¹Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY
²Rosentiel School of Marine and Atmospheric Research, Miami University, Miami, FL
³Princeton University, Princeton, NJ

Contents

• ABBREVIATIONS AND ACRONYMS
• ABSTRACT
• 1. BACKGROUND INFORMATION
• 2. DESCRIPTION OF THE EXPEDITION
• 2.1 R/V Nathaniel B.Palmer Technical Details
• 2.2 R/V Nathaniel B.Palmer S04I Cruise Information
• 2.3 Brief Cruise Summary
• 3. DESCRIPTION OF VARIABLES AND METHODS
• 3.1 Hydrographic Measurements
• 3.2 Total CO₂ Measurements
• 3.3 Total Alkalinity Measurements
• 3.3.1 Titration Total Alkalinity, Total CO₂, and pH Measurement Results
• 3.4 Discrete pCO₂ Measurements
• 3.4.1 Comparison of observed and calculated pCO₂
• 3.4.2 Comparison of observed and calculated total alkalinity
• 3.5 Radiocarbon Measurements
• 3.5.1 Comparison of observed and calculated pCO₂
• 4. HOW TO OBTAIN THE DATA AND DOCUMENTATION
• 5. REFERENCES

ABBREVIATIONS AND ACRONYMS

ALACE - autonomous Lagrangian circulation explorer
AMS - Accelerator Mass Spectrometry
¹⁴C - radiocarbon
CDIAC - Carbon Dioxide Information Analysis Center
CFC - chlorofluorocarbon
CO₂ - carbon dioxide
CRM - certified reference material
CTD - conductivity, temperature, and depth sensor
DMSO - dimethylsulfoxide
DOE - U.S. Department of Energy
GC - gas chromatograph, gas chromatography
GO - General Oceanic
JGOFS - Joint Global Ocean Flux Study
LADCP - lower acoustic Doppler current profiler
LDEO - Lamont-Doherty Earth Observatory
NDP - numeric data package
NOAA - National Oceanic and Atmospheric Administration
NOSAMS - National Ocean Sciences AMS Facility
NSF - National Science Foundation
NTA - normalized total alkalinity
ODF - oceanographic data facility
ORNL - Oak Ridge National Laboratory
pCO₂ - partial pressure of CO₂
PU - Princeton University
QA - quality assurance
QC - quality control
RSMAS - Rosentiel School of Marine and Atmospheric Research
R/V - research vessel
SIO - Scripps Institution of Oceanography
SOMMA - single-operator multi-parameter metabolic analyzer
TALK - total alkalinity
TAMU - Texas A&M University
TCO ₂ - total carbon dioxide
UH - University of Hawaii
UM - University of Miami
UW - University of Washington
WHOI - Woods Hole Oceanographic Institution
WHP - WOCE Hydrographic Program
WOCE - World Ocean Circulation Experiment

ABSTRACT,

Takahashi, T., F. Millero, R. Key, D. Chipman, E. Peltola, S. Rubin, C. Sweeney, and S. Sutherland, 2005. Determination of Carbon Dioxide, Hydrographic, and Chemical Parameters during the R/V Nathaniel B. Palmer Cruise in the Southern Indian Ocean (WOCE Section S04I, 3 May - 4 July, 1996), ed. A. Kozyr. ORNL/CDIAC-150, NDP-086. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, 50 pp.

 

This report discusses the procedures and methods used to measure total carbon dioxide (TCO₂), total alkalinity (TALK), and partial pressure of CO₂ (pCO₂) at hydrographic stations during the cruise of research vessel (R/V) Nathaniel B. Palmer in the Southern Indian Ocean on the S04I Section as a part of the Joint Global Ocean Flux Study (JGOFS)/World Ocean Circulation Experiment (WOCE). The carbon-related measurements were sponsored by the U.S. Department of Energy (DOE). The expedition started in Cape Town, South Africa, on May 3, 1996, and ended in Hobart, Australia, on July 4, 1996. Instructions for accessing the data are provided.

