Dataset: GP17-ANT carbonate system data

All samples were collected and analyzed following best practices guidelines (Dickeson et al. 2007). Dissolved inorganic carbon (DIC) was 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. The total alkalinity (TA) and pH_T were 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 the 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 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 TA and pHt, the rubber stopper was inserted and an aluminum seal was crimped on top. Once sealed, all bottles were gently inverted several times in order to mix the mercuric chloride. The entire collection process was completed as quickly as possible, but with care, to minimize any gas exchange. After collection, DIC/TA bottles were placed in plastic crates with protective foam and stored in a refrigerated shipping van until arrival in port. The samples were shipped back to the laboratory without temperature control and stored in the laboratory at room temperature until analysis. The pH_T samples were immediately placed in a 20-degree Celsius (°C) water bath to equilibrate the temperature before analysis.

The pH_T at 20°C was analyzed within 6 hours of collection. The samples were allowed a minimum of 2 hours for temperature equilibration before analysis was started. The pH_T 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-centimeter (cm) quartz micro-volume 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 pH_T, 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 -0.0649 and 0.0781, 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 0.0013 ± 0.0008 (N=27). The mean and standard deviation of CRM (Batch 199) were 7.9115 ± 0.0013 (N =15), and for TRIS (Woosley Lab Batch 5) 8.21514 ± 0.0016 (N = 17).

The DIC/TA samples were stored in a refrigerated van on deck after collection, and shipped (without temperature control) to the land-based laboratory at MIT (via USAP in Port Hueneme, California), and then stored at room temperature in a closet until analysis. Analyses were performed between 8 and 20 months after the end of the cruise. All TA samples were analyzed first. Then DIC was analyzed with several analyses of TA being performed immediately afterwards from the DIC bottle as a check on the sample collected for TA.

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 (Orbo Tube, Millipore-Sigma, Inc.) 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 station were analyzed to assess precision. The mean and standard deviation of the absolute difference between duplicates was 1.6 ± 1.1 (N = 29). CRM Batch 216 was used, the overall mean difference from the certified (measured - certified) value was -2.55 ± 1.47 (N = 51).

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 was performed at 20°C. A sample of approximately 100 to 130 grams (g) is weighed and added to a clean, dry water-jacketed beaker with a stir bar, and a cap with an Ecotrode electrode (Metrohm, AG), 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 (450 rpm) 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 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 station were analyzed to assess precision. In addition, several bottles collected for DIC were also analyzed for TA as a check and to provide additional duplicate measurements. These samples were analyzed immediately after analyzing DIC (generally within 15-20 minutes). 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 2.43 ± 1.65 (N = 153). The mean and standard deviation of the CRM Batch 216 difference from the certified value (measured - certified) was -0.10 ± 2.09 (N = 116).