The absorbance ratio of the indicator dye used was adjusted to R = 1.6 (pH ∼ 7.9) in the laboratory before the cruise. Small changes in sample pH (measurement perturbations; Clayton and Byrne, 1993) are created by the addition of titrant to samples. Dye perturbation was previously quantified using samples collected from profiles over a range of pH values. For each perturbation determination, ∆pH was defined as ∆pH = pHfinal – pHinitial, where pHinitial is the total scale pH taken after a single titrant addition, and pHfinal is the total scale pH after a second titrant addition.

An equation developed using this perturbation data was used to correct pH measurements: pH^0_T = -0.0042 · pH - 0.0326 (1.3). Where pH is the raw pHT measurement and pH0 is the perturbation-corrected pHT measurement.

All spectrophotometric pH measurements were tentatively flagged if the baseline shifted more than 0.002 absorbance units. A series of five spectra were averaged for each determination, and samples were rerun if the overall standard deviations were higher than 0.0004. This process was repeated until the standard deviation of multiple readings was within 0.0004. Absorbance values were saved so that the quality criteria could be evaluated in the future.

Perturbation-corrected pHT measurements were reported along with their associated quality-control flags. pHT was reported at the measurement temperature of approximately 25 ºC, with the exact measurement temperature specified as an average of temperature measurements at the beginning and end of each set of samples.

The spectrophotometric AT measurement titration procedure is outlined in Liu et al. (2015).

The sample bottle thermostatted to room temperature (∼24℃) was wiped clean and placed inside the bottle holder on the stir plate. The bottle cap was removed, and excess seawater that may have moved into the flanged bottleneck was rinsed into the sample bottle with a small amount (1–2 mL) of MilliQ water. A small stir bar was added to the bottle, and the acid tube from the Dosimat titrator was inserted just below the sample liquid’s surface to ensure that the HCl was well mixed with the seawater before reaching the light path at the lower part of the bottle. 

After a reference absorbance was recorded, 0.1 mL of 10 mM bromocresol purple BCP stock solution (R ∼ 0.3) was added and mixed. Acid was then delivered to the sample bottle, with the rate of acid addition being guided by continuous spectrophotometric monitoring of the seawater pH. The titrations were terminated at R = 0.070 (pHT ~ 4.2).  

A plastic syringe was used to remove ∼60 mL of the acidified sample so that the sample did not overflow and to decrease purging time. A fritted glass purging tube was placed into the bottle, positioning it above the light path, and the sample was purged of CO2 with N2 until R stabilized (i.e., ± 0.002 units in 1 min). The final solution temperature was measured and used with the absorbance ratio to calculate the final pHT of the sample.

Spectrophotometric measurements of AT were calculated using the following equation: A_T = {[HCl]_AV · V_A - [H+]_ASW · M_ASW}/M_SW (2.1). Where [HCl]AV is the HCl concentration of the acid titrant in volumetric units, and VA is the volume of the added acid titrant (mL). 

MSW is the sample mass obtained from volumetric measurements and solution densities (SOP 12, Dickson et al., 2007). Sample bottles were calibrated (as a function of temperature) in the laboratory prior to the cruise following methods outlined in Section 3.2 in Liu et al. (2015). On board the ship, sample fill temperature was used to calculate the mass of the collected seawater (SOP 12, Dickson et al., 2007): M_SW(t_s) = V(t_s) · ᵨ(sw,t_s) (2.2)

where seawater density (ρ(sw, ts)) as a function of salinity and temperature was calculated according to Millero and Poisson (1981), and V(ts) is the volume of the sample at fill temperature (SOP 12, Dickson et al., 2007, Liu et al., 2015, Eqs. 7 – 11).

The gravimetric MA term (MASW = MSW + MA) was calculated by measuring the added volume and density (σA) of the acid (MA = VA σA). 

The acid concentration ([HCl]AV) was determined by titrating the acid with a seawater Certified Reference Material (CRM) with known AT and solved for [HCl]AV in Eq. (2.1). CRM Batch #197 was obtained from Dr. Andrew Dickson of the Scripps Institution of Oceanography and used throughout this cruise (Salinity = 33.529; AT = 2256.77 ± 0.90 μmol kg-1). 

The titrant acid (0.2 N) was prepared from concentrated HCl and adjusted to an ionic strength of 0.7 M using NaCl. [HCl]AV was calculated to be 0.1934 N at 23°C (N = 7).

The [H+]ASW term in Eq. (2.1) provides proton concentrations directly in terms of total moles per kg of seawater (pHT = −log[H+]ASW). Solution pHT was measured spectrophotometrically from absorbance ratio measurements (R) at λ2A and λ1A (Liu et al., 2015; Hudson-Heck et al., 2021): pH_T = -log(K^T_2 · e_2) + log((R - e_1)/(1-Re_4)) (2.3).

Where is the indicator’s equilibrium constant, and the symbols e1, e2, and e4 refer to indicator molar absorbance ratios at wavelengths λ1 and λ2. Sample pH was calculated using the indicator bromocresol purple (BCP). Absorbances were measured using at λ1 = 432 nm and λ2 =589 nm, and an additional wavelength of 700 nm (λ3) to correct for baseline changes ().  The remaining terms in Eq. (2.3) can be calculated using equations from Hudson-Heck et al. (2021) over a range of salinity from 0 to 40 and temperatures from 278.15 to 308.15K. 

• e_1 = 0.00049 (2.4a).
• e_4 = -7.101 · 10^-3 + 7.674 · 10^-5·T + 1.361 · 10^-5·S (2.4b).
• -log(K^T_2·e_2) = 4.981 - 0.1729·S^0.5 + 0.09406·S + 0.373S^1.5 + 0.000907·S^2 + 310.1/T - 17.03S^1.5 /T - 0.058S^1.5·ln(T) - 0.0005658S^0.5T (2.4c)”

The sample pH was calculated using the recorded R value and the final measured temperature.

The acid concentration of the titrant was calibrated using a CRM (Batch 197, certified AT value: 2256.77 ± 0.90 μmol kg-1). At the start of the cruise and for each CTD cast, a CRM was titrated before or after analyzing collected samples to check acid concentration. A total of 7 CRMs were measured, with an average [HCl]AV of 0.1934 ± 0.0001 N and measured AT value of 2256.77 ± 1.03 μmol kg-1.  

For each CTD cast, duplicates were collected from two Niskin bottles. The average standard deviation of the duplicates was 0.53 μmol kg-1 (N=12). All sample data was saved so that the quality criteria could be evaluated in the future. AT measurements were reported along with their associated quality-control flags.

Omega_calcite was calculated from pH and Total Alkalinity using CO2SYS (MATLAB v3.1.1) (Lewis and Wallace, 1998), using the total pH scale and K1, K2 constants from Hansson (1972, 1973) and Mehrbach et al. (1973), refit by Dickson and Millero (1987).