Turnover of Carbon in the Biosphere

As has been pointed out earlier, terrestrial ecosystems are the main sink of carbon dioxide due to the photosynthesis process. The present bulk of living organisms is confined to land and their mass (on dry basis) amounts to 1880 x 109t. The average carbon concentration in the dry matter of terrestrial vegetation is 46% and, consequently, the carbon mass in the land vegetation is about 865 x 109t.

In accordance with various estimates, the oceanic biomass of photosynthetic organisms contains 1.7 x 109t of organic carbon, Co. In addition, we have to include a large number of consumers. This gives 2.3 x 109t of Co. Totally, the oceanic organic carbon is equal to 4.0 x 1091 or about 0.5% from that in land biomass.

Moreover, a substantial amount of dead organic matter as humus, litterfall and peat is also present in the terrestrial soil cover. The mass of forest litter is close to 200 x 109t, mass of peat is around 500 x 10 t, and that of humus is 2400 x 10 t. Recalculation of this value for organic carbon amounts to 1550 x 109 t.

However, the greatest amount of carbon in the form of hydrocarbonate, HCO^T (38 600 x 109t) is contained in the ocean, 10 times higher than the total carbon in living matter, atmosphere, and soils.

Thus, in the terrestrial ecosystems the least amount of carbon is monitored in living biomass, followed by dead biomass and atmosphere.

The mass distribution of carbon in the Earth's crust is of interest for understanding of the global biogeochemis-try of this element. These values are shown in Table 1. One can see that carbon from carbonates (Cc) is the major form. The Cc/Co ratio is about 5 for the whole Earth's crust as well as for its main layers (sedimentary, granite, and basalt) and crustal types: continental, subcontinental, and oceanic. However, for the latter this ratio is higher.

The sedimentary layer of the Earth's crust is the main carbon reservoir. The Cc and Co concentrations in the sedimentary layer are by an order of magnitude higher than in granite and basalt layers oflithosphere. The volume of sedimentary shell is about 0.10 from the crust volume; however, this shell accounts for 75% of both carbonate and organic carbon. Dispersed organic matter (kerogen) contains most of the Co mass. Localized accumulation of Co in oil, gas, and coal deposits are of secondary importance. It has been estimated that the oil/gas fields amount to 2 00 x 109t of carbon, and the coal deposits contain 600 x 109t, totally 800 x 109t. This is by three orders of magnitude less than the carbon mass of dispersed organic matter in the sedimentary shell. The general carbon distribution between reservoirs is shown in Table 2.

Thus, there are two major reservoirs of carbon in the Earth: carbonate and organic compounds. It should be stressed that both are of biotic origin. Nonbiotic carbonates, for instance, from volcanoes, are the rare exception of the rule. A connecting link between the carbonate and organic species is CO2, which serves as an essential starting material for both the photosynthesis of organic matter and the microbial formation of carbonates.

Atmospheric CO2 provides a link between biological, physical, and anthropogenic processes. Carbon is exchanged between the atmosphere, the ocean, the terrestrial biosphere, and, more slowly, with sediments and sedimentary rocks. The faster components of the cycle are shown in Figure 3.

The component cycles (Figure 3) are simplified and subject to considerable uncertainty (cf. Table 2, for example). In addition, this figure presents average values. The riverine flux, particularly the anthropogenic portion, is currently very poorly qualified and is not shown here. While the surface sediment storage is approximately 150 x 109t, the amount of sediment in the bioturbated and potentially active layer is of order of 400 x 109t. Evidence is accumulating that many of the key fluxes can fluctuate significantly from year to year (e.g., in the terrestrial sink and storage). In contrast to the static view conveyed by figures such as this one, the carbon system is

Table 1 Mass distribution of carbon in the Earth's crust

Average concentration (%)

Earth's compartments

Mass (10181) CO2 Cc

Mass (10151)

Earth's compartments

Mass (10181) CO2 Cc

o

Total Earth's crust

28.5

1.44

0.38

0.07

409

108

20

128

5.4

Continental type including:

