the preservation of less soluble calcite, acting as a deep-sea, carbonate version of galvanization. Here, we investigate the role of aragonite dissolution on the early diagenesis of calcite-rich sediments using a novel 3D, micrometric-scale reactive-transport model combined with 3D, X-ray tomography structures of natural aragonite and calcite shells. Results highlight the important role of diffusive transport in benthic calcium carbonate dissolution, in agreement with recent work. We show that, locally, aragonite fluxes to the seafloor could be sufficient to suppress calcite dissolution in the top layer of the seabed, possibly causing calcite recrystallization. As aragonite producers are particularly vulnerable to ocean acidification, the proposed galvanizing effect of aragonite could be weakened in the future, and calcite dissolution at the sediment-water interface will have to cover a greater share of CO2 neutralization.SERENAT

Respiration Patterns in the Dark Ocean

In the dark ocean, respiring organisms are the main sink for dissolved oxygen. The respiration rate in a given seawater volume can be quantified through dissolved oxygen drawdown or organic matter consumption as a function of time. Estimates of dissolved oxygen utilization rates (OUR) abound in the literature, but are typically obtained using proxies of questionable accuracy, often with low vertical resolution, and neglecting key regions such as the Southern and Indian oceans. Respiration rates based on particulate (POC) or dissolved (DOC) organic carbon are also sparsely observed and for DOC are unavailable in many regions. Consequently, the relative contributions of POC or DOC as a respiration substrate in the dark ocean are unknown. Here, we use recent datasets of true oxygen utilization, seawater age, and DOC to derive OUR and DOC consumption-rate profiles in 10 oceanic regions. We demonstrate that although DOC and POC consumption rates are globally consistent with OUR, they underestimate OUR in the deep, suggesting strong oxygen utilization at the seafloor. In the abyss, we find a negative correlation of the DOC consumption rate with seawater age, suggesting that DOC reactivity decreases along the deep branch of the conveyor circulation. Our results highlight that benthic organisms are sensitive to perturbations in the surface production of organic matter and to large-scale circulation changes that affect its supply to the abyss

Future directions for deep ocean climate science and evidence-based decision making

Introduction: A defining aspect of the Intergovernmental Panel on Climate Change (IPCC) assessment reports (AR) is a formal uncertainty language framework that emphasizes higher certainty issues across the reports, especially in the executive summaries and short summaries for policymakers. As a result, potentially significant risks involving understudied components of the climate system are shielded from view. Methods: Here we seek to address this in the latest, sixth assessment report (AR6) for one such component—the deep ocean—by summarizing major uncertainties (based on discussions of low confidence issues or gaps) regarding its role in our changing climate system. The goal is to identify key research priorities to improve IPCC confidence levels in deep ocean systems and facilitate the dissemination of IPCC results regarding potentially high impact deep ocean processes to decision-makers. This will accelerate improvement of global climate projections and aid in informing efforts to mitigate climate change impacts. An analysis of 3,000 pages across the six selected AR6 reports revealed 219 major science gaps related to the deep ocean. These were categorized by climate stressor and nature of impacts. Results: Half of these are biological science gaps, primarily surrounding our understanding of changes in ocean ecosystems, fisheries, and primary productivity. The remaining science gaps are related to uncertainties in the physical (32%) and biogeochemical (15%) ocean states and processes. Model deficiencies are the leading cited cause of low certainty in the physical ocean and ice states, whereas causes of biological uncertainties are most often attributed to limited studies and observations or conflicting results. Discussion: Key areas for coordinated effort within the deep ocean observing and modeling community have emerged, which will improve confidence in the deep ocean state and its ongoing changes for the next assessment report. This list of key “known unknowns” includes meridional overturning circulation, ocean deoxygenation and acidification, primary production, food supply and the ocean carbon cycle, climate change impacts on ocean ecosystems and fisheries, and ocean-based climate interventions. From these findings, we offer recommendations for AR7 to avoid omitting low confidence-high risk changes in the climate system

Calcite dissolution kinetics at the sediment-water interface in an acidifying ocean

