2. DESCRIPTION OF THE EXPEDITION

2.1 R/V Ronald H. Brown: Technical Details and History

The NOAA ship R/V Ronald H. Brown, a state-of-the-art oceanographic and atmospheric research platform, is the largest vessel in the NOAA fleet. With its highly advanced instruments and sensors, R/V Ronald H. Brown travels worldwide supporting scientific studies to increase our understanding of the worlds oceans and climate. Commissioned on July 19, 1997, in its home port of Charleston, South Carolina, Ronald H. Brown has sailed in the Pacific, Atlantic, and Indian Oceans. The ship was named in honor of former Secretary of Commerce Ronald H. Brown, who was killed in a plane crash on April 3, 1996, while on a trade mission to Bosnia. R/V Ronald H. Brown is operated by NOAA Marine and Aviation Operations and carries a complement of 6 NOAA Corps officers, 20 crew members, and a maximum of 33 scientists. Table 2.1. provides a detailed description of the ship.

2.2 R/V Ronald H. Brown A16S_2005 Cruise Information

2.3 Parameters Measured, Institution, and Responsible Investigators

2.4 Participating Institutions

3. DESCRIPTION OF VARIABLES AND METHODS

3.1 Hydrographic Measurements

Samples for CFCs, helium isotopes (³He), oxygen (O₂), hydrochlorofluorocarbon (HCFCs), partial pressure of CO₂ (pCO₂), TCO₂, hydrogen ion activities (pH), TALK, radiocarbon (Δ¹⁴C), tritium, DOC, chromophoric dissolved organic matter (CDOM), particulate inorganic/organic carbon (PIC/POC), salinity, and nutrients were drawn in this sequence from a CTD sampling package containing thirty-six 12-L Bullister bottles. A detailed descriptions of methods for the CTD data, LADCP data and biooptical data are given in Wanninkhof and Doney (2005). Oxygen, nutrient, and salinity samples were taken from all bottles. Oxygen draw temperature readings were commenced after station 25. For the other parameters, not all stations or all bottles were sampled. The stations at full degrees of latitude (odd numbered stations) were generally completely sampled for CFCs, TCO₂, pH, and TALK, with partial sampling for DOC and CDOM. The half-degree stations were partially sampled for HCFC, PIC/POC, CFCs, TCO₂, pH, and TALK. Discrete pCO₂ profiles were obtained at every two degrees. ³He, Δ¹⁴C, and tritium were sampled at different intervals (primarily at full-latitude stations).

A total of 4174 bottle salinity samples were taken during the cruise, of which 127 were flagged as questionable and 3 as bad. Two samples were lost during analysis. 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 hours and typically not more than 24 hours from the time of sampling. Samples were run on a Guildline 8400B Laboratory Salinometer, serial number 60843. The salinometer had been last calibrated at Guildline in January of 2004. IAPSO Standard Seawater was used to standardize the instrument. The software used (ASALW) was developed at Scripps Institution of Oceanography. As per the instructions provided in the software, the cell was rinsed at least two times with sample at a relatively fast flow rate; the flow was adjusted to a slower rate for the final fill, and a reading was taken. The cell was drained and slowly filled for a second reading. If the two readings agreed within 0.00005, the values were accepted; otherwise, an additional reading was required. PSS-78 salinity (UNESCO, 1981) was calculated. Corrections were applied to the data for differences between beginning and ending standards.

Samples for dissolved oxygen analyses were drawn from 12-L Bullister bottles into calibrated 140-mL iodine titration flasks using Tygon tubing with silicone adapters that fit over the petcock to avoid contamination of DOC samples. Bottles were rinsed twice and filled from the bottom, then overflowed three volumes (care was taken to avoid entraining any bubbles). One-mL of MnCl₂ and one-mL of NaOH/NaI were added, then the flask was stoppered and shaken. Deionized water was added to the neck of each flask to create a water seal. The flasks were stored in the lab in plastic totes at room temperature for 12 hours before analysis. A total of 4659 samples were analyzed during the cruise, of which 37 samples were flagged as questionable and 4 as bad. Three samples were not reported. The whole-bottle titration technique of Carpenter (1965) was performed with modifications by Culberson et al. (1991), but with a more dilute solution of thiosulfate (10 g/L). Dissolved oxygen analyses were performed with a Monterey Bay Aquarium Research Institute (MBARI)-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 a 386 PC running the oxygen program written by Gernot Friedrich (Friederich et al. 1984). Thiosulfate was dispensed by a Dosimat 665 fitted with a 5.0-mL burette. The autotitrator and Dosimat performed well.