The TCO₂ was measured in discrete water samples using the Lamont-Doherty Earth Observatory (LDEO) coulomteric system with an overall precision of ±1.7 µmol/kg. TALK was determined by potentiometric titration with an overall precision of ±1.7 µmol/kg. During the S04I cruise pCO₂ was also measured using the LDEO equilibrator-gas chromatograph system with a precision of 0.5% (including the station-to-station reproducibility) at a constant temperature of 4.0°C

The R/V Nathaniel B. Palmer S04I data set is available free of charge as a numeric data package (NDP) from the Carbon Dioxide Information Analysis Center. The NDP consists of the oceanographic data files and this printed documentation, which describes the contents and format of all files as well as the procedures and methods used to obtain the data.

Keywords: carbon dioxide, TCO₂, TALK, partial pressure of CO₂, carbon cycle, coulometry, potentiometry, hydrographic measurements, World Ocean Circulation Experiment.

1. BACKGROUND INFORMATION

The World Ocean Circulation ExperimentWorld Hydrographic Program (WOCE-WHP) was a major component of the World Climate Research Program. The primary goal of WOCE was to understand the general circulation of the global ocean well enough to be able to model its present state and predict its evolution in relation to long-term changes in the atmosphere. The need for carbon system measurements arose from the serious concern over the rising atmospheric concentrations of carbon dioxide (CO₂). Increasing atmospheric CO₂ may intensify the earths natural greenhouse effect and alter the global climate.

Although CO₂-related measurementsspecifically, total CO₂ (TCO₂), total alkalinity (TALK), partial pressure of CO₂ (pCO₂), and pHwere not official WOCE measurements, a coordinated effort was supported as a core component of the Joint Global Ocean Flux Study (JGOFS). This effort received support in the United States from the U.S. Department of Energy (DOE), the National Oceanic and Atmospheric Administration (NOAA), and the National Science Foundation (NSF).

This report discusses results of the research vessel (R/V) Nathaniel B. Palmer expedition along the WOCE Section S04I. The cruise started from Cape Town, South Africa, on May 3, 1996, and disembarked at Hobart, Australia, on July 4, 1996. The ship occupied 108 stations along 60-62°S latitude from 20°E to 120°E meridian, plus two north-to-south sections south from this line to the Antarctic continental shelf (Fig. 1).

 

 

 

 

This cruise had the following objectives:

• To better constrain the fluxes of carbon, both organic and inorganic, in the Southern Ocean and to place these fluxes into the context of the contemporary global carbon cycle. There is a great interest in understanding the CO₂ system in the oceans because of the so-called greenhouse effect of CO₂ on the climate. Approximately 30% of CO₂ added to the atmosphere from the burning of fossil fuels is thought to be going into the oceans.
• To identify the factors and processes which regulate the magnitude and variability of primary productivity, as well as the fate of biogenic materials.
• To determine how the Southern Ocean has responded in the past to naturally occurring climate changes.
• To monitor and predict the response of the Southern Ocean to global climate change.

 

The measurements along S04I section provide a rare look at the far south Indian Ocean in an oceanographically important time of year, when the sea surface is cooling and dissolved gases and other properties are being mixed downward.

2. DESCRIPTION OF THE EXPEDITION

2.1 R/V Nathaniel B. Palmer Technical Details

The R/V Nathaniel B. Palmer is a large icebreaker in the service of the U.S. National Science Foundation. It is tasked with extended scientific missions in the Antarctic. The vessel carries a helicopter and about four dozen scientists on expeditions that last for months. The vessel is named after the first American credited with sighting Antarctica. This R/V, built for the NSF and launched in 1992, is chartered and operated by the Raytheon Company. Table 1 provides a detailed description of the ship.

