Ocean alkalinity enhancement through restoration of blue carbon ecosystems

Limiting global average surface warming to well below 2 °C requires sustained atmospheric CO2 removal alongside rapid emissions reductions. Natural CO2 removal, including restoration of coastal blue carbon ecosystems (BCEs) such as mangroves and seagrasses, offers climate benefits with substantial ecological and economic co-benefits. BCEs, while covering ~0.5% of the seafloor, may contribute up to half of organic carbon burial in ocean sediments due to high sequestration rates. However, BCEs also influence local carbonate chemistry: elevated organic matter inputs enhance aerobic and anaerobic respiration (for example, sulfate reduction), altering sedimentary CO2, porewater pH, alkalinity, and dissolved inorganic carbon (DIC). Increased alkalinity production within porewaters can drive additional marine CO2 uptake, storing legacy anthropogenic carbon on effectively permanent (>1,000 years) timescales. Although alkalinity export from shallow marine ecosystems has been increasingly studied, a systematic evaluation of how BCE restoration could increase alkalinity export is lacking. Moreover, enhanced carbonate precipitation can lower pH and potentially cause CO2 outgassing. The study uses a stochastic modelling approach to evaluate how restoring seagrass and mangrove ecosystems reshapes benthic redox dynamics to drive ocean alkalinity enhancement and associated CO2 uptake, building a time-dependent model of marine sediment biogeochemistry that simulates C, N, Fe, and S cycling across a wide range of conditions.

Prior work highlights BCEs’ disproportionate role in organic carbon burial despite minimal seafloor coverage and their broader ecosystem services. Studies have documented BCE impacts on carbonate chemistry via enhanced respiration processes affecting pH, alkalinity, and DIC. There has been growing attention to alkalinity export from shallow marine settings and the role of coupled carbonate dissolution/reprecipitation in carbonate-rich sediments (for example, Bahamas). Some research indicates that calcification in seagrass systems can drive CO2 emissions that exceed organic carbon sequestration under certain conditions. However, alkalinity production coupled to processes like sulfate reduction and pyrite formation has been identified as an overlooked blue carbon component. The literature reveals a gap: no systematic assessment of increased alkalinity export associated with BCE restoration, as well as uncertainty regarding the balance between alkalinity production and local calcification in determining net CO2 fluxes.

The authors developed a time-dependent reaction-transport model of marine sediments representing seagrass and mangrove environments, simulating coupled C, N, Fe, and S cycling. The model includes production of alkalinity via anaerobic respiration (for example, sulfate reduction) and dissolution/precipitation of calcium carbonate as emergent responses to redox and pH changes. It also accounts for acidity production from oxidation of reduced species (e.g., sulfide), advection (bio-irrigation, plant-mediated transport, tidal pumping), methane production and oxidation (aerobic and anaerobic), and diffusion/bio-diffusion. The governing equations describe solute and solid-phase transport with burial velocities, bio-diffusion, bio-irrigation, and reaction terms. Organic matter degradation is represented as a multi-stage transformation (POC→DOC→DIC): POC decay follows a Middelburg-type power-law reactivity (log10 k = −0.95 log10 t − 0.81) with oxygen correction; DOC→DIC mineralization follows a Monod scheme with half-saturation constant for DOC degradation. Root-zone releases of oxygen and DOC are parameterized with Gaussian-distributed fluxes across depth grid points. Boundary conditions: Dirichlet concentrations for dissolved species at the sediment-water interface, mixed-type flux conditions for solids at the interface, and zero-gradient conditions at depth; initial conditions are zero, with restoration scenarios initialized from steady-state baseline profiles without vegetation. Carbonate system speciation and pH are computed from DIC and alkalinity. Model validation used observed depth profiles (oxygen, DIC, alkalinity, pH, sulfide, TOC) from seagrass (Bahamas) and multiple mangrove sites with limited calibration; root-zone organic matter flux was the only tuned parameter within observed ranges. A stochastic simulation framework varied environmental boundary conditions and key parameters (reaction rates, burial fluxes of organic/inorganic phases, bottom-water DIC/alkalinity, extent of advection/irrigation) across literature-constrained ranges (uniform or log-uniform) for 1,000 realizations, with and without pre-formed (allochthonous) carbonate to assess the role of carbonate dissolution in replenishing alkalinity. CO2 removal was estimated from the change in total benthic alkalinity flux pre- vs post-restoration corrected for surface ocean buffering using an uptake efficiency (δCO2) of 0.75–0.85, based on re-equilibration of the carbonic acid system using WOA2018 temperature and salinity fields. These efficiency estimates assume thermodynamic and air–sea gas-exchange equilibrium and thus represent upper bounds on effective ingassing.