Dissolved nutrient (phosphate, silicic acid, nitrate, and nitrite) samples were drawn in 40-mL HDPE Boston Round sample bottles that had been stored in 10% HCl and rinsed four to five times with sample before filling. A replicate was always drawn from the deep bottle for analysis on the subsequent station. All samples were brought to room temperature prior to analysis. A separate analytical run was conducted at each station (except for the shallowest stations). An analytical run consisted of blanks and working standards, old working standards, deep water from the previous station, samples analyzed from deep to surface, replicate analysis of the four deep samples and any problem samples, and finally the working standards and blanks. The blanks were deionized water, and the standards were a zero standard in Low Nutrient Seawater (LNSW) and a high standard. Linearity of the autoanalyzer was checked every ten days, and corrections for non-linearity were applied during final data reduction.

All nutrients were measured using automated continuous flow analysis with a segmented flow and colorimetric detection. The four-channel autoanalyzer was customized using components from various systems. The major components were an Alpkem 301 sampler, two 24-channel Ismatek peristaltic pumps, four Thermo-Separation monochrometers, and custom software for digitally logging and processing the chromatographs. Glass coils and tubing from the Technicon Autoanalyzer II were used for analysis of phosphate, and micro-coils from Alpkem were used for the other three analyses. The detailed methods are described by Gordon et al. (1993). Because pump tubing destined for the cruise was lost in transit, some of the pump tube sizes suggested in the manual had to be modified. Pump tubes were changed four times during the expedition.

Silicic acid was analyzed using a modification of Armstrong et al. (1967). An acidic solution of ammonium molybdate was added to a seawater sample to produce silicomolybic acid. Oxalic acid was added to inhibit a secondary reaction with phosphate. Finally, the reduction with ascorbic acid formed the blue compound silicomolybdous acid. The color formation was detected using a 6-mm flowcell at 660 nm. The use of oxalic acid and ascorbic acid (instead of tartaric acid and stannous chloride as suggested by Gordon et al.) was to reduce the toxicity of our waste stream.

Nitrate and nitrite analyses were also modified from Armstrong et al. (1967). Nitrate was reduced to nitrite in a cadmium column, then formed into a red azo dye by complexing nitrite with sulfanilamide and N-1-naphthylethylenediamine. The color formation was detected using a 6-mm flow cell at 540 nm. The same technique was used to measure nitrite (excluding the reduction step), but the color formation was detected using a 10-mm flow cell at 540 nm.

Phosphate analysis was based on the technique of Bernhardt and Wilhelms (1967). An acidic solution of ammonium molybdate was added to the sample to produce phosphomolybdic acid, and this was reduced to the blue compound phosphomolybdous acid following the addition of hydrazine sulfate. The reaction was heated to 55°C to bring the reaction to completion, and color formation was detected using a 10-mm flow cell at 815 nm.

3.2 Total CO₂ Measurements

Samples for TCO₂ measurements were drawn according to procedures outlined in the Handbook of Methods for CO₂ Analysis (DOE 1994) from 12-L Bullister bottles into 540-mL Pyrex bottles using Tygon tubing with a silicone adapter on the petcock to avoid contamination of DOC samples. Bottles were rinsed and filled from the bottom, leaving 5 mL of headspace; care was taken not to entrain any bubbles. After 0.2 mL of saturated HgCl₂ solution was added as a preservative, the sample bottles were sealed with glass stoppers lightly covered with Apiezon-L grease and were stored at room temperature for a maximum of 12 hours prior to analysis.

TCO₂ samples were collected at every degree from all depths (typically 36 for standard deep water stations) with three replicate samples. Some samples were also collected at every half-degree. The replicate seawater samples were taken from the surface, 1000 m, and bottom Bullister bottles and run at different times during the cell. The first replicate of the bottom water was used at the start of the cell with fresh coulometer solution, and the first of the 1000-m replicates was run in the middle of the cell after about 12 mg of C were titrated. The second one of the bottom replicates was run at the end of the cell after about 25 mg of C were titrated. A new coulometer cell was started with the second of the 1000-m replicates and the first of the surface replicates. In the middle of this cell, the second of the surface replicates was run and the first one of the surface duplicates of a partial station. The second of the partial station duplicates was run at the end of this cell. No systematic difference between the replicates was observed. There was no systematic dependency of results with an amount of carbon titrated for a particular cell. A total of 2482 samples for TCO₂ were collected and analyzed during the cruise.