Carbon dioxide (CO2), produced and released to the atmosphere by human activities, has been accumulating in the oceans for approximately two centuries and will continue to do so well beyond the end of this century if emissions are not curbed. One direct consequence of CO2 build-up in the atmosphere and its transfer to the ocean is the acidification of seawater. Calcite, a mineral secreted by many organisms living in the surface ocean to produce their shells and skeletons, covers a large part of the seafloor and acts as a natural anti-acid, neutralizing CO2. Thus, a precise knowledge of the kinetics of this dissolution reaction is necessary to predict the ocean recovery time towards its pre-acidification state once anthropogenic CO2 emissions are curbed. This thesis combines laboratory experiments, oceanographic measurements and model outputs to explore and unravel the mechanisms that control calcite dissolution at the sediment-water interface (SWI) on the seafloor in the context of current anthropogenic ocean acidification. Using a newly developed temperature-controlled rotating-disk reactor, as well as a stirred reactor, we were able to measure the rate of dissolution of simulated and natural sediment disks of variable calcite content under environmental and hydrodynamic conditions representative of deep-sea benthic environments. These experiments revealed that, in contrast to the reigning paradigm that calcite dissolution kinetics in seawater is surface reaction-controlled and of high order, calcite dissolution at the SWI and under deep-sea conditions is linearly dependent on the undersaturation state of the overlying seawater with respect to calcite and controlled by the presence of a diffusive-boundary layer (DBL) above the sediment bed. Therefore, irrespective of the mineralogy and sediment properties, the dissolution rate is simply a function of the saturation state of the overlying seawater with respect to calcite, the calcite content of the sediment, and hydrodynamics at the seafloor. From these observations, using a compilation of seawater chemical variables in the global ocean, corresponding sediment calcite content and rain rates, as well as recently modeled bottom current velocities, we have been able to identify the loci of current anthropogenic calcite dissolution and its rate. We found that significant anthropogenic dissolution of calcite at the seafloor currently occurs in the western North Atlantic, where the bottom waters are youngest, and at various hot spots in the southern Atlantic, Indian and Pacific Oceans. Using model projections for the 21st century, under a “business as usual” scenario, we found that while seawater will become more corrosive to this mineral, calcite dissolution at the seafloor will decrease in intensity because bottom currents will slow down and the amount of calcite particles delivered to the seafloor will diminish. These results indicate that the neutralization of human-made CO2 by calcite dissolution at the seafloor may take longer than previously thought. These findings are of critical interest to the scientific community and the public, as these results have far reaching implications for ocean acidification mitigation, to the fate of benthic communities living under increasing environmental stress, and to geologists contemplating both present and past records of ocean acidification.Le dioxyde de carbone (CO2), produit et libéré à l’atmosphère par les activités humaines, s’est accumulé dans les océans depuis environ deux siècles et continuera ainsi bien après la fin du siècle si les émissions actuelles ne sont pas freinées. Une des conséquences directes de cette accumulation de CO2 dans l’atmosphère et son transfert aux océans est l’acidification de l’eau de mer. La calcite, un minéral secrété par de nombreux organismes vivant à la surface des océans afin de former leur coquille ou squelette, recouvre une large partie du plancher océanique et agit comme un antiacide naturel, neutralisant le CO2. De ce fait, une connaissance précise de la cinétique de cette réaction de dissolution est requise afin de prédire le temps nécessaire à l’océan de retrouver son état pré-acidification une fois que les émissions de CO2 anthropiques ralentiront. Cette thèse rassemble les résultats d’expériences en laboratoire, des données océanographiques et des sorties de modèles afin d’explorer les mécanismes contrôlant la vitesse de dissolution de la calcite à l’interface eau-sédiment (IES) sur les fonds océaniques, et dans le contexte actuel d’acidification des océans. En utilisant un nouveau réacteur à disque rotatif, sous température contrôlée, ainsi qu’un réacteur mélangé, nous avons été capables de mesurer les taux de dissolution de disques de sédiment contenant des quantités variées de calcite, sous des conditions hydrodynamiques et environnementales représentatives des environnements benthiques profonds. Ces expériences ont montré que, au contraire du paradigme selon lequel la cinétique de dissolution de la calcite dans l’eau de mer est contrôlée par des processus à la surface des grains et d’ordre élevé, la dissolution de la calcite à l’IES et sous des conditions d’océan profond est une fonction linéaire de l’état de sous-saturation de l’eau de mer surnageante par rapport à la calcite, et contrôlée par la présence d’une couche limite de diffusion (CLD) au-dessus du sédiment. Par conséquent, indépendamment de la minéralogie ou des propriétés des sédiments, la vitesse de dissolution est simplement une fonction de l’état de saturation de l’eau de mer par rapport à la calcite, du contenu en calcite des sédiments, et des conditions hydrodynamiques au niveau du plancher océanique. A partir de ces observations, une compilation de variables chimiques de l’eau de mer à travers l’océan global, des teneurs en calcite des sédiments et de taux d’accumulation de calcite, ainsi que des vitesses de courant de fond récemment modélisées, nous avons identifié les lieux où la dissolution anthropique de calcite a lieu, ainsi que sa vitesse. Nous avons trouvé qu’une dissolution significative de calcite s’effectuait présentement au niveau du plancher océanique dans l’Atlantique nord, où les eaux de fond sont jeunes, ainsi qu’à divers points chauds dans le sud des océans Atlantique, Indien et Pacifique. Par la suite, en utilisant des prédictions de modelés pour le 21ème siècle, sous un scenario de « statut quo », nous avons déterminé que bien que l’eau de mer deviendra davantage corrosive pour la calcite, la dissolution de ce minéral au plancher océanique allait décroitre en intensité parce que les courants de fond ralentiront, et que la quantité de particules de calcite livrée au plancher océanique diminuera. Ces résultats indiquent que la neutralisation du CO2 d’origine humaine par la dissolution de calcite du plancher océanique pourrait prendre plus de temps que prévu. Ces trouvailles sont d’un intérêt capital pour la communauté scientifique aussi bien que pour le grand publique, car elles ont d’importantes implications pour l’atténuation de l’acidification des océans, le devenir des communautés benthiques vivant sous des conditions environnementales qui se dégradent, et pour les géologues étudiant à la fois les enregistrements sédimentaires présents et passés de l’acidification des océans

