To mitigate the impact of excess atmospheric carbon dioxide (CO2), humans need to reduce CO2 emissions and remove CO2 from the atmosphere. According to the Intergovermental Panel on Climate Change, global efforts to reduce greenhouse gas emissions are still not on track to avoid surpassing the 1.5 °C threshold of temperature increase by the end of the century (de Coninck et al., 2018; UNFCC, 2022). Since reducing CO2 emissions may not be sufficient to meet the 2015 Paris Agreement Goals (National Academies of Sciences, Engineering, and Medicine, 2022), strategies for marine Carbon Dioxide Removal (mCDR) are being investigated to aid in the uptake and storage of CO2. The Earth has balanced its own carbon storage and acidity on geological timescales (i.e., 100-1000 ka) through mineral weathering and mineral precipitation (Berner et al., 1983; Berner and Berner, 1997; Holland, 1978; Penman et al., 2020; Walker et al., 1981). While there may be more than enough calcium carbonate (CaCO3) in ocean sediments to neutralize all anthropogenic CO2 (Archer, 1996; Sulpis et al., 2018), this recovery could take >10 000 years (Zachos et al., 2005) given slow dissolution kinetics and ocean mixing rates. Ocean Alkalinity Enhancement (OAE) is a mCDR method that can potentially speed up the process of oceanic CO2 uptake for human relevant timescales. A thorough assessment of the scientific background and considerations for various forms of OAE has been presented in Renforth and Henderson (2017). The proposed risks and co-benefits of increasing surface ocean alkalinity based on different deployment strategies have been theorized in Bach et al. (2019).
There are still many technical challenges to overcome before OAE can be scaled to remove gigatons of CO2 (Eisaman et al., 2023; Renforth, 2025). The impacts of different alkaline feedstocks and concentrations on ocean chemistry and the ecosystem are yet to be completely understood. The successful long-term sequestration of CO2 relies on many factors - the rate of air-sea gas exchange, the loss of alkalinity to re-precipitation of carbonate minerals or even the loss of an unequilibrated plume as it subducts below the surface (Lunstrum et al., 2026). Increasing alkalinity could change the cycling of inorganic carbon in the ocean by increasing the likelihood of CaCO3 precipitation or decreasing natural CaCO3 dissolution and thereby affecting the associated organic carbon cycling pumps (Bach, 2024; Falkowski et al., 2000; Ridgwell and Zeebe, 2005; Riebesell et al., 2000; Zondervan et al., 2001). Limiting secondary precipitation of CaCO3 is important for OAE because there is release of CO2from the precipitation of CaCO3 (Eq. 1) - which reduces the efficiency of OAE.
Another potential outcome of OAE is for organic matter to be ballasted to the deep ocean by aggregating on newly formed particulate inorganic carbon (PIC) (Riebesell et al., 2009). The natural global production of particulate organic carbon (POC) is known to be 4-10 times higher than PIC production (Barker et al., 2006); although this ratio is highly variable in time and space (Berelson et al., 2007; Dunne et al., 2007). Renforth and Henderson, (2017) hypothesized that if the POC production remains higher than PIC production even under OAE, then the CO2 emissions from CaCO3 production by marine calcifiers could be offset. This highlights the need for us to assess whether enhanced calcification after OAE will contribute to CO2 production or if it might further sequester organic carbon by enhancing POC production or by aggregating and ballasting POC via enhanced PIC (Bach et al., 2019).
Given the variety of compounds available to increase alkalinity (Renforth and Henderson, 2017), their composition and deployment style will determine the magnitude of change in Omega (Ω = the saturation state) and likelihood of CaCO3 precipitation. This applies to both the near field impacts at the point of injection and the far field impacts after the alkalinity has been dispersed. Hereafter, we shall reference Ω in terms of Ωaragonite as this is the morphotype that precipitates more readily in present-day seawater (Morse et al., 2007). Many studies have observed increased carbonate mineral precipitation, as demonstrated by the loss of alkalinity (TA) and dissolved inorganic carbon (DIC) after the addition of sodium hydroxide (NaOH), quicklime/slaked lime (CaO/Ca(OH)2), and even sodium carbonate (Na2CO3). Precipitation was observed in both short term (<5 d) (Hartmann et al., 2023; Hashim et al., 2025; Moras et al., 2022; Subhas et al., 2022), and longer term (∼ 20 d) perturbation experiments at a range of mineral saturation states (Moras et al., 2022, 2024; Paul et al., 2025; Ringham et al., 2024; Suitner et al., 2024). Yet, OAE experiments conducted with cultures of coccolithophores have noted no change to calcification rates after additions of calcium and bicarbonate ([Ca2+] + [HCO3-]) (Gately et al., 2023). The experimental conditions of observed CaCO3 precipitation will be discussed to provide further context to the results presented in this study.
The examples of existing perturbation experiments come from natural waters around Spain, North Pacific, North Atlantic gyres, and Australian coastal waters. To this rapidly growing body of research, we present the changes of organic and inorganic carbon inventory and partioning during 4 d OAE experiments conducted using coastal California waters from Los Angeles (LA) Harbor. The waters offshore from Los Angeles are richly characterized by decades of regional studies (https://dornsife.usc.edu/spot/publications-and-projects/, https://calcofi.org/publications/peer-reviewed-publications/, last access: 5 March 2026). Yet waters from within LA Harbor contain blends of offshore ecosystems modified by nearshore processes. In our study, we use three different approaches to elevate Ωaragonite by: (i) increasing pH (using two different bases) and (ii) adding [Ca2+] + [HCO3-] to simulate accelerated weathering of limestone. In constraining the resultant changes to the amount of PIC, POC and DOC production, we conduct a carbon budget analysis that helps define how alkalinity may change the natural cycling of organic and inorganic carbon.