The TCO₂ analytical equipment was set up in a seagoing laboratory van. The analysis was done by coulometry with two analytical systems (called AOML-1 and AOML-2) used simultaneously on the cruise. Each system consisted of a coulometer (UIC, Inc.) coupled with a single operator multi-parameter metabolic analyzer (SOMMA) inlet system developed by Kenneth Johnson (Johnson et al. 1985, 1987, 1993; Johnson 1992) now retired from Brookhaven National Laboratory (BNL). In the coulometric analysis of TCO₂, all carbonate species are converted to CO₂ (gas) by addition of excess hydrogen ion (acid) to the seawater sample, and the evolved CO₂ gas is swept into the titration cell of the coulometer with pure air or compressed nitrogen, where it reacts quantitatively with a proprietary reagent based on ethanolamine to generate hydrogen ions. In this process, the solution changes from blue to colorless, which triggers a current through the cell and causes coulometrical generation of OH ions at the anode. The OH ions react with the H⁺, and the solution turns blue again. A beam of light is shone through the solution, and a photometric detector at the opposite side of the cell senses the change in transmission. Once the percent transmission reaches its original value, the coulometric titration is stopped, and the amount of CO₂ that enters the cell is determined by integrating the total charge during the titration.

The coulometers were calibrated by injecting aliquots of pure CO₂ (99.995%) by means of an 8-port valve outfitted with two sample loops with known gas volumes (AOML-1: 1.9951 mL at 25.05°C and 0.9807 mL at 25.10°C; AOML-2: 2.0018 mL at 25.09°C and 0.9949 mL at 25.06°C) bracketing the amount of CO₂ extracted from the water samples for the two AOML systems.

The stability of each coulometer cell solution was confirmed three different ways: the Certified Reference Material (CRM), Batch 66, supplied by Dr. A. Dickson of SIO, was measured at the beginning and the middle, gas loops in the beginning and at the end, and the duplicate samples at the beginning, middle, and end of each cell solution (Fig 3.1., Table3.1.). The coulometer cell solution was replaced after 25 mg of carbon was titrated, typically after 912 hours of continuous use.

The pipette volume was determined prior to the cruise by taking aliquots at known temperature of distilled water from the volumes. The weights with the appropriate densities were used to determine the volume of the pipettes (AOML1: 28.716 mL at 20.00°C, AOML2: 22.547 mL at 20.00°C).

Calculation of the amount of CO₂ injected was according to the CO₂ handbook (DOE 1994). The concentration of CO₂ ([CO₂]) in the samples was determined according to:

where Cal. Factor is the calibration factor, Counts is the instrument reading at the end of the analysis, Blank is the counts/minute determined from blank runs performed at least once for each cell solution, Run Time is the length of coulometric titration (in minutes), and K is the conversion factor from counts to mol.

The instrument has a salinity sensor, but all TCO₂ values were recalculated to a molar weight (µmol/kg) using density obtained from the CTDs salinity sensor. The TCO₂ values were corrected for dilution by 0.2 mL of HgCl₂ used for sample preservation. The total water volume of the sample bottles was 540 mL. The correction factor used for dilution was 1.00037. A correction was also applied for the offset from the CRM. This correction was applied for each cell using the CRM value obtained in the beginning of the cell. The results underwent initial quality control on the ship using property plots: TCO₂ vs.Depth, TCO₂ vs. Potential Temperature, TCO₂ vs. AOU, TCO₂ vs. NO₃; TCO₂ vs. SiO₃, TCO₂ vs. PO₄, TCO₂ vs. TALK, and TCO₂ vs. pH. Also contour plots of TCO₂ vs. LAT and Depth were used to analyze the quality of the data.

Fig. 3.1. Results of the duplicate TCO₂ samples collected during the R/V Ronald H. Brown cruise along the Atlantic Ocean section A16S_2005.

The overall performance of the instruments was good during the cruise. The air purifier supplying carrier and pneumatic gas malfunctioned at station 24. Compressed tanks of ultra-high purity nitrogen gas were used thereon. At the same time, soda lime traps used to scrub any CO₂ from the carrier gas were removed from the air/N₂ line, since they developed cracks over time and also appeared to release CO₂ in pulses into the carrier. A coulometer was replaced during the test cast runs. It did not find an endpoint and did not stop counting. A number of pinch valves failed and were replaced; some cell caps began to leak, and some electrode leads broke. Finally, the Orbo tubes (filled with silica gel to absorb possible acid vapors) tended to break and leak and were not used after station 109 on either system.

Due to concerns about the large amount of water used for a TCO₂ sample and use of grease on the stoppers that could contaminate samples for dissolved organic matter (DOM), comparison tests were performed with samples drawn in 250-mL borosilicate bottles with ground glass stoppers stored under cold water with the regular sampling procedures outlined above. The results are shown in Table 3.2.

Table 3.2. Test results of different sample bottle sizes for TCO₂ measurements