Carbon cycle fluxes between reservoirs

Organic carbon and inorganic carbon are the two major forms present in soil-water-plant components of wetland ecosystem. Carbon cycle in this system can be depicted as a storage of carbon in major reservoirs, which serve as either a source or a sink, and flux between reservoirs (Figure 5.3). The reservoirs of carbon in a wetland can be grouped as follows ... 

The most common way in which the global carbon budget is calculated and analyzed is through simple diagrammatical or mathematical models. Diagrammatical models usually indicate sizes of reservoirs and fluxes (Figure 1). Most mathematical models use computers to simulate carbon flux between terrestrial ecosystems and the atmosphere, and between oceans and the atmosphere. Existing carbon cycle models are simple, in part, because few parameters can be estimated reliably. 

Although the largest reservoirs of carbon are found in the lithosphere, the fluxes between it and the atmosphere, hydrosphere, and biosphere are small. It follows that the turnover time of carbon in the lithosphere is many orders of magnitude longer than the turnover times in any of the other reservoirs. Many of the current modeling efforts studying the partitioning of fossil fuel carbon between different reservoirs only include the three "fast" spheres the lithosphere s role in the carbon cycle has received less attention. 

Just as important as evaluating the performance of the nuclear fuel cycle, one must also consider the size of the fluxes and reservoirs of the carbon cycle. Present CO2 emissions from fossil fuels and the production of cement are estimated to be 6.3 + 0.4 GtC/y emissions related to changes in land use (e.g., deforestation) are 1.6 0.8 GtC/y (Schimel et al. 2001). At present, the reduction of C02 emissions that can be attributed to the use of nuclear power is 0.5 GtC/y. Thus, the uncertainties in the major fluxes in the carbon cycle are approximately the same as the present impact of nuclear power on C02 emissions (Sarmiento Gruber 2002). To quote from Falkowski et al. (2000), Our knowledge is insufficient to describe the interactions between the components of the Earth system and the relationship between... 

In these models the increase in the number of factors considered is clearly observed, as is the respective increasing adequacy that accompanies them. One of the first and sufficiently complete models of the global C02 cycle is the model proposed by Bjorkstrom (1979) which takes into account the dynamic interaction between carbon reservoirs in the biosphere and fluxes between them. For the first time, a unit for the World Ocean was realistically represented. In this unit the ocean is... 

Note once more that the land, ocean, and sediment reservoirs are not at steady state because of man s activities. It is also the case because we are dealing with such small differences in values between large reservoir sizes and fluxes, that the natural system may not be balanced today and may not have been in steady state prior to humans coming on the scene. Thus, we have a real problem when we try to assess the baseline for the carbon cycle against which inputs from human activities can be evaluated. 

This chapter summarizes selected aspects of our current understanding of the organic carbon (OC) cycle as it pertains to the modem ocean, including underlying surficial sediments. We briefly review present estimates of the size of OC reservoirs and the fluxes between them. We then proceed to highlight advances in our understanding that have... 

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). 

This chapter is an attempt to give an account of the fundamental aspects of the carbon cycle from a global perspective. An outline of the details we shall encounter is shown in Fig. 11-1. After a presentation of the main characteristics of carbon on Earth, four sections follow a section about the carbon reservoirs within the atmosphere, the hydrosphere, the biosphere, and the lithosphere a section covering the most important fluxes between the reservoirs a section giving brief accounts of selected models of the carbon cycle and a final section describing cultural influences on the carbon cycle today. 

The preindustrial carbon cycle in Fig. 6.1 is shown as a steady state system, with inputs balancing outputs for each reservoir. Human activity, particularly since the industrial revolution in the mid-1700s, has had a significant influence on the size of fluxes between some of the carbon reservoirs. Since industrialization there has been an imbalance between uptake and release of C02, as can... 

Both, (a) changes in the isotopic composition of the hydrological cycle due to, e.g., climate variations and (b) changes in the carbon and oxygen fluxes between the reservoirs therefore can induce changes in the oxygen isotopic composition of atmospheric CO, and Oj. Dynamically, after a perturbation a new atmospheric steady state of the oxygen isotope ratios establishes within a few years in the case of 0/ 0 in CO, and within about 1200 years in... 

Carbon fluxes between ocean atmosphere reservoir and carbonate, and atmosphere and organic carbon are estimated as 12.5 x 10 mol my and 3.2 X 10 mol my, respectively. Hydrothermal CO2 flux from mid-oceanic ridges is estimated to be (1-2) x 10 mol my from rate of seawater and hydrothermal solution cycling at mid-oceanic ridges and CO2 concentration of hydrothermal solution. This flux is smaller than that between ocean-atmosphere reservoir and carbonate reservoir. CO2 flux by hydrothermal solution associated with back arc volcanism today is estimated as which is higher or similar to hydrothermal CO2 flux from mid-oceanic ridges (Shikazono 2003). 

The cycles of carbon and the other main plant nutrients are coupled in a fundamental way by the involvement of these elements in photosynthetic assimilation and plant growth. Redfield (1934) and several others have shown that there are approximately constant proportions of C, N, S, and P in marine plankton and land plants ("Redfield ratios") see Chapter 10. This implies that the exchange flux of one of these elements between the biota reservoir and the atmosphere - or ocean - must be strongly influenced by the flux of the others. 

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.

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