Carbon reservoirs, residence times

The global rate of carbon burial is 50 x 10 mol per year, four parts carbonate and one part organic carbon. The residence time of the combined surface carbon and carbonate reservoirs is 67,000 years. During this time, a carbon atom is in CO2 for 66,000 years and in the biomass for 1000 years. It gets oxidized and reduced 165 times. This example indicates that organic carbon burial cannot be... 

Fluxes are linear functions of reservoir contents. Reservoir size and the residence time of the carbon in the reservoir are the parameters used in the functions. Between the ocean and the atmosphere and within the ocean, fluxes rates are calculated theoretically using size of the reservoir, surface area of contact between reservoirs, concentration of CO2, partial pressures of CO2, temperature, and solubility as factors. The flux of carbon into the vegetation reservoir is a function of the size of the carbon pool and a fertilization effect of increased CO2 concentration in the atmosphere. Flux from vegetation into the atmosphere is a function of respiration rates estimated by Whittaker and Likens (79) and the decomposition of short-lived organic matter which was assumed to be half of the gross assimilation or equal to the amount transferred to dead organic matter. Carbon in organic matter that decomposes slowly is transferred... 

The dynamics of these models depend strictly on carbon fluxes, but the fluxes are poorly measured or are calculated from carbon reservoir size and assumptions about the residence time of the carbon in the reservoir. In addition, model fluxes are linear functions while in reality few, if any, probably are linear. 

Each part of the carbon cycle acts as a reservoir for carbon atoms a place where carbon enters, resides for some time, and then leaves. Each reservoir has its own characteristics. The amount of carbon present, the length of time carbon remains, the way carbon enters and exits, and the reactions and roles of carhon atoms vary for each reservoir. 

From Figure 9.1, it can be seen that the major form of carbon in the atmosphere is C02(g), constituting over 99% of atmospheric carbon. Carbon dioxide makes up 0.035% by volume of atmospheric gases, or 350 ixatm = 350 ppmv. The atmosphere has a mass of CO2 that is only 2% of the mass of total inorganic carbon in the ocean, and both of these carbon masses are small compared to the mass of carbon tied up in sediments and sedimentary rocks. Therefore, small changes in carbon masses in the ocean and sediment reservoirs can substantially alter the CO2 concentration of the atmosphere. Furthermore, there is presently 3 to 4 times more carbon stored on land in living plants and humus than resides in the atmosphere. A decrease in the size of the terrestrial organic carbon reservoir of only 0.1% y-1 would be equivalent to an increase in the annual respiration and decay carbon flux to the atmosphere of nearly 4%. If this carbon were stored in the atmosphere, atmospheric CO2 would increase by 0.4%, or about 1 ppmv y-l. The... 

To the extreme right of Figure 9.1 is the major gaseous reservoir of carbon in the atmosphere—CO2. The residence time (x) of about 6 years means that the gas should be reasonably well-mixed in the atmosphere, but there is a small gradient in mean concentration from the northern hemisphere to the southern. The residence time is calculated with respect to total organic productivity (although the system is not at steady state and X changes with time) ... 

The initial two moles of organic carbon are both transformed into inorganic carbon. One mole is precipitated as a mineral, with carbon being trapped inside the calcium carbonate crystals, i.e. stored on a long-term time scale. The other mole can be released into the atmosphere and reused for phototrophy. Therefore, the oxalate-carbonate pathway constitutes a true carbon sink because one out of two moles of organic carbon is stored in a mineral state with a long residence time, whereas the other one returns to the atmospheric reservoir. 

However, the fraction of the CH4 isotope in the methane produced does not follow that of the CO but increases at a much slower rate, as shown in Fig. 10 for the same conditions as in Fig. 1. There is a reservoir of adsorbed intermediate (CH ) through which the C passes irreversibly, and the residence time is = NcuJRcu or cHyTORcH4 (r = 0/r in simplified notation). The intermediate is hydrogenated via surface H to CH4, which does not accumulate on the surface. The process through which the carbon passes can be shown by... 

The evolution of the isotopic compositions of carbon-bearing substances in uncontaminated systems where carbon is derived from carbonate minerals and soil CO2 is bounded between two limiting cases (i) open systems, where carbonate reacts with water in contact with a gas phase having a constant Pco, and (ii) closed systems, where the water is isolated from the CO2 reservoir before carbonate dissolution (Deines et al., 1974 Clark and Fritz, 1997). Both of the extremes assume water residence times long enough for significant isotope exchange between the gas and the aqueous phase to take place. 

