The Ocean-Atmosphere CO 2 Cycle

The preceding discussion shows that there are two competing processes that affect the spatial distribution of CO2 in the ocean. The biological pump transports carbon fixed by

photosynthesis as organic matter, plus CaCO3 , from surface to deep waters where they undergo

re-dissolution. This sets a gradient in composition between the surface and deep layers. On the other hand, the ocean circulation system mixes surface and deep waters together on a time scale of 1000-2000 years, and serves to equalize any composition differences between the two layers created by the biological pump. We may think of the ocean circulation system as a physical pump that operates in both directions, unlike the one-way biological pump.

These features are easier to visualize in a box model such as that in Figure 8.10. This represents the atmosphere, ocean surface layer and deep layer, and the ocean sediments as boxes. The arrows connecting the boxes represent fluxes of carbon moving from one box to another. The fluxes shown in the figure are given in units of Gt of carbon per year. Superimposed on the Figure are estimates of the current rate of production of excess CO2 by the combustion of fossil

fuels. Surface Ocean Deep Ocean Sediments 18.9 16.8 2.16 0.05 2.11 Burial Volcanic gases 0.05 Atmosphere Biological Pump Physical Pump 5.5 2.4 Fossil Fuels (Gt C per year)

Figure 8.10 Idealized box model for the ocean-atmosphere carbon cycle. Fluxes are in Gt of C per year, where 1 Gt = 109 metric tonnes = 1012 kg.

Interpretation of the figure requires information about the amounts of carbon held in the different reservoirs. This is shown in Table 8.2 in units of Gt. To estimate the turnover time for a particular reservoir, divide the reservoir size in Gt from Table 8.2 by the correspnding flux value shown in the Figure. For example, CO2 in the atmosphere is increasing at a rate of 3.3 Gt/yr from

fossil fuel burning, and the pre-industrial atmosphere contained 600 Gt of CO2. Therefore the

turnover time for fossil fuel CO2 input is 600 / 3.3 = 180 years. The interpretation of this value is

that fossil fuel CO2 inputs will, in the absence of any other change, double the atmospheric CO2

concentration in a time scale of 180 years.

Table 8.2. Comparison of the amounts (in Gt of carbon) in different reservoirs of the global carbon cycle.

Carbon Reservoir Amount (Gt C) Recoverable fossil fuels 4,200 Soil organic matter 1200 Terrestrial biomass 1200 Ocean, inorganic carbon 40,000 Ocean, organic matter 4800 Ocean, living biomass 8.4 Atmosphere (1993) 820 Atmosphere (1750) 600 Atmospheric increase 220

Features of the Ocean-Atmosphere Cycle

Table 8.2 shows that oceanic carbon inventory is dominated by inorganic carbon (40,000 Gt). Of this quantity, about 2 % or 800 Gt is located in the surface layer. Therefore its size is comparable to the atmosphere, with which it maintains equilibrium through gas exchange of CO2.

Although not shown in Figure 8.10, this gas exchange flux is about 8-12 Gt/yr across the global ocean surface.

As indicated in the Table, the terrestrial and oceanic biomasses are very different in size. However, their importance in terms of carbon flux is about equal. This is because the average member of the oceanic biomass is a unicellular planktonic organisms having a very short lifetime (days). By contrast, the average terrestrial plant is a tree having a lifespan measured in years or decades. Although not shown in Figure 8.10, the gross rate of CO2 fixation by marine

escapes the surface layer as sinking biogenic debris. The remainder undergoes respiration in the surface layer where it was originally formed, and thus has no net effect on the carbon cycle.

The carbon cycle, as presented in Figure 8.10, is largely internal. The small loss of carbon from the deep ocean to marine sediments, which is in the form of both residual organic matter and CaCO3, is more or less balanced by the input of fresh CO2 by volcanic gases.

