Carbon dioxide ocean-atmosphere exchange

The oceans mediate some important carbon fluxes. The exchange of carbon dioxide between ocean and atmosphere has been studied extensively, since the prevailing view is that fossil fuel derived CO2 not remaining in the atmosphere has entered the oceans. To appraise the ocean-atmosphere exchange, we make use of the radiocarbon distribution in the oceans. All C is produced in the atmosphere hence all radiocarbon in the oceans must have entered through the air-sea interface. Under a steady-state assumption, the net influx of C must be balanced by the total decay within the oceans. Using our knowledge of the C distribution in the oceans (Fig. 

Carbon dioxide is constantly exchanged between the ocean and atmosphere. Each year the ocean and atmosphere exchange about 350 Gt CO2, with a net ocean uptake currently of about 8 Gt CO2. Because of this exchange, questions arise as to how effective ocean sequestration will be at keeping the CO2 out of the atmosphere. Specifically, is the sequestration permanent, and if not, how fast does the CO2 leak back to the atmosphere. Because there has been no long-term CO2 direct-injection experiment in the ocean, the long-term effectiveness of direct CO2 injection must be predicted based on observations of other oceanic tracers (e.g., radiocarbon) and on computer models of ocean circulation and chemistry. 

An important example of non-linearity in a biogeochemical cycle is the exchange of carbon dioxide between the ocean surface water and the atmosphere and between the atmosphere and the terrestrial system. To illustrate some effects of these non-linearities, let us consider the simplified model of the carbon cycle shown in Fig. 4-12. Ms represents the sum of all forms of dissolved carbon (CO2, H2CO3, HCOi" and... 

Keeling, C. D. (1973a). The carbon dioxide cycle. Reservoir models to depict the exchange of atmospheric carbon dioxide with the oceans and land plants. In "Chemistry of the Lower Atmosphere" (S. Rasool, ed.), pp. 251-329. Plenum Press, New York. 

Revelle, R. and Suess, H. E. (1957). Carbon dioxide exchange between atmosphere and ocean, and the question of an increase of atmospheric CO2 during the past decades, Tellus 9,18-27. 

As a simple example of a global geochemical simulation, consider the exchange of carbon dioxide between the ocean and the atmosphere. The... 

Fig. 2-1. The exchange of carbon dioxide between the atmosphere and an infinite oceanic reservoir.

Carbon dioxide plays a key role in climate, in biological processes, in weathering reactions, and in marine chemistry. I shall next describe how the partial pressure of this gas in the atmosphere may be calculated. Because there is a rapid exchange of carbon dioxide between ocean and atmosphere, we must consider the fate of dissolved carbon. 

The radiocarbon ratio also evolves very rapidly from its initial value of -50 to an average value of about -8 per mil. This evolution is not a consequence of evaporative concentration but, instead, of an approach to equilibrium with atmospheric carbon dioxide. Average surface seawater contains significantly less radiocarbon than does the atmosphere because its isotopic composition is affected by exchange with the deep ocean as... 

The balance between calcium carbonate production and dissolution is the major pH buffering mechanism of seawater over periods of time at least on the order of thousands of years ( ). The atmospheric carbon dioxide reservoir is less than 2 percent the size of the seawater reservoir ( ) and there is active exchange between these two reservoirs across the air-water interface. Consequently, the carbon dioxide content of the atmosphere and accumulation of calcium carbonate in the deep oceans are closely coupled. 

Etcheto J. and Merlivat L. (1988) Satellite determination of the carbon-dioxide exchange coefficient at the ocean-atmosphere interface—a 1st step. J. Geophys. Res. 93, 15669-15678. 

In the atmosphere CO2 is affected by processes that operate at different time scales, including interaction with the silicate cycle (see Chapter 2), dissolution in the oceans, and annual cycles of photosynthesis and respiration (see also Section 3). The relative effect of these processes is described below in the consideration of the whole carbon biogeochemical cycle and environmental aspects of biogeochemistry. Here, it is important to note that carbon dioxide is not reactive with other atmospheric species its MRT is 3 years (Figure 4). This value is largely determined by exchange with seawater (see Section 2). 

The key point for an Earth Systems approach to the Earth is that models of geochemical cycles provide the means whereby exchange processes can be quantified. This quantification is element-specific and is a function of the timescale under consideration. It provides the basis for calculating the steady state abundances of particular substances such as oxygen or carbon dioxide in the atmosphere, or sulfate in the ocean. Since these abundances are related to fluxes which relate to biogeo-chemical and physical processes, we can gain an understanding of the controls on key variables in the Earth system. 

The flow of carbon in the marine part of the biochemical subcycle involves buffering by the large reservoir of dissolved C (39 T t), most of which is in the form of dissolved inorganic carbon (DIC). Whereas terrestrial plants take up carbon dioxide directly from the atmosphere in gaseous form, aquatic plants utilize the C02 dissolved in water, and it is for this reason that the photosynthesis and respiration flux arrows in Fig. 6.1 do not point directly to marine biota. A dynamic equilibrium exists whereby C02 molecules are constantly exchanging between the atmosphere and oceans (where it... 

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