Carbon reservoir masses

Table 11-1. Geochemical Carbon Reservoirs Mass Contents in PgC (1012 kg)...

In past times the total mass of stored organic carbon may have been larger or smaller than it is now, depending on the past climate. Let us define as the norm the amount of carbon presently stored, M, and define a time dependent factor, M/M, by which the organic carbon reservoir may be increased (M/M > 1) as in a lush, tropical coal age, or decreased (M/M < 1) as in an ice age, where M is the mass of organic carbon at the time in the past when the material was alive. We assume that the total amount of atmospheric carbon dioxide has always remained the same (which may or may not be true). 

Variations in content of some important carbon compounds are schematically demonstrated in Fig. 2.11 The two most important carbon reservoirs on Earth, marine carbonates and the biogenic organic matter, are characterized by very different isotopic compositions the carbonates being isotopically heavy with a mean 5 C-valne around 0%c and organic matter being isotopically light with a mean 5 C-value aronnd -25%c. For these two sedimentary carbon reservoirs an isotope mass balance mnst exist such that ... 

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... 

Figure 10.31. Schematic diagram of a three-box (reservoir) model of a closed-system geochemical cycle of a substance (e.g., carbon). The reservoir masses are designated Mi, M2, and M3, and the rates of transfer (fluxes) of a substance between boxes are shown as Fy, where i and j = 1,2,3, but i j. The mass balances for the three reservoirs are given by the three differential equations, kjj are first-order rate constants (units of 1/T) and T is time.

Figure 10.39. Perturbation of the CO2-O2 geochemical cycle illustrated in Figure 10.38. In this case, the perturbation was a doubling of the carbon flux related to marine productivity. Changes in reservoir masses and the carbon flux to the seafloor are shown as a function of time. (After Garrels et al., 1976.).

Figure 10.42. A quantitative box model of the carbonate-silicate geochemical cycle. Reservoir masses are in units of 1018 moles, and fluxes in units of 1018 moles per million years. Comparison with Figure 10.32 gives some idea how flux values and portrayal of the cycle have changed during the last decade and a half. (After Lasaga et aJ., 1985.)...

FIGURE 5.2 Schematic illustration of the modern "deep Earth" carbon cycle showing the main Earth carbon reservoirs and the pathways between them. The important fluxes are listed in Table 5.3. The mass of carbon in each reservoir is given in giga-tonnes of carbon (1 Gt = 1015g). Figure adapted from Killops and Killops (2005). 

FIGURE 22.6 Six-compartment model of (he global carbon cycle (Schmitz 2002). Values shown for reservoir masses (Mh in Pg C) and fluxes (F,y, in Pg C yr l) represent those for the preindustrial steady state. 

A long-term correlation of sulfur and carbon ratios is implied if the system remains closed. From Reaction (2), the fraction of carbonate in the carbon reservoirs is low when the fraction of sulfur in the sulfate reservoir is high. The mass... 

The quantity of dissolved organic carbon in the oceans has been estimated to be about 1018 g and constitutes one of the major reservoirs of organic carbon. Although large in total mass, the concentration of organic carbon in seawater is low (typically 0.5 -1.5 mg C per litre). 

In turn, the concentration of C02 in the atmosphere depends on the mass of the biosphere and its rate of decay after death, and on the carbonic-anhydrase concentrations in the sea surface. In future predictions of the rate of increase of C02 partial pressure in the atmosphere due to burning fossil fuels, it will be important to include the interaction of the atmospheric C02 with the bio-organic reservoir and the catalyzation of its absorption into the sea by means of the action of carbonic-anhydrase dissolved in sea water, considerations which have not been taken into account in past computations. 

Then the atmospheric reservoir is enriched by about 2.7 percent multiplied by the ratio of the mass of stored organic carbon to its mass today. That is the enrichment E is given by,... 

It is often difficult to define precisely the elemental flux from a system to another as a product of a mass flux of a carrier multiplied by a concentration in this carrier. For instance, the flux of carbon from the biosphere to the atmosphere is not adequately represented by a carrier flux since carbon dioxide escapes directly to the air. We therefore have to resort to a direct formulation in terms of total quantities (amounts, e.g., in tons, kilograms or moles) and fluxes. Denoting Mt the total quantity of the species under consideration in the reservoir i and the flux of the same species from reservoir i to reservoir j, we note that... 

Figure 5. A plot of A Mg vs. 5 Mg for terrestrial Mg materials. Within best estimates of uncertainties (cross) all of the data lie in the region bounded by equilibrium and kinetic mass fractionation laws. Waters, carbonates, and organic Mg (chlorophyll) have higher A Mg values than mantle and crustal Mg reservoirs represented by mantle pyroxene, loess, and continental basalts. The difference in A Mg values is attributable to episodes of kinetic mass fractionation.

The high precision with which Mg isotope ratios can be measured using MC-ICPMS opens up new opportunities for using Mg as a tracer in both terrestrial and extraterrestrial materials. A key advance is the ability to resolve kinetic from equilibrium mass-dependent fractionation processes. From these new data it appears that Mg in waters is related to mantle and crustal reservoirs of Mg by kinetic fractionation while Mg in carbonates is related in turn to the waters by equilibrium processes. Resolution of different fractionation laws is only possible for measurements of Mg in solution at present laser ablation combined with MC-ICPMS (LA-MC-ICPMS) is not yet sufficiently precise to measure different fractionation laws. 

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