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7 Ocean Alkalinity Enhancement
Pages 181-208

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From page 181...
... Adding alkalinity via natural or enhanced weathering is counteracted by the precipitation of carbonate, which reduces alkalinity and, in today's ocean, is driven almost entirely by calcifying organisms. For example, on geologic timescales, the dissolution of alkaline silicate minerals plays a major role in restoring ocean chemistry via addition of alkalinity to the ocean and conversion of CO 2 into other dissolved inorganic carbon (DIC)
From page 182...
... 7.2 KNOWLEDGE BASE The ocean absorbs up to ~30 percent of the CO2 that is released to the atmosphere through physicochemical processes and, as atmospheric CO2 increases, equivalent levels of CO2 are measurable in the surface ocean within timescales of months to years, causing ocean acidification. Briefly, as CO2 diffuses in seawater, it combines with H2O to form carbonic acid (H2CO3)
From page 183...
... to coastal waters through pipelines and ships, and to open-ocean waters through ships; monitoring of the carbonate system through autonomous measurements on buoys and flow through systems aboard ships; potential impacts of alkalinity addition.
From page 184...
... On timescales longer than millennia, carbonate compensation adjusts the alkalinity of the global ocean, counteracting any changes in atmospheric CO2. Carbonate chemistry conditions are critical to the functioning and ecological fitness of calcareous organisms, and this is exemplified in two water basins of contrasting carbonate properties: the Black Sea, with carbonate chemistry conditions comparable to those in the open ocean, and the Baltic Sea, which harbors low-alkalinity waters (Müller et al., 2016)
From page 185...
... OAE has potential benefits over other CDR schemes although empirical data are necessary to determine the effectiveness, risks, and side effects. Advantages include "permanent" CO2 sequestration on timescales of millenia or longer in the absence of processes that remove the added alkalinity; not requiring long-term storage of large quantities of CO2; and, in addition to its application as a CDR method, possible lessening of some of the effects of ocean acidification.
From page 186...
... 2) -- in the surface ocean.
From page 187...
... bars were generated using 1,000 bins across the horizontal axis scale to generate frequency of carbon capture potential for each rock type. Given that the total size of the samples varied (n = 10,839, 8,747, 10,337, 5,431, and 3,226, for granite, andesite, basalt, kimberlite, and peridotite, respectively)
From page 188...
... . It appears that because of very low dissolution rates per unit surface area, silicate minerals in general need to be pulverized to ≤1 µm to dissolve on relevant timescales as elevated ocean pH slows dissolution rates (GESAMP, 2019)
From page 189...
... However, the two main approaches to the study of chemical weathering in the field (geochemical) and laboratory (individual mineral dissolution rates)
From page 190...
... . Fate of Mineral Particles in the Surface Ocean A concern with the deployment of particles as one OAE approach (adding alkalinized seawater following mineral dissolution is also an option)
From page 191...
... ; (3) the amount and type of organic matter that leads to aggregation of mineral particles, for example, the adsorption of dissolved organic matter onto dust particles or the effect of colloidal-size particles and gels on particle aggregation (Alldredge and Silver, 1988; Engel, 2000; Armstrong et al., 2001; Francois et al., 2002; Klaas and Archer, 2002; van der Jagt et al., 2018)
From page 192...
... . The main challenge is that mineral particles must be ground to a very small size to avoid the rapid sinking of particles out of the euphotic zone before being dissolved, to ensure rapid CO2 uptake at the surface (Meysman and Montserrat, 2017)
From page 193...
... For coastal regions, ecosystems particularly susceptible to ocean acidification and upwelling zones could perhaps be good initial candidates for trials. For open-ocean deployments, in addition to new dedicated vessels, the use of existing vessels with modifications to transport and deploy mineral, when possible, would be financially, logistically, and energetically desirable.
From page 194...
... Licensed by Creative Commons CC BY 4.0. Transportation All OAE approaches require the extraction, processing, and transport of rock.
From page 195...
