Dataset: DIC, TA, pH, DO from nGOM cruises conducted aboard the R/V Pelican throughout 2017, 2018 and July 2019.

DIC and TA

Measure of DIC using NDIR method and of TA using Gran titration. DIC and TA were measured using the instruments from Apollo Scitech Inc. Briefly, for DIC analysis, samples were analyzed at room temperatures. Each seawater sample (0.75 mL) was acidified using phosphoric acid and the evolved CO2 gas was extracted and carried by pure N2 gas to an infrared CO2 detector (Li-Cor 6262) for quantification. TA was determined on 25 mL seawater sample by potentiometric titration, using 0.1 M hydrochloric acid and an open-cell titration system. All TA samples were analyzed in pre-thermostated (25 °C) glass cells. For each DIC or TA sample, sub-samples were sequentially analyzed 2 or 3 times until we obtained two replicates with a precision within 0.1%. The average of the two values is reported. The precision of both the TA and DIC measurements was +/- 2 umol/kg. The accuracies of the TA and DIC measurements were determined by routine analysis of certified reference material (CRM) provided by A. G. Dickson, Scripps Institution of Oceanography.

DO_spec

Winkler titration was used for DO analysis. Samples were drawn from Niskin bottles directly into 60 ml BOD bottles and pickled using manganese chloride and sodium iodide/sodium hydroxide. Iodine liberated by acidifying pickled sample was then measured spectrophotometrically using Genesis 30 (Thomas Scientific) spectrophotometer at 466 nm. Blank absorbance from sample turbidity was obtained by adding a few drops of sodium thiosulfate to the sample solution and subtracted from sample absorbance. Calibration was performed by spiking known amounts of potassium iodate.

Error on DO was from the uncertainty of measuring absorbance (0.001), which is equivalent of 0.7 uM. Samples which had blank absorbance exceeding 5% of sample absorbance were flagged.

pH_elec

pH samples were drawn in 60 ml glass bottles and temperature equilibrated at 25 oC. An Orion Combination electrode connected to a pH meter (Orion Star A211) was used to measure the potential (EMF, mV) generated by the H+ ions. EMF was calibrated using three NBS buffer solutions at pH 4.01, 7.0, and 10.01 purchased from Fisher Scientific. Probe was kept immersed in the sample until the EMF stabilized. Two EMF readings at a difference of 1 minute were obtained for each sample and average value used with calibration to calculate the pH.

Precision on pH is estimated from the standard deviation of the mean of two EMF readings. Samples where such deviation exceeded 0.16% of the mean EMF are flagged.