Sea surface carbonate mapping data of 10 cruises in the Yellow Sea during 2011 to 2018

Depth profiles of temperature (after the International Temperature Scale of 1990) and salinity (after the practical salinity scale of 1978) were collected at 43–72 stations, with calibrated conductivity-temperature-depth/pressure (CTD) recorders (SBE19+ in October 2017 and SBE911+ during other surveying periods, Sea-Bird Co., USA) aboard R/V Beidou (October 2017) and R/V Dongfanghong 2 (other surveying periods). Sea surface samples for dissolved oxygen (DO), DIC, pH, and total alkalinity (TAlk) determinations were obtained at a water depth of 1–6 m (depending on field state of the sea) using rosette samplers fitted with Niskin bottles of 5–12 L (General Oceanics Inc., USA), which were mounted with the CTD units.

Water samples for DO analyses were collected, fixed, and titrated aboard following the classic Winkler titration procedure. The DO saturation (DO%) was calculated from the field-measured DO concentration and the DO concentration at equilibrium with the atmosphere, as per the Benson and Krause (1984) equation and the field-measured atmospheric pressure (recorded with an onboard barometer).

Following our earlier studies, water samples for DIC and TAlk were stored in 60 mL borosilicate glass bottles (for DIC) and 140 mL high-density polyethylene bottles (for TAlk), mixed with 50 μL saturated HgCl₂, sealed with screw caps and stored at room temperature. This procedure has been favorably evaluated earlier. There were no differences between the measurement results from our procedure and those from the widely used sampling procedure recommended by Dickson et al. (2007). The water samples were unfiltered but allowed to settle before measurement. DIC was determined by infrared detection following acid extraction of a 0.5–0.9 mL sample with a Kloehn^® digital syringe pump. TAlk was measured by Gran acidimetric titration on a 15–25 mL sample with another Kloehn^® digital syringe pump, using a precision pH meter and an Orion^® 8102BN Ross electrode for detection. Certificated Reference Materials from A. G. Dickson’s lab were used for quality assurance during analyses, achieving a precision level of ±2 μmol kg^–1.

Water samples for pH analyses were collected in 140 mL brown borosilicate glass bottles using a bubble-free procedure similar to that used for DO, except for being preserved with 50 μL HgCl₂ and measured within 6 h of sampling using a precision pH meter. The electrode was calibrated by three NIST (National Institute of Standards and Technology)-traceable buffers (pH = 4.01, 7.00, and 10.01 at 25.0 °C; Thermo Fisher Scientific Inc., USA). The overall uncertainty of our field-measured pH data was estimated to be 0.01 pH units.

Seawater fugacity of CO₂ (fCO₂) was mostly calculated from sea water temperature, salinity, DIC and TAlk, using the calculation program CO2SYS.xls (Pelletier et al., 2015), which is an updated version of the original CO2SYS.EXE (Lewis and Wallace, 1998). During our July 2016 and March–April 2018 cruises, however, some DIC data were unavailable due to sample storage failures. For those two cruises, fCO₂ and DIC values were calculated from field-measured pH and TAlk data, using the same calculation program. During calculation, we used the Millero et al. (2006) dissociation constants of carbonic acid and the Dickson (1990) dissociation constant of HSO₄^– ion. Phosphate and silicate concentrations required by the program were replaced by zero.

To assess the quality and internal consistency of this data set, the calculated fCO₂ from DIC and TAlk versus calculated fCO₂ from pH and TAlk, calculated fCO₂ (from DIC and TAlk) versus field-measured pCO₂, and calculated DIC from pH and TAlk versus field-measured DIC were compared. To examine the possible existences of organic alkalinity in those bloom-associated waters, we also calculated TAlk values from DIC and pH data. For fCO₂, the calculated fCO₂ values (from DIC and TAlk) were consistent with the measured pCO₂ at a deviation level of ±5%, which was reasonably higher than the overall uncertainty of our field-measured pCO₂ of 1%. Furthermore, the two sets of calculated values were mostly consistent with each other at a deviation level of ±10%. For DIC and TAlk, most measured data and calculated results were consistent with each other at deviation levels of ±15 μmol kg^–1, reasonably higher than the precision of DIC and TAlk determinations (±2 μmol kg^–1). These comparisons suggested that the measured data and calculated results of the carbonate system parameters were reliable.