Inorganic carbon in the ocean: Basics - MIT OpenCourseWare · 2020-01-04 · Why study carbonate in...
Transcript of Inorganic carbon in the ocean: Basics - MIT OpenCourseWare · 2020-01-04 · Why study carbonate in...
Inorganic carbon in the ocean: Basics
IntroductionEquilibrium and Measurement
Biogeochemical Processes and the CO2 system-- organic matter formation / destruction-- CaCO3 precipitation / dissolution
Why study carbonate in the ocean?(stating the obvious)
Life on earth is carbon-based!
pH is a key master variable for many processes - and carbonateequilibria play an important role in determining ocean pH
HCO3- is the 3rd most concentrated anion in the ocean
The marine carbon cycle is a key factor in determining the fate ofanthropogenic CO2
…
And if the pursuit of knowledge isn’t enough:MC&G students: I guarantee you’ll have a carbonate question
on your general exam!
Carbon cycling in the ocean(an overview)
Land
Atmosphere
Ocean
CO2
CO2 + H2O ⇔ H2CO3
H2CO3* ⇔ H+ + HCO3
−
HCO3− ⇔ H+ + CO3
2−
CO2 ⇒ Corg
Ca2+ + CO32− ⇒ CaCO3(s)
Corg ⇒ CO2
CaCO3(s) ⇒ Ca2+ + CO32−
Corg ⇒ CO2
CaCO3(s) ⇒ Ca2+ + CO32−
sediments
HCO3-
Earth surface carbon cycle - Time Scales
Increasing…
Surface ocean~ atmosphere
Deep ocean :large reservoir!
Largest reservoir:long time scales
Sundquist, 1993
Atmosphere420 18 ka590 A.D. 1750750 A.D. 1990
1
10
100
1,000
10,000
100,000
1,000,000
Foss
il fu
els 4
,000
Terrestrialbiosphere
560
Active Soils1,100
Old Soils500
Ocean Surface 900
Deep Ocean36,400
Reactive Marine Sediments3,000
90,000,000Earth's Crust
6 1 to 3 60 2 to 3 70
0.6 0.4
0.2 0.2
Tim
e re
quire
d to
affe
ct a
tmos
pher
e (y
ears
)
Earth Surface Carbon Cycle - Time Scales
Figure by MIT OCW.
Dissolved Carbonate Equilibria
ΣCO2 = H2CO3*[ ]+ HCO3
−[ ]+ CO32−[ ]
K1 =H+[ ]HCO3
−[ ]H2CO3
*[ ]
K2 =H+[ ]CO3
2−[ ]HCO3
−[ ]
K1, K2 known…
-- 3 equations, 5 unknowns…
Specify (or measure) 2 things - ∑CO2, pH … to get :
2.0x10-3
1.5
1.0
0.5
0.0
Conce
ntr
atio
n (
mole
s/kg
)
1210864pH = - log(H
+)
HCO3-
CO32-H2CO3*
Seawater pH
10-8
10-7
10-6
10-5
10-4
10-3
10-2
Conce
ntr
atio
n (
mole
s/kg
)
1210864pH = - log(H
+)
HCO3-
CO32-H2CO3*
Seawater pH
K1 =H+[ ]HCO3
−[ ]H2CO3
*[ ] ----> log K1{ }= log H+{ }+ logHCO3
−[ ]H2CO3
*[ ]⎧ ⎨ ⎪
⎩ ⎪
⎫ ⎬ ⎪
⎭ ⎪
Speciation as ƒ(pH) on log-log plot:A “Bjerrum” Plot
Are ∑CO2 and pH an ideal pair to measure?Sometimes, but note:
Water mass ApHA
∑CO2A
Water mass BpHB
∑CO2B
50 : 50 mix of A and B
∑CO2(mix)=0.5*∑CO2(A) + 0.5*∑CO2(B)
** Not true for pH !! **
How about a quantity that is related to the major ion composition?
The charge balance in seawater:
An apparent excess of positive charge of ~ 2.2 mmol / kg
What are the missing anions?