... . Despite increasing efforts to minimize the negative impacts of mining on local communities, an expansion of the mining industry raises concerns not only about the environmental impact but also the local social landscape with implications for sustainability.
From page 196...
... However, the impact of ocean alkalinization (which increases Ω, pH, and levels of CO32− and HCO3− and decreases CO2) is largely unknown because most of the literature has focused on environmental responses associated with ocean acidification (decreasing Ω, CO32−, and pH and increasing CO2 and HCO3−)
From page 197...
... . There are, however, concerns about the potential negative impact of Ni and other metals leached during mineral dissolution on marine organisms and how micronutrient addition will affect the health and structure of marine communities.
From page 198...
... . Impacts of Biota on Weathering In addition to the impacts of weathering on marine biota, organisms can accelerate mineral dissolution and act as catalysts for alkalinization.
From page 199...
... . The transit of mineral particles through the gut of benthic fauna such as lugworms under high enzymatic activity and low pH, combined with mechanical abrasion during ingestion and digestion, appear to increase silicate mineral dissolution rates (Mayer et al., 1997; Needham et al., 2004; Worden et al., 2006)
From page 200...
... 7.7 VIABILITY AND BARRIERS Co-benefits The main co-benefits of OAE are the potential mitigation of ocean acidification, which would have a positive impact on many organisms, particularly the CaCO3-producing community, and the potential for fertilization via the addition of metals such as iron. Assessing the impact of OAE deployments on calcification and the ecological fitness of calcifying organisms and other functional groups requires transitioning from laboratory experimentation and mesocosm-based trials to field deployments to determine the complexity of factors affecting the inorganic carbon chemistry.
From page 201...
... . Processes that facilitate accelerated mineral dissolution typically have the largest energy demand within a supply chain.
From page 202...
... Governance and Social Dimensions The legal framework for ocean CDR is discussed in Chapter 2. Many of the international and domestic laws discussed in that chapter could apply to OAE.
From page 203...
... 7.9 RESEARCH AGENDA Field trials are urgently needed in both coastal and open-ocean waters to monitor mineral dissolution kinetics, the dynamics of the DIC system, biogeochemical and biological impacts, and carbon sequestration potential, and to assess technological readiness and determine environmental and societal impacts of OAE both in marine systems and on land. Research consortia and philanthropic endeavors in coastal waters such as the nonprofit Project Vesta that proposes the EW of ground olivine on beaches to increase coastal carbon capture or the planning of pilot studies by the European Union-funded OceanNETs consortium, which includes offshore mesocosm experiments to assess the ecological impacts of OAE, are already under way.
From page 204...
... Given that the surface ocean is supersaturated with common carbonate minerals, they cannot be directly added to the ocean. To facilitate mineral dissolution, the reaction of carbonate minerals with elevated CO2 has been suggested, initially for reducing fossil fuel emissions, but potentially for CDR by coupling with direct air capture or biomass energy carbon capture and storage.
From page 205...
... Uncertainty is high for CDR possible impacts. Efficacy High Confidence What is the confidence level that this approach will remove Need to conduct field deployments to assess CDR, atmospheric CO2 and lead to net increase in ocean carbon alterations of ocean chemistry (carbon but also metals)
From page 206...
... Energy Land, silicate rock 50–150 100–1,000 kWh/t Coast, silicate rock No data ~100–1,000 kWh/t Ocean, silicate rock No data 100–1,000 kWh/t Ocean liming 70–130 5 GJ/t and 150–300 kWh Accelerated weathering of limestone 10–40 (Opex) No data Ocean, carbonate addition to upwelling 20 (Opex)
From page 207...
... most of the work will be lab and and readiness level of OAE approaches mesocosm based. (including the development of pilot-scale What are the conditions leading to facilities)
From page 208...
... , what are the most appropriate locations and depths to deploy alkalinity sensors? NOTE: Bold type identifies priorities for taking the next step to advance understanding of ocean alkalinity enhancement as an ocean CDR approach.


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