Hydrographic Measurements
Samples for CFCs, helium isotopes (3He, 4He), oxygen (O2), hydrochlorofluorocarbon (HCFCs), TCO2, TALK, radiocarbon (Δ14C) and δ13C, tritium, DOC, salinity, and nutrients were drawn in this sequence from a conductivity, temperature, and depth (CTD) sampling package containing thirty-six 12-L Niskin bottles. A detailed description of methods for the CTD data, LADCP data, and bio-optical data are given in the cruise reports at CCHDO Web site for Section P16S_2005 and for Section P16N_2006.
Section P16S_2005
Salinity
Two Guildline Autosal Model 8400A salinometers (S/N 57-396 & S/N 48-266/backup), located in the aft hydro lab, were used for allsalinity measurements. The salinometers were modified by the 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. They were set to 21°C for stations 1 - 18 and 25 - 34 analyses, then switched to 24°C for stations 19 - 24 and 35 - 11. The salinity analyses were performed after samples had equilibrated to laboratory temperature, usually within 8 - 26 h after collection. The salinometers were standardized for each group of analyses (usually 1 - 3 casts, up to ~84 samples) using at least two fresh vials of standard seawater per group. Salinometer measurements were made by computer, with the software prompting the analyst to change samples and flush.
A total of 3,699 salinity measurements were made and approximately 220 vials of standard sea water (SSW) were used during the cruise. An additional 547 samples were taken by the Trace Metals group and analyzed by Shipboard Technical Support (STS)/ODF. Salinity samples were drawn into 200-mL Kimax high-alumina borosilicate bottles, which were rinsed three times with sample prior to filling. The bottles were sealed with custom-made plastic insert thimbles and Nalgene screw caps. This assembly provides very low container dissolution and sample evaporation. Prior to sample collection, inserts were inspected for proper fit and loose inserts replaced to ensure an airtight seal. The draw time and equilibration time were logged for all casts. Laboratory temperatures were logged at the beginning and end of each run. Salinity was calculated for each sample from the measured conductivity ratios. The difference (if any) 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 data. The corrected salinity data were then incorporated into the cruise database. The estimated accuracy of bottle salinities run at sea is usually better than ±0.002 relative to the particular standard seawater batch used. The 95% confidence limit for residual differences between the bottle salinities and calibrated CTD salinity relative to SSW batch P-144 was ±0.0055 for all salinities, and ±0.0018 for salinities deeper than 1000 dB. Three adjustments other than bath temperature changes were made to the Autosal during the cruise. After station 20 salinity was run, it was discovered that the amplifier gain for proper balance between suppression ranges had not been adjusted. This was changed, and stations 1 - 20 salinities were recalculated. A minor adjustment was made to the Autosal before station 47, and maintenance was performed on the air pump before station 92 was run. The temperature in the salinometer laboratory varied from 17.8 to 24.0°C during the cruise. The air temperature change during 80 of the 110 sample runs was less than ±0.4°C, and 25 runs had a temperature difference of ±0.5°C to ±0.9°C. International Association for the Physical Sciences of the Ocean (IAPSO) standard seawater (SSW) Batch P-144 was used to standardize all salinity measurements.
Oxygen
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 computer. 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 (~0.012 mol/L) and thiosulfate solution (~55 gm/L). Pre-made liquid potassium iodate standards were run once a day approximately every 4 stations, unless changes were made to system or reagents. Reagent/distilled water blanks were determined every day or more often if a change in reagents required it to account for presence of oxidizing or reducing agents. The auto-titrator performed well.
A total of 3,892 oxygen measurements were made during this cruise. Samples were collected for dissolved oxygen analyses soon after the rosette was brought on board. Using a Tygon and silicone drawing tube, nominal 125 mL volume-calibrated iodine flasks were rinsed 3 times with minimal agitation, then filled and allowed to overflow for at least 3 flask volumes. The sample drawing temperatures were measured with a small platinum resistance thermometer embedded in the drawing tube. These temperatures were used to calculate µmol/kg concentrations and as a diagnostic check of bottle integrity. Reagents were added to fix the oxygen before the bottles were sealed. 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 - 2 h of collection, and the data were incorporated into the cruise database.
