Dataset: Thermogravimetric analysis (TGA) for precipitate from shipboard ocean alkalinity enhancement (OAE) experiments in the North Atlantic on R/V Atlantic Explorer cruise AE2320 in September 2023

Experiments involved the addition of NaOH solution prepared by weighing ACS grade NaOH in the lab prior to the cruise (R/V Atlantic Explorer AE2320) 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.

A TA Instruments Q600 simultaneous thermal analyzer was used for thermogravimetric analysis and differential scanning calorimetry of mineral precipitates from Experiment D. About 35 mg was heated from room temperature to 1100°C with a heating rate of 10°C per minute in a N2 atmosphere. The N2 flow rate was 50 mL.min-1. Routine measurements of standard materials supplied by the manufacturer indicate that the temperature is accurate to 1 degree Celsius (°C) and the weight change to 0.5 μg.