Geology Reference
In-Depth Information
during the day, a reaction with NO
3
may be important at night.
Relatively low levels are associated with air masses derived from con-
tinental areas, owing to the enhanced concentrations of oxidants. While
oceanic venting rates are dependent upon a number of meteorological
and oceanographic conditions, there is no question that the marine
photic zone acts as the major source of DMS to the overlying tropo-
sphere.
The oxidation of DMS in the atmosphere (see also Chapter 2) could
yield several products, namely dimethylsulfoxide (DMSO), methane-
sulfonate (MSA) or sulfate. Insofar as aerosol formation is concerned,
the two key products are MSA and SO
4
2
. Atmospheric particles in the
sub-micron size range exert a significant influence on the earth's climate.
The effect can be manifested via three mechanisms. Firstly, the particles
themselves may enhance backscatter of solar radiation. Secondly, they
act as CCN promoting cloud formation and so increasing the earth's
albedo. Thirdly, such clouds affect the hydrological cycle. The evidence
for such a biofeedback mechanism limiting global warming remains
circumstantial.
4.2.3 Nutrients
Although several elements are necessary to sustain life, traditionally in
oceanography the term ''nutrients'' has referred to nitrogen (notably,
but not exclusively, nitrate), phosphorus (usually as phosphate), and
silicon (as silicate). The rationale for this classification was that analyt-
ical techniques had long been available that allowed the precise deter-
mination of these constituents despite their relatively low
concentrations. They were observed to behave in a consistent and
explicable manner, but quite differently to the major constituents in
seawater.
The distributions of these three nutrients are determined by biological
activity. Nitrate and phosphate become incorporated into the soft parts
of organisms. As evident in the modified carbon fixation equation of
Redfield
18
given below
106CO
2
þ
16NO
3
þ
HPO
3
4
þ
122H
2
O
þ
18H
þ
Ð
C
106
H
263
O
110
N
16
P
þ
138O
2
The uptake of these nutrients into tissues occurs in constant relative
amounts. The ratio (i.e., Redfield ratio) for C/N/P is 106:16:1. Silicate is
utilised by some organisms, particularly diatoms (phytoplankton) and
radiolaria (zooplankton), to form siliceous skeletons. Such skeletons
