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

log10c = x

c = 10x

Logarithmetic:

Could seawater originate by evaporation of river water?

River Water ≠ Sea Water

Mainly Ca2+/HCO3- Mainly Na+/Cl-

Both the composition and key ratios are differentppm = mg kg-1

What Controls the Composition of Rivers?

Weathering of limestone is considered a congruent reaction (all solid dissolves)CaCO3(s) + CO2(g) + H2O = Ca2+ + 2 HCO3

-

1 2Weathering 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 = 1/2 Al2Si2O5(OH)4(s) + K+ + HCO3

- + 2H4SiO4

* With these reactions you could calculate how much CO2(g) is consumed by weathering

Weathering of rocks

(orthoclase feldspar) (kaolinite)

Variability in Erosion Among Continents

Europe, North Americaand Asia are more calcareous continents.

Most of variability dueto Ca2+ and HCO3

- whichcome from weathering ofcarbonate rock

SO42- and Cl- come

from aerosols and weathering of evaporite rocks (e.g. Salt or NaCl).

Na+, K+, Mg2+, SiO2

come from weatheringsilicate rocks

Evaporation of River water

pH = -log (H+)

Examples:Mono Lake, CASoap Lake, WA

Makes a Na, HCO3, CO3 brine. pH is very basic.

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 reactionsSinks - 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+, SO4

2-)

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 fromthe volcanoes and bases from the rocks.

Sillen suggested that the following phases were at equilibrium.

If these phases at equilibrium at constant T and Cl, then the SW composition is fixedand it could only change if temperature or Cl- changed. Equilibrium constants not known.

Kaolinite, illite, chlorite, montmorillonite and phillipsite are types of clay minerals

Mass Balance approaches

Mackenzie and Garrels 1966 proposed that the input from riverswas balanced by removal to sediments but they had to invokea 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 Na90% of Mg100% of K90% 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 pastandThe 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.

What is the origin of seawater’s composition?SourcesRivers??Mid-Ocean Ridges??Other?? Aerosols

SinksSediments??Mid-Ocean Ridges??Other?? Aerosols

Kinetic Model of Seawater - A Mass Balance Approach

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 riversmain sink over geological time = evaporites = controlled by tectonics, geometry of marginal seasresidence 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 crustThus control is mass balance: VrCr = Vhydro (Csw – Cexit fluid)

for Mg2+ , Cexit fluid = 0thus: 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 estuariesb) Ca/HCO3 regulation by calcium carbonate equilibria

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)

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

Group III (e.g. nutrients (Si, P, C, N) and trace metals

Internal cycling can be described by the simple 2-box ocean modelThe main balance is input from rivers and removal as biological debris to sediments

Input from rivers = removal to sedimentsVrCr = f B

where f is the fractionof 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

South Atlantic-Sayles (1979)

But if fluxes are real therewould be more solid phase Mgthan observed!

The long-term global carbon balance

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

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

H4SiO4 vs Mg in a “Black Smoker” at 21°N

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

Weathering SusceptibilitiesMinerals 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.

Mg

Alk

East Pacific Rise , from Von Damm et al., (1985)

East Pacific Rise, continued

SO4

350C 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