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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?

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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 thecomposition 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


log10c = x

c = 10x

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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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Weathering of rocks

Weathering of limestone is considered a congruent reaction (all solid dissolves)

CaCO3(s) + CO2(g) + H2O = Ca2+ + 2 HCO3-


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


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





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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+)


Mono Lake, CA

Soap Lake, WA

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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 froma 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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So, an equilibrium approach doesn’t work.

The composition of seawater has changed in the past


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 - AMass Balance Approach

What is the origin of seawater’s composition?



Mid-Ocean Ridges??

Other?? Aerosols



Mid-Ocean 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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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-). What’s left is Cl-, Na+, Ca2+, K+, Fe2+

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Hydrothermal End-Member (350°C)(from Von Damm et al (1985)

from site at 21° N (Hanging Garden)

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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:

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

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But – the problem with this approach is that not all HT flow is 350°C!

  • Three Categories of Hydrothermal Flow

  • 350°C Black Smokers - 0.5 x 1013 kg y-1

  • 10°C Axial - 440 x 1013 kg y-1

  • 10°C 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 flow is 350

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 flow is 350

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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Pore Water Gradients in flow is 350

Marine Sediments

But if fluxes are real there

would be more solid phase Mg

than observed!

South Atlantic-Sayles (1979)

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The long-term global carbon balance flow is 350

CaCO3(s) + CO2(g) + H2O = 2HCO3- + Ca2+

2HCO3- + Ca2+ = CaCO3(s) + CO2(g) + H2O

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H flow is 3504SiO4 vs Mg in a “Black Smoker” at 21°N

Used to obtain end-member concentrations for 350°C vents

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Weathering Susceptibilities flow is 350

Minerals Weather at Different Rates

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Chemical Weathering and the Geological Carbon Cycle flow is 350

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, continued flow is 350


350C vents have no Mg, SO4 or HCO3. What’s 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