Cations
Na+
K+
Mg2+
Sr2+
Ca2+
[ci] [ci][qi] [qi]
Total
467.8
53.3
10.3
9.9
0.1
467.8
106.5
20.6
9.9
0.2
605.0
Anions
Cl-
F-
Br-SO4
2-
Total
545.5 545.5
28.2
0.8
0.1
56.4
0.8
0.1
602.8
Concentrations, [ci] (mmol kg-1), and charge concentrations, [qi] = zi [ci] (mmol kg-1),of conservative ions in seawater at S = 35.
.
. .
..
Figure by MIT OCW.
The Alkalinity
The missing anions: HCO3-, CO3
2-, B(OH)4-
The conjugate bases of the weak acids,H2CO3 and B(OH)3
HA ⇒ H+ + A−
acid conjugatebase
Alkalinity (mol/kg) = the amount of strong acid that must be added to a1 kg (sea)water sample to make its pH equal to that of the second equivalence point of the dissolved CO2 system, pH = 4.3
Total Alkalinity: seawater, pH = 8
“Practical Alkalinity”:
PA = HCO3
−⎡ ⎣ ⎢
⎤ ⎦ ⎥ + 2 CO3
2−⎡ ⎣ ⎢
⎤ ⎦ ⎥ + B OH( )4
−⎡
⎣ ⎢
⎤
⎦ ⎥ + OH−⎡
⎣ ⎢ ⎤ ⎦ ⎥ − H+⎡
⎣ ⎢ ⎤⎦⎥
20001770
1600
1200
800
400
0
Con
trib
utio
n to
PA
(µm
ol k
g-1 )
[HCO3]-
Σ = PA = 2300 µmol kg-1
438
866 -0.01
2[CO3 ]-2 [B(OH)4]-
[OH ]- -[H+]
Figure by MIT OCW.
What does the definition mean?
10-14
10
-12
10
-10
10
-8
10
-6
10
-4
Conce
ntr
atio
n (
mol/
kg)
12108642pH
H2CO3 HCO3-
CO32-
B(OH)3B(OH)4
-
Bjerrum Plotfor seawater
pH = 8
The titration of seawater with a strong acid (HCl)
8
7
6
5
4
3
pH
1.00.80.60.40.20.0Va (ml HCl added)
A titration of seawaterwith HCl
Start
8
7
6
5
4
3
pH
0.80.60.40.20.0Va (ml HCl added)
pH
dpH / dVa
titration endpoint atinflection of pH vs Va curve
10-14
10
-12
10-10
10
-8
10-6
10-4
Conce
ntr
atio
n (
mol/
kg)
12108642pH
H2CO3 HCO3-
CO32-
B(OH)3B(OH)4
-
Bjerrum Plotfor seawater
10-12
10
-10
10
-8
10
-6
10
-4
10
-2
Conce
ntr
atio
n (
mol/
l)
12108642pH
2*[CO32-] + [HCO3
-] + [B(OH)4-] + [OH-]
H+
The H+ at which
alkalinity --> 0
At the titration endpoint,
H+[ ]= 2 CO32−[ ]+ HCO3
−[ ]+ B OH( )4−[ ]+ OH−[ ]
That is, the endpointIs the point at whichAlk = 0
So: the moles of acidadded to reach theendpoint = Alk
Concentrations during the titration:
TA = HCO3
−⎡ ⎣ ⎢
⎤ ⎦ ⎥ + 2 CO3
2−⎡ ⎣ ⎢
⎤ ⎦ ⎥ + B OH( )4
−⎡
⎣ ⎢
⎤
⎦ ⎥ + OH−⎡
⎣ ⎢ ⎤ ⎦ ⎥ − H+⎡
⎣ ⎢ ⎤⎦⎥
Alkalinity: A precise definitionDickson, 1981 ; 19994
“The total alkalinity of a natural water is thus defined as the number of moles of hydrogen ionequivalent to the excess of proton acceptors (bases formed from weak acids with a dissociation constant K <= 10-4.5 at 25°C and zero ionic strength) over proton donors (acidswith K > 10-4.5) in one kilogram of sample.”
Remember:
(1) predominant speciesat pH = 4.5 does not contribute to Alkalinity
(2) How do you estimate pK for an acid from this diagram?
