Mineral Formation during Shipboard Ocean Alkalinity Enhancement Experiments in the North Atlantic

Abstract. Ocean alkalinity enhancement (OAE) is a carbon dioxide (CO₂) removal approach that involves the addition of alkaline substances to the marine environment to increase seawater buffering capacity and allow it to absorb more atmospheric CO₂. Increasing seawater alkalinity leads to an increase in the saturation state (Ω) with respect to several minerals. This may trigger mineral precipitation, consuming the added alkalinity and decreasing the overall efficiency of OAE. To explore mineral formation due to alkalinity addition, we present results from shipboard experiments in which an aqueous solution of NaOH was added to unfiltered seawater collected from the surface ocean in the Sargasso Sea. Alkalinity addition ranged from 500 to 2000 µmol.kg^-1 and the carbonate chemistry was monitored through time by measuring total alkalinity (TA) and dissolved inorganic carbon (DIC), which were used to calculate Ω. The amount of precipitate and its minerology were determined throughout the experiments. Mineral precipitation took place in all experiments over a timescale of hours to days. The dominant mineralogy of precipitate is aragonite with trace amounts of calcite and brucite. Aragonite crystallite size increases and its micro-strain decreases with time, consistent with Ostwald ripening. The precipitation rate (r) in our experiments and those of other calcium carbonate (CaCO₃) precipitation OAE studies correlates with aragonite saturation state (Ω_A), and the resulting fit of log₁₀(r) = n × log₁₀(Ω_A-1) + log (k) yields a reaction order n = 2.16 ± 0.5 and a rate constant k = 0.15 ± 0.09 µmol.hr^-1. The reaction order is comparable to that derived from previous studies, but the rate constant is an order of magnitude lower, which we attribute to the fact that our experiments are unseeded, and thus precipitation occurs (pseudo)homogenously whereas previous studies used aragonite seeds that act as nuclei for precipitation. Observable precipitation was delayed by an induction period, the length of which is inversely correlate with the initial Ω. Mineral precipitation occurred in a runaway manner, decreasing TA to values below that of seawater prior to alkalinity addition.

This study demonstrates that the highest risk of mineral precipitation is immediately following alkalinity addition and before dilution and CO₂ uptake by seawater, both of which lowers Ω. Aragonite precipitation will decrease OAE efficiency, because aragonite is typically supersaturated in surface ocean waters. Thus, once formed, aragonite essentially permanently removes the precipitated alkalinity from the CO₂ uptake process. Runaway mineral precipitation also means that mineral precipitation following OAE may not only decrease OAE efficiency at should be avoided by keeping Ω below the threshold of precipitation and quantifying its sequestering CO₂ but could render this approach counterproductive. As such, mineral precipitation consequences on OAE efficiency if it occurs. Lastly, in order to be able to quantitatively determine the impact of mineral precipitation during OAE, a mechanistic understanding of precipitation in the context of OAE must be developed.