The history of large-scale CO observations in the ocean date back to the 1970s and 1980s. Measurements of the partial pressure of CO (pCO), total dissolved inorganic carbon (DIC) and total alkalinity (A) were made during the global Geochemical Ocean Sections (GEOSECS) expeditions between 1972 and 1978, the Transient Tracers in the Oceans (TTO) North Atlantic and Tropical Atlantic Surveys in 198183, the South Atlantic Ventilation Experiment (SAVE) from 19881989, the French Southwest Indian Ocean experiment, and numerous other smaller expeditions in the Pacific and Indian Oceans in the 1980s. These studies provided marine chemists with their first view of the carbon system in the global ocean.
These data were collected at a time when no common reference materials or standards were available. As a result, analytical differences between measurement groups were as large as 29 µmol kg for both DIC and A, which corresponds to more than 1% of the ambient values. Large adjustments had to be made for each of the data sets based on deepwater comparisons at nearby stations before individual cruise data could be compared. These differences were often nearly as large as the anthropogenic CO signal that investigators were trying to determine (Gruber et al., 1996). Nevertheless, these early data sets made up a component of the surface ocean pCO measurements for a global climatology and also provided researchers with new insights into the distribution of anthropogenic CO in the ocean, particularly in the Atlantic Ocean.
At the onset of the Global Survey of CO in the Ocean (Figure 2), several events took place in the United States and in international CO measurement communities that significantly improved the overall precision and accuracy of the large-scale measurements. In the United States, the CO measurement program was co-funded by the Department of Energy (DOE), the National Oceanic and Atmospheric Administration (NOAA) and the National Science Foundation (NSF) under the technical guidance of the U.S. CO Survey Science Team. This group of academic and government scientists adopted and perfected the recently developed coulometric titration method for DIC determination that had demonstrated the capability to meet the required goals for precision and accuracy. They advocated the development and distribution of certified reference materials (CRMs) for DIC, and later for A, for international distribution under the direction of Andrew Dickson of Scripps Institution of Oceanography (see sidebar). They also supported a shore-based intercomparison experiment under the direction of Charles Keeling, also of Scripps. Through international efforts, the development of protocols for CO analyses were adopted for the CO survey. The international partnerships fostered by JGOFS resulted in several intercomparison CO exercises hosted by France, Japan, Germany and the United States. Through these and other international collaborative programs, the measurement quality of the CO survey data was well within the measurement goals of ±3 µmol kg and ±5 µmol kg, respectively, for DIC and A.
Figure 2. The Global Survey of CO in the Ocean: cruise tracks and stations occupied between 1991 and 1998.
Several other developments significantly enhanced the quality of the CO data sets during this period. New methods were developed for automated underway and discrete pCO measurements. An extremely precise method for pH measurements based on spectrophotometry was also developed by Robert Byrne and his colleagues at the University of South Florida. These improvements ensured that the internal consistency of the carbonate system in seawater could be tested in the field whenever more than two components of the carbonate system were measured at the same location and time. This allowed several investigators to test the overall quality of the global CO data set based upon CO system thermodynamics. Laboratories all around the world contributed to a very large and internally consistent global ocean CO data set determined at roughly 100,000 sample locations in the Atlantic, Pacific, Indian and Southern oceans (Figure 2). The data from the CO survey are available through the Carbon Dioxide Information and Analysis Center (CDIAC) at Oak Ridge National Laboratory as Numeric Data Packages and on the World Wide Web (http://cdiac.esd.ornl.gov/home.html). Taro Takahashi and his collaborators have also amassed a large database of surface ocean pCO measurements, spanning more than 30 years, into a pCO climatology for the global ocean (Takahashi et al., 2002). These data have been used to determine the global and regional fluxes for CO in the ocean.
|Reference Materials For Oceanic CO2 Measurements|
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