-1
-1.5
-2
-3
-4
-4
-5
-6-6
-7
-8
-2.5
-3.5
-4.5
-5.5
-6.5
-7.5
2 4 6 8 10 12
H+
SO42-
CO2
NH3
B(OH)3
2-CO3
2-HPO43-PO4
OH-
F-HF
pH
2 4 6 8 10 12pH
-HCO3
-HSO4B(OH)4
-
H3SiO4-
H2PO4-
H3PO4
H4SiO4
NH4+
Log
[con
cent
ratio
n (m
ol k
g-1)]
Log
[con
cent
ratio
n (m
ol k
g-1)]
Figure by MIT OCW.
Alkalinity
So we have:Carbonate Alkalinity: CA = [HCO3
-] + 2[CO32-]
“Water Alkalinity” = [OH-] - [H+]
“Borate Alkalinity” = [B(OH)4-]
For most applications, can use:
Alk = [HCO3-] + 2[CO3
2-] + [OH-] - [H+] + [B(OH)4-]
But in some applications, you need to add minor contributors, such as
NH3, PO43-, HPO4
2-, H3SiO4-, HS- , …
•Remember, when you titrate a seawater sample to the 2nd CO2 endpoint,The result is “Titration Alkalinity”, which includes all conjugate bases of weakAcids in the sample. The approximation in the Alk expression above is theNeglect of the contribution of minor constituents to the result.
Measurements: Summary
∑CO2
(i) Titration + curve fitting(ii) Acidify ; strip CO2 ; Measure CO2 by gas chromatography,Coulometry, IR analyzer
Calibration: reference seawater (Dickson)
± 2 µmol/kg
Alkalinity (i) Titration + curve fitting(ii) Gran titration
Calibration: reference seawater (Dickson)
± 2 µmol/kg
pH (i) Electrochemical measurement(ii) pH-sensitive dyes
Calibration: buffers for seawater
±0.002 pH units
The “more-than-I-ever-wanted-to-know” section :pH scales
(1) Laboratory chemistry: the NBS pH scale
pH = − logaH + ** Defined relative to low-ionic strength buffers --abandoned for measurements in seawater
(2) Seawater: An analysis of “[H+]” in seawater yields[H+]free + [HSO4
-] + [HF]
[HF] is small; in order to determine [H+]free, need to know K for HSO4
- dissociation WELL.
“They” have decided it is better to define a new pH scale:
The “Total pH scale” :
HT+ = H +⎡
⎣ ⎢ ⎤ ⎦ ⎥ free
+ HSO4−⎡
⎣ ⎢ ⎤ ⎦ ⎥
Or the “seawater pH scale” :
HT+ = H +⎡
⎣ ⎢ ⎤ ⎦ ⎥ free
+ HSO4−⎡
⎣ ⎢ ⎤ ⎦ ⎥ + HF[ ]
pH scales, cont.
So that:
(1) pH measurements are reported on the pHT scale(2) equilibrium constants are reported on the pHT scale
Can convert between the scales with KHSO4 and KHF
**The difference between pHsws and pHT is ~ 0.01 unit ~ 2% at pH 8The difference between pHNBS and pHT is ~ 0.1 unit ~ 20% at pH 8
The precision of good pH measurements is ~ 0.002 pH units
On equilibrium constantsfor calculations in seawater
Consider the reaction: HCO3− ⇔ H + +CO3
2−
In an “ ideal” solution: K =H +⎡
⎣ ⎢ ⎤ ⎦ ⎥ CO3
2−⎡ ⎣ ⎢
⎤ ⎦ ⎥
HCO3−⎡
⎣ ⎢ ⎤ ⎦ ⎥
In an “ideal” solution, all concentrations --> 0, so there are no ion-ion or ion-solvent interactions.
In a real solution, these interactions are important, so…
On equilibrium constantsfor calculations in seawater
Consider the reaction: HCO3− ⇔ H + +CO3
2−
In an “ ideal” solution: K =H +⎡
⎣ ⎢ ⎤ ⎦ ⎥ CO3
2−⎡ ⎣ ⎢
⎤ ⎦ ⎥
HCO3−⎡
⎣ ⎢ ⎤ ⎦ ⎥
In an “ideal” solution, all concentrations --> 0, so there are no ion-ion or ion-solvent interactions.