 

 

2.2 R/V Nathaniel B. Palmer S04I Cruise Information

 

Ship Name	Nathaniel B. Palmer
EXPOCODE	320696_3
WOCE section	S04I
Ports of call	Cape Town, South Africa; Hobart, Australia
Dates	May 3 - July 4, 1996
Funding support	NSF, DOE
Chief scientist	Dr. Thomas Whitworth III, Texas A&M University

 

 

Parameters measured, institution, and responsible investigators

 

Parameter	Institution	Responsible Personnel
CTD	SIO	J. Swift
Bottle sal., oxy. nutrients	SIO	J. Swift
ALACE floats	SIO	R. Davis
LADCP	UH	E. Firing/P. Hacker
CFCs	LDEO/ UW	W. Smethie/M. Warner
Current meters	TAMU	W. Nowlin/T. Whitworth
Transmissometer	TAMU	W. Gardner
Tritium, He	LDEO	P. Schlosser
14C	PU	R. Key
TCO2 , pCO2	LDEO	T. Takahashi
TALK	RSMAS/UM	F. Millero
ALACE = autonomous Lagrangian circulation explorer CTD = conductivity, temperature, and depth sensor TAMU = Texas A&M University SIO = Scripps Institution of Oceanography

 

 

Participating institutions

 

SIO	Scripps Institution of Oceanography
TAMU	Taxes A&M University
LDEO	Lamont-Doherty Earth Observatory
UW	University of Washington
PU	Princeton University
UH	University of Hawaii
RSMAS/UM	Rosentiel School of Marine and Atmospheric Research, University of Miami

 

 

2.3 Brief Cruise Summary

The S04I section constituted the Indian Ocean portion of WOCE line S04, a meridional circumnavigation of Antarctica at a nominal latitude of 60°S. This segment covered the longitudes from 20°E to 120°E. R/V Nathaniel B. Palmer departed Cape Town, South Africa, on May 3, 1996. The cruise track proceeded south to near 58°S and 20°E where station occupations began on May 16. The first station line was run southeast to Gunnerus Ridge, about 50 miles south of the ice edge (see Fig.1). During the transit to station 1 and continuing to 58°S, 17 autonomous Lagrangian circulation explorer (ALACE) floats were launched. Stations across the Enderby Abyssal Plain trended east-northeast from 66°S at 33°E to 61°S at 83°E on the Kerguelen Plateau. A line of stations (35-42) was made north from the 500-m isobath on the continental slope at 53°E, and three self-reporting current meters were deployed along the slope. A line of stations (65-72) extending east from the crest of the Kerguelen Plateau was made at about 59°S, and three more current meters were placed in the boundary current on the eastern flank of the plateau. On June 8, after station 72, science operations were suspended for seven days when R/V Nathaniel B. Palmer was diverted to Mirnyi Station in the Davis Sea to deliver emergency food supplies.

On June 14, R/V Nathaniel B. Palmer left Mirnyi and began a line of stations (73-86) from the shelf break of the Davis Sea to Kerguelen Plateau. One current meter was placed near the 3000-m isobath north of the Antarctic Continental Slope, and two were deployed at the southern end of Kerguelen Plateau. The zonal line of stations at a nominal latitude of 62°S was resumed at 90°E. Ice conditions and fuel and time considerations necessitated 45-mile station separation for most of the final 22 stations. The last station, No. 108, was sampled at 120°E on June 27, and Nathaniel B. Palmer then transited directly to Hobart, Australia, arriving on July 7, 1996. For more information on the cruise please see the chief scientist cruise report at http://whpo.ucsd.edu/data/onetime/southern/s04/s04i/s04ido.txt.

 

 

3. DESCRIPTION OF VARIABLES AND METHODS

3.1 Hydrographic Measurements

The basic hydrography program consisted of salinity, dissolved oxygen, and nutrient (nitrite, nitrate, phosphate, and silicate) measurements made from bottles taken on CTD/rosette casts, plus pressure, temperature, salinity, and dissolved oxygen from CTD profiles. Overall, 109 CTD/rosette casts were made at 108 stations, usually to within 5-15 meters of the bottom. Station 2 cast 1 was aborted at the surface because of signal failure at 322 m on the down-cast. Water was found inside the CTD case; after repairs, station 2 cast 2 was successfully accomplished. Hydrographic casts were performed with a rosette system consisting of a 36-bottle rosette frame [Oceanographic Data Facility (ODF)], a General Oceanics (GO) 36-place pylon (Model 2216) and thirty-six 10-L PVC bottles (ODF). Underwater electronic components consisted of an ODF-modified NBIS Mark III CTD (ODF #3) and associated sensors, SeaTech transmissometer [Texas A&M University (TAMU)] and Benthos pinger (Model 2216).

