OCE/ATM/ESS 588, Winter 2003
THE CARBON CYCLE AND MARINE CARBONATE CHEMISTRY - Lecture 2
Overheads
I. The Global Carbon Cycle
The major reservoirs and fluxes of carbon in the Earth.
Values are in 1015 g C or 1015 g C yr-1 (1015 g is called Gigatons, Gt or Petagrams, Pg)
Carbon Reservoirs (exchangeable on time scales of 1000s of years or less) and Fluxes. The data are
updated from the compilations of Pilson (1998), Siegenthaler (1986) and the reverences therein. Partition of
anthropogenic fluxes for the 1990s is from the IPCC report (2001).
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1015 g C or
Reservoirs (Gt):
1015 g C yr-1
Atmosphere: CO2 (288 ppm in 1850)
612
(369 ppm in 2000)
784
Oceans:
Biota
1-2
DOC
700
Org C in sediments (1 meter)
1,000
DIC
37,300
Terrestrial: Biota
600-1000
Soil Humus (1 meter)
1,500
Fossil Fuels (identified reserves), gas
44
oil
90
coal, oil sands and shales
3440
Fluxes (Gt yr-1):
Net Primary Production, Ocean
Land
Atmosphere-Ocean exchange
Net C export from the surface ocean
Sedimentation of Org. C. in the ocean
Anthropogenic Changes (Gt or Gt yr-1):
Cumulative Changes (Gt):
Atmospheric Increase 1850 – 1996
Fossil Fuels Burnt
1860 – 1996
From Plants and Soils oxidized 1860 - 1994
Partitioning of Anthropogenic Fluxes (1990s) (Gt yr -1)
Fossil Fuel and Cement Production
Atmosphere Accumulation
Uptake by Terrestrial Biosphere
Ocean Uptake
50
60
78
8-15
0.2
159
258
162
6.3  0.4
3.2 ± 0.1
1.4  0.7
1.7  0.5
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The next four figures and one table are from the IPCC report (2001)
____ Large fluxes active on short time scales = (730 Pg / 120 Pg yr-1 = 6.1 yr)
------ Fluxes of CaCO3 important on much longer time scales
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GPP (Gross Primary Production)
 Autotrophic Respiration
NPP (Net Primary Production)
 Heterotrophic Respiration
NCP (Net Community Production)
At steady state NCP = Organic Carbon Export
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II. The Marine Carbonate System
The carbonate system is defined by the Alkalinity (A) , Dissolved Inorganic Carbon
(DIC) and the chemical equilibria among the carbonate species
A. The alkalinity of seawater is the sum of the concentrations of anions that accept
protons at the pH of seawater.
The concentrations of the species that make up the alkalinity of seawater at pH = 8.2. (without
the contribution of silicate and phosphate).
Species
-log C
Concentration
% of Alkalinity
-1
-1
mmol kg
meq kg
HCO32.75
1.77
1.77
74
2CO3
3.58
0.26
0.52
22
B(OH)44.00
0.10
0.10
4
OH
5.20
0.01
0.01
0.4
Total Alkalinity (AT) = 2.40 meq kg-1
Note that the species of carbonate and Borate make up > 99% of the AT
-
Carbonate + Borate Alkalinity
AC&B = [HCO3-] + 2[CO32-] + [B(OH)4-]
A change in alkalinity is brought about only by the addition or subtraction of
charged species. For example precipitation or dissolution of CaCO3 changes the alkalinity
by 2 eq for every 1 mole of CaCO3 reacted:
CaCO3(s)  Ca2+ + CO32B. Dissolved Inorganic Carbon (DIC) (sometimes called “total CO2”, CO2)
DIC is the sum of the dissolved inorganic carbon species
DIC = [HCO3-] + [CO32-] + [CO2]
Changes in this quantity are brought about by:
(a) addition of fossil fuel CO2;
(b) by organic matter synthesis and degradation:
CH2O(o.m.)  CO2 + O2
( c) by CaCO3 precipitation and dissolution (via CO32-):
CaCO3(s)  Ca2+ + CO32-
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C. The Carbonate system of equations consist of the Alkalinity, DIC and chemical
equilibrium among the carbonate species:
(1)
AC&B = [HCO3-] + 2[CO32-] + [B(OH)4-]
(2)
DIC = [HCO3-] + [CO32-] + [CO2] (mol kg-1)
(3)
K1' 
K
(4)
(5)
'
2
HCO H 