Carbon leaves the oceanic surface layer through both the physical pump (sinking of surface water in polar areas) and through the one-way biological pump. The flux figures show that about 10 % of the total carbon flux is accounted for by the biological pump. This is consistent with the fact that the total carbon inventory (CT) of the surface layer is about 10 % lower than the deep

ocean (Figure 8.3). Interestingly, for the limiting macro-nutrients nitrate and phosphate, transport out of the surface layer is almost completely accounted for by the biological pump. The turnover times for carbon removal from the surface layer are relatively short

yr 42 8 . 16 16 . 2 800 T = + = [8.14]

Carbon leaves the deep layer primarily through the physical pump, as the carbon-enriched deeper waters complete the oceanic circulation cycle and return to the surface. Much of this takes place in specific areas of deep water upwelling, especially on the western boundaries of the major continents. Because this water contains very high concentrations of the macro-nutrients, upwelling areas are generally extremely fertile. A good example is the coast of Peru, home to a (sometimes) massive anchovy fishing industry. This return of macro-nutrients to the surface layer is what supports their net productivity, i.e. the fixation of carbon that becomes lost in the form of sinking biological material. Oceanographers often refer to this as new production to distinguish it from the much larger total production that goes through the complete photosynthesis-respiration cycle without leaving surface waters. The latter, as already mentioned, has no net effect on the carbon cycle budget.

The turnover time for loss of carbon from the deep layer is yr 2200 05 . 0 9 . 18 200 , 41 T = + = [8.15]

This value is consistent with measurements of the 14C radiocarbon content of CO2 taken from

deep waters of the ocean, which reach as high as several thousand years in the North Pacific. It also provides a perspective on how long it will take for recently-released fossil fuel CO2 to reach

equilibrium with the deep ocean.

Of the 2.16 Gt/yr of carbon transported into deep waters as sinking biological material, 2.11 Gt/yr or 98 % undergoes re-dissolution and remains within the oceanic cycle. Only 2 % escapes to be buried on sediments. A small fraction of this carbon eventually becomes fossil fuel deposits, probably less than 1 % (0.0005 Gt/yr). This allows a crude comparison of the rate of formation of fossil fuels with the rate at which we are presently consuming them. The consumption rate is 3.3 + 2.4 = 5.7 Gt/yr, which is many orders of magnitude faster than the natural production rate.

Reference to Table 8.2 shows that the Earth system contains about 4,200 Gt of recoverable fossil fuels, of which perhaps 90 % still remain untouched. Indeed, at the present rate of consumption, these reserves would last

yr 660 4 . 2 3 . 3 90 . 0 200 , 4 T = + × = [8.16]

However, 660 years is not very long, even in the context of the brief span of human history. Obviously other energy sources must be found.

Figure 8.10 also shows that the fluxes of CO2 generated by fossil fuel burning (and other

human activities such as cement manufacture, gas flaring and deforestation) are of the same order of magnitude as the natural fluxes in the ocean atmosphere system. Of the 5.7 Gt/yr of carbon as CO2 released, only about a third (2.4 Gt/yr) manages to get into the ocean system. The rest

“backs up” in the atmosphere, causing increasing concentrations of CO2. The rate of uptake of

atmospheric CO2 by the ocean is simply not fast enough to cope with the onslaught produced by

the burning of carbon-containing fuels.

Since the surface layer reservoir occupies only a small fraction of the total oceanic carbon inventory, it is clear that the entry of 2.4 Gt/yr of additional carbon to the ocean system from fossil fuels is significant in relation to the natural internal fluxes. Obviously, penetration of recent fossil fuel CO2 into some of the deep parts of the ocean must be involved. Indeed, this can be

demonstrated by very careful detailed studies of the CT properties of deep waters. Increased CT

consistent with uptake of fossil fuel CO2 can be found in the deep waters at the start of the

circulation system having the youngest radiocarbon age. However, the measurement problems are severe, as the following argument shows. The total carbon released as fossil fuels since the mid- 1700’s is about 400 Gt, of which about half now remains in the atmosphere. Some of the ‘missing’ 200 Gt has probably ended up in increased terrestrial biomass, but it is likely that most of this 200 Gt has ended up somewhere in the deep ocean. However, although large, 200 Gt represents only about 0.5 % of the total inorganic carbon held in the ocean. Were it not for the fact that only the youngest deep waters have been affected by fossil fuel CO2, it would not be

technically possible to measure the increase!