Dissolved Oxygen:

Samples were collected and preserved for Winkler dissolved oxygen analysis following standard protocols (Langdon, 2010). Supplementary samples collected for this project were collected in duplicate  into volume-calibrated flasks and were titrated onboard the ship within 24-48 hours of collection using a custom-built Winkler titrator with automated potentiometric end point detection (control software available: Nicholson et al., 2023).

Precision of the sample collection and analysis procedure is determined by agreement between replicate measurements from the same Niskin bottle. Median agreement of replicates across both cruises was 0.12% (0.4 μmol/kg).

Accuracy of sample measurements depends on standardization of the sodium thiosulfate titrant based on a reference standard. The sodium thiosulfate titrant used on each cruise was determined by standardization with a 0.01N potassium iodate reference solution from Fujifilm WAKO chemicals. Lab-prepared potassium iodate standards, measured routinely throughout both cruises to verify titration accuracy and stability, were verified and adjusted by measurements against the WAKO standard.

Dissolved oxygen discrete measurements collected and analyzed by the OOI program as part of their routine operations are labeled as Oxygen1. The supplementary dissolved oxygen discrete measurements collected for this project are labeled as Oxygen2 and Oxygen3. Intercomparison across samples collected from the same Niskin bottles by the two groups yielded an offset of 0.5±1.5 μmol/kg for AR30-03 (June 2018) and 0.4±0.6 μmol/kg for AR35-05 (August 2019).

Dissolved Inorganic Carbon and Total Alkalinity:

Samples were collected for dissolved inorganic carbon (DIC) and total alkalinity (TA) analysis following standard protocols (Dickson et al., 2007). Samples were collected into either 250 mL or 500 mL borosilicate glass bottles and preserved with saturated mercuric chloride (100 μL in 250 mL bottles, 200 μL in 500 mL bottles) for later analysis.

Samples were analyzed at the Boston College Marine Biogeochemistry Laboratory. DIC was analyzed using an Apollo SciTech AS-C6L DIC Analyzer and TA was analyzed using an Apollo SciTech AS-ALK2 TA Analyzer. Both DIC and TA were measured from each sample bottle. All DIC measurements were made on the day the bottle was opened for analysis, and TA measurements were made within the same week. DIC and TA instruments were calibrated daily and monitored throughout each analysis session by measuring Certified Reference Materials (Andrew Dickson, UCSD).

Analytical replicates were measured for all samples such that after analytical outliers (>SD) were removed, all samples for both DIC and TA retained at least two replicate measurements (median number of replicates for DIC = 3; median number of replicates for TA = 5). Analytical precision was determined for each sample as the standard deviation of analytical replicates. Mean analytical precision for all DIC samples in this dataset is 0.8 μmol/kg. Mean analytical precision for all TA samples in this dataset is 2.9 μmol/kg. Individual samples are flagged as questionable (QC flag = 3) if the analytical precision is >8 umol/kg. Individual samples for DIC are also flagged as questionable (QC flag = 3) if CRMs run prior and subsequent to the sample in question differ by >8 μmol/kg.