1. Baltic Sea Research Institute Warnemünde
The Baltic Sea CO2 system and carbon cycle
Bernd Schneider
Baltic Sea Research Institute Warnemünde
Germany

2. CO2 in seawater: Chemical equilibria and characteristic variables
Part 1
CO2 in seawater: Chemical equilibria and characteristic variables
CO2 solubility;
CO2 reactions with water;
the buffer (re-equilibration) reaction;
CO2 partial pressure;
total CO2;
pH;
alkalinity;

3. CO2 solubility in seawater:
atmosphere
CO2 solubility constant, ko [ (mol/kg)/atm ]:
CO2: 380 ppm (0.038 %)
pCO2 = 380 µatm
seawater
But: In most cases seawater is not at equilibrium with CO2 in the atmosphere!

4. CO2 reactions with seawater:
Definition:

5. CO2 equilibria in seawater:
Almost spontaneous equilibration!
k1 ( S=7 )
k2( S=7 )

6. Main reaction (buffer reaction):
CO2 re-equilibration after physically (temperature, salinity) and/or biologically
(production or decomposition of organic matter