Hydrographic Measurements

Samples for CFCs, helium isotopes (3He), oxygen (O2), hydrochlorofluorocarbon (HCFCs), TCO2, TALK, radiocarbon (Δ14C), tritium, DOC, chromophoric dissolved organic matter, TDN, salinity, and nutrients were drawn in this sequence from a conductivity, temperature, and depth (CTD) sampling package containing thirty-six 12-L Bullister bottles. A detailed description of methods for the CTD data, lowered acoustic Doppler current profile data, and bio-optical data are given in the cruise reports at for the sections A20_2003 and A22_2003.

In all, 2,530 bottle salinity samples were taken during the cruise along A20 section and 2,493 samples measured along section A22. Samples were drawn from the 12-L Bullister bottles into 250-mL Kimax borosilicate bottles. The bottles were rinsed at least three times before being filled to approximately 220 mL. A plastic insert and Nalgene cap were used to seal the sample in the bottle. At the conclusion of sampling, the time was noted and samples were placed into the salinometer lab so they could equilibrate to room temperature. Samples were analyzed after a period of at least 10 h and typically not more than 24 h from the time of sampling. Two Guildline Autosal Model 8400A salinometers (S/N 57-263 and 57-266) were used on section A20, and a single Guildline Autosal Model 8400A salinometer (S/N 48-266) was used on section A22. They were located in the forward analytical lab and were used for measuring salinity on all stations. The salinometers were modified by SIO Oceanographic Data Facility (ODF) to contain an interface for computer-aided measurement. The water bath temperatures were set and maintained at a value near the laboratory air temperature 24°C for the entire leg. For more information on salinity measurements, see the A20_2003 and A22_2003 cruise reports.

Dissolved oxygen analyses were performed with an ODF-designed automated oxygen titrator using photometric end-point detection based on the absorption of 365-nm wavelength ultra-violet light. The titration of the samples and the data logging were controlled by PC software. Thiosulfate was dispensed by a Dosimat 665 buret driver fitted with a 1.0-mL buret. ODF used a whole-bottle modified-Winkler titration following the technique of Carpenter (1965) with modifications by Culberson et al. (1991), but with higher concentrations of potassium iodate standard (~0.012N) and thiosulfate solution (~65 gm/l). Samples were collected soon after the rosette was brought on board using a Tygon and silicone drawing tube. Nominal 125-mL volume-calibrated iodine flasks were rinsed three times with minimal agitation, then filled and allowed to overflow for at least three flask volumes. The sample draw temperature was measured with a small platinum resistance thermometer embedded in the drawing tube. Reagents were added to fix the oxygen before stoppering. The flasks were shaken twice (10-12 inversions) to ensure thorough dispersion of the precipitate, once immediately after drawing, and then again after about 20 min. The samples were analyzed within 1-6 h of collection, then the data were incorporated into the cruise database. A total of 2,503 oxygen measurements were made during section A20 and 2487 oxygen measurements were made during section A22. For more information on salinity measurements, see the A20_2003 and A22_2003 cruise reports.

Nutrient analyses (phosphate, silicate, nitrate and nitrite) were performed on an ODF-modified 4-channel Technicon AutoAnalyzer II, generally within 1 h after sample collection. Occasionally, samples were refrigerated up to 4 h at ~4°C. All samples were brought to room temperature prior to analysis. The methods used are described by Gordon et al. (1992). The analog outputs from each of the four colorimeter channels were digitized and logged automatically by computer (PC) at 2-s intervals. Silicate was analyzed using the technique of Armstrong et al. (1967). An acidic solution of ammonium molybdate was added to a seawater sample to produce silicomolybdic acid, which was then reduced to silicomolybdous acid (a blue compound) following the addition of stannous chloride. Tartaric acid was also added to impede phosphate (PO4) color development. The sample was passed through a 15-mm flowcell and the absorbance measured at 660 nm. A modification of the Armstrong et al. (1967) 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 15-mm flowcell and the absorbance measured at 540 nm. The same technique was employed for nitrite analysis, except the cadmium column was bypassed, and a 50-mm flowcell was used for measurement. Phosphate was analyzed using a modification of the Bernhardt and Wilhelms (1967) technique. An acidic solution of ammonium molybdate was added to the sample to produce phosphomolybdic acid and was then reduced to phosphomolybdous acid (a blue compound) following the addition of dihydrazine sulfate. The reaction product was heated to ~55°C to enhance color development, then passed through a 50-mm flowcell and the absorbance measured at 820 nm. In all, 2,540 nutrient samples were analyzed along section A20 and 2,497 nutrient samples were analyzed during section A22. For more information on salinity measurements, see the A20_2003 and A22_2003 cruise reports.

Last Modified: 2017-08-18 13:09 UTC