Thiosulfate normalities were calculated from each standardization and corrected to 20°C. The 20°C normalities and the blanks were plotted versus time and were reviewed for possible problems. The sample drawing temperature thermometer during this leg was functional and calibrated at the beginning of the expedition. A noisy endpoint was occasionally acquired during the analyses, usually due to small water bath contaminations. These endpoints were checked and recalculated using STS/ODF-designed software. The blanks and thiosulfate normalities for each batch of thiosulfate were smoothed (linear fits) in four groups during the cruise and the oxygen values recalculated. Oxygen flask volumes were determined gravimetrically with degassed deionized water to determine flask volumes at the STS/ODF chemistry laboratory. This is done once before using flasks for the first time and periodically thereafter when a suspect volume is detected. The volumetric flasks used in preparing standards were volume-calibrated by the same method, as was the 10 mL Dosimat buret used to dispense standard iodate solution.
Nutrients
Nutrient analyses (phosphate, silicate, nitrate and nitrite) were performed on an ODF-modified 4‑channel Technicon AutoAnalyzer II, generally within 1 - 2 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 at 2-second 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 PO4 color development. The sample was passed through a 15-mm flow cell and the absorbance was measured at 660nm.
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 flow cell 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 flow cell 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, 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 flow cell where the absorbance was measured at 820 nm.
A total of 3806 nutrient samples were analyzed during the cruise. An additional 547 samples were taken by the Trace Metals group and analyzed by STS/ODF. Nutrient samples were drawn into 45 mL screw-capped Nalgene "Oak Ridge" centrifuge tubes. The tubes were cleaned with 10% HCl and rinsed with sample 2 - 3 times before being filled. Standardizations were performed at the beginning and end of each group of analyses (typically one cast, up to 36 samples) with an intermediate concentration mixed nutrient standard prepared prior to each run from a secondary standard in a low-nutrient seawater matrix. The secondary standards were prepared aboard ship by diluting primary standard solutions. Dry standards were pre-weighed at the laboratory at ODF, and transported to the vessel for dilution to the primary standard. Sets of seven different standard concentrations were analyzed periodically to determine any deviation from linearity as a function of absorbance for each nutrient analysis. A correction for non-linearity was applied to the final nutrient concentrations when necessary. A correction for the difference in refractive indices of pure distilled water and seawater was periodically determined and applied where necessary. In addition, a "deep seawater" high nutrient concentration check sample was run with each station as an additional check on data quality. The pump tubing was changed 3 times. After each group of samples was analyzed, the raw data file was processed to produce another file of response factors, baseline values, and absorbencies. Computer-produced absorbance readings were checked for accuracy against values taken from a strip chart recording. The data were then added to the cruise database. Nutrients, reported in micromoles per kilogram, were converted from micromoles per liter by dividing by sample density calculated at 1 atm pressure (0 db), in situ salinity, and a per-analysis-measured laboratory temperature.
Primary standards for silicate (Na2SiF6) and nitrite (NaNO2) were obtained from Johnson Matthey Chemical Co.; the supplier reported purities of >98% and 97%, respectively. Primary standards for nitrate (KNO3) and phosphate (KH2PO4) were obtained from Fisher Chemical Co.; the supplier reported purities of 99.999% for each. The efficiency of the cadmium column used for nitrate was monitored throughout the cruise and ranged from 99 - 100%. No major problems were encountered with the measurements. The temperature of the laboratory used for the analyses ranged from 21.6°C to 25.8°C, but was relatively constant during any one station (±1.5°C).
Section P16N_2006
Salinity
A single Guildline Autosal Model 8400A salinometer (S/N 48-266), located in the forward analytical lab, was used for all salinity measurements during Legs 1 and 2 of Repeat Section P16N_2006. The salinometer was modified by SIO/ODF to contain an interface for computer-aided measurement. The water bath temperature was set and maintained at a value near the laboratory air temperature (24°C). The salinity analyses were performed after samples had equilibrated to laboratory temperature, usually within 6 - 8 h after collection. The salinometers were standardized for each group of analyses (usually 1-2 casts, up to ~40 samples) using at least two fresh vials of standard seawater per group. Salinometer measurements were made by computer, with the software prompting the analyst to change samples and flush.
A total of 1,692 salinity measurements were made and ~100 vials of SSW were used during Leg 1 of the cruise and 3,250 salinity measurements were made and ~200 vials of SSW were used during the Leg 2. Salinity samples were drawn into 200 mL Kimax high-alumina borosilicate bottles, which were rinsed three times with sample prior to filling. The bottles were sealed with custom-made plastic insert thimbles and Nalgene screw caps. The temperature in the salinometer laboratory varied from 21 to 24°C, during the cruise. The air temperature change during any particular run varied from -1.2 to +2.2°C. The laboratory air temperature (21°C) was significantly lower than the bath temperature (24°C) for the first 7 casts of Leg 1. The estimated accuracy of bottle salinities run at sea was better than ±0.002 on both legs relative to the particular standard seawater batch used. The 95% confidence limit for residual differences between the bottle salinities and calibrated CTD salinity relative to SSW batch P-145 was ±0.010 for all salinities, and ±0.0035 for salinities collected in low gradients.