In a real solution: K =γ H + γCO3
2− H +⎡ ⎣ ⎢
⎤ ⎦ ⎥ CO3
2−⎡ ⎣ ⎢
⎤ ⎦ ⎥
γ HCO3− HCO3
−[ ]
The γ are “activity coefficients”, and are generally <1 forions in solution because of electrostatic interactions between ions and between and ions and solvent .In addition, some ions (esp. CO3
2-
) form complexes with other ions in seawater.
Equilibrium constants for calculations in seawater
For a solution with the ionic strength of seawater (I = ∑ci zi2 ~ 0.7), it is difficult to
calculate the γ values. Therefore:“Constant ionic medium” equilibrium constants are used. They are:
-- measured in solutions with the same proportions of major ions as seawater
-- measured as functions of T and S-- pressure dependence is calculated from thermodynamic data
So:
K '=H +⎡
⎣ ⎢ ⎤ ⎦ ⎥ T
CO32−⎡
⎣ ⎢ ⎤ ⎦ ⎥ T
HCO3−⎡
⎣ ⎢ ⎤ ⎦ ⎥ T
The “T” denote “total” concentrations, including complexes…And remember, for H+, the “T” denotes the “total”pH scale.
These “seawater” constants areavailable from the literature as ƒ(T,S,P)
Working with the carbonate system in seawater
ΣCO2 = H2CO3*⎡
⎣ ⎢ ⎤ ⎦ ⎥ + HCO3
−⎡ ⎣ ⎢
⎤ ⎦ ⎥ + CO3
2−⎡ ⎣ ⎢
⎤ ⎦ ⎥
Alk = HCO3−⎡
⎣ ⎢ ⎤ ⎦ ⎥ + 2 CO3
2−⎡ ⎣ ⎢
⎤ ⎦ ⎥ + B OH( )4
−⎡
⎣ ⎢
⎤
⎦ ⎥ + OH−⎡
⎣ ⎢ ⎤ ⎦ ⎥ − H+⎡
⎣ ⎢ ⎤ ⎦ ⎥ ± minor components
TB = B OH( )3[ ]+ B OH( )4−⎡
⎣ ⎢
⎤
⎦ ⎥ = cons tant × Salinity
K1 =H+⎡
⎣ ⎢ ⎤ ⎦ ⎥ HCO3
−⎡ ⎣ ⎢
⎤ ⎦ ⎥
H2CO3*⎡
⎣ ⎢ ⎤ ⎦ ⎥
K2 =H+⎡
⎣ ⎢ ⎤ ⎦ ⎥ CO3
2−⎡ ⎣ ⎢
⎤ ⎦ ⎥
HCO3−⎡
⎣ ⎢ ⎤ ⎦ ⎥
KB =H+⎡
⎣ ⎢ ⎤ ⎦ ⎥ B OH( )4
−⎡
⎣ ⎢
⎤
⎦ ⎥
B OH( )3[ ] KW = H+⎡
⎣ ⎢ ⎤ ⎦ ⎥ OH−⎡ ⎣ ⎢
⎤ ⎦ ⎥
Measure ∑CO2, Alk, S ==>7 equations in 7 unknowns
==> you can solve for speciation!
Useful approximations for back-of-the-envelopecalculations
At seawater pH,
∑CO2 ~ [HCO3-] + [CO3
2-]
Alk ~ [HCO3-] + 2 [CO3
2-]
And
[CO32-] ~ Alk - ∑CO2
10-14
10
-12
10
-10
10
-8
10
-6
10
-4
Conce
ntr
atio
n (
mol/
kg)
12108642pH
H2CO3 HCO3-
CO32-
B(OH)3B(OH)4
-
Bjerrum Plotfor seawater
pH = 8
The effect of biogeochemical processes on ∑CO2 and Alk
(1) The incorporation of CO2 into organic matter (I.e., production of o.m.)
