Geoscience Reference
In-Depth Information
Alkalinity
Sulfates and
chlorides
Silica
Figure 7.12
Compositional distribution of the dissolved charge of rivers among its three main components:
silica, alkalinity (mainly carbonates), and chlorides and sulfates derived mostly from evaporites.
These compositions are closely controlled by the nature of the substrate (granitic cratons,
7.9 Elements of marine chemistry
Rivers carry their dissolved load to the ocean. It is practical to group all of this added
mineralization under the heading of erosion-related alkalinity flux. Many elements do not
pass the filter of estuaries; as discussed above, at this point, fresh water and salt water mix,
causing a substantial decrease in the dielectric constant of the water. Massive precipita-
tion of colloids takes place, with the adsorption of many elements such as the transition
elements, rare-earths, etc. This phenomenon is amplified by the richness of estuaries in
organic matter.
The ocean surface is characterized by abundant life, sustained throughout the depth of
water where light can penetrate (known as the photic zone, some 50 m deep) by primary
photosynthetic production by algae. This primary productivity allows other planktonic
forms or larger organisms to develop by grazing and predation. As indicated above, organic
matter concentrates light isotopes, particularly
12
C relative to
13
C and
14
N relative to
15
N,
and the intensity of this productivity can be measured with the increased
13
C and
15
Nof
δ
δ
surface water compared with deep water (
Figs. 7.13
and
7.14
)
.
Biological activity consumes the surplus alkalinity supplied by rivers, notably in the
form of CaCO
3
, which is one of the major constituents of the hard parts of the micro-
fauna (foraminifera). Biological activity is limited only by the availability of other elements
required for tissue construction (P, N), for metabolism (Fe), and for locomotion and defense
(SiO
2
). It is a remarkable fact that oceanic organisms, including phyto- and zooplank-
ton, consume these constituents in fairly constant proportions, particularly the C : N : P
proportions which are universally 115 : 15 : 1 in organic matter (Redfield ratio). These
proportions are believed to correspond approximately to a fairly general reaction, the
