Dataset: CO2 system data
Deployment: PE18-09

Discrete seawater samples and depth, temperature and salinity data were acquired using a SeaBird Electronics rosette, equipped with a CTD (Sea-Bird Electronics SBE 911 plus). Sampling for carbonate system parameters (TA, DIC and pH) commenced immediately after the recovery of the rosette to ship’s deck and employed the standard procedures described by (Dickson et al., 2007), and was followed by sampling for nutrients (TON, SRP, Silica). Samples were kept at room temperature until their analysis. Nutrient samples were collected into 15 ml plastic vials, to which 100 μL of 0.1 M HCl were added.
Total alkalinity measurements were conducted at Wang’s laboratory at WHOI by a modified Gran titration procedure (Cai et al., 2010; Huang et al., 2012). The titrations were conducted potentiometrically with an automated titrator (AS-ALK2; Apollo SciTech) using an open-cell configuration and a ROSSTM combination electrode (Thermo Fisher Scientific) at a controlled temperature. The titrant (HCl, 0.07 M) concentration was determined by titration of Certified Reference Material (CRM) provided by Dr. A.G. Dickson at the Scripps Institution of Oceanography. The precision and accuracy of total alkalinity measurements is better than ± 2 μmol kg-1.
 The DIC concentrations were measured at Wang’s laboratory at WHOI, using a NDIR-based DIC autoanalyzer (AS-C3, Apollo SciTech), calibrated with CRM. The NDIR-based DIC analyzer (AS-C3, Apollo SciTech) uses N2 to purge CO2 from a known volume of acidified seawater sample. The CO2 in the resulting gas stream is dried using a Nafion membrane trap, which is constantly flushed with a counter-flow of dried room air. Then, the CO2 content of the gas stream is quantified by a NDIR CO2 analyzer (LI-7000, LICOR). This instrument has a precision and accuracy of better than ±2.0 μmol kg−1.
Discrete pH samples were measured in Wang’s lab at WHOI using 10 cm path-length optical cells and analyzed at 25°C within a few weeks after sampling. Discrete pH measurements were based on the conventional spectrophotometric procedure using purified m-cresol purple as an indicator and 10 cm path-length optical cells (Dickson et al., 2007; Liu et al., 2011) on a HP 8453 spectrophotometer, with a precision of ±0.0004 and an accuracy of 0.001-0.002 pH units. 
Nutrient (Nitrate, soluble reactive phosphate and silicic acid) measurements were conducted at Lazar’s laboratory at the Inter-University Institute for oceanography in Eilat, Israel. The analysis will be conducted colorimetrically, using a Flow Injection Analyzer (FIA, LACHAT Instruments Quik-Chem 8500) (Grasshoff et al., 2009). Accuracy was obtained by calibration against a commercial, high-concentration standard (Merck).  

Instruments:
Discrete seawater samples and depth, temperature and salinity data were acquired using a SeaBird Electronics rosette, equipped with a CTD (Sea-Bird Electronics SBE 911 plus).