Two Guildline Autosal Model 8400A salinometers were available for measuring salinities. The salinometers were modified by ODF and contained interfaces for computer-aided measurement. Autosal No.55-654 was used to measure salinity on all stations. Its water bath temperature was set and maintained at 24°C for all runs except stations 32-39, where the bath temperature was set at 21°C. The salinity analyses were performed when samples had equilibrated to laboratory temperature, within 7-28 hours after collection. The salinometer was standardized for each group of analyses (typically one cast, usually 36 samples) using two fresh vials of standard seawater per group. A computer (PC) prompted the analyst for control functions such as changing sample, flushing, or switching to read mode. At the correct time, the computer acquired conductivity ratio measurements and logged results. The sample conductivity was redetermined until readings met software criteria for consistency. Measurements were then averaged for a final result.

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 burette driver fitted with a 1.0-mL burette. 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 (approximately 0.012N) and thiosulfate solution (50 gm/L). Carbon disulfide was added to the thiosulfate as a preservative. Standard solutions prepared from pre-weighed potassium iodate crystals were run at the beginning of each session of analyses, which typically included from 1 to 3 stations. Nine standards were made up during the cruise and compared to ensure that the results were reproducible, and to preclude the possibility of a weighing or dilution error. Reagent/distilled water blanks were determined to account for presence of oxidizing or reducing materials.

Nutrient analyses (phosphate, silicate, nitrate, and nitrite) were performed on an ODF-modified 4-channel Technicon AutoAnalyzer II, generally within a few hours after sample collection. Occasionally samples were refrigerated up to a maximum of 8 hours at 2°C to 6°C. All samples were brought to room temperature prior to analysis. The methods used are described by Gordon et al. (1993). The analog outputs from each of the four channels were digitized and logged automatically by computer (PC) 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 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, 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.

 

3.2 Total CO₂ Measurements

The coulometric analysis system used to measure the total CO₂ (TCO₂) concentration in seawater samples during this cruise was the same as described in Chipman et al. (1993) and Takahashi et al. (1998), except for the way the sample was introduced. The system consists of a Model 5011 coulometer, manufactured by UIC Inc., Jolliet, IL, and a sample introduction/CO₂ extraction system of a Lamont-Doherty Earth Observatory (LDEO) CO₂ Group design. It differs from the single-operator multi-parameter metabolic analyzer (SOMMA) system used by other participants of the DOE/CO₂ program. In the LDEO system, the titration cell was maintained at a constant temperature (about 25°C) using a water-jacketed cell to prevent drifts of the titration end point. The temperature of the water circulating through the jacket was kept at a constant temperature by a temperature-controlled heat exchanger system. A precisely known volume of seawater sample was introduced manually into a CO₂ extraction vessel using a calibrated syringe instead of the automated pipette used by the SOMMA system. The syringe is a hand-ground Pyrex glass medical syringe with two firm reference stops which allow the quantitative sampling of seawater. Our experience with syringe and pipette methods is that accumulated coatings on the glass surfaces from repeatedly being filled with and emptied of seawater would not significantly affect the volume contained of either, but that it will affect the volume delivered by the passively drained pipette in the SOMMA system. The positive displacement of the plunger of a syringe keeps the delivered volume constant, even in the presence of surface coatings. Additionally, since the water sample in the syringe has no air space, changes in TCO₂ due to gas exchange with air in the head space are eliminated. Unlike the SOMMA, our system allows multiple samples from the same bottle. We can test the reproducibility of either certified reference material (CRM) or seawater samples by taking two or more samples with the syringes. For this reason, the total number of analyses for CRM on the LDEO system was three times greater than if SOMMA were used.

