Lecture 9 – What Controls the
Composition of Seawater
Seawater is salty! Why?
How is the composition of river water different from seawater?
What controls the composition of riverwater?
What happens when you evaporate riverwater?
What controls the composition of seawater?
Could Chemical Equilibrium reactions control the composition of the Ocean?
What is meant by the Kinetic Model of Seawater?
How does the Mass Balance Control work?
Sources - Rivers, Mid-Ocean Ridges (MOR)
Sinks – Sediments, MOR
Observed Mean Ocean Concentrations – large range
Logarithmetic:
log10c = x
c = 10x
Could seawater originate by evaporation of river water?
River Water ≠ Sea Water
Mainly Ca2+/HCO3ppm = mg kg-1
Mainly Na+/Cl-
Both the composition and key ratios are different
What Controls the Composition of Rivers?
Weathering of rocks
Weathering of limestone is considered a congruent reaction (all solid dissolves)
CaCO3(s) + CO2(g) + H2O = Ca2+ + 2 HCO31
2
Weathering of alumino-silicate minerals to clay minerals are examples
of incongruent reactions (solid partially dissolves)
silicate minerals + CO2(g) + H2O == clay minerals + HCO3- + 2 H4SiO4 + cation
1
1
2
A specific reaction written in terms of CO2(g)
KAlSi3O8(s) + CO2(g) + 1 1/2H2O
(orthoclase
= 1/2 Al2Si2O5(OH)4(s) + K+ + HCO3- + 2H4SiO4
feldspar)
(kaolinite)
* With these reactions you could calculate how much CO2(g) is consumed by
weathering
Variability in Erosion Among Continents
Europe, North America
and Asia are more
calcareous continents.
Most of variability due
to Ca2+ and HCO3- which
come from weathering of
carbonate rock
SO42- and Cl- come
from aerosols and
weathering of evaporite
rocks (e.g. Salt or NaCl).
Na+, K+, Mg2+, SiO2
come from weathering
silicate rocks
Evaporation of
River water
Makes a Na, HCO3, CO3 brine.
pH is very basic.
pH = -log (H+)
Examples:
Mono Lake, CA
Soap Lake, WA
Mono Lake, California
Tufa Towers
Equilibrium approaches – Some History
Goldschmidt (1933)
igneous rock (0.6kg) + volatiles (1kg) === seawater (1 L) + sediments (0.6kg) + air (3 L)
Sillen (1959, 1961)
Sources - Weathering reactions
Sinks - Reverse weathering reactions
Organizational framework:
Gibbs Phase Rule
f=c+2–p
f = degrees of freedom
(variables like T,P, concentrations, e.g. Na+, Cl-, Ca2+, SO42-)
c = components (ingredients, e.g., HCl, NaOH, MgO))
p = phases at equilibrium
(domains of uniform composition, e.g. gas, liquid, pure solids)
Sillen: Nine component model (C = 9)
Acids: HCl, H2O, CO2 Bases: KOH, CaO, SiO2, NaOH, MgO, Al(OH)3
The ocean chemistry results from a giant acid-base titration. Acids from
the volcanoes and bases from the rocks.
Sillen suggested that the following phases were at equilibrium.
Kaolinite, illite, chlorite, montmorillonite and phillipsite are types of clay minerals
If these phases at equilibrium at constant T and Cl, then the SW composition is fixed
and it could only change if temperature or Cl- changed. Equilibrium constants not known.
Mass Balance approaches
Mackenzie and Garrels 1966 proposed that the input from rivers
was balanced by removal to sediments but they had to invoke
a reverse weathering hypothesis for which there was (and still is)
little evidence.
The river inputs are given below (total amount for 108 y).
For a steady state ocean, these have to be removed.
Mackenzie and Garrels (1966) American Journal of Science, 264, 507-525
Mackenzie and Garrels (1966)
A Chemical Mass Balance for Seawater
Still need to remove:
15% of Na
90% of Mg
100% of K
90% of SiO2
42% of HCO3
Specific reverse weathering type reactions
proposed to remove excess ions.
Newly formed clays would equal 7% of sedimentary mass.
Most clays are detrital-reflecting continental sources
chlorite in deep-sea sediments
detrital = particles of rock derived from pre-existing rock by weathering and erosion
illite in deep-sea sediments
So, an equilibrium approach doesn’t work.
The composition of seawater has changed in the past
and
The phases suggested do not appear to be at equilibrium
But there is some evidence that such reactions do occur
– especially in near shore sediments
So reverse weathering not totally eliminated!
But maybe not for an equilibrium ocean.
Kinetic Model of Seawater - A Mass Balance Approach
What is the origin of seawater’s composition?
Sources
Rivers??
Mid-Ocean Ridges??
Other?? Aerosols
Sinks
Sediments??
