Background Information

A dramatic increase in the atmospheric concentration of carbon dioxide (CO2) has been directly observed over the past 30 years (Keeling et al. 1989). Although the present growth rate of CO2 in the atmosphere is about 3.4 gigatons (1015 g) of carbon per year (GtC/yr), the present estimate of anthropogenic CO2 sources to the atmosphere is much larger [approximately 6 GtC/yr (IPCC 1990)]. Most of the CO2 (77%) released to the atmosphere originates from the combustion of fossil fuels (Marland and Rotty 1984). The remainder is estimated to originate from deforestation and changing land use patterns (IPCC 1990). Since the growth rate of CO2 in the atmosphere is less than the rate of carbon release, some of the anthropogenic carbon released to the atmosphere must be absorbed by either the terrestrial biosphere or the oceans.

The oceans have been suggested as a repository for carbon dioxide from the atmosphere through a chemical and biological pathway. Carbon dioxide dissolves readily in the surface oceans. The dissolved gas reacts with water to form carbonic acid which rapidly dissociates to bicarbonate and carbonate ions. Ocean plants can also draw down CO2 , some of which is converted to particulate matter which sinks out of surface waters.

The uncertain role of the ocean in absorbing anthropogenic carbon from the atmosphere has stimulated interest in the cycling and fate of carbon in the sea. The Pacific Marine Environmental Laboratory (PMEL) made measurements on 5 cruises (12 legs) in the Pacific and Indian Oceans (Fig. 1) to determine the concentrations of trace gases in the atmosphere and in surface seawater. These cruises were conducted from 1986-1989 aboard U.S. National Oceanographic and Atmospheric Administration (NOAA) Research Vessels (R/V) Oceanographer and Discoverer and the Russia Far East Hydrometeorological Institute R/V Akademik Korolev. Table 1 presents the summary of the cruises.

Table 1. Summary of PMEL Cruises 1986 to 1989. The distance given is the distance over which data were collected along the cruise track.

Year Cruise Section Research Vessel Ports Dates Distance (km) Number of Samples
1986 EPOCS Oceanographer Balboa - Honolulu 21 May-20 June 12,172 637
1986 RITS/CO2 Oceanographer Honolulu-Kodiak 1 July-23 July 7,709 461
1987 SAGA II, Transit Akademik Korolev Hilo-Kuril Trench 1 May-8 May 4,503 218
1987 SAGA II, Leg 1 Akademik Korolev Kuril Trench-Wellington 8 May-9 June 11,441 633
1987 SAGA II, Leg 2 Akademik Korolev Wellington-Singapore 12 June-6 July 11,912 501
1987 TEW-3 Oceanographer Townsville-Kwajalein 13 July-27 July 3,491 297
1987 RITS/CO2 Oceanographer Kwajalein-Seattle 29 July-28 August 10,520 669
1988 RITS/CO2 Oceanographer Dutch Harbor-Am. Samoa 6 April-5 May 8,648 668
1988 EPOCS Oceanographer Am. Samoa-Honolulu 9 May-4 June 8,492 547
1989 RITS/CO2, Leg 1 Discoverer Seattle-Easter Island 5 February-1 March 7,097 398
1989 RITS/CO2, Leg 2 Discoverer Easter Island-Papeete 4 March-2 April 11,365 686
1989 RITS/CO2, Leg 3 Discoverer Papeete-Seattle 7 April-20 April 7,773 298
Total 105,123 6,013

The purpose of this report is to present the CO2 results and to show the calculations which have been used to convert the raw values to fugacities in units of microatmospheres (µatm).


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