18.1

1.48

0.40

0.08

267

72

14

86

5.1

Sedimentary layer

1.8

9.57

2.61

0.50

177

48

9

57

5.3

Granite layer

6.8

0.81

0.22

0.05

55

15

3

18

5.0

Basalt layer

9.4

0.37

0.10

0.02

35

9.4

1.9

11

5.0

Subcontinental type

4.3

1.37

0.36

0.07

58

16

3

19

5.3

Oceanic type

6.1

1.35

0.36

0.05

82

21

3

24

7.0

Earth's sedimentary shell

2.4

12.4

3.37

0.62

297

81

15

96

5.4

Phanerozoic sedimentary deposits

1.3

15.0

4.08

0.56

194

53

7

60

7.5

Table 2 The major global carbon reservoirs

Reservoirs C(109t)

Atmosphere, CO2 1680 Global land

Vegetable biomass prior to human activity 1150 (estimates)

Present natural vegetable biomass 900 Soil cover

Forest litterfall 100

Peat 250

Humus 1200

Total 1550 Ocean

Photosynthetic organisms 1.7

Consumers 2.3

Soluble and dispersed organic matter 2100

Hydrocarbonate ions in solution 38539

Total 40643 Earth's crust

Sedimentary shell, Co 15000000

Sedimentary shell, Cc 81 000000

Continental granite layer, Co 4000 000

Continental granite layer, Cc 18 000 000

Total 118000000

Total present global C mass 118 044 773

clearly dynamic and coupled to the climate system on seasonal, interannual, and decadal timescale.

Thus, the obvious discrepancies between data shown in Tables 2 and 3 as well as in Figure 3 and above-mentioned discussion in the text are related to both the uncertainties in data sources and different authors' speculations on the topic. At the state of the art of present knowledge, one cannot make more precise estimates of carbon fluxes and pools at the global scale.

The carbonate formation and photosynthesis have to be considered as two general processes in the global activity of living matter over geological history of the Earth. The Cc-to-Co mass ratio may specify the 'growth limit' of living matter at sequential stages of Earth's geological history over the period of 3.5-3.8 billion years. This ratio tends to decrease regularly with the last 1.6 billion years. The Cc/Co ratio was 18 in the sedimentary layers of the Upper Proterozoic period (1600-750 million years); that of the Paleozoic (570-400 million years), 11; of the Mesozoic (235-66 million years), 5.2; and of the Cainozoic (66 million years to the present), 2.9. The never interrupted increase in the relative content of organic matter in the ancient stream loss provides evidence for a

Figure 3 The global carbon cycle, showing the reservoirs (in 109tyr 1) relevant to the anthropogenic perturbation as annual averages over the period 1980-89.

Table 3 Net primary production of the Earth's major ecosystems

Global ecosystem zone Area (106 km2) Plant mass (1091) C-NPP(1091)

Table 3 Net primary production of the Earth's major ecosystems

Polar

8.1

13.8

1.3

Coniferous forests

23.2

439.1

15.2

Temperate

22.5

278.7

18.0

Subtropical

24.3

323.9

34.6

Tropical

55.9

1347.1

102.5

Total land

133.9

2402.1

171.6

Lakes and rivers

2.0

0.04

1.0

Glaciers

13.9

0

0

Total continents

149.3

2402.5

172.6

Oceans

361.0

0.2

60.0

Earth total

510.3

2402.7

232.6

progressively increasing productivity of terrestrial photo-synthetic organisms. This provides also the proof for the growing importance ofglobal terrestrial ecosystems in the fixation of CO2. Apparently, the increasing productivity of land vegetation would be the major sink of CO2 under the increasing content of this green-house gas in the atmosphere; however, the role of increasing input of nitrogen, for instance, with atmospheric deposition, has to be considered. Moreover, both carbonate formation and the photosynthesis of organic matter share in the common tendency for removal from the atmosphere of CO2 continually supplied from the mantle. Consequently, these processes take part in the global mechanisms for maintaining the present low concentration of carbon dioxide in the Earth's gas shield, which is an essential parameter in the greenhouse effect.

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