Dataset: Carbonate system data from the US GETORACES GP17-OCE cruise on R/V Roger Revelle (RR2214) across the South Pacific and Southern Oceans from December 2022 to January 2023

All samples were collected and analyzed following best practices guidelines (Dickson et al. 2007). The total alkalinity (TA) and dissolved inorganic carbon (DIC) were collected in 250 milliliter (mL) borosilicate glass reagent bottles and sealed using Apiezon L grease and a rubber band held in place with a plastic hose clamp. TA and DIC were analyzed from the same bottle. The pHt was collected in 150 mL borosilicate glass serum bottles and sealed with a butyl-rubber cap and aluminum seal. All bottles were filled in the same manner. A silicone tube was attached to the niskin valve. A small amount of water was added to the bottle and the bottle was swirled to cover all surfaces and dumped out. This rinse was repeated a total of three times. Then the tube was placed near the bottom of the bottle and filled. The tube was tapped against the bottom to dislodge any bubbles, and care was taken to overflow any bubbles. Once full, the water was allowed to overflow for at least half the volume of the bottle (as estimated by time to fill the bottle) and all visible bubbles were gone. Overflow water was used to rinse the caps. With the water still flowing, the tube was gently, but quickly, removed leaving the bottle full to the brim. A pipette was then used to remove a precise amount of water leaving ~1% headspace once bottle was capped. After removing the water for headspace, a saturated mercuric chloride solution was added to a total volume of 0.04% of the sample. The DIC/TA was then capped with a glass stopper with apiezon L grease, twisting the cap to ensure even distribution of grease, creating an airtight seal. A rubber band and hose clamp were then placed on the cap to keep it in place. For pHt, the rubber stopper was inserted and an aluminum seal was crimped on top. Once sealed, bottles were gently inverted several times in order to mix the mercuric chloride. After collection, DIC/TA bottles were placed in plastic crates with protective foam and stored at room temperature for shipping back to the laboratory at the end of the cruise. The pHt samples were immediately placed in a 25 degree Celsius water bath to equilibrate the temperature before analysis.

The pHt at 25°C was analyzed within 6 hours of collection. The samples were allowed approximately 2 hours for temperature equilibration before analysis was started. The pHt was then measured spectrophotometrically with purified metacresol purple (mCP) dye (Woosley lab batch 4) obtained from the laboratory of Robert H. Byrne (University of South Florida), using a custom-designed automated system similar to that of Carter et al. (2013). The instrument uses a 10 mL Kloehn syringe pump to draw the sample from the bottle, rinse the flow-through 10 cm quartz spectrophotometric cell (Starna, Inc.), add and mix the mCP indicator, and finally rinse the cell after analysis. An Agilent 8454 UV/VIS spectrophotometer was used to take the blank and full spectra with mCP. The absorbances at 434, 578, 730, and 488 nanometers (nm) were used for calculations. The equations of Liu et al. (2011) were used to calculate pHt, and the isobestic absorbance at 488 nm was used to determine the indicator perturbation adjustment following the method described in Carter et al. (2013). The dye perturbation slope and intercept were -0.0732 and 0.071, respectively. Duplicate samples, Certified reference material provided by the laboratory of Andrew G. Dickson (University of California, San Diego), and TRIS buffers prepared according to Paulsen and Dickson (2020) were used to check precision and "accuracy". The mean absolute difference between duplicate samples was 0.0017 ± 0.0016 (N=50). The mean and standard deviation of CRM (Batch 179) was 8.0920 ± 0.0020 (N =41), and for TRIS (Woosley Lab Batch 5) 8.0949 ± 0.0022 (N = 58).

The DIC/TA samples were stored in the science hold at room temperature after collection, shipped to the land-based laboratory at MIT, and then stored at room temperature in a closet until analysis. Analyses were performed between 15 and 20 months after the end of the cruise. DIC was analyzed first then handed off to an analyst for TA. Both analyses were always performed on the same day and generally within 10-15 minutess of each other.

DIC was analyzed using a custom-designed DIC extractor (DICE) built by NOAA PMEL (Pacific Marine Environmental Laboratory). It is a modern version of the original SOMMA system (Johnson 1992) and follows the methods described in Dickson et al. (2007) in SOP 2. Analysis is performed at 20°C. The instrument uses a calibrated pipette to precisely dispense the volume of sample into a stripper chamber where 8.5% phosphoric acid had been added. The acid converted all the DIC to CO2 gas. A pure N2 carrier gas then carried the evolved CO2 through a condensor to remove water vapor followed by a silica gel to remove any organic acids and finally into the coulometer (UIC, Inc.) for detection. The instrument was calibrated at the start of each coulometer cell (1 per day) with a blank, 2 pure (99.999% CO2) gas loops, each run at least twice, and certified reference material (CRM) provided by the laboratory of Andrew G. Dickson (University of California, San Diego). Sample values were adjusted to the CRM value for that day using a constant offset from the certified value. Two duplicates per cast were analyzed to assess precision. The mean and standard deviation of the absolute difference between duplicates was 2.5 ± 2.3 (N = 44). Two batches of CRM were used, the overall mean difference from the certified value was -2.5 ± 4.1 (N = 118), the Batch 199 values were -2.6 ± 4.4 (N = 102) and Batch 216 were -1.8 ± 0.8 (N = 16).

TA was analyzed using an instrument custom-designed and built by the laboratory of Andrew G. Dickson (UCSD) and described in Dickson et al (2003, 2007). Analysis is performed at 20°C. A sample of approximately 100 - 130 grams (g) is weighed and added to a clean, dry water jacketed beaker with a stir bar, and a cap with a Metrohm ECotrode electrode, thermometer, acid line, and air line is placed on top. A computer-controlled dosimat then adds enough acid to reach a pH of ~3.5. The sample is then stirred and bubbled with lab air for 300 seconds to drive off evolved CO2. Then, 15-20 fine additions of 50 microliters (µL) of acid are added, and voltages and temperatures are recorded. Once complete, a non-linear least squares fitting method is used to calculate the TA as well as calibrate the E0 of the electrode. The fitting method is described in Dickson et al. (2003). A CRM was analyzed at the beginning and end of each day. Sample values were NOT adjusted to the certified value. Two duplicates per cast were analyzed to assess precision. The acid was ~0.1 N HCl prepared in ~0.6 M NaCl to match the approximate ionic strength of seawater. The exact concentration was calibrated by borax titration following the method of Kolthoff (1926). The mean and standard deviation of the absolute difference between duplicates was 2.3 ± 2.2 (N = 43). The mean and standard deviation of the CRM difference from certified value was 2.25 ± 2.22 (N = 118). CRM batches 189, 199, and 216 were used.