Limits and CO2 equilibration of near-coast alkalinity enhancement

Abstract

Abstract. Ocean Alkalinity Enhancement (OAE) has recently gained attention as a potential method for negative emissions at gigatonne scale, with near-coast OAE operations being economically favorable due to proximity to mineral and energy sources. In this paper we study critical questions which determine the scale and viability of OAE: Which coastal locations are able to sustain a large flux of alkalinity at minimal pH and ΩArag (aragonite saturation) changes? What is the interference distance between adjacent OAE projects? How much CO2 is absorbed per unit of alkalinity added? How quickly does the induced CO2 deficiency equilibrate with the atmosphere? Using the LLC270 (0.3deg) ECCO global circulation model we find that the steady-state OAE rate varies over 1–2 orders of magnitude between different coasts and exhibits complex patterns and non-local dependencies which vary from region to region. In general, OAE in areas of strong coastal currents allow the largest fluxes and depending on the direction of coastal currents, neighboring OAE sites can exhibit dependencies as far as 400 km or more. We found that within relatively conservative constraints set on ∆pH or ∆Omega, most regional stretches of coastline are able to accommodate on the order of tens to hundreds of megatonnes of negative emissions within 300 km of the coast. We conclude that near-coastal OAE has the potential to scale globally to several GtCO2/yr of drawdown with conservative pH constraints, if the effort is spread over the majority of available coastlines. Depending on the location, we find a diverse set of equilibration kinetics, determined by the interplay of gas exchange and surface residence time. Most locations reach an uptake-efficiency plateau of 0.6–0.8mol CO2 per mol of alkalinity after 3–4 years, after which there is little further CO2 uptake. The most ideal locations, reaching an uptake of around 0.8 include north Madagascar, San Francisco, Brazil, Peru and locations close to the southern ocean such as Tasmania, Kerguelen and Patagonia, where the gas exchange appears to occur faster than the surface residence time. Some locations (e.g. Hawaii) take significantly longer to equilibrate (up to 8–10 years), though can still eventually achieve high uptake. If the alkalinity released advects into regions of significant downwelling (e.g. around Iceland) up to half of the OAE potential can be lost to bottom waters.

A biogeochemical model of mineral-based ocean alkalinity enhancement: impacts on the biological pump and ocean carbon uptake

“…The encounter rate of zooplankton with marine aggregates in the model is parameterized as a function of net primary production, consistent with observations from the modern oceans (e.g. [19,20]). Any discrete zooplankton-aggregate encounter event can lead to either ingestion or fragmentation, the likelihood and results of which are parameterized based on fragmentation experiments with tethered Euphausia pacifica [22,27] and a power law relationship for fragment production [22].…”

“…Although there have been important steps forward in understanding the regional and global impacts of idealized alkalinity addition to surface oceans [10,[14][15][16][17][18][19], there are at present no estimates of the CDR potential and environmental impacts of mineral-based ocean alkalinity enhancement that are grounded in marine particle dynamics. Specifically, although previous work has sought to explore the impacts of imposed dissolved alkalinity fluxes to the surface ocean or has inverted prescribed atmospheric pCO 2 trajectories for required surface ocean alkalinity fluxes, no previous work has mechanistically modeled the dynamics of alkalinity release from potential mineral feedstocks as they interact with the marine particle factory-the complex set of physical and chemical processes that control the aggregation, disaggregation, and settling of marine particles in the ocean.…”

Electrolytic seawater mineralization and how it ensures (net) carbon dioxide removal

“…56 Using a model that considers ocean circulation, i.e., ECCO (Estimating the Circulation and Climate of the Ocean) LLC270 physical fields, 57 and constraints of ΔpH = 0.1 and ΔΩaragonite = 0.5, regions within 300 km of the coast can accommodate 100s of megatons of atmospheric CO2 removal. 58 This shows that simple near-coastal alkalinity discharge, such as that proposed herein, can scale to several Gt per year if spread over available coastlines. 2.…”

Electrolytic Seawater Mineralization and the Mass Balances That Demonstrate Carbon Dioxide Removal

“…For context, the critical saturation ratio, Ω, for runaway aragonite precipitation is reported to be 5 (SI = 0.69) . Using a model that considers ocean circulation, i.e., ECCO (Estimating the Circulation and Climate of the Ocean) LLC270 physical fields, and constraints of ΔpH = 0.1 and ΔΩ aragonite = 0.5, regions within 300 km of the coast can accommodate 100s of megatons of atmospheric CO 2 removal . This shows that simple near-coastal alkalinity discharge, such as that proposed herein, can scale to several Gt per year of CDR if spread over available coastlines …”