Dataset: DIC, TA, calculated pH and carbonate saturation state in the summer bottom water in North Gulf of Mexico from 2006 to 2017

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 (0.5 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.

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.

pH_spec

Discrete sample pH_spec was measured spectrophotometrically at 25 degrees Celsius on the total pH scale using purified M-Cresol Purple purchased from R. Byrne at the University of South Florida [Clayton and Byrne, 1993; Liu et al., 2011]. The spectrophotometric pH (25 oC, total scale) was calculated from the ratio of the measured absorbances of mCP at wavelengths l2 = 578 nm and l1 = 434 nm following the dissociation constants proposed by Liu et al 2011].

pHcal[at25C] and Omegacal[at25C]

pHcal[at25C] and Omegacal[at25C] was calculated using the CO2SYS program and the measured DIC and TA as the input pair (van Heuven, 2011). Silicate and phosphate concentrations were assigned as 0 in CO2SYS. The coefficients for pH and Ω calculation were selected as: K1 and K2 value from Lueker et al. (2000); KHF- was from Dickson & Riley (1979); KHSO4 was from Dickson (1990) and BT (total boron) was from Uppström (1974).