Dataset: X-ray diffraction (XRD) raw data and Rietveld refinement results for precipitates from shipboard ocean alkalinity enhancement (OAE) experiments in the North Atlantic on R/V Atlantic Explorer cruise AE2320 in September 2023

Experiments were conducted during a research expedition aboard the R/V Atlantic Explorer in the Sargasso Sea (31º40'00''N, 64º10'00''W) at the Bermuda Atlantic Time Series site (BATS) from September 5 to 11, 2023. The experiments were performed in opaque 5L Cali-5-Bond(TM) multi-layer foil bags (Calibrated Instruments), placed in a flow-through incubator where surface seawater was continuously flowed to maintain a constant temperature of approximately 27 degrees Celsius (ºC). Each bag was filled with approximately 3 ± 0.02 liters (L) of unfiltered seawater using a rubber hose that was flushed with water to remove air bubbles, ensuring that no air entered the bags. Bags were rinsed 3 times with seawater before filling and sealing with the Luer-fitted stopcock. The mixed layer at the study site was approximately 40 meters (m) thick, and seawater for the experiment was collected from the upper 10 m. The bags were allowed to float freely in the incubator and they moved continuously due to the boat movement.

Experiments involved the addition of NaOH solution prepared by weighing ACS grade NaOH in the lab prior to the cruise in a plastic Falcon tube that was capped and sealed with parafilm tape. During the cruise, DI water was added to make up stock NaOH solutions with a final concentration of 1 M. The NaOH solution was pipetted into the seawater-filled bags through the Luer-fitted stopcock. Because NaOH contributes only alkalinity but not DIC, seawater in the experiments was out of equilibrium with the atmosphere, which was intended to simulate conditions immediately following alkalinity addition to seawater during OAE deployments.

In total, 5 experiments were conducted. The first experiment (experiment A) was a control with no alkalinity addition. In the second, third, and fourth experiments (B, C, and D), alkalinity was enhanced by 500, 1000, and 2000 micromoles per kilogram (µmol/kg) respectively. The fifth experiment (E) represents a set of "sacrificial" time series experiments whereby 9 bags were prepared similar to other experiments and alkalinity was enhanced by 1000 µmol/kg in each one of them, but each bag was sequentially opened and filtered in order to evaluate the precipitate mineralogy through time. In experiment E, water samples for TA and DIC measurements were taken only at the end of the experiment. The experiments were run for approximately 5 days.

Experiments were quenched by filtering all remaining seawater through 47 mm 0.8 µm polycarbonate filters using a peristaltic pump. Filters were rinsed with deionized and purified water (18.2 MΩ), dried at 55ºC, and stored cool and in the dark. The precipitates were then scraped off the filters and analyzed for mineralogy with X-ray diffraction.