Dataset: Carbonate chemistry

Conductivity-Temperature-Depth sensor (CTD, Seabird SBE 911plus) and a SBE43 dissolved oxygen (DO) sensor was used to collect discrete seawater samples for analysis of total dissolved inorganic carbon (DIC), total alkalinity (TA), pH, DO, nutrients (nitrate, phosphate, and silicate), and salinity. Seawater samples were collected for pH, DIC, TA, and nutrients at all 34 stations for all sampling depths. Salinity and DO samples were collected at all stations at every third depth for calibrations of the conductivity and DO sensors on the CTD Rosette. Samples for DO were collected first from the Niskin bottles, followed by pH, DIC/TA (in the same bottle), nutrients, and salinity. Standard protocols were followed for sampling procedures [Dickson et al., 2007]. DIC and TA samples were collected into 250 ml Pyrex borosilicate bottles after being filtered with a 0.45 µm in-line capsule filter (Farrwest Environmental Supply, Texas). Each sample was poisoned with 100 µL of a saturated mercuric chloride solution for preservation [Dickson et al., 2007] and sealed with a ground-glass stopper coated with APIEZON®-L grease and was secured with a rubber band applied to the bottle top.

Dissolved oxygen (DO) samples were collected from CTD Niskin bottles into 150 mL brown glass tincture bottles. Sodium iodide-sodium hydroxide mixture and manganese chloride were added sequentially to the sample immediately after collection and mixed thoroughly. One mL of sulfuric acid was added and mixed into the sample prior to measurement. DO samples were measured after temperature equilibration, within 3 days, on an automated dissolved oxygen titrator developed at Woods Hole Oceanographic Institution [Knapp et al. 1990]. The system is based on the Winkler technique. This method has an accuracy of 1.0 µmol kg-1 and precision of 0.2 µmol kg-1. The titrator was standardized with sodium thiosulfate for each group of analyses, usually about 30 samples. A custom procedure was developed and implemented to detect concentrations of dissolved oxygen < 90 µmol kg-1. In situ temperature was used to calculate potential density in order to convert from mL L-1 to µmol kg-1 for bottle samples. Ideally, the conversion temperature should be the temperature measured at the time of sample drawing, but those were not measured. Bottle values were used to calibrate CTD values.

Salinity samples were collected into 250 mL square cross-sectioned, borosilicate glass bottles and measured on an Autosal Salinometer (Guildline 8400B). PSS-78 salinity was calculated from measured conductivity ratios. Measurements were performed after samples had equilibrated to laboratory temperature, within 3 days after collection. The machine was standardized for each group of analyses, approximately 50 samples. International Association for the Physical Sciences of the Oceans (IAPSO) Standard Seawater Batch P-152 was used for standardization. This method has an accuracy of ±0.003 and precision of ± 0.0002. Bottle analyses were used to calibrate CTD salinity values.

DIC was analyzed using an Apollo SciTech DIC auto-analyzer (Model AS-C3), which uses a nondispersive infrared (NDIR) method. The sample is acidified with a 10% phosphoric acid in 10% sodium chloride solution, and CO2 is purged with high purity nitrogen gas and measured by a LI-COR 7000 infrared analyzer. Certified Reference Material (CRM) from Dr. A. Dickson at Scripps Institution of Oceanography was used to calibrate the DIC auto-analyzer at least once daily. In addition, CRM was measured as a sample every few hours to gauge and correct any potential drift. The precision and accuracy of the instrument was ~ ±2.0 µmol kg-1.

TA was measured with an Apollo SciTech alkalinity auto-titrator (Model AS-ALK2), a Ross combination pH electrode and a pH meter (ORION 3 Star) to perform a modified Gran titration [Wang and Cai, 2004]. The electrode and concentration of hydrochloric acid was calibrated every day. The CRMs were also measured as samples every few hours to correct any potential small drift. The accuracy and precision of the instrument was about ± 2.0 µmol kg-1. 

Seawater pH samples were collected directly into 10 cm cylindrical optical cells via silicone tubing and thermostated to 25.0 ± 0.1°C for at least an hour before measurement. Samples were analyzed using an Agilent 8453 UV-VIS spectrophotometer and meta-cresol purple (m-CP) as the indicator [Dickson et al., 2007]. Sample pH was measured on the total scale. Measurements of pH were corrected for indicator perturbation and indicator impurity [Clayton and Byrne, 1993; Dickson et al., 2007; Liu et al., 2011; Yao et al., 2007]. The average of combined corrections was 0.0026 ± 0.0046. The instrument accuracy and precision was 0.002 and 0.001, respectively. 
The nutrient samples were filtered through 0.2 µm filters, collected into vials that had been cleaned with 10% hydrochloric acid and were immediately frozen after collection. Samples were analyzed at the University of California at Santa Barbara Marine Science Institute. Nutrient data from NH1208 were found to have relatively larger scatter than the previous P17N cruise data. This may be due to storage issues that involved freezing and possible melting of samples. 

PACIFICA recommended adjustments for CLIVAR P17N 2001 are shown below along with deep isopycnal crossover results for NH1208 2012 cruise adjusted to the CLIVAR 2001 PACIFICA-corrected data. These values are listed in the data file as ‘deepcorr’ values. Salinity, DIC, and TA factors are additive, while oxygen and silicate factors are multiplicative. Units for all parameters are in µmol kg-1 except for salinity, which is measured on the practical salinity scale.

Cited References:
Clayton, T. D., and R. H. Byrne (1993), Spectrophotometric seawater pH measurements: total hydrogen ion concentration scale calibration of m-cresol purple and at-sea results, Deep. Res., 40(10), 2115-2129, doi:10.1016/0967-0637(93)90048-8.
Dickson, A., C. Sabine, and J. Christian (2007), Guide to best practices for ocean CO2 measurement, PICES Spec. Publ.,191.
Knapp, George P., Marvel C. Stalcup, and Robert J. Stanley (1990), Automated oxygen titration and salinity determination. Woods Hole Oceanographic Institution.
Liu, X., M. C. Patsavas, and R. H. Byrne (2011), Purification and characterization of meta-cresol purple for spectrophotometric seawater ph measurements, Environ. Sci. Technol., 45(11), 4862-4868, doi:10.1021/es200665d.
Wang, Z. A., and W.-J. Cai (2004), Carbon dioxide degassing and inorganic carbon export from a marsh-dominated estuary (the Duplin River): A marsh CO2 pump, Limnol. Oceanogr., 49(2), 341-354, doi:10.4319/lo.2004.49.2.0341.
Yao, W., X. Liu, and R. H. Byrne (2007), Impurities in indicators used for spectrophotometric seawater pH measurements: Assessment and remedies, Mar. Chem., 107(2), 167-172, doi:10.1016/j.marchem.2007.06.012.