Samples for TCO₂ analysis were drawn from the Niskin bottles of the rosette casts directly into 250-mL glass reagent bottles with ground standard-taper stoppers, sealed with silicone grease and pressed in using two strong rubber bands. Immediately after sample collection, 200 µL of 50%-saturated mercuric chloride solution was added to prevent biological alteration of the TCO₂. A small head space (~5 mL) was left in the bottle to prevent thermal expansion of the water from causing a leak or breaking the bottle. Samples were normally analyzed within 24 hours of collection. For analysis, a water sample was sucked into a syringe, and a calibrated volume (19-20 mL) of water sample was introduced into a CO₂ extraction chamber through a rubber septum. The mass of the seawater sample delivered was determined from the density of seawater, calculated using the measured salinity, the temperature at the time of injection, and the International Equation of State of Seawater. Prior to the expedition, the volume of each sampling syringe between two reference stops was determined by repeatedly weighing aliquots of distilled, deionized water dispensed. The measurements were corrected for the buoyancy of air displaced by the water, amounting to about 0.1% of the weight of the water. The volume was then computed using the density of pure water at the temperature of the measurement. Repeated measurements gave a precision of ±0.03% or better.

The seawater sample in the extraction vessel was acidified with ~1 mL of 8.5% phosphoric acid introduced through a sidearm of the extraction chamber. The evolved CO₂ was stripped from the sample and transferred into the electrochemical cell of the CO₂ coulometer by a stream of CO₂-free air. In the coulometer cell, the CO₂ was quantitatively absorbed by a solution of ethanolamine in dimethylsulfoxide (DMSO). Reaction between the CO₂ and the ethanolamine formed the weak hydroxyethylcarbamic acid. The pH change of the solution associated with the formation of this acid resulted in a color change of the thymolphthalein pH indicator in the solution. The color change, from deep blue to colorless, was detected by a photodiode which continuously monitored the transmissivity of the solution. The electronic circuitry of the coulometer, in detecting the change in the color of the pH indicator, caused an electrical current to flow through the cell, generating hydroxyl (OH^-) ions from a small amount of water in the solution. The OH^- generated, then titrated the acid, returning the solution to its original pH and color, at which point the current flow was stopped. The product of current passed through the cell and time was related by the Faraday constant to the number of moles of OH^- generated, and hence to the number of moles of CO₂ absorbed to form the acid. A thermostated, double-walled titration cell was used during titration to eliminate the shifting of the titration endpoint due to a change in the temperature of the cell solutions.

The coulometer was calibrated using research grade CO₂ gas (99.998% pure) introduced into the carrier gas line upstream of the extraction chamber alternately using two fixed-volume sample loops on a gas-sampling valve. The loops were vented to the atmosphere, and the ambient atmospheric pressure in the laboratory was measured using a high-precision electronic barometer with an accuracy of better than 0.05%. The loop temperatures were measured to ±0.05°C with a thermometer calibrated against one traceable to the NIST. The non-ideality of CO₂ was incorporated into the computation of the loop contents. Prior to the expeditions, the volumes of the loops were determined by the difference in weight of the loop injection valve assembly when empty and when filled with water. Repeated measurements gave a precision of ±0.02%. During the expedition the coulometer was calibrated several times a day using this gas-sampling system.

The calibration factor, which represents the ratio between the number of moles of CO₂ in the loop and the reading of the coulometer, changes during the use of a titration cell. Depending on the condition of the solution in the titration cell, this factor varies around the ideal ratio of unity by a few tenths of a percent. It commonly starts from less than unity when the cell solution is new and increases to greater than unity as increasing amounts of carbon are titrated. This change can be represented by a quadratic equation relating values of calibration factor with the total amount of carbon titrated in a given cell (see Fig. 2). The CO₂ concentration in each seawater sample was corrected using a factor estimated from the equation fit to the calibration data for each cell. Generally a cell had to be cleaned and filled with fresh solution after about 40 samples. After this number the cell began to behave erratically with unreliable analytical results.

