Effects of Ocean Alkalinity Enhancement in deep and bottom water formation regions on the 21st century CO2 uptake under low and high emission pathways.

Subduction regions play an important role in transferring carbon from the surface to the deep ocean and sequestering it on a multi-decadal to centennial timescale. Hence, we test the hypothesis that a Carbon Dioxide Removal (CDR) method, namely Ocean Alkalinity Enhancement (OAE) based on olivine addition, is more efficient in deep and bottom water formation region in terms of enhancing the ocean CO₂ uptake.

Using an ocean-only setup of the physical-biogeochemical model FESOM2.1-REcoM3, we quantify the responses to the spatially uniform and continuous addition of olivine (alkalinity, silicic acid and iron) over the period of 2030-2100 under the SSP1-2.6 and SSP3-7.0 emission scenarios in a global (3 Pg olivine/yr) and a regional application (0.22 Pg olivine/yr). For the regional case, we deposit olivine in the major deep and bottom water formation areas of the Southern Ocean, in the Labrador Sea and the Norwegian Sea.

Under the SSP1-2.6 (SSP3-7.0) scenarios, CO₂ uptake increases by 1.2 (1.3) Pg C/yr by the end of the 21^st century in the global case, whereas it increases by 0.2 (0.2) Pg C/yr in the regional case. The area of uniform olivine deposition is significantly smaller in the regional case compared to the global case, yet the regional OAE has a 2.3-fold higher CDR potential compared to the global OAE in both emission scenarios. The high CDR potential in the regional case is largely (80%) attributed to enhanced biological activity resulting from nutrient fertilization in the Southern Ocean, while only 20% is due to enhanced alkalinity. However, the nutrient effect decreases over time. Furthermore, nutrient addition promotes small-phytoplankton calcification in global and regional OAE cases, leading to lower surface alkalinity by the end of the century. Interestingly, CDR potential of adding alkalinity only (without nutrients) is also 4% and 6.6% higher in the subduction regions than in global OAE under the SSP1-2.6 and SSP3-7.0 scenarios. Overall, the subduction regions, hence show higher CDR efficiency in both cases (alkalinity+nutrients, only alkalinity addition). This effect is two orders of magnitude larger when nutrients are included as this essentially includes Southern Ocean iron fertilization.