Pump nutrients and salts along the US GEOTRACES East Pacific Zonal Transect from the R/V Thomas G. Thompson TN303 cruise in the tropical Pacific from Peru to Tahiti during 2013 (U.S. GEOTRACES EPZT project) (NCEI Accession 0278316)

Acquisition Description:
Methodology (extracted from the TN303 cruise report):
Salinity and nutrient samples were taken from the surface fish and from Niskins attached to the wire at each Deep and Mid McLane pump cast.

Nutrient analyses (phosphate, silicate, nitrate+nitrite, nitrite) were performed on a Seal Analytical continuous-flow AutoAnalyzer 3 (AA3). The analytical methods used are described by Gordon et al., Hager et al., and Atlas et al.

Silicate: Silicate was analyzed using the technique of Kirkwood et al. An acidic solution of ammonium molybdate was added to a seawater sample to produce silicomolybdic acid, which was then reduced to a silicomolybdenum blue complex following the addition of ascorbic acid. Oxalic acid was also added to impede PO4 color development. The sample was passed through a flowcell and the absorbance measured at 660nm.

Nitrate + Nitrite: A modification of Armstrong et al. procedure was used for the analysis of nitrate and nitrite. For the nitrate analysis, the seawater sample was passed through a cadmium reduction column where nitrate was quantitatively reduced to nitrite. Sulfanilamide was introduced to the sample stream followed by N-(1-naphthyl)ethylenediamine dihydrochloride which coupled to form a red azo dye. The stream was then passed through a flowcell and the absorbance measured at 520nm. The same technique was employed for nitrite analysis, except the cadmium column was not present.

Phosphate: Phosphate was analyzed using the Murphy and Riley technique. An acidic solution of ammonium molybdate and antimony potassium tartrate was added to the sample to produce phosphomolybdic acid, then reduced to phosphomolybdous acid (a blue compound) following the addition of ascorbic acid. The reaction product was heated to ~37 degrees C to enhance color development, then passed through a flowcell and the absorbance measured at 880nm.

Salinity: A single Guildline Autosal 8400B salinometer (S/N 69-180) located in the hydro lab was used for all salinity measurements. Samples were analyzed after they had equilibrated to laboratory temperature, usually within 12-29 hours after collection. The salinometer was standardized for each group of analyses, 36 to 72 samples, using two fresh vials of standard seawater per group. Normal standardization procedures included flushing the cell at least 2 times with a fresh vial of IAPSO Standard Seawater (SSW, Batch P-155), setting the flow rate as low as possible during the last fill, and monitoring the STD dial setting. Samples were run using 2 flushes before the final fill. The computer determined the stability of a measurement and prompted for additional readings if there appeared to be drift. Salinity samples were drawn into 200 ml Kimax high-alumina borosilicate bottles, which were rinsed three times with the sample prior to filling. The bottles were sealed with custom-made plastic insert thimbles and kept closed with Nalgene screw caps. PSS-78 salinity [UNES81] was calculated for each sample from the measured conductivity ratios. The difference between the initial vial of standard water and the next one run as an unknown was applied as a linear function of elapsed run time to the measured ratios. The corrected salinity data were then incorporated into the cruise database. Data processing included double checking that the station, sample and box number had been correctly assigned, and reviewing the data and log files for operator comments. The salinity data were compared to CTD salinities and were used for shipboard sensor calibration.