Geology Reference
In-Depth Information
water exits the Mediterranean Sea at depth and can be identified as a
distinctive layer within the North and South Atlantic.
4.2 SEAWATER COMPOSITION AND CHEMISTRY
4.2.1 Major Constituents
The major constituents in seawater are conventionally taken to be those
elements present in typical oceanic water of salinity 35 that have a
concentration greater than 1 mg kg
1
, excluding Si, which is an impor-
tant nutrient in the marine environment. The concentrations and main
species of these elements are presented in Table 1. One of the most
significant observations from the Challenger expedition of 1872-1876
was that these major components existed in constant relative amounts.
As already explained, this feature was exploited for salinity determina-
tions. Inter-element ratios are generally constant, and often expressed as
a ratio to Cl% as shown in Table 1. This implies conservative behaviour,
with concentrations depending solely upon mixing processes, and in-
deed, salinity itself is a conservative index.
Because of this behaviour, individual seawater constituents can be
utilised for source apportionment studies in non-marine environments.
For instance, an enrichment factor (EF) for a substance X is defined as
X
=
Na
þ
ð
Þ
sample
EF
X
¼
ð
4
:
2
Þ
X
=
Na
þ
ð
Þ
seawater
An EF of 1 indicates that the substance exists in comparable relative
amounts in the sample and in seawater, thereby giving a good indication
Table 1 Chemical species and concentrations of the major elements in seawater
Concentration
(mol L
1
)
For S
¼
35
(g kg
1
)
Ratio to chlorinity
(Cl
¼
19.374%)
Element
Chemical species
Na
1
4.79
10
1
5.56
10
1
Na
10.77
Mg
21
5.44
10
2
6.66
10
2
Mg
1.29
Ca
21
1.05
10
2
2.13
10
2
Ca
0.4123
K
1
1.05
10
2
2.06
10
2
K
0.3991
Sr
21
9.51
10
5
4.20
10
4
Sr
0.00814
Cl
5.59
10
1
9.99
10
1
Cl
19.353
SO
4
2
, NaSO
4
2.89
10
2
4.67
10
2
S
0.905
C (inorganic) HCO
3
,CO
3
2
2.35
10
3
1.42
10
2
0.276
Br
8.62
10
4
3.47
10
3
Br
0.673
B(OH)
3
, B(OH)
4
4.21
10
4
2.30
10
3
B
0.0445
F
, MgF
1
7.51
10
5
7.17
10
5
F
0.00139
Source: based on Dyrssen and Wedborg, 1974.
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