The Sensitivity of the Marine Carbonate System to Regional Ocean Alkalinity Enhancement

Abstract

Ocean Alkalinity Enhancement (OAE) simultaneously mitigates atmospheric concentrations of CO2 and ocean acidification; however, no previous studies have investigated the response of the non-linear marine carbonate system sensitivity to alkalinity enhancement on regional scales. We hypothesise that regional implementations of OAE can sequester more atmospheric CO2 than a global implementation. To address this, we investigate physical regimes and alkalinity sensitivity as drivers of the carbon-uptake potential response to global and different regional simulations of OAE. In this idealised ocean-only set-up, total alkalinity is enhanced at a rate of 0.25 Pmol a-1 in 75-year simulations using the Max Planck Institute Ocean Model coupled to the HAMburg Ocean Carbon Cycle model with pre-industrial atmospheric forcing. Alkalinity is enhanced globally and in eight regions: the Subpolar and Subtropical Atlantic and Pacific gyres, the Indian Ocean and the Southern Ocean. This study reveals that regional alkalinity enhancement has the capacity to exceed carbon uptake by global OAE. We find that 82–175 Pg more carbon is sequestered into the ocean when alkalinity is enhanced regionally and 156 PgC when enhanced globally, compared with the background-state. The Southern Ocean application is most efficient, sequestering 12% more carbon than the Global experiment despite OAE being applied across a surface area 40 times smaller. For the first time, we find that different carbon-uptake potentials are driven by the surface pattern of total alkalinity redistributed by physical regimes across areas of different carbon-uptake efficiencies. We also show that, while the marine carbonate system becomes less sensitive to alkalinity enhancement in all experiments globally, regional responses to enhanced alkalinity vary depending upon the background concentrations of dissolved inorganic carbon and total alkalinity. Furthermore, the Subpolar North Atlantic displays a previously unexpected alkalinity sensitivity increase in response to high total alkalinity concentrations.

Limits and CO2 equilibration of near-coast alkalinity enhancement

“…A number of previous studies have used ocean circulation models combined with a carbon cycle model to estimate the carbon uptake potential of various hypothetical OAE scenarios (Köhler et al, 2013;González and Ilyina, 2016;Feng et al, 2017;Ilyina et al, 2013;Keller et al, 2014;Burt et al, 2021;Tyka et al, 2022) Some of these studies investigate very high rates of alkalinity injection to test the limits of OAE. Ilyina et al (2013) simulated alkalinity addition on the order of 2.8Pmol/yr (for an approximate uptake of 50 GtCO 2 /yr).…”

“…Following the addition of some quantity ∆Alk to seawater, the ocean will begin taking up CO 2 , eventually reaching a maximum η CO2 (t = ∞) ≈ 0.8 (depending on the exact state of the carbonate system) (Renforth and Henderson, 2017;Tyka et al, 2022). However, the equilibration kinetics of this equilibration are known to vary spatially due to differences in the gas exchange timescales and the surface residence time of CO 2 deficient water (Jones et al, 2014;Burt et al, 2021). We thus conducted simulated experiments with short, localized pulse injections, followed by tracking of the total excess alkalinity and DIC relative to a reference simulation as done previously with a much coarser model (Tyka et al, 2022).…”

The Availability of Limestone and Other Raw Materials for Ocean Alkalinity Enhancement

“…Logistically, minerals would most likely be added at discrete locations because an even distribution over entire ocean regions seems unfeasible (Kohler et al, 2013), potentially leading to hotspots of impact, depending on how minerals are added, on the type of material added, and on the attenuation/mixing in the surface ocean relative to dissolution rate of the mineral. Burt et al (2021) show different carbon-uptake potentials in different regions of the world' oceans, mainly driven by the surface pattern of total alkalinity, but the limitation of the modeling experiments (i.e., little detail in the biological feedbacks of OAE), as well as differences with the results of previous works (Ilyina et al, 2013;Lenton et al, 2018), due to different modeling of the physical circulation and marine carbonate system states, suggest caution in defining single regions as most suitable for OAE.…”

“…Burt et al. (2021) show different carbon‐uptake potentials in different regions of the world’ oceans, mainly driven by the surface pattern of total alkalinity, but the limitation of the modeling experiments (i.e., little detail in the biological feedbacks of OAE), as well as differences with the results of previous works (Ilyina et al., 2013; Lenton et al., 2018), due to different modeling of the physical circulation and marine carbonate system states, suggest caution in defining single regions as most suitable for OAE.…”

Microbial ecosystem responses to alkalinity enhancement in the North Atlantic Subtropical Gyre

“…The biological effects of OAE approaches have been understudied (Bach et al, 2019;National Academies of Sciences Engineering and Medicine, 2022). The point-source nature of alkalinity introduction to the ocean means that temporal and regional gradients will exist in neutralization effectiveness (Burt et al, 2021) and in the extent of impact (Butenschön et al, 2021;Mongin et al, 2021). OAE may not simply restore ocean chemistry, or exactly reverse the biogeochemical effects of ocean acidification (Zickfeld et al, 2021).…”