Geoscience Reference
In-Depth Information
the electrostatic field is transmitted in a medium is known as the Debye length
λ
D
and, in
an electrolyte solution, can be shown to be:
C
ε
RT
e
2
I
λ
D
=
(7.31)
with e being the charge of the electron and
C
a constant. In this equation,
I
is the ionic
strength
2
c
i
z
i
of the solution, where
c
i
and
z
i
stand for the concentration and charge,
respectively, of ion
i
. The potential at the distance
r
of an ion considered as a point charge
will be:
e
−
r
/λ
D
V
(
r
)
=
V
(
0
)
(7.32)
The energy stored in the shell around the ion is an integral part of the total energy budget
and therefore affects the thermodynamic properties of ions in solutions with respect to
uncharged solutes. This effect is described in chemistry textbooks as the Debye-Hückel
theory.
Adding salt to a solution also has another effect, which we have already met: it reduces
the Debye length and therefore the thickness of the charged fringe around ions and
molecules and lets small particles approach each other at short distance and coalesce. This
is know as flocculation and is responsible for the removal of iron and other elements from
estuaries.
7.6 Biological activity
Biological activity has conditioned the chemistry of the ocean and the atmosphere since
earliest times. It is characterized by exploitation of the environment to manufacture soft and
hard body parts, and to maintain the metabolism of organisms. The primary plant producers
extract carbon from the surrounding medium, atmospheric CO
2
in the case of land plants,
and carbonates dissolved in seawater in the case of algae, and reduce it with solar energy
(photosynthesis). The higher elements of the prey-predator chain, the food chain, recycle
the primary reduced carbon. Organisms run on real reduced-carbon batteries. Respiration
provides the oxygen for combustion and eliminates the CO
2
produced. While the reduced
carbon, in the form of carbohydrates and lipids, acts as a repository of energy, it is also
used to build soft parts, notably proteins. In addition to carbon, the production of organic
matter requires nitrogen, phosphorus, and trace elements (particularly Fe, Mg, Zn), which
are essential for manufacturing many vital enzymes.
For locomotion or defense, organisms form hard parts, based essentially on silica SiO
2
for diatoms, calcite CaCO
3
for foraminifera and invertebrates, and on phosphate for ver-
tebrates. Carbonate precipitation consumes alkalinity. In the seas, calcite and silica are the
main biogenic components of sediments and stand in contrast to the detrital material carried
by rivers and the wind, and which is composed largely of clays, hydroxides, and quartz.