Inorganic blue carbon sequestration

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Control of CaCO3 dissolution at the deep seafloor and its consequences

Prediction of the neutralization of anthropogenic CO2 in the oceans and the interpretation of the calcite record preserved in deep-sea sediments requires the use of the correct kinetics for calcite dissolution. Dissolution rate information from suspended calcite-grain experiments consistently indicates a high-order nonlinear dependence on undersaturation, with a well-defined rate constant. Conversely, stirred-chamber and rotating-disc dissolution experiments consistently indicate linear kinetics of dissolution and a strong dependence on the fluid flow velocity. Here, we resolve these seeming incongruities and establish reliably the kinetic controls on deep-sea calcite dissolution. By equating the carbonate-ion flux from a dissolving calcite bed, governed by laboratory-based nonlinear kinetics, to the flux across typical diffusive boundary layers (DBL) at the seafloor, we show that the net flux is influenced both by boundary layer and bed processes, but that flux is strongly dominated by the rate of diffusion through the DBL. Furthermore, coupling that calculation to an equation for the calcite content of the seafloor, we show that a DBL-transport dominated model predicts lysoclines adeptly, i.e., CaCO3 vs ocean depth profiles, observed across the oceans. Conversely, a model with only sediment-side processes fails to predict lysoclines in all tested regions. Consequently, the past practice of arbitrarily altering the calcite-dissolution rate constant to allow sediment-side only models to fit calcite profiles constitutes confirmation bias. From these results, we hypothesize that the reason suspended-grain experiments and bed experiments yield different reaction orders is that dissolution rates of individual grains in a bed are fast enough to maintain porewaters at or close to saturation, so that the exact reaction order cannot be measured accurately and dissolution appears to be linear. Finally, we provide a further test of DBL-transport dominated calcite dissolution by successfully predicting, not fitting, the in-situ pH profiles observed at four stations reported in the literature

Control of CaCO₃ dissolution at the deep seafloor and its consequences

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