An Introduction to Environmental Chemistry

Atmosphere

• well mixed

Surface ocean layer

• well mixed

Deep ocean

Vertical

diffusion

Diffusion

First order exchange _

fluxes proportional to

concentration in upstream

reservoir

Vertical

circulation

Fig. 1The main elements of a simple model for ocean CO 2 uptake.

Box 7.1 Simple box model for ocean carbon dioxide uptake

In order to calculate how much

anthropogenic carbon dioxide (CO 2 ) the

oceans can take up from the atmosphere, it is

often necessary to construct a model of the

system. The simplest of these models divide

the oceans into a series of boxes (numbering

from a few to several hundred), with water

containing its dissolved carbon (C) flowing

between them. The main elements of such

models are shown in Fig. 1.

For the relatively well-mixed atmosphere

and surface-ocean boxes, the carbon flow

between them is assumed to be proportional

to their carbon content. Within the deep

ocean, where the circulation is much more

sluggish, vertical mixing is often modelled as

a diffusion process. In addition, the model

can include a simple circulation with direct

input to the ocean bottom from the surface,

balanced by upward water movement

throughout the deep ocean, to represent

convective processes. The spatial and depth

distribution of radioactive substances, such as

the isotope^14 C (see Section 2.8) (produced

both by cosmic rays in the atmosphere and

from the detonation of nuclear devices in the

248 Chapter Seven

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