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Calcite Dissolution Kinetics at the Sediment-Water Interface in an Acidifying Ocean

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Calcite Dissolution Kinetics at the Sediment-Water Interface in an Acidifying Ocean Calcite dissolution kinetics at the sediment-water interface in an acidifying ocean Thesis by Olivier Sulpis Department of Earth and Planetary Sciences McGill University, Montreal July 2019 A thesis submitted to McGill University in partial fulfillment of the requirements of the degree of Doctor of Philosophy © Olivier Sulpis, 2019 Acknowledgements It is perhaps the most challenging part of writing this thesis to express my gratitude to all of those who helped, supported and inspired me all along the PhD road, but I will give it a try. First and foremost, I would like to thank my supervisor, Alfonso Mucci, for his wise guidance, ongoing support and encouragements during these four years. Al gave me the freedom to pursue my own path, while putting me back on track when I needed. I would also like to thank Bernard Boudreau for his precious advice and creative thinking that drove much of the research outputs presented here. I have learned so much from Al and Bernie, and they will remain a true scientific inspiration for the rest of my career. None of this work would have been possible without the help of my colleague and friend Claire Lix, who laid the foundations of the work on which this PhD is based. Many thanks go to Don Baker, who always came up with unexpected scientific ideas, encouraging words and made me discover the teaching experience. Carolina Dufour, who welcomed me in her group, and made me discover the wonderful world of models, is heartily thanked. For fruitful discussions and for their availability, I would like to thank Jeanne Paquette, Peter Douglas and Olivia Jensen. Thanks also go to Don Baker and Burke Hales for their careful and insightful review of this thesis. And to the McGill Earth and Planetary Sciences Department, I would like to express my gratitude for financial support. To all of those who shared their ideas, data, samples or elbow grease, through email exchanges, conference meetings or phone calls, thank you infinitely. I am lucky to be a part of this welcoming oceanographic community. In particular, thanks go to Jess Adkins, who hosted me in his research group at Caltech, for a very enjoyable summer stay, for sharing his passion for science and always finding time for constructive discussions. I am also wholly indebted to John Trowbridge, for his invaluable help in understanding bottom flows. I thank Katherine Gombay, for her time and curiosity. She made the story of a dissolving seafloor accessible and understandable to all. For encouraging me to pursue my studies, and develop an interest in science, I thank my earlier supervisors, Sylvain Pichat, Amaelle Landais and Pierre Deschamps. To Carlos, Sanita, Sanoop, Gloria, Shane, Duncan and, above all, Ashley, thank you for supporting me through the ups and downs of the first year, for the musical interludes, deep conversations and everlasting memories. To Monika, Longbo, Becky, Charlie, Jenny, Alexis, Pascale, Noah, Rob, Jethro, Jiaxin, Saad, Louise, Deneyn and Mathilde, my time at McGill would not have been as fun without you, and I will miss you dearly. Thanks go to Kamila, Cloé, Greg, Domitille, Fabien, Romain and Marie-Emilie, who encouraged me to give a try to the Canadian experience, and to all my friends from Bourges, Lyon and Marseille. I am happy to move back to the old continent, closer to you. Although they must still be wondering what I am doing studying the oceans, I would like to thank my parents for their love and ongoing support. I dedicate this thesis to my grandfather, James. i Abstract 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. Over the past decades, numerous studies of calcium carbonate (CaCO3) mineral dissolution kinetics have been performed in seawater. Despite this ongoing effort, the mechanisms controlling the dissolution rate of the two most common CaCO3 minerals and polymorphs (calcite and aragonite) in seawater are still poorly understood and large discrepancies exist between results of in-situ and laboratory studies, most of which have been carried out under conditions (e.g., mineral suspensions) that are not representative of processes taking place at the seafloor. 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. The DBL is a thin layer of fluid at the interface with a solid surface, in which frictional forces cause molecular diffusion to become the dominant mode of solute transport and whose thickness is inversely correlated with the turbulence of the fluid above or the speed of bottom currents. 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 ii 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. The amount of anthropogenic CO2 currently neutralized each year by seafloor or water-column calcite dissolution is negligible, less st than 1% of the current CO2 emission rate, and will remain so throughout the 21 century. 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. iii Résumé 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. Durant les décennies passées, de nombreuses études de la cinétique de dissolution des carbonates de calcium (CaCO3) ont été effectuées dans l’eau de mer. Malgré cet effort continu, les mécanismes contrôlant la dissolution des deux plus communs minéraux et polymorphes du CaCO3 (calcite et aragonite) dans l’eau de mer ne sont toujours pas bien compris, et il existe de larges différences entre les résultats d’études in situ et en laboratoire, dont la plupart ont été effectuées sous des conditions (par exemple, des minéraux gardés en suspension) qui ne sont pas représentatives des processus ayant lieu au niveau du plancher océanique. 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.
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