Measurements of sample pH made using the mvMICA were made on the total scale (i.e., pHT = – log[H+]T). Spectrophotometric pH was measured using the indicator meta Cresol Purple (mCP).

At the beginning of each set of samples, surface seawater without indicator was injected into the submerged optical cell to obtain a reference spectrum. Due to volume limitations, fresh surface seawater collected from the ship’s underway system was used as a reference, as it had properties similar to porewater samples. After a reference spectrum was taken, a syringe with sample and dye was quickly mixed and injected into the pH cell through the Luer lock port of the pH channel. The input port was checked for bubbles before sample injection to prevent bubble entrapment. The exit port, a two-way Luer shutoff valve, was closed immediately after sample injection. After thermal equilibration of the sample was reached (usually 1 minute), the signal stabilized, and absorbance measurements were obtained at 434, 578, and 730 nm. The pHT was calculated according to Müller and Rehder (2018).

Measurements made using the mvMICA were conducted at 20℃. A Fisher Scientific Traceable Digital Thermometer (±0.05℃) monitored the water bath temperature. The bottom water salinity was measured by the CTD for each station and used in calculations as sample salinity.

Duplicate pH samples were collected at each station. Replicate pH measurements of fill water had an average standard deviation of 0.0023 (N = 8). The measurement of pH is calibration-free. With the small sample volume measurements, dye perturbation was not considered. All absorbance measurements were tentatively flagged if the baseline shifted more than 0.005 absorbance units or if there were visible signal issues. Absorbance values were saved so that the quality criteria could be evaluated in the future. In the data set, pHT measurements were reported along with their associated quality-control flags. The pHT was reported at the measurement temperature of 20ºC.

Sample AT was measured spectrophotometrically after equilibrating with CO2 gas using bromocresol purple (BCP) as indicator dye. Samples were inserted into the 1-cm cuvette holder partially submerged inside the mvMICA. After the lid was closed, a reference (baseline) spectrum was obtained. Absorbances were measured using at λ1 = 432 nm and λ2 =589 nm, and an additional wavelength of λ3 = 700 nm to correct for baseline changes. 10 μL of 2 mM BCP stock solution (R ≃ 1) was added to the cuvette with an Eppendorf pipette.

The cuvette cover with its attached CO2 equilibration tube and vent port cutouts was placed on the cuvette, and CO2 bubbling was initiated. The CO2 concentration used in AT measurements for this cruise was measured to be 20% using an onboard Picarro Cavity Ring-Down Spectrometer (G2131-i) with a Liaison autosampler. CO2 was bubbled through the upper portion of the spectrophotometric cell while absorbance measurements were monitored. Once stabilized (∼ 4 minutes), R was recorded along with temperature and sample information. BCP absorbance ratios are then used to calculate AT (μmol kg-1) using the equation:

log(A_T + [H+]_T) = log(K_0 · K’_1) + E + pH_T (4.1)

where pHT = –log[H+]T on the total pH scale and is measured spectrophotometrically using Eqs. (2.3) and (2.4) (Clayton and Byrne, 1993; Liu et al., 2011; Hudson-Heck et al., 2021).

The K0 term in Eq. 4.1 is the Henry’s law constant (K0), which is calculated as a function of temperature and salinity as 273.15 ≤ T ≤ 313.15 K and 0 ≤ S ≤ 40 (Weiss, 1974, Millero, 1995)

ln(K_0) = -60.2409 + 93.4517(100/T) + 23.3585·ln(T/100) + S[0.023517 – 0.023656(T/100) + 0.0047036(T/100)^2] (4.4)

The dissociation constant of carbonic acid, , is calculated using the characterization of Roy et al. (1993) (5 ≤ S ≤ 45 and 273.15 ≤ T ≤ 318.15 K).

ln(K’_1) = 2.83655 – 2307.1266/T – 1.5529423·ln(T) + (-0.20760841 – 4.0484/T) · S^0.5 + 0.08468345S – 0.00654208S^1.5 + ln(1-0.001005S) (4.5)

E is a temperature-dependent empirical constant that includes known constants and the fugacity of CO2 (fCO2). The constant is determined by measuring the absorbances of a certified reference material (CRM) after equilibration with CO2. The procedure for determining E is outlined in section 4.4.

The calibration constant, E, in Eq. (4.1) was determined using a standard CRM with a well-characterized AT (Batch # 197). 1 mL of CRM was pipetted into a 1-cm pathlength cuvette. The same measurement procedure outlined in section 4.3 was followed. After equilibration, absorbance ratios were used with the known AT value of the CRM to calculate E using the equation:

E = log(A_T + [H+]_T) – log(K’_1·K_0) – pH_T (4.6)

The calibration constant, E, was determined to be -1.180 with a standard deviation of 0.003 (N = 54).

The AT of CRMs was measured using the mvMICA at the beginning and end of each set and after every 10 – 15 samples to monitor instrument performance and check the calibration constant. The calibration constant, E, was determined at the start of each sample set and used in Eq (4.1) to calculate sample AT. The average AT of CRM batch #197 (Salinity = 33.529; certified AT value: 2256.77 ± 0.90 μmol kg-1) using the average calibration constant (-1.180 ±0.003) was measured to be 2256.81 ± 13.87 μmol kg-1 (N = 54).

Duplicate samples were collected at each station. All absorbance measurements were tentatively flagged if the baseline shifted more than 0.005 absorbance units or if there were visible signal issues. Absorbance values were saved so that the quality criteria could be evaluated in the future. AT measurements and associated quality-control flags were reported in the data set. The AT was reported at the measurement temperature of 20ºC.

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).