Marine biologists are increasingly concerned about the impact of human-induced climate change on the oceans. In response, they have been exploring various strategies to address this issue, one of which is ocean carbon dioxide removal (CDR). Ocean alkalinity enhancement is a specific approach to CDR that involves introducing pulverized minerals or dissolved alkali into the ocean surface to increase its capacity to uptake carbon dioxide. However, the consequences of this method are still largely unknown.
In a recent report published in Science Advances, James A. Gately and his team at the University of California, Santa Barbara, conducted a study to investigate the effects of limestone-inspired alkalinity on the bioecology of two phytoplankton functional groups: the coccolithophore Emiliania huxleyi and the diatom Chaetoceros sp. Emiliania huxleyi is responsible for large-scale calcium carbonate production, while Chaetoceros sp. is a silica producer in modern oceans. The research found that both species showed no significant changes in their growth rate and elemental ratios when exposed to limestone-inspired alkalization. The study also revealed insights into the biogeochemical and physiological responses to ocean alkalinity enhancement through the observation of abiotic precipitation, which removed nutrients and alkalinity from the solution.
The motivation for the study of ocean alkalinity enhancement stems from the goals set at the 2015 Paris Agreement. In order to limit global temperature rise, researchers and industry leaders proposed incorporating carbon dioxide removal approaches in the ocean, alongside emission reductions. The target is to remove 9 gigatons of CO2 annually. Ocean alkalinity enhancement, also known as ocean alkalinization and accelerated weathering, offers a potential solution by increasing carbon storage and mitigating ocean acidification.
Ocean alkalinity enhancement involves mimicking the natural process of rock weathering to restore the alkalinity of the ocean. By increasing the total alkalinity, carbon dioxide can be permanently removed, offering a quasi-natural method to restore ecosystems in fragile habitats like coral reefs that are affected by oceanic acidification. The study conducted by Gately and his colleagues focused on the biogeochemical and physiological responses to limestone-inspired alkalinity enhancement using representative species.
During the experiments, Gately and his team observed that the starting conditions for both Emiliania huxleyi and Chaetoceros sp. fell within the predicted range of scenarios. The total alkalinity varied between the two species due to their different nutritional uptake, and pH values also increased, with a greater extent in the biotic experiments. The researchers examined the nutrient evolution and physiological and biogeochemical responses of the two species. The growth rates remained relatively unchanged and comparable to those in treatments with moderate and high total alkalinity.
The experiments conducted by James Gately and his team shed light on the biological responses to changes in seawater carbonate chemistry and pH caused by ocean alkalinity enhancement. Adding alkalinity to the surface ocean can sequester carbon dioxide into various forms of dissolved inorganic carbon, promoting the uptake of atmospheric carbon dioxide and assisting in global carbon cycling.
While the initial findings showed minimal effects on the physiology and biochemistry of the coccolithophore and diatom, it is important to acknowledge that the experiments only focused on two species. The long-term effects of alkalinity on a broader range of species may differ. Further research should explore the potential ecosystem impacts of ocean alkalinity enhancement and evaluate the risks associated with additional carbon dioxide removal technologies. It is crucial to fully understand the consequences before implementing large-scale ocean alkalinity enhancement projects.
Ocean alkalinity enhancement shows promise as a potential solution to address human-induced climate change on the oceans. However, the long-term impacts on marine ecosystems remain unclear. Through continued research and evaluation, scientists can gain a better understanding of the unknown consequences, allowing for more informed decision-making regarding the implementation of ocean carbon dioxide removal approaches.
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