3

CO2 
CO H 

HCO 
2
3


3
KH 
CO2 
fCO2
(Note: the fugasity of CO2, fCO2 is nearly the same as the pCO2 and they are used
interchangeably. In seawater fCO2 ~ 0.995 pCO2)
Borate Equilibria:
(6)
(7)
BT = [B(OH)4-] + [B(OH)3] (BT is the total borate concentration)
KB
B(OH ) H 


4

B(OH ) 3 
- The equilibrium constants are strong functions of temperature and pressure and minor
function of salinity so T, S and depth must be known to use K1', K2' KB' and KH'
- BT is conservative in seawater and has a constant ratio to Salinity
- Since equilibrium constants are known, there are 9 unknowns and 7 equations. All
carbonate parameters can be calculated if two are known. Often AC&B and DIC are measured.
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D. The pH dependence of the carbonate and borate species distribution is a result of the
acid base behavior of these weak acids. The AT and DIC and the equilibria control the pH of
seawater.
Notice:
- at Surface water pH (8.1-8.2) HCO3- dominates DIC, CO32- is next and CO2 is
relatively unimportant.
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E. A Matlab carbonate system program to calculate fCO2 from AC&B and DIC as a
function of T, S and depth. The program is on the web (co3eq.m)
co3eq(t,s,alk,dic)
Problems: Titration of Seawater with fossil fuel CO2
1. How much has the DIC, pH and [CO32-] of surface seawater changed by the addition
of CO2?
Given:
T = 20 C; S = 35 ppt; AC&B = 2300 eq kg-1; DIC (preindustrial) = 1970 mol kg-1
From the Program:
DIC
(mol kg-1)
1970
2000
2020
2030
2040
fCO2
(ppm)
280
323
356
374
393
pH
8.17
8.12
8.08
8.07
8.05
[CO32-]
(mol kg-1)
231
211
198
192
192
2. If the total fossil fuel added to date were well mixed in the ocean, what would be the
fCO2 of the atmosphere?
Given:
total fossil fuel added = 258 Gt; total DIC in seawater = 37,300 Gt
The increase in DIC is:
258 / 37,300 = 0.0069  0.0069 X 1970 mol kg-1 = 13.6 mol kg-1
Using the program:
For AC&B = 2300 eq kg-1; DIC = 1984 mol kg-1  fCO2 = 299
3. If all known fossil fuel reservoirs were burned and become well mixed in the sea,
how much of the C will be in the atmosphere and ocean?
Given: known fossil fuel reservoirs ~ 3440 Gt
Assuming this all accumulated in the ocean, the DIC increase would be (an approximation!):
3440 / 37,300 = 0.092 X 1970 mol kg-1 = 182 mol kg-1
Using the program:
for AC&B = 2300 eq kg-1; DIC = 2152 mol kg-1  fCO2 = 743 ppm
Atm increase in Gt:
(742 / 288 -1) X 612 Gt = 967 Gt
Ocean increase in Gt:
(2152 / 1970 -1) X 37,300 = 3445 Gt
Fraction in the sea:
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3445 / (3445 + 967) = 0.78
(A second iteration using 2440 x 0.78 for the amount in the ocean gives a fraction of 0.80.)
References:
IPCC (2001) Climate Change 2001: the scientific basis, Contribution of Working Group I in the
Third Assessment Report of the Intergovernmental Panel on Climate Change (Houghton
et al., eds.), Cambridge University Press, Cambridge, U.K. and New York, NY, USA,
881 pp.
Pilson, M. (1998) An Introduction to the chemistry of the sea, Prentice Hall, N.Y. , 431p.
Siegenthaler, U. (1986) Carbon dioxide: Its natural cycle and anthropogenic perturbation, in The
Role of Air-Sea Exchange in Geochemical Cycling (Buat-Menard, ed.), Ridel
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