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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