For the purpose of quality control of total CO₂ determinations, Scripps Institution of Oceanography (SIO) CRM, Batch 31, was run through our analytical system at sea as unknowns. The shipboard analyses compare with the SIO manometric analyses shown in Table 2.

 

Table 2. Comparison of LDEO shipboard analyses with SIO shore-based manometric analyses

CRM Batch No.	SIO Manometric TCO2 Shore (µmol/kg)	LDEO Coulometric TCO2 Sea (µmol/kg)	Difference (SIO − LDEO) (µmol/kg)
31	1876.57±1.27 (N=10)*	1878.31±1.12 (N=294)	-1.74

^*N = number of analyses.

 

Since more than one sample was taken from each CRM bottle and analyzed, number of analyses exceeded the number of the CRM bottles supplied. The ± values indicate one standard deviation from the mean. Figure 3 shows the results graphically. The results of the shipboard analyses that were made using a high-purity CO₂ gas standard are 1.7 µmol/kg greater than the results of the SIO manometric analyses. The difference is, however, within the combined standard deviation for each set of measurements, and it is well within two standard deviations of each set. The TCO₂ values listed in this report are not adjusted for this difference. The documentation for the CRM batch provided by Andrew Dickson is reproduced in Table 3.

 

Fig. 2. Change in the coulometer calibration factor as a function of the amount of CO2 titrated. The change is expressed as a ratio of the moles of CO2 found divided by the moles of CO2 injected.

Fig. 3. The results of TCO₂ analyses of the certified reference solutions using the coulometer during the expedition.

Table 3. Specification of the CRM batch 31 (November 25, 1996)

Reference Material Batch 31

Bottled on August 16, 1995

Original certificate issued December 22, 1995

Total alkalinity values certified August 16, 1996

Certified Values

Salinity 32.899

Total Dissolved Inorganic Carbon

Mean: 1876.57 µmol/kg

Standard Deviation: 1.27 µmol/kg

Number of Analyses: 10

Total Alkalinity*

Mean: 2130.33 µmol/kg

Standard Deviation: 0.79 µmol/kg

Number of Analyses: 17 (on 5 bottles)

*These values were not measured when the batch was certified, but are based on measurements on archived samples.

Information values for nutrient levels measured at the time of bottling

Phosphate: 0.27 µmol/kg

Silicate: 1.30 µmol/kg

Nitrite: 1.34 µmol/kg

Nitrate: 2.59 µmol/kg

Note: Nutrient levels may have changed on storage, their stability has not been examined.

