Carbon cycle fluxes

Feedbacks may be affected directly by atmospheric CO2, as in the case of possible CO2 fertilization of terrestrial production, or indirectly through the effects of atmospheric CO2 on climate. Furthermore, feedbacks between the carbon cycle and other anthropogenically altered biogeochemical cycles (e.g., nitrogen, phosphorus, and sulfur) may affect atmospheric CO2. If the creation or alteration of feedbacks have strong effects on the magnitudes of carbon cycle fluxes, then projections, made without consideration of these feedbacks and their potential for changing carbon cycle processes, will produce incorrect estimates of future concentrations of atmospheric CO2. 

Figure 5.1. Tentative model of global carbonate cycle. Fluxes are in units of 1012 moles C y1 as (Ca,Mg)C03. aAgegian et al. (1988) bMeybeck (1979) cSmith (1978) and Milliman (1974) defBroecker and Peng (1982) SBroecker and Peng...

WoUast R. (1991) The coastal organic carbon cycle fluxes, sources, and sinks. In Physical, Chemical, and Earth Sciences Research Report, vol. 9, pp. 365—381. 

Figure 44 Tentative model of global ocean carbonate cycle. Fluxes are in units of lO molCyr as (Ca,Mg)C03 and represent estimates of fluxes averaged over the most recent glacial-interglacial transition (after Mackenzie and Morse, 1992).

Wollast R (1991) The coastal organic carbon cycle Fluxes, sources and sinks. In Mantoura RFC, Martin MJ, Wollast R (eds.) Ocean Margin Processes in Global Change. Dahlem Workshop Reports. Chichester, Wiley Interscience, UK, pp.365-381. 

Renewable carbon resources is a misnomer the earth s carbon is in a perpetual state of flux. Carbon is not consumed such that it is no longer available in any form. Reversible and irreversible chemical reactions occur in such a manner that the carbon cycle makes all forms of carbon, including fossil resources, renewable. It is simply a matter of time that makes one carbon from more renewable than another. If it is presumed that replacement does in fact occur, natural processes eventually will replenish depleted petroleum or natural gas deposits in several million years. Eixed carbon-containing materials that renew themselves often enough to make them continuously available in large quantities are needed to maintain and supplement energy suppHes biomass is a principal source of such carbon. 

The magnitude and fate of coastal-zone biological production is a major unknown in the global carbon cycle. Since river nutrient flux into these regions may be altered with C02-induced climate change, it is important that generation and fate of coastal-zone production be better understood. 

Figure 1. The global carbon cycle. Estimates of reservoir size and annual fluxes are from Post et al. (4), Vegetation carbon reservoir was estimated from latest carbon density estimates. All values except the atmospheric reservoir are approximate only. All values are in gigatons. Fluxes are next to the arrows and are in gigatons ear.

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. 

As an application of the turnover time concept, let us consider the model of the carbon cycle shown in Fig. 4-3. This diagram is different from the one used in the chapter on the carbon cycle (Chapter 11), because it serves our purposes better for this chapter. The values given for fhe various fluxes and burdens are very similar to the corresponding figure in Chapter 11 (Fig. 11-1). 

Fig. 4-3 Principal reservoirs and fluxes in the carbon cycle. Units are 10 g (Pg) C (burdens) and PgC/yr (fluxes). (From Bolin (1986) with permission from John Wiley and Sons.)...

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

Fig. 10-16 A simple schematic of the carbon cycle. Part (a) is the pre-industrial case, and part (b) shows the contemporary reservoirs and fluxes, in Pg C and Pg C/yr, respectively (Pg C = lO g C). This diagram of the carbon cycle is similar to those presented in Chapters 4 and 11. (Reprinted by permission from Nature (1993). 365 119-125, Macmillan.)...

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. 

Fig. 11-18 A four-box model of the global carbon cycle. Reservoir inventories are given in moles and fluxes in mol/yr. The turnover time of CO2 in each reservoir with respect to the outgoing flux is shown in brackets. (Reprinted with permission from L. Machta, The role of the oceans and biosphere in the carbon dioxide cycle, in D. Dryssen and D. Jagner (1972). "The Changing Chemistry of the Oceans," pp. 121-146, John Wiley.)...

Kindermann, J., Wiirth, G., Kohlmaier, G. H. and Badeck, F.-W. (1996). Interannual variations of carbon exchange fluxes in terrestrial ecosystems. Global Biogeochem. Cycles 10, 737-755. 

Shikazono, N. and Kashiwagi. H. (1999) Carbon dioxide flux due to hydrothermal venting from back-arc basin and island arc and its influence on global carbon dioxide cycle. 9th Annual V.M. Gohl.schmidt Conference, August 22-27. Harvard, Abstr., p. 272. 

In Chapter 3, hydrothermal and volcanic gas fluxes from submarine back-arc basins and island arc are estimated. These fluxes are compared with midoceanic ridge hydrothermal fluxes. Particularly, hydrothermal flux of CO2 is considered and the influences of this flux on global long-term carbon cycle and climate change in Tertiary-Quaternary ages are discussed in Chapter 4. 

A major opportunity to test the use of " Th as a proxy for POC flux arose with the Joint Global Ocean Flux Study (JGOFS). JGOFS had as a central goal a better understanding of the ocean carbon cycle, including the flux of POC leaving the euphotic zone. Process studies were carried out in the Atlantic Ocean, Pacific Ocean, Arabian Sea and Southern Ocean. " Th profiles were obtained as a part of each process study. 

Since the biospheric growth rate depends, among other factors, on the C02 supply, it is probable that the C02 increase induces, at least for part of the biosphere, an increased growth rate ("C02 fertilization"). A simple concept to take this into account is the introduction of a biota growth factor e if the atmospheric C02 pressure increases by p percent, the C02 flux to the biosphere increases by zp percent. Typically, values for e between 0 and 0.5 have been used in carbon cycles models [26,41]. 

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