Lecture 4 What Controls the

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Lecture 4 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
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Observed Mean Ocean Concentrations large range
Logarithmetic
log10c x c 10x
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Could seawater originate by evaporation of river
water?
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River Water ? Sea Water
Mainly Na/Cl-
Mainly Ca2/HCO3-
ppm mg kg-1
Both the composition and key ratios are different
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What Controls the Composition of Rivers?
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Weathering of rocks
Weathering of limestone is considered a congruent
reaction (all solid dissolves) CaCO3(s)
CO2(g) H2O Ca2 2 HCO3-
1 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
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2 A specific reaction written
in terms of CO2(g) KAlSi3O8(s) CO2(g) 1
1/2H2O
1/2 Al2Si2O5(OH)4(s) K HCO3-
2H4SiO4? With these reactions you could
calculate how much CO2(g) is consumed by
weathering
(orthoclase feldspar)
(kaolinite)
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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
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Evaporation of River water
Makes a Na, HCO3, CO3 brine. pH is very basic.
pH -log (H)
Examples Mono Lake, CA Soap Lake, WA
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Mono Lake, California
Tufa Towers
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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)
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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.
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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
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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
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Specific reverse weathering type reactions
proposed to remove excess ions.
Newly formed clays would equal 7 of sedimentary
mass.
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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
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illite in deep-sea sediments
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Pore Water Gradients in Marine Sediments
But if fluxes are real there would be more solid
phase Mg than observed!
South Atlantic-Sayles (1979)
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So, an equilibrium approach doesnt 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.
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Kinetic Model of Seawater - A Mass Balance
Approach
What is the origin of seawaters
composition? Sources Rivers?? Mid-Ocean
Ridges?? Other?? Aerosols Sinks Sediments?? Mid-O
cean Ridges?? Other?? Aerosols
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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)
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The long-term global carbon balance
CaCO3(s) CO2(g) H2O 2HCO3- Ca2
2HCO3- Ca2 CaCO3(s) CO2(g) H2O
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Mass Balance Model Modern Version. Includes
ridge crest processes.
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How about mid-ocean ridges??
350ºC vents have no Mg2, SO42- or alkalinity
(HCO3-). Whats left is Cl-, Na, Ca2, K, Fe2
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Sites of Hydrothermal Vents on Mid-Ocean Ridges
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Hydrothermal End-Member (350C)(from Von Damm et
al (1985)
from site at 21 N (Hanging Garden)
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H4SiO4 vs Mg in a Black Smoker at 21N Used to
obtain end-member concentrations for 350C vents
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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
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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.
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Group II (e.g. Ca, Na) (e.g. the remaining
cations with long residence times) Consider the
charge balance for seawater 2Ca2 Na
2Mg2 K HCO3- Cl- 2SO4
2- or rearranged 2Ca2 Na -
HCO3- Cl- 2SO42- - 2Mg2 -
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
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But the problem with this approach is that not
all HT flow is 350C!

• Three Categories of Hydrothermal Flow
• 350C Black Smokers - 0.5 x 1013 kg y-1
• 10C Axial - 440 x 1013 kg
  y-1
• 10C Off Axis - 630 x 1013 kg y-1
• River Flux (Global) - 3500 x 1013 kg y-1

from Emerson and Hedges (p. 55)
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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)
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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!
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Weathering Susceptibilities Minerals Weather at
Different Rates
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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.
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East Pacific Rise , from Von Damm et al., (1985)
Mg
Alk
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East Pacific Rise, continued
SO4
350?C vents have no Mg, SO4 or HCO3. Whats left
is Cl, Na, Ca, K, Fe
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Hydrothermal Vent Compositions German and Von
Damm (2004) Treatise on Geochemistry, Vol. 6,
The Oceans and Marine Geochemistry, Elsevier