Organic carbon reservoir

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). 

Interactions between the Carbonate-Carbon Reservoir and Organic Carbon 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... 

Table I - Summary results of Non-living Organic Carbon Reservoirs and Rates of Change (excluding crops and major forest areas).

DesMarais D. J. (1994) Tectonic control of the crustal organic-carbon reservoir during the Precambrian. Chem. Geol. 114(3-4), 303-314. 

Figure 2. The marine carbon cycle. Reservoir sizes, flux estimates, and isotopic values are presumed to be representative of the Phanerozoic. The short-term carbon cycle is enclosed within the dashed box. The dashed line from the short-term carbon cycle to the organic reservoir represents the slow leak of biosphere organic carbon that supplies the long-term organic carbon reservoir. Reservoir sizes in Gmoles fluxes in Gmole/ky.

Figure 1 Major global reservoirs Involved in active production, exchange and cycling of organic carbon. Reservoir sizes are shown in Gt carbon (1 GtC = 10 g C). Numbers in parentheses are based on 1980s values numbers without parentheses are estimates of the pre-anthropogenic values. Fluxes primarily mediated by biological reactions are shown with dashed arrows physical transport processes are shown with solid arrows. (Modified after Siegenthaler and Sarmiento (1993) and Hedges and Oades (1997).)...

The late Cenozoic was therefore a time of unusually high organic carbon deposition rates, leading to an increase in the size of the sedimentary organic carbon reservoir. The organic subcycle thus acted as a carbon sink over the course of the Himalayan uplift. There are two possible causes of this evolution. 

The size of the crustal reservoirs allows an imbalance of this magnitude to persist on a 100 m.y. time scale with limited observable effect. For example, it would take 1260-2670 m.y. for this oxygen loss to produce the buried organic carbon reservoir. Given that my estimates have some slop and that subduction of organic carbon involves the vagaries of tectonics and biology, 1 am not particularly concerned about an instantaneous modem imbalance. 

The dissolved organic carbon reservoir in natural waters represents a very significant component of the carbon cycle but reliable quantitative determination of this parameter has proved to be difficult particularly in seawater. Commercially available systems are better suited to freshwaters though obtaining accurate results is still not a trivial exercise and great care must be taken to ensure that blanks are satisfactorily and consistently low and that precision remains consistent. 

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