The carbonate chemistry of the Western Mediterranean during the OCEAN CERTAIN 2015 cruise.

In Summer 2015 (4-31 August), CNR-ISMAR carried out an oceanographic field-study in the Western Mediterranean Sea, on board of R/V MINERVA UNO. Sampling stations consisted in 7 transects, that spanned from Sicily Channel to Ligurian sea, Catalan sea, and Balearic Basin, dividing the area in sub-basins. 92 stations were visited totally. The dataset includes 550 discrete data of carbonate chemistry (pH-total scale and Total Alkalinity), concentrations of dissolved oxygen, and basic hydrological data (temperature, salinity and density). Methods: At each station, pressure (dbar), temperature (°C), and conductivity(mS/cm) were measured with a CTD SBE 911 plus General Oceanics Rosette System, equipped with 24 12-litres Niskin Bottles. Salinity (S, psu) and depth (m) were calculated by Sea-Bird Scientific routines. Seawater samples (n = 550) for the determination of biogeochemical parameters were collected from the Niskin bottles. Samples for dissolved oxygen (DO) were drawn in 60-mL BOD bottles and treated with Winkler reagents immediately after collection. For the determination of pH on the total hydrogen ion scale at 25 °C (pHT25), the samples were drawn after DO samples into 10 cm long cylindrical glass cells and analyzed spectrophotometrically. For the determination of total alkalinity (TA; ?mol kg-1), the samples were collected in 300 ml borosilicate bottles, poisoned with mercuric chloride, tightly closed and stored in the dark at a temperature similar to the in situ one (4-25 °C). DO samples were analyzed by the Winkler method (Grasshoff et al., 1999) using an automated Metrohm 798 MPT Titrino potentiometric titration system (CV = 0.17 % at 210 µmol L-1). pHT25 was measured on board, within 24 h after the sampling, using the spectrophotometric method with m-cresol purple as indicator (Clayton and Byrne, 1993). The precision was ±0.002 units (n = 3), accuracy and stability of the method were checked daily with reference seawater certified for TA and TCO2 (n = 34, CRM batch 146 provided by Prof A. G. Dickson, Scripps, 210 California). TA was determined by potentiometric titration in an open cell with a difference derivative readout (Hernandez-Ayon et al., 1999). The average precision was ±2.0 ?mol kg-1 (n = 86 duplicate samples) and the accuracy was checked daily by the titration of certified reference seawater (n = 59, CRM batch 146)