3.3 Total Alkalinity Measurements

The titration system for TALK determination used during expedition along WOCE section S04I was similar to the one used in our earlier studies (Millero et al. 1993). The systems consisted of a titrator (Metrohm, model 665 Dosimat) and pH meter (Orion, model 720A). Both instruments were controlled by computer. The electrodes used to measure the emf of the sample during a titration consisted of a ROSS glass pH electrode (Orion, model 810100) and a double junction Ag, AgCl reference electrode (Orion, model 900200) with an inner filling of 0.7 m NaCl. The temperature of both the acid titrant in a water-jacketed burette and the seawater sample in a water-jacketed cell were controlled to a constant temperature of 25 ± 0.1°C with a constant temperature bath (Neslab, model RTE 221). The Plexiglas water-jacketed cells used during the cruise were similar to that used by Bradshaw et al. (1988) except with a larger volume (about 200 cm3) to increase the precision. This cell had a fill and drain valve, which increased the reproducibility of the volume of sample contained in the cell. The volumes of the cells used at sea were determined in the laboratory before and after the cruise by several titrations using seawater of known TALK. The volumes of the cells did not change during the cruise. The nominal volumes of all the cells were about 200 cm³ and the values were determined to ±0.03 cm³ (cell 18 - 204.34 cm³, cell 19 - 203.76 cm³). A Lab Windows-C program was used to run the titration and record the volume of the added acid and the emf of the electrodes using RS232 interfaces. Seawater samples were titrated by adding HCl to exceed the carbonic acid end point. During a typical titration the emf readings are recorded after the readings become stable (±0.09 mV), and then a volume of acid is added to change the voltage to a pre-assigned increment (13 mV). In contrast to the delivery of a fixed volume increment of acid, this method gives an even distribution of data points in the range of rapid increase in the emf near the endpoint. A full titration (25 points) takes about 20 minutes. Using two systems, a 36-bottle station cast can be completed in 6 hours. Calibrations of the burettes of the Dosimat with water at 25°C indicate that the systems deliver 3.000 cm³ (the value for a titration of seawater) to a precision of ±0.0004 cm³. This uncertainty results in an error of ±0.4 µmol/kgin TALK and TCO₂. Corrections due to a small error in the volume delivery have been applied to our titration data. The precision of the values of TALK on a CRM (Dr. Andrew Dickson, Marine Physical Laboratory, La Jolla, California) is ±2 µmol/kg (Lee and Millero 1995).

Standard acid was made by preparing a single large 55-gal batch of ~0.25 M HCl acid by dilution of concentrated HCl, AR Select^® Mallinckrodt. The acid was prepared in 0.45 M NaCl to yield a total ionic strength similar to seawater of salinity 35.0 (I~0.7 M). The acid was standardized by a coulometric technique (Taylor and Smith, 1959; Marinenko and Taylor, 1968). The acid is titrated with OH^- generated coulometrically at a Pt electrode. The current is recorded as a function of time with data acquisition measuring the voltage across a standard 1-ohm resistor. An Orion combination electrode was used to measure the pH of the solution during the titration. The endpoint is determined from the plot of the concentration of H⁺ and OH^- in the solution as a function of the coulombs delivered. The precision of the coulometric titration was 0.002 %. Sub-samples of the acid were sent to Dr. Andrew Dicksons laboratory in La Jolla, California, for an independent laboratory determination of the molarity. TALK titrations on the same seawater using different acids calibrated by this technique agree to within ±2 µmol/kg. The calibrated concentration of the acid used during this cruise was 0.24793 ±0.00009 M HCl. The acid was bottled in 500-cm³ glass bottles for field use.

The TALK of seawater was evaluated from the proton balance at the alkalinity equivalence point, pH_equiv = 4.5, according to the exact definition of TALK (Dickson 1981).

 

TALK = [HCO₃⁻] + 2[CO₃²⁻] + [B(OH)₄⁻] + [OH⁻] + [HPO₄²⁻] + 2[PO₄³⁻] + [SiO(OH)₃⁻] + [HS⁻] + [NH₃] − [H+] − [HSO4⁻] − [HF] − [H₃PO₄]

 

The full titration is used to evaluate TALK from a given experiment. This is accomplished with a program patterned after that developed by Dickson (1981), Johansson and Wedborg (1982), and Dickson and Goyet (DOE 1994). The program determines pH, E* (for the electrode), TALK, TCO₂, and pK₁ for carbonic acid in the solution (Na₂CO₃, TRIS or seawater). The program used the Levenberg-Marquardt nonlinear least-squares algorithm to perform the calculations. The program assumes that the nutrients are negligible in the calculation. This does not affect the calculated TALK, which includes the alkalinity due to nutrients. The concentration of the effect of nutrients on TALK, however, must be considered when calculating the carbonate alkalinity ([HCO₃^-] + 2 [CO₃^2-]).

