Global Carbon Fluxes

A specific feature of these two major biogeochemical cycles of carbon is their openness, which is related to the permanent removal of some carbon from the turnover 

HC03 ion mass exchange between land and troposphere  

Transport of oceanic airborne HCOj ions to land 0.003 

Over the period of 570 million years, 71.3 x lO tons of carbonate-bound carbon and 8.1 x lO tons of dispersed organic carbon became buried in sedimentary rocks. Dobrovolsky (1994) has considered that the increased influx of carbon dioxide at the periods of active volcanism was attended by an overall climatic warming up and a reduced temperature contrast between high and low latitudes. Possibly, the wide 

Analyses of ice cores from Vostok, Antarctica, have provided new data on natural variations ofC02 and CH4 levels over the last 220,000 years (Barnola et al, 1991  

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Regional transfer of carbon in 2000 due to production of and trade in crops, wood, and paper constituted 0.72 GtC yr 1. The pure global carbon flux at the atmosphere-ocean boundary in 1995 was estimated at 2.2 GtC (—19% +22%) with an interannual variability of about 0.5 GtC. The greatest extent of C02 flux oscillations in the system can be observed in the equatorial Pacific. 

This model also works well for the Archaean, with its likely higher mantle heat flow and smaller continental mass and is consistent with calculations which show that there was a much higher flux of hydrothermal fluid into the early oceans compared to the present day (Section 5.4.2). In addition, it is consistent with the many observations of carbonate alteration in Archaean mafic and ultramafic rocks (Rose et al., 1996 Nakamura Kato, 2004). Nakamura and Kato (2004) proposed a global carbon flux of 3.8 x 1013 mol/yr into the ocean floor at 3.46 Ga, an order of magnitude greater than the modern carbon flux of veined MORB (Table 5.3). If these Archaean fluxes are used in Zahnle and Sleep s (2002) ingassing-outgassing... 

Figure 3 Global carbon reservoirs and annual fluxes. Units are gigatons of carbon in the reservoirs and Gt C yr for fluxes...

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. 

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. 11-1 Major reservoirs and fluxes of the global carbon cycle, including time scales. Numbers given are Pg C (1 Pg C = lO g C) Pg C/yr, respectively. (After Sundquist, 1993.)...

Box models have a long tradition (Craig, 1957b Revelle and Suess, 1957 Bolin and Eriksson, 1959) and still receive a lot of attention. Most work is concerned with the atmospheric CO2 increase, with the main goal of predicting global CO2 levels during the next hundred years. This is accomplished with models that reproduce carbon fluxes between the atmosphere and other reservoirs on time... 

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.)...

Raich, J. W. and Schlesinger, W. H. (1992). The global carbon dioxide flux in soil respiration and its relationship to vegetation and climate, Tellus, Ser. B, 44,81-99. 

Sano and Williams (1996) calculated present-day volcanic carbon flux from subduction zones to be 3.1 x 10 mol/year based on He and C isotopes and C02/ He ratios of volcanic gases and fumaroles in circum-Pacific volcanic regions. Williams et al. (1992) and Brantley and Koepenich (1995) reported that the global CO2 flux by subaerial volcanoes is (0.5-2.0) x lO mol/m.y. and (2-3) x 10 mol/m.y. (maximum value), respectively. Le Guern (1982) has compiled several measurements from terrestrial individual volcanoes to derive a CO2 flux of ca. 2 x 10 mol/m.y. Le Cloarec and Marty (1991) and Marty and Jambon (1987) estimated a volcanic gas carbon flux of 3.3 X 10 mol/m.y. based on C/S ratio of volcanic gas and sulfur flux. Gerlach (1991) estimated about 1.8 x 10 mol/m.y. based on an extrapolation of measured flux. Thus, from previous estimates it is considered that the volcanic gas carbon flux from subduction zones is similar to or lower than that of hydrothermal solution from back-arc basins. 

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. 

P. J. Kuikman, Quantification of carbon fluxes in grassland. Report Nr. 410 100 047 of the Dutch National Research Programme on Global Air Pollution and Climate Change. RIVM, Bilthoven, p. 52 (1996). 

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. 

Liu et al. (2008) for the current global flux of 0.13 Pg C yr1 to the continental aquatic ecosystem. In effect, based on the above estimates, the flux of C to groundwater could account for 2% to 12% of the missing carbon sink in the global carbon budget. 

FIGURE 14.11 Summary of global carbon cycle. Amount (in gigatons of C = 109 metric tons = fO1 g of C). Reservoirs are shown in parentheses, and fluxes (gigatons of C per year) are indicated by arrows. Note that the time scales associated with the various processes vary (adapted from IPCC, 1996). 

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