• Across essentially all simulated conditions, restoration of mangroves and seagrasses increases sedimentary alkalinity production and enhances benthic alkalinity fluxes to surface waters, driving net atmospheric CO2 uptake.
• Restoration shoals oxygen penetration and increases anaerobic respiration, boosting alkalinity production. In carbonate-rich settings, increased acidity production (notably via sulfide oxidation) reduces pH and CaCO3 saturation near the surface, enhancing carbonate dissolution and further augmenting alkalinity export.
• Seagrass restoration: estimated atmospheric CO2 uptake of approximately 0.1–0.9 tCO2 ha−1 yr−1, with higher values in carbonate-rich systems; scaling to a theoretical restoration range of −8 to 25 million ha yields −0.8 to 23 MtCO2 yr−1 potential removal.
• Mangrove restoration: larger CO2 removal potential of approximately −1 to 17 tCO2 ha−1 yr−1; combined with a theoretical restoration potential of −9 to 13 million ha gives −9 to 221 MtCO2 yr−1 potential removal.
• Collectively, restoration-driven alkalinity enhancement in mangrove and seagrass ecosystems could, under theoretical upper-limit assumptions, permanently capture roughly ~1% of total fossil fuel CO2 emissions.
• Methane: modelled increases in CH4 production under restoration are relatively small in CO2-equivalent terms compared with enhanced alkalinity-driven CO2 uptake, implying minor offsets to net removal under typical cases.
• Model ecosystem budget analysis indicates alkalinity production generally exceeds local calcification rates, making these ecosystems a net sink for CO2 in almost all cases examined.
• Salt marsh sediments also show compatible geochemical profiles in the model, suggesting additional potential for CO2 uptake via restoration (quantification deferred to future work).

The study demonstrates that restoring blue carbon ecosystems can drive durable (>1,000 years) atmospheric CO2 removal by enhancing benthic alkalinity fluxes, complementing and potentially surpassing the reliability of organic carbon burial, which is susceptible to remobilization. The increased organic matter inputs and altered redox dynamics in restored systems promote anaerobic respiration and localized acidity generation, expanding alkalinity export and, after surface ocean re-equilibration, stimulating air-to-sea CO2 flux. Importantly, the modelling indicates that alkalinity production typically outcompetes local calcification, so restored seagrass and mangrove systems act as net CO2 sinks rather than sources. While surface ocean buffering reduces the translation of alkalinity to CO2 uptake, the corrected estimates still indicate significant removal potential, especially for mangroves. Environmental stressors such as warming, ocean acidification, and eutrophication may sustain or even enhance carbon capture by increasing organic matter delivery and diffusive alkalinity fluxes, and BCEs may help buffer coastal carbonate chemistry. The findings support incorporating alkalinity-based CO2 removal into restoration planning, regulation, and potential carbon market mechanisms, recognizing variability among sites and the need for robust monitoring and modelling to quantify project-specific outcomes and uncertainties.

Restoration of mangrove and seagrass ecosystems can enhance benthic alkalinity fluxes and drive effectively permanent atmospheric CO2 removal. Mangroves, in particular, show high per-area removal potential due to greater organic matter release and trapping, with carbonate-rich settings further amplifying alkalinity via dissolution. The study adds an overlooked inorganic carbon pathway to the benefits of blue carbon restoration and provides a framework to estimate CO2 removal accounting for surface ocean buffering. Future work should: (1) extend analyses to salt marshes and other BCEs; (2) develop coupled modelling–empirical approaches for site-specific quantification and uncertainty bounds; (3) evaluate interactions with environmental stressors; and (4) integrate alkalinity-based CO2 removal into economic and policy incentives to support restoration and protection efforts.

• CO2 uptake efficiencies (0.75–0.85) and surface ocean response assume thermodynamic and gas-exchange equilibrium, providing upper bounds on effective ingassing; real-world kinetics and mixing may reduce realized uptake.
• Global removal potentials (including ~1% of fossil emissions) reflect theoretical upper limits and substantial restoration extents that may be constrained by economic and societal factors.
• Results are model-based with parameter variations; local heterogeneity (e.g., calcification rates, carbonate availability, advection, electron acceptor supply) may yield different outcomes at specific sites.
• Nitrous oxide fluxes were not explicitly modeled; although typically a small component in BCEs, they contribute uncertainty to full greenhouse gas budgets.
• Methane emissions are included but with model assumptions; empirical validation of CH4 responses to restoration across diverse settings is needed.
• Organic carbon burial benefits are uncertain due to potential remobilization; conclusions emphasize alkalinity-driven permanence but depend on alkalinity production exceeding local calcification.