Carbonate Geochemistry - Welcome to the MGG …mgg.rsmas.miami.edu/groups/sil/Carbonate...
Transcript of Carbonate Geochemistry - Welcome to the MGG …mgg.rsmas.miami.edu/groups/sil/Carbonate...
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Carbonate GeochemistryCarbonate Geochemistry
Everything useful you need to know about carbonate geochemistry
A Short Course VU March, 2009 Peter Swart University of Miami
Seawater
• Salinity– Concentration of all salts
– 1000 gms of seawater evaporated to dryness gives 35 gms of saltsgives 35 gms of salts
– 35 ppt, gm/kg, psu (practical salinity units)
– Also defined as conductivity, but does not include non-polar material.
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Seawater:cations
• Most Common cations in seawater
• Ca 2+
• Mg 2+
• Na +
• K +
• Mainly present as uncomplexed form
Anions
• Chloride, Cl-
• Bicarbonate, carbonate,HCO3-, CO3
2-
• Sulfate, SO42-
Molality of cations in seawater
• Sodium– Concentration = 10,773 ppm= 10773/23= 467 mM
= 0.467 M
• Magnesiumg– 1294 ppm = 1294/24 = 55 mM = 0.055 M
• Potassium– 399 ppm = 399/39 = 10 mM = 0.010 M
• Calcium– 420 ppm = 420/40 = 10 mM = 0.010 M
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Anions
• Chloride– 19344 ppm = 19344/35.5 = 546 mM = .546 M
• Sulfate2717 2717/94 28 M 0 028 M– 2717 ppm = 2717/94 = 28 mM = 0.028 M
• Bicarbonate– 142 ppm = 142/61 = 2.2 mM = 0.0022
Other Important Ions
• Strontium– 8 ppm, 8/87.62 = 93 M = 0.000093 M
• Bromine67 67/80 835 M 0 000835 M– 67 ppm, 67/80 = 835 M = 0.000835 M
• Na .31
• Cl .23
• Mg .15
• SO4 12
• 467
• 546
• 55
• 28
Riverwater (mM) Seawater (mM)
• SO4 .12
• Ca 0.37
• K 0.036
• HCO3 .87
• 28
• 10
• 10
• 2
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Conservative and non-Conservative
• Elements which generally do not participate in chemical reactions are called conservative– Na– Cl– K– Br
• Chemical reactions can include– Calcification, photosynthesis, respiration
Non-conservative
• Non-conservative elements include– C
– Ca
SO 2-– SO42-
– Mg
– Sr
– Fe, NO3-, NH4
-
Ca 2+ + CO32- = CaCO3
At equilibrium Ksp = CaCO3/ [Ca 2+ ] [CO32-]
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Concentrations of elements are in moles per kilogram (molal)
Ion molal product (IMP) of two elements = the concentration of one times the concentration of the
otherother
For CaCO3 the IMP = 0.011 x 0.0002
The effective molality is called the activity
The activity is only equal to the molality in an infinitely dilute solution
In other solutions the activity is related to the molality through the activity coefficient
So that:
= m
= activity coefficient
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Definitions
• Ksp = solubility product
• IMP= Ion Molar Product
• IAP = Ion Activity Product
• Activity= molality * activity coefficient
• Activity = effective concentration
• Ionic strength= I= ½ (m1c12+m2c2
2.mncn2)
Ionic Strength of Seawater
• I = ½(.546*1+ .467*1 + 0.028*4 + 0.01*4 + 0.01*1+ 0.055*4 + 0.002*1)
• Seawater or any solution must remain electrically neutralelectrically neutral
• mNa*c^2+ mK*c^2+mCa*c^2+mMg*c^2= mCl-*c^2+mSO4*c^2+mHCO3-*c^2
Thermodynamic Saturation Thermodynamic Saturation State of CaCOState of CaCO33
= [= [CaCa2+2+] [CO] [CO3322--]/Ksp(t,S,P)]/Ksp(t,S,P)
> 1> 1 SupersaturationSupersaturation
= 1= 1 EquilibriumEquilibrium
< 1< 1 Under SaturationUnder Saturation
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0 1 2 3 4 5
(m)
0
1000
2000
AragoniteD
epth
3000
4000
5000
6000
N. AtlanticN. Pacific
C a lc ite
0 2 4 6
(m)
0
1 0 0 0
2 0 0 0
Dep
th (
3 0 0 0
4 0 0 0
5 0 0 0
6 0 0 0
N . A tla n ticN . P a c if ic
Water
• H2O = H+ + OH-
• KH2O = 10 -14 = [H+][OH-]/ H2O
• pK = negative log = 14
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pH
7.4 7.6 7.8 8.0 8.2H
(m
)
0
1000
2000
DE
PT
H
3000
4000
5000
N. AtlanticN. Pacific
pCO2 (atm)
200 400 600 800 1000 1200 1400 1600
(m)
0
1000
2000
DE
PT
H
3000
4000
5000
N. AtlanticN. Pacific
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L A T IT U D E
-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70
pH
in
sit
u
8.0
8.0
8 .1
8 .1
8 .2
8 .2
A tlan tic
P acific
L A T IT U D E
-70 -60 -50 -40 -30 -20 -10 0 10 20 30
pH
in
sit
u
7.6
7.7
7 .8
7 .9
8 .0
8 .1
8 .2
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1
2
1.462 3
2
{ }10
o
H
H COK
pCO
6.3531
2 3
{ }{ }10
{ }O
HCO HK
H CO
3
2 3{ }H CO
10.3332 2
3
{ }{ }10
{ }
CO HK
HCO
2 2 32 oH O pCO H CO
1.462 3
2
{ }10
o
H
H COK
pCO
2 3 3oH CO HCO H
6.3531
2 3
{ }{ }10
{ }O
HCO HK
H CO
21log * 356.3094 .06091964 21834.37 / 126.8339log 1684915 /K T T T T
10 33{ }{ }CO H
23 3HCO CO H
10.3332 2
3
{ }{ }10
{ }
CO HK
HCO
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1.462 3
2
{ }10
o
H
H COK
pCO
Take logs
2 3 2log{ } 1.46 logoH CO pCO
g
1.462 3
2
{ }10
o
H
H COK
pCO
Combine
2 3 2{ }OHH CO K pCO
6.3531
2 3
{ }{ }10
{ }O
HCO HK
H CO
1 23{ }HK K pCO
HCOH
2 3 2{ } HH CO K pCO
3 2log{ } 7.81 logHCO pCO pH
1 23{ }HK K pCO
HCOH
Combine
10.3332 2
3
{ }{ }10
{ }
CO HK
HCO
1 2 23{ }
2HK K K pCO
COH
Take logs
3 2log{ } 18.34 log 2CO pCO pH
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5
10
3 2log{ } 18.34 log 2CO pCO pH
3 2log{ } 7.81 logHCO pCO pH
2 3 2log{ } 1.46 logoH CO pCO
-25
-20
-15
-10
-5
0
5
0 5 10 15
pH
Co
nc
en
tra
tio
n
HCO3
CO3
H2CO3
120%
-25
-20
-15
-10
-5
0
5
10
0 5 10 15
pH
Co
ncen
tratio
n
HCO3
CO3
H2CO3
0%
20%
40%
60%
80%
100%
0 5 10 15
pH
Concentration
HCO3
CO3
H2CO3
22 3 3 3
oDIC mH CO mCO mHCO
70%80%90%
100%
ion
Bjerrum Diagram
0%10%20%30%40%50%60%
0 5 10 15
pH
Concentrati
HCO3
CO3
H2CO3