 

3.3.1 Titration Total Alkalinity, Total CO₂, and pH Measurement Results

 

Our effort during this cruise was to provide reliable TALK measurements on all the collected samples. Measurements on CRM (batch No. 31) were made before and during the cruise to monitor the performance of the titrators. A summary of the titration results for TALK, TCO₂, and pH are given in Table 4. Deviations from the mean of TALK, TCO_2, and pH for CRM over the course of the cruise are given in Figs. 4, 5, and 6. The values of TALK of cell 18 (2130 ± 2.3 µmol/kg) and cell 19 (2129 ± 3.5 µmol/kg) determined at sea were in good agreement with the laboratory results (2128 ± 0.09 µmol/kg). Measurements made on 47 CRM samples using the two cells at sea also indicate that the systems have a reproducibility of ±2.9 µmol/kg in TCO₂ and ±0.005 in pH.

The potentiometric measurements of pH at sea were 0.017 ± 0.005 higher than the laboratory spectroscopic values while the TCO₂ values were 10 ± 3 µmol/kg higher. These differences are due to the non-Nernstian response of the electrodes (Millero et al. 1993). Since the differences are highly reproducible, we have adjusted all our pH and TCO₂ measurements using these corrections.

Two batches of surface seawater were also collected to check the precision of the two titration systems during the cruise. TALK, TCO_2, and pH differences of cell 18 and cell 19 over the course of the cruise using this reference surface water are shown in Figs. 7, 8, and 9. The results show an agreement of ± 1.8 µmol/kg in TALK, ±1.7 µmol/kg in TCO_2, and ± 0.006 in pH throughout the cruise.

The performance of the titrators was also checked by analyzing duplicate samples from the same Niskin bottles for surface water and water collected from 800 meters. The duplicates were analyzed on the same cell and on different cells. The deviations of TALK, TCO₂ and pH over the course of the cruise are shown in Figs. 10, 11, and 12. The results showed an agreement of ±1.5-1.6 µmol/kg for TALK, ±1.3-1.7 µmol/kg for TCO₂, and ±0.005-0.008 for pH.

TALK and total carbon dioxide values of all the surface water samples are plotted in Fig. 13 for the stations occupied. A linear relationship between TALK and salinity is shown in Fig. 14 (TALK = 70.4 + 65.7 S, σ = ±5 µmol/kg). The Normalized TALK (NTA = TALK*S/35) and TCO₂ (NTCO₂ = TCO₂*S/35) are plotted versus station in Fig. 15. The values of NTA = 2370 ± 5 µmol/kg and NTCO₂ = 2230 ± 9 µmol/kg for the surface waters do not show large variations. They seem, however, to be higher on the first north-to-south section at 63-65°S latitude and 52-54°E longitude (stations 38-41). The salinity range is very narrow in the waters of Antarctica (33.8-34.3), compared with, for example, the values of North Atlantic waters (34.8-37.5). This narrow range is partly responsible for the uniform values of TALK and TCO₂ for the surface waters.

All the measurements for TALK, total carbon dioxide, and pH vs. depth are presented in Figs. 16, 17, and 18. The CO₂ properties of the deep waters were quite uniform (TALK =2355 ±10 µmol/kg, TCO₂ = 2240 ±10 µmol/kg and pH = 7.60 ± 0.02). Lower values of pH were obtained at 62-63°S latitude and between the longitudes 68-70°E, 85-93°E, and 104-118°E (the same stations that have slightly higher TCO₂). The TALK data are combined with the partial pressure of carbon dioxide (pCO₂) and total carbon dioxide (TCO₂) measurements, which were performed by Dr. Taro Takahashis research group from Lamont-Doherty Earth Observatory, Columbia University, to fully characterize the CO₂ system in these waters in future work.

 

Table 4. Summary of the CRM Measurements.

 

	Salinity	TALKa	TCO2b	pHc	N
Laboratory measurements of CRM Batch 31	32.899	2128±0.9	1828±0.9	7.984±0.001	6
Measurements of CRM Batch 31 at sea
Cell 18		2130±2.3	1887±3.0	7.968±0.005	27
Cell 19		2129±3.5	1887±2.2	7.965±0.004	20

^aFrom weight titrations;

^bStandard values assigned to the batch;

^cFrom spectrophotometric measurements.