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, which may trigger mineral precipitation, consuming the added alkalinity and thus 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 mineralogy were determined throughout the experiments. Mineral precipitation took place in all experiments over a timescale of hours to days. The dominant precipitate phase is aragonite with trace amounts of calcite and magnesium hydroxide (MgOH2, i.e., 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 OAE-related calcium carbonate precipitation studies correlate with the aragonite saturation state (ΩA), and the resulting fit of log10(r) = n × log10 (ΩA−1) + log10 (k) yields a reaction order n=2.15 ± 0.50 and a rate constant k=0.20 ± 0.10 µmol h−1. The reaction order is comparable to that derived from previous studies, but the rate constant is 1 order of magnitude lower, which we attribute to the fact that our experiments are unseeded compared with previous studies that used aragonite seeds which act as nuclei for precipitation. Observable precipitation was delayed by an induction period, the length of which is inversely correlated with the initial Ω. Mineral precipitation occurred in a runaway manner, decreasing TA to values below those 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 lower 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 sequestering CO2 but could also render this approach counterproductive. As such, mineral precipitation should be avoided by keeping Ω below the threshold of precipitation and quantifying its consequences for 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.