The availability of limestone and other raw materials for ocean alkalinity enhancement


The work assesses the availability and localizations of different raw materials suitable for alkalinization enhancement (OAE), like limestone, olivine, magnesite and brucite, since several billion tons of rocky materials are needed to achieve meaningful results for carbon sequestration through OAE. Resources of carbonates are immense and widespread around all continents. Availability of pure carbonates is still very large (outcrop area 4.1 million km2) and is not a constraint for the large-scale development of OAE. Outcrops of pure carbonates within 10 km from the coastline and below bare ground or scrub/shrub, preferred for the logistics of exploitation, account for about 70,000 km2, and could provide about 5.000 Gt of limestone. These values increase by a factor of 3 and 8 within 50 and 100 km from the coastline, respectively. Potential resources of olivine, less easily identifiable from the geological data, are estimated in the order of a few hundred billion tons and could provide only a minor contribution to ocean-based carbon removal strategies. A comparison with the current level of world extraction of mineral raw materials is also provided. The annual production of limestone, estimated to be more than 6.6 Gt from deposits scattered all around the world, is about 9% of the world production of mineral raw materials (around 44 Gt y-1), and is of the same order of magnitude as coal (7.3 Gt y-1). The annual productions of magnesite (29 Mt y-1), olivine (8.4 Mt y-1) and brucite (1.5 Mt y-1) are two orders of magnitude lower.

Plain Language Summary

The ongoing acidification of the oceans due to the increase of carbon dioxide concentration in the atmosphere and the consequent increased uptake by the sea can be tackled by adding alkaline compounds under controlled conditions. Since this promotes a number of reactions in the carbonate system which ultimately results in a drawdown of atmospheric CO2, the enhancement of ocean alkalinity can also contribute to the global effort of limiting global warming, and it is widely studied in the literature among the carbon dioxide removal technologies. A limiting factor can be the worldwide availability of alkaline materials to be used for such a scope. The paper shows that the global availability of pure carbonates is vast, although unevenly distributed, and it could largely satisfy the requirement for large-scale development of ocean alkalinization. The required up-scaling of limestone production compared to the current level is much lower than what would be needed for other raw materials that could be used, such as olivine, magnesite and brucite. Moreover, a large fraction of pure limestone resources that could be exploitable is located nearby the coastline, in areas without or with low vegetation cover, which favours the logistics related to the exploitation.

Caserini S., Storni N. & Grosso M., 2022. The availability of limestone and other raw materials for ocean alkalinity enhancement. Global Biogeochemical Cycles 36: e2021GB007246. doi: 10.1029/2021GB007246. Article.

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