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Lecture 2 - Major Ions in Sea Water. What is the composition of seawater? What defines Major Ions? What are their concentrations? What are their properties?. How are the major ions of seawater defined? What are the major ions? Elements versus species?

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Lecture 2 - Major Ions in Sea Water

What is the composition of seawater?

What defines Major Ions?

What are their concentrations?

What are their properties?

How are the major ions of seawater defined?

What are the major ions?

Elements versus species?

moles versus grams – conversions (See E&H Table 1.2)

How are the major ions of seawater defined?

ans: major ions contribute to salinity (e.g. 35.000‰)

salinity can be determined to 0.001 ppt = 1 ppm = 1 mg kg-1

Elements versus species?

e.g., Na is an element Na+ is a species (cation)

S is an element SO42- is a species (anion)

What are the major ions? n = 11

ans: cations = Na+ > Mg2+ > Ca2+ ~ K+ > Sr2+

anions = Cl- >> SO42- > HCO3- > Br- > F-

neutral = B(OH)3°

moles versus grams – conversions (See E&H Table 1.2)

1 mol = 6.02 x 1023 atoms

mol kg-1 = g(solute)/kg (water)

g(solute)/mol. wt.

1 molNaCl = 1 mol Na+ + 1 molCl-

From Pilson


Na+ > Mg2+ > Ca2+ > K+> Sr2+


Cl- >>SO42- >HCO3-> Br->F-



Si and gases

Liverpool and NIO


Some major ions are conservative.

These are Na+, K+, Cl-, SO42-, Br-, B(OH)3and F-.

What does this mean? conservative.

Q. How do you demonstrate this?

What are the consequences?

Do conservative major ions have a constant

concentration in the ocean? Q

Law of Constant Proportions(major ion/S‰ = constant)

Knudsen equation ( S = 0.030 + 1.8050 Cl‰)

The Law breaks down in estuaries, evaporite basins,

hydrothermal vents. Q

Some Major Ions are non-conservative


Ca2+, Mg2+, Sr2+, Dissolved Inorganic Carbon (HCO3-)

Non-conservative behavior due to:

biological production

hydrothermal ridge crest solutions

river water (as in estuaries)

Nutrient Like Profiles

Superposition of vertical

biological flux on

horizontal circulation

Results in low surface water and high

deep water concentrations.

Results in higher concentrations in

the older deep Pacific than the

younger deep Atlantic

Example: Comparison of vertical profiles of nutrients

from the Atlantic and Pacific



Shallow remineralization

Soft parts

Deep remineralization

Hard parts

Non-Conservative Major Elements

Calcium (Ca)

Ca = 0.1 / 10.2 =

+1.0 % with depth


CaCO3 (s) = Ca2+ + CO32-

(from de Villiers, 1999)

Sr – also increases with depth (~2%) and N. Atl to N. Pac

Distributions similar to PO4 (excellent correlation)

But why? The mineral phase Celestite (SrSO4) produced by Acantharia

protozoa is proposed as the transport phase.

Acantharia shell and cyst

Acantharia are marine planktonic


Examples from sediment traps at Bermuda

Inverse Mg – Ca Relationship

from EPR at 17S; 113W

(from de Villiers, 1999)

Note significant variability in Mg

(normalized to S = 35)!

In this case ~1% variability.

Hydrothermal Origin??

River water ≠ seawater

HCO3- > Cl-

Ca2+ > Na+

density of seawater
Density of Seawater


See Millero 1993

S = 35.000

Q. How is salinity measured?

Q. Why does salt make water

more dense?

What is salinity? What are  and σ? What are their units?

surface density isopycnal outcrops
Surface density, isopycnal outcrops

Waters will move mostly along surfaces of constant density.

sea surface salinity
Sea Surface Salinity

Q. Why does surface salinity vary? DS = 30 to 37

Review questions from OCN210

1. How is the salinity of seawater defined? Units?

(see editorial by Millero (1993) at the end of this Lecture).

2. What techniques have been used to measure the salinity of seawater?


3. How does salinity vary in the surface ocean?

4. What controls this variability?

annual average surface salinity
Annual average surface salinity

What processes influence surface salinity?

Can salinity be changed away from the surface?


Is salinity making the water column stable? Where and where not?

annual average surface temperature
Annual average surface temperature

Identify influences of the wind-driven circulation on surface temperature

potential temperature
Potential Temperature

Temperature must be responsible for stratification. But everywhere?

Identify the influence of the wind-driven circulation.