Geology Reference
In-Depth Information
Table 3 The net global fluxes of some trace gases across the air/sea interface
Global air-sea direction
a
Flux magnitude (g yr
1
)
Gas
10
12
10
13
CH
4
þ
6
10
15
CO
2
6
10
12
N
2
O
þ
10
10
{ CCl
4
¼
B
{ CCl
4
0
5
10
9
{ CCl
3
F
¼
B
{ CCl
3
F
0
3-13
10
11
CH
3
I
þ
100
90
10
12
CO
þ
4
2
10
12
H
2
þ
2
10
9
Hg
þ
B
a
þ
sea to air;
air to sea;
¼
no net flux.
Source: from Chester, 1990.
1
gradient determines the direction of the flux, into or out of the ocean.
Net global fluxes for some gases are presented in Table 3. The atmos-
phere serves as the source of material for conservative gases, especially
those of anthropogenic origin, but several gases produced in situ by
biological activity evade from the ocean.
4.2.2.2 Oxygen. Oxygen is a non-conservative gas and a typical
oceanic profile is shown in Figure 8. The concentration varies through-
out the water column, its distribution being greatly influenced by
biological activity. The generalised chemical equation for carbon fixa-
tion is often given as
nCO
2
þ
nH
2
O
(CH
2
O)
n
þ
nO
2
During photosynthesis this reaction proceeds to the right, thereby
producing organic material, as designated by (CH
2
O)
n
, and O
2
. The
surface waters become equilibrated with respect to atmospheric O
2
, but
they can get supersaturated during periods of intense photosynthetic
activity. Respiration occurs as the above reaction proceeds to the left
and O
2
is consumed. Photosynthesis is obviously restricted to the photic
zone in the upper ocean and ordinarily exceeds respiration. However,
the relative importance of the two processes changes with depth. The
oxygen compensation depth is the horizon in the water column at which
the rate of O
2
production by photosynthesis equals the rate of respira-
tory O
2
oxidation.
Below the photic zone, O
2
is utilised in chemical and biochemical
oxidation reactions. As evident in Figure 8, the concentration diminishes
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