The Geochemistry of the Earth - Thanatia: The Destiny of the Earth's Mineral Resources - page 112

times of more than one million years. The salinity of oceanic water is about 35
parts per thousand by mass but variations in a range of 33 to 38 have been observed
(Henderson, 1982). Such fluctuations result from a number of physical processes
that control the salt content of seawater such as temperature, rainfall, ice melt or
land runoff.
Not all seawater substances have an origin stemmed in the planetary crust.
Indeed, the sea is a huge reservoir for many atmospheric substances including carbon
dioxide. Through the air-sea interaction, all the air's constituents can be expected
to find their way into the ocean. These dissolved gases are classified into four general
groups (Idyll, 1970):

The first group contains the inert gases: nitrogen, argon, helium, neon, xenon
and krypton. These gases enter the oceans through the air-sea interface or
through the introduction of aerated water from land runoff.
The second group is solely composed by oxygen, coming from the same sources
as the first group, in addition to marine photosynthesis.
The third group also contains only one member, carbon dioxide. This gas is
introduced into the sea through the carbon cycle. Specific sources of carbon
dioxide include the atmosphere, land runoff and the ocean floor. Worthy of note
is the oceanic acidification caused by an increase in dissolved carbon dioxide
stemming from an anthropogenic origin. According to Hall-Spencer et al. (2008)
and Caldeira and Wickett (2005), the surface ocean pH may lead to a drop of
up to 0.5 units between 1900 and 2100.
The fourth group is simply the collection of all the remaining gaseous ingredients
found in seawater. Its sources are chiefly air pollution, usually from industry
and non-photosynthetic chemical reactions. Hydrogen sulphide resulting from
the reduction of sulphate in the absence of oxygen is one example.

Table 5.5 The average composition of
seawater. Adapted from Millero (1996)
Substance Concentration, mg=g

Cl

19.351

Na +

10.784

Mg 2+

1.284

SO 2
4

2.713

Ca 2+

0.412

K +

0.399

HCO 3

0.107

Br

0.067

Sr 2+

0.008

CO 2
3

0.048

B(OH) 4

0.003

F 0.013

B(OH) 3 0.009

Sum 35.198