Background Information

The cruise of research vessel (R/V) Knorr along the World Ocean Circulation Experiment (WOCE) designated sections A20 and A22 from Woods Hole, Massachusetts, to Port of Spain, Trinidad, and back to Woods Hole were the part of the Climate Variability (CLIVAR) Carbon Dioxide and Repeat Hydrography Section Project, sponsored by the National Oceanic and Atmospheric Administration (NOAA) and National Science Foundation (NSF). The goal of the Repeat Hydrogaphy Project is to measure decadal changes in circulation, heat and fresh water budgets, and carbon inventory in the ocean. The cruises repeat a subset of the WOCE Hydrographic Program (WHP) and Joint Global Ocean Flux Study (JGOFS) lines occupied in each major ocean basin in the 1990s.

The WOCE/WHP program is driven by the need to monitor the increases in carbon dioxide (CO₂) in the ocean and provide the necessary data to support continuing model development that will lead to improved forecasting skills for oceans and global climate. During the 1990s, the WOCE/JGOFS provided a full-depth baseline data set against which to measure future changes. By integrating the scientific needs of programs requiring measurement of the full water column, major synergies and cost savings are achieved. These measurements are of importance both for major research programs, such as CLIVAR and the U.S. Global Climate Research Project Ocean Carbon and Climate Change Program, and for operational activities such as the Global Ocean Observation System and the Global Climate Observing System. As outlined in the program documentation, one component of a global observing system for the physical climate/CO₂ system should include periodic observations of hydrographic variables, CO₂ system parameters, and other tracers. The large-scale observation component of the Ocean Carbon and Climate Change Program needs systematic observations of the invasion of anthropogenic carbon in the ocean that is superimposed on a variable natural background. The five topical areas addressed by the CO₂/CLIVAR repeat hydrography program are:

• carbon system studies;
• heat and freshwater storage and flux studies;
• deep and shallow water mass and ventilation studies;
• calibration of autonomous sensors; and
• data for model calibration.

R/V Knorr cruise 173 was conceived to reoccupy two meridional hydrographic sections in the western North Atlantic as part of the CLIVAR/Global Carbon Program of repeat hydrography. Section A20, which lies nominally along 52° 20' W, was sampled during leg 1 of the cruise. The return leg to Woods Hole reoccupied section A22 along 66° W ( Fig. 1). Meridional hydrographic sections near 52° W had been made on three occasions prior to this cruise: in the 1950s, 1980s, and in 1997. The sampling plan for the 2003 occupation of both sections was simply to make a full-depth hydrographic station at each site sampled in 1997 (see NDP-082). The extremely tight station spacing at the northern end of the sections done in 1997 was relaxed slightly in 2003.

This data report focuses on the measurements of total carbon dioxide (TCO₂), total alkalinity (TALK), dissolved organic carbon (DOC), chlorofluorocarbons (CFC-11 and CFC-12), carbon-14 (¹⁴C), nitrate (NO₃), nitrite (NO₂), phosphate (PO₄), silicate (SiO₄), salinity, and dissolved oxygen (O₂).

The methodology, instrumentation, and standardization of these parameters have improved significantly during the WOCE/JGOFS era. Notable developments include the release of manuals detailing the analytical methods and operating protocols (DOE 1994, PICES 2007). Certified reference materials (CRMs) are now available for TCO₂ and TALK, analyses for which are run interspersed with samples to determine calibration offsets. For this cruise, the TALK and TCO₂ values were adjusted to account for the small difference between the CRMs run at sea and the certified value determined at Scripps Institution of Oceanography (SIO). The TCO₂ coulometers were calibrated daily by injecting aliquots of pure CO₂ (99.995%). The stability of each coulometer cell solution was confirmed with the analyses of several CRMs each day.

Instrumentation has improved in the last decade. Alkalinity measurements can be done with better precision through automation and close checks of the response of electrodes. Burettes are independently calibrated, and the preparation of titrant (hydrochloric acid) has undergone improved quality control and standardization (Millero et al. 1998). The TCO₂ measurements are done by coulometry, a precise integrative method. During the cruise two single-operator multiparameter metabolic analyzers (SOMMAs) (Johnson et al. 1999) PMEL-1 and PMEL-2 were used for analyses, which facilitated a sample throughput of up to 80 per day. Oxygen measurements were performed by Winkler titrations (Carpenter 1965) with photometric endpoint detection (Friederich, Sherman, and Codispoti 1984).

The data underwent careful quality assurance and quality control both during and after the cruise. The precision of the measurements was determined from duplicate sampling and comparison of data from deep water, where little variability is expected. Outliers in the data were flagged based on several methods using prior knowledge of the trends and known relationships between parameters. Depth profiles for each parameter were scrutinized for outliers. When deviations were observed, other parameters were assessed to determine whether they showed deviations as well.

This report describes procedures and methods for hydrographic measurement and the analytical procedures, calculations, and assessment of precision for nutrient, oxygen, TCO2, TALK, CFCs, ¹⁴C, and DOC measurements.