Mid-Ocean Ridges??
Other?? Aerosols
Residence Time 
 = mass / input or removal flux = M / Q
=M/S
Q = input rate (e.g. moles y-1)
S = output rate (e.g. moles y-1)
[M] = total dissolved mass in the box (moles)
Mass Balance Model – Modern Version.
Includes ridge crest processes.
How about mid-ocean ridges??
350ºC vents have no Mg2+, SO42- or alkalinity (HCO3-). What’s left is Cl-, Na+, Ca2+, K+, Fe2+
Sites of Hydrothermal Vents on Mid-Ocean Ridges
Hydrothermal End-Member (350°C)(from Von Damm et al (1985)
from site at 21° N (Hanging Garden)
Kinetic model of seawater – mass balance model
Main input and removal fluxes for major ions in seawater (from McDuff and Morel, 1980)
Note: Vr = 4.55 x 1016 L y-1 Vr/Vhydro = 300 Volume of ocean = 1.37 x 1021 L
Group Ia – Cl
short term cycle = aerosols and rivers
main sink over geological time = evaporites
= controlled by tectonics, geometry of marginal seas
residence time is so long (~100 My) that changes are hard to see.
Group Ib – Mg, SO4, probably K
input from rivers ; main sink through ocean crust
Thus control is mass balance:
VrCr = Vhydro (Csw – Cexit fluid)
for Mg2+ , Cexit fluid = 0
thus: Csw = ( Vr / Vhydro ) Cr
= 300 Cr
The dominant control is Vhydro, thus tectonics.
Group II (e.g. Ca, Na) (e.g. the remaining cations with long residence times)
Consider the charge balance for seawater:
2[Ca2+] + [Na+] + 2[Mg2+] + [K+] = [HCO3-] + [Cl-] + 2[SO4 2-]
or rearranged:
2[Ca2+] + [Na+] - [HCO3-] = [Cl-] + 2[SO42-] - 2[Mg2+] - [K+]
This side is controlled by tectonics
Therefore this sum is also controlled by tectonics
The controls on the relative proportions of elements on the left hand side
are complicated but include:
a) Ca/Na ion exchange in estuaries
b) Ca/HCO3 regulation by calcium carbonate equilibria
But – the problem with this approach is that not all HT flow is 350°C!
Three Categories of Hydrothermal Flow
•350°C Black Smokers
•10°C Axial
•10°C Off Axis
•River Flux (Global)
- 0.5 x 1013 kg y-1
- 440 x 1013 kg y-1
- 630 x 1013 kg y-1
- 3500 x 1013 kg y-1
from Emerson and Hedges (p. 55)
Group III (e.g. nutrients (Si, P, C, N) and trace metals
Internal cycling can be described by the simple 2-box ocean model
The main balance is input from rivers and removal as biological debris to sediments
Input from rivers = removal to sediments
VrCr = f B
where f is the fraction
of biogenic flux that is
buried (escapes remineralization)
Summary
Salinity of seawater is determined by the major elements.
Early ideas were that the major composition was controlled by equilibrium chemistry.
Modern view is of a kinetic ocean controlled by sources and sinks.
River water is main source – composition from weathering reactions.
Evaporation of river water does not make seawater.
Reverse weathering was proposed – but the evidence is weak.
Sediments are a major sink. Hydrothermal reactions are a major sink.
Still difficult to quantify!
Pore Water Gradients in
Marine Sediments
But if fluxes are real there
would be more solid phase Mg
than observed!
South Atlantic-Sayles (1979)
The long-term global carbon balance
CaCO3(s) + CO2(g) + H2O = 2HCO3- + Ca2+
2HCO3- + Ca2+ = CaCO3(s) + CO2(g) + H2O
H4SiO4 vs Mg in a “Black Smoker” at 21°N
Used to obtain end-member concentrations for 350°C vents
Weathering Susceptibilities
Minerals Weather at Different Rates
Chemical Weathering and the Geological Carbon Cycle
1. CO2 is removed by weathering of silicate and carbonate rocks on land.
2. The weathering products are transported to the ocean by rivers where they
are removed to the sediments.
3. When these sediments are subducted and metamorphosed at high T and P,
4. CO2 is returned to the atmosphere.
Ittekkot (2003) Science 301, 56
For more detail see Berner (2004) The Phanerozoic Carbon Cycle: CO2 and O2. Oxford Press, 150pp.
East Pacific Rise , from Von Damm et al., (1985)
Mg
Alk
East Pacific Rise, continued
SO4
350C vents have no Mg, SO4 or HCO3. What’s left is Cl, Na, Ca, K, Fe
Hydrothermal Vent Compositions – German and Von Damm (2004)
Treatise on Geochemistry, Vol. 6, The Oceans and Marine Geochemistry, Elsevier