Oxygen
Dissolved oxygen analyses during Leg 1 were performed with an Friederich (MBARI)-designed automated oxygen titrator. During Leg 2 of the cruise dissolved oxygen analyses were performed with a Monterey Bay Aquarium Research Institute (MBARI)-designed automated oxygen titrator with photometric end-point detection based on the absorption of 365nm wavelength ultra-violet light. The titration of the samples and the data logging were controlled by computer. Thiosulfate was dispensed by a Dosimat 665 buret driver fitted with a 1.0 mL buret. A whole-bottle modified Winkler titration following the technique of Carpenter 1965 with modifications by Culberson et al. 1991 was used. Pre-made liquid potassium iodate standards were run every other day approximately every 4 stations, unless changes were made to the system or reagents. Reagent/distilled water blanks were determined every other day or more often if a change in reagents required accounting for the presence of oxidizing or reducing agents.
A total of 1,442 oxygen measurements were made during Leg 1 and 1,536 measurements were made during Leg 2 of the cruise. Samples were collected for dissolved oxygen analyses soon after the rosette was brought on board. Using a Tygon and silicone drawing tube, nominal 125 mL volume-calibrated iodine flasks were rinsed 3 times with minimal agitation, then filled and allowed to overflow for at least 3 flask volumes. The sample drawing temperatures were measured with a small glass bead thermistor thermometer embedded in the drawing tube. These temperatures were used to calculate µmol/kg concentrations, and as a diagnostic check of Niskin bottle integrity. Reagents were added to fix the oxygen before the samples were stoppered. 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 - 4 h of collection, and the data were incorporated into the cruise database. Thiosulfate normalities were calculated from each standardization and corrected to 20°C. Oxygen flask volumes were determined gravimetrically with degassed deionized water at AOML.
In addition to the photometric end-point technique, samples from several stations during Leg 2 were analyzed using an amperometric detection method (Culberson and Huang, 1987) for comparison. This was done to test the amperometric detection method for future standard use. The difference between the two techniques was on average <1 µmol/kg.
Nutrients
Nutrient samples were collected from the Niskin bottles in acid-washed 25 mL linear olyethylene bottles after three complete seawater rinses and analyzed within 1 h of sample collection. Measurements were made in a temperature-controlled laboratory (20±2°C). Concentrations of nitrite, nitrate, phosphate, and silicate were determined using an Alpkem Flow Solution Auto-Analyzer aboard the ship. During the Section P16N_2006 cruise ~3000 samples were analyzed along with their standards and baseline samples.
Nitrite was determined by diazotizing with sulfanilamide and coupling with N-1 naphthyl ethylenediamine dihydrochloride to form an azo dye. The color produced is measured at 540 nm (Zhang et al. 1997a). Samples for nitrate analysis were passed through a castom-made cadmium column (Zhang et al. 2000), which reduced nitrate to nitrite; the resulting nitrite concentration was then determined as described above. Nitrate concentrations were determined from the difference of nitrate + nitrite and nitrite.
Phosphate in the samples was determined by reacting with molybdenum (VI) and antimony (III) in an acidic medium to form an antimonyphosphomolybdate complex at room temperature. This complex was subsequently reduced with ascorbic acid to form a blue complex, and the absorbance was measured at 710 nm.
Silicate in the sample was analyzed by reacting the aliquot with molybdate in a acidic solution to form molybdosilicic acid. The molybdosilicic acid was then reduced by ascorbic acid to form molybdenum blue (Zhang et al. 1997b). The absorbance of the molybdenum blue was measured at 660 nm. Stock standard solutions were prepared by dissolving high purity standard materials (KNO3, NaNO2 , KH2PO4 and Na2SiF6) in deionized water. Working standards were freshly made at each station by diluting the stock solutions in low nutrient seawater. The low nutrient seawater used for the preparation of working standards, determination of blank, and wash between samples was filtered seawater obtained from the surface of the Gulf Stream. Standardizations were performed prior to each sample run with working standard solutions. Two or three replicate samples were collected from the Niskin bottle sampled at deepest depth at each cast. The relative standard deviation from the results of these replicate samples was used to estimate the overall precision obtained by the sampling and analytical procedures. The precisions of these samples were 0.04 µmol/kg for nitrate, 0.01 µmol/kg for phosphate, and 0.1 µmol/kg for silicate.
