Geoscience Reference
In-Depth Information
Rivers
Evaporation
Surface water
Particles
Thermocline
Mixing
Particle
dissolution
Deep water
Sediments
Figure 7.16
chromatography, we saw that the reactivity of some ions (Cd, Ba, Fe, rare-earths, Th) rel-
ative to biological particulate matter rich in nitrate, phosphate, and silica accelerates their
transfer from the surface to the ocean floor, even if they are not directly involved in the
biological cycle. Such ions therefore display a nutrient-like character with depletion in the
surface water and regeneration in deep water.
The waste matter and carrion produced by surface organisms sink through the ocean
where they are recycled by other life forms. In the layer of water down to 100-1000 m,
known as the thermocline because temperature varies very quickly, nitrogen, phosphorus,
and other nutrients are redissolved. This process is accompanied, of course, by the con-
sumption of oxygen (respiration), which is therefore depleted at such depths relative to the
remainder of the ocean (oxygen minimum), and by the release of CO
2
.
Some 95% of organic matter is recycled in this way before it sinks to great depths.
Nutrients are recycled through the thermocline many times before being incorporated into
the sediment. The importance of recycling has been demonstrated by Broecker and Peng
(1982) in a simple but illustrative way (
Fig. 7.16
). Let us consider an ideal ocean made
of two layers, one on top of the other; surface water and deep water, separated by the
thermocline. Elements are carried from the continents to the sea by rivers and are removed
from the deep ocean by sedimentation. Let us call
F
river
the input of river water (kg m
−
3
s
−
1
) into the ocean and
C
river
the concentration (mol kg
−
1
) of river water in element
i
.
F
river
is compensated by evaporation so that the amount of surface water remains unchanged. We
call
F
mix
the amount of water exchanged per unit time between the surface and deep-water
layers, and
C
deep
and
C
surface
the concentration of element
i
in deep and surface water,