CO2 --> “Corg” + O2
∆∑CO2 = -1∆Alk = 0
… and a small (usually negligible) increase in Alk due to NO3- assimilation, ∆Alk = (16/106))* mol Corg formed
(2) The precipitation of CaCO3
Ca2+ + CO32- --> CaCO3
∆∑CO2 = -1∆Alk = -2
How do these processes affect the ability of the ocean to take up CO2 from the atmosphere? Taking into account Henry’s law relating [CO2] in the water to the PCO2 in equilibrium withthe water,
CO2[ ]= KH PCO2System approaches equilibrium afterPerturbation: lower ocean [CO2] -->ocean absorbs CO2
Contours of [CO2] vs. Alk and ∑CO2
2450
2400
2350
2300
2250
Tot
al A
lkal
inity
230022502200215021002050TCO2
90
80 70
60
50
50
40
30
30 20
20
20
10
form Corg
form CaCO3
o.m. formation:decrease [CO2]
CaCO3 pptn:increase [CO2]
Biogeochemical processes and the carbonate system
How do decomposition and dissolution processes affect the carbonate system?
(3) Organic matter dissolution… reverses (1),Corg + O2 --> CO2 + …
∆∑CO2 = +1∆Alk = 0
(and, as before, there’s a small drop in Alk due to NH3 oxidation to NO3-)
(4) CaCO3 dissolution… reverses (2),CaCO3 --> Ca2+ + CO3
2-
∆∑CO2 = +1∆Alk = + 2
How do these processes affect the [CO32-] of seawater as it “ages” during its transit through the
deep sea?
Contours of [CO32-]
2450
2400
2350
2300
2250
Tot
al A
lkal
inity
230022502200215021002050TCO2
280 260
240
220
200 180
160
140 120
120 100
100
80
80
60
40
Contours of [CO32-]
Dissolve CaCO3
oxidize Corg
Oxidize o.m.:decrease [CO3
2-]
Dissolve CaCO3:increase [CO3
2-]
Question: what can you use this contour plot for if you have actual data?
0
12
3
45
6
7
DEP
TH (k
M)
Geosecs North Atlantic North of Equator
1.9 2.0 2.1 2.2 2.3 2.4
TCO2 (10-3 M/kG)
0
1
2
3
4
5
672.20 2.25 2.30 2.35 2.40 2.45 2.50
Talk (10-3 EO/KG)
DEP
TH (k
M)
Geosecs North Atlantic North of Equator
0
1
23
4
5
6
7
DEP
TH (k
M)
Geosecs North Atlantic North of Equator
1000 200 300CO3= (10-6 M/kG)
0
1
2
3
4
5
6
7
DEP
TH (k
M)
Geosecs North Pacific North of Equator
1000 200 300
CO3= (10-6 M/kG)
0
1
2
3
4
5
6
7
DEP
TH (k
M)
Geosecs North Pacific North of Equator
2.32.22.12.01.9 2.4
TCO2 (10-3 M/kG)
0
1
2
3
4
5
6
7
DEP
TH (k
M)
Geosecs North Pacific North of Equator
2.20 2.25 2.30 2.35 2.40 2.45 2.50
Talk (10-3 EO/kG)
OBSERVATION
NORTH ATLANTIC NORTH PACIFIC
Alk
TCO2
CO32-
Figure by MIT OCW.
Using data and theory to calculate relative ratesof organic matter oxidation and carbonate dissolution
see Broecker and Peng, Tracers in the Sea
North Indian andPacific Bottom Waters
1 Mole CaCO3
1 Mole ORG C
1 Mole CaCO3
4 Moles ORG C
15∆ALK∆ΣCO2
= =12
12 105
2 - .93
15∆ALK∆ΣCO2
∆ΣCO2N (µmol/kg)
ALK
N (µ
eq/k
g)
= =15
45 105x 2- .29
Antarctic SurfaceWater
Temperate SurfaceWater
Antarctic DeepWaters
North Atlantic DeepWater (2.5 oC)
x
2500
2450
2350
22501900 2000 2100 2200 2300 2400 2500
2400
2300
DeepWater
Surface Water
WSBWCPDWNIBWNPBWNADW
ANTARCTICS.INDIANS.PACIFICN.PACIFICS.ATLANTICN.ATLANTIC
Figure by MIT OCW.
And…
We’ve shown how biogeochemical processes affect ∑CO2 and Alk in the ocean…We’ve started to show how distributions of ∑CO2 and Alk can be usedTo learn about biogeochemical processes…
We’ve not discussed the biological and chemical processes that control
formation and decomposition of organic matter
formation and dissolution of CaCO3
Those discussions will follow…