The atmosphere is an important conveyor belt for many pollutants. The atmosphere reacts most sensitively to anthropogenic disturbance because proportionally it represents a much smaller reservoir than land and water furthermore, the residence times of many constituents of the atmosphere are smaller than those that occur in the other exchange reservoirs. Water and atmosphere are interdependent systems. Many pollutants, especially precursors of acids and photooxidants, originate directiy or indirecdy from the combustion of fossil fuels. Hydrocarbons, carbon monoxide, and nitrogen oxides released by thermal power plants and, above all by automobile engines, can produce, under the influence of sunlight, ozone and other photooxidants. 

Example 4.13. Carbon-14 as a Tracer for Oceanic Mixing In a simplified two-box model of the ocean, the warm waters and the cold waters may tie subdivided into two well-mixed reservoirs—an upper one a few hundred meters in depth and a lower one of 3200 m depth. The Cj content of the upper and lower reservoirs (corresponding to the Pacific) are, respectively, 1.98 x 10 mol liter and 2.44 x 10 mol liter, whereas the C/C ratios for uppsr and lower reservoirs are, respectively, 0.92 x 10 and 0.77 x 10 mol/ mol. Estimate from this information the rate of vertical mixing and the residence time of the water in the deep sea (Broecker, 1974). 

Figure 15.18. Comparison of global reservoirs and their residence times (t in years) (Example 15.3). The reservoirs of the atmosphere, of surface fresh waters, and of living biomass are significantly smaller than the reservoirs of sediment and marine waters and are thus more susceptible to distuibance. For example, the combustion of fossil fuel (from the reservoir of organic carbon in sediments) will have an impact on the smaller reservoirs CO2 in the atmosphere will be markedly enlarged. This combustion also fixes some N2 to NO and NO2 sulfur, associated with the organic carbon, introduces CO2 into the atmosphere. These nitrogen and sulfur compounds are washed out relatively rapidly into soil and aquatic ecosystems. The total groundwater reservoir may be twice that of surface fresh water but, however, is less accessible. (From Stumm, 1986.)...

Figure 15.19 gives a description of the global carbon cycle. The inventories of the various reservoirs were already given in Table 4.1, where we noticed that the atmosphere is a relatively small reservoir with large fluxes, so that the residence time of C in the atmosphere is only a few years. The carbon system is not at steady state. Because of fossil fuel combustion and possibly also because of deforestation, the inventories of C in the atmosphere and hydrosphere are increasing. The flux related to fossil fiiel combustion is nearly 1 % of the total atmospheric CO2 reservoir. The flux due to land use is more con-troversal but is probably 1-2 X 10 mol C . As the summary at the bottom... 

The true reservoir size for carbon dioxide, however, is larger than its concentration, [CO2], because of the carbonate system reactions. The ratio of the reservoir of DIG that exchanges carbon to the CO2 gas reservoir in the mixed layer is ADIC / A[C02]. Thus, the residence time for carbon in the surface ocean with respect to gas exchange from Eq. (11.23) is ... 

The corresponding example of reservoirs is depicted in Table 1. This Table shows current estimates of the most important carbon reservoirs and averaged residence times for carbon in these pools. The lithosphere is the reservoir for the bulk of global carbon, with about 20% of this being in the form of fossil organic fuels. 

From Fig. 6.1 it can be seen that the annual net primary production for land plants and marine plants is similar (c.60 and 40Gt, respectively), although the biomass of terrestrial plants is much greater than that of marine plants. This is an important demonstration of the fact that biomass is not necessarily a guide to productivity. There is another difference between the marine and terrestrial parts of the biochemical subcycle the residence time of C in the main reservoirs. From Fig. 6.1 it can be seen that the residence time of carbon in the terrestrial biota is c.5.5 years (i.e.600/110yr), and c.26 years (1600/60.6 yr) in soil organic matter. In contrast, the residence time of C in marine phyto-planktonic biomass is only c.2 weeks (1.5/40 yr), but c.338 years (39 000/115.3 yr) in oceanic dissolved carbon. 

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