Andrew G. Dickson
Scripps Institution of Oceanography
University of California, San Diego,
La Jolla California USA
High-quality measurements of carbon dioxide (CO) in the ocean have been an integral part of JGOFS. Despite their importance for understanding the oceanic carbon cycle, measurements made by different groups were rarely comparable in the past. A significant contribution of U.S. JGOFS has been to produce and distribute reference materials for oceanic CO measurements. These materials are stable substances for which one or more properties are established sufficiently well to calibrate a chemical analyzer or to validate a measurement process.
Our laboratory at Scripps Institution of Oceanography (SIO), established in 1989 with U.S. JGOFS support from the National Science Foundation (NSF), has prepared over 50 separate batches of reference material and has distributed more than 25,000 bottles of this material to scientists in 33 laboratories in the U.S. and 58 facilities in 24 other countries. The reference materials have been used both as a basis for collaborative studies and as a means of quality control for at-sea measurements. Although most JGOFS field studies are over, we are still distributing more than 2,000 bottles per year and demand is again growing.
To prepare the reference material, we sterilize a batch of seawater, equilibrate it to a virtually constant partial pressure of CO and deliver it for bottling. For each batch, surface seawater collected on ships of opportunity and stored in our laboratory is pumped into a holding tank using filters and a sterilizing unit to reduce contamination. When the holding tank is nearly full, mercuric chloride is added as a biocide. The seawater is then recirculated for a few days to ensure complete mixing and enable some gas exchange with filtered air that is pumped through the head-space of the tank. Finally, aliquots of the seawater are pumped through an ultraviolet sterilizing unit and a 0.1 µm filter and into clean 500-mL glass bottles. These are sealed with grease and labeled.
Figure. A 500 ml glass bottle containing certified reference material for oceanic CO measurements.
Random samples from each batch of reference material are analyzed over a period 23 months for both total dissolved inorganic carbon (C) and total alkalinity (A), and the results are used to certify the batch before it is distributed. From the start, we used a high-quality method for the determination of C in which a weighed amount of seawater is acidified and the CO extracted under vacuum, purified and determined manometrically. These analyses are carried out in the laboratory of C.D. Keeling at SIO using equipment originally developed for the calibration of gases for atmospheric CO measurements.
By 1996, we had also developed an accurate method for the measurement of A using a two-stage potentiometric, open-cell titration with coulometrically analyzed hydrochloric acid. Once this latter method was being employed routinely to certify new batches of reference material, we used it retrospectively to analyze archived samples from earlier batches.
The uncertainties of these analyses used for certification are ±1.5 µmol kg in C and ±2 mol µkg in A, and our reference materials have been shown to be stable for more than 3 years. We are now working on providing pH values on future reference materials, as well as values for C.
As part of U.S. JGOFS, the Department of Energy (DOE) supported measurements of ocean CO system parameters on sections of the WOCE Hydrographic Programme one-time survey. The CO Survey Science Team adopted the use of our reference materials as soon as they were available in early 1991 and continued to use them on subsequent cruises. Measurements made on reference materials while at sea were used to ascertain data quality on these expeditions and are thought to have contributed substantially to the overall high quality of the resultant data set.
A further indication that the use of reference materials has improved oceanographic data quality can be seen by examining the degree of agreement between measurements for deep water masses obtained where two separate cruises intersect. For cruises where reference materials were available, measurements of C in deep water now typically agree to within 2 µmol kg. This is in sharp contrast to the problems encountered over the years with earlier ocean carbon data sets, where adjustments of as much as 1520 µmol kg were fairly common. The high-quality data sets now available provide a resource for synthesis and modeling that makes it possible to put together a coherent global view of the oceanic carbon cycle.
Figure. George Anderson filling bottles with sterilized seawater for use as a reference material.
This work was supported by NSF through grants OCE8800474, OCE9207265, OCE9521976 and OCE-9819007 and by DOE through Pacific Northwest National Laboratory subcontract No. 121945 and grant DEFG0392ER61410. This work was encouraged early on by Neil Andersen, then at NSF, and has benefited from advice from C.D. Keeling, the members of the DOE CO Survey Science Team and colleagues from the National Oceanic and Atmospheric Administration. I should also like to thank Justine Afghan and George Anderson, who carried out most of the technical work involved in this project, as well as Guy Emanuele and Peter Guenther from the Carbon Dioxide Research Group at SIO, who performed the C analyses.
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