Lecture 2

Lecture 2 - Major Ions in Sea Water
Why do we care about the major ions?
What is the composition of seawater?
What defines Major Ions?
What are their concentrations?
What are their properties?
controls (salinity and temperature)
Density of Seawater
σ = (ρ - 1) 1000
if ρ = 1.0250 gm/cm3
then σ = 25.0
σ  as S 
σ  as T 
Q. Why?
What is salinity? What are  and σ? What are their units?
Q. How is salinity measured?
1. gravimetric
2. analyze all the ions and sum
3. relative to halogens (Cl- + Br- + I-)
using Knudsen equation from 1911 (n = 9 samples)
S ‰ = 0.030 + 1.8059 Cl ‰ (or gms per kg)
4. Conductivity UNESCO, 1981 Defined the Practical Salinity Scale (PSS)
See Millero 1993
S = 35.000 (don’t use units like PSU)
Surface density, isopycnal outcrops
Waters will move mostly along surfaces of constant density.
Sea Surface Salinity
Q. Why does surface salinity vary? DS = 30 to 37
What are broad patterns and what controls salinity?
Evaporation and Precipitation Effects on
Surface Salinity
All original salinity signatures acquired at the sea surface
Modified in the ocean interior by mixing.
Becomes tracers for water masses.
Salinity Cross Section in Altantic Ocean
Salinity Cross Section (Pacific Ocean)
Sea Surface Temperature – Annual Average
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- > Fneutral = B(OH)3°
written as main species
1 mol = 6.02 x 1023 atoms
mol kg-1 = g(solute)/kg (water)
g(solute)/mol. wt.
1 mol NaCl = 1 mol Na+ + 1 mol Cl-
molar (M) mol / ltr H2O
mol / kg H2O
mol / kg SW (H2O + salt)
Q Why??
10-12 (Q How many atoms?)
We want to make a solution with Na+ = 468.96 mmol/kg
from NaCl (table salt)
From Pilson
Na+ > Mg2+ > Ca2+ > K+> Sr2+
Cl- >>SO42- >HCO3-> Br->FB(OH)3
Q. mol balance
Q. charge balance
Si and gases
Liverpool and NIO
What are the properties of the major ions?
Some major ions are conservative.
These are Na+, K+, Cl-, SO42-, Br-, B(OH)3 and 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‰)
More recently (S‰ = 1.8065 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)
DCa = 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??
Black Smoker Fluids, East Pacific Rise , from Von Damm et al., (1985)
River water ≠ seawater
HCO3- > ClCa2+ > Na+
Example of using seawater ratios:
From Christner et al (2014) Nature, 512, 310
“A microbial ecosystem beneath the West Antarctic ice sheet”
Crustal and seawater components to Subglacial Lake Whillans (SLW) waters
The weathering products probably came from sulfide oxidation, carbonation reactions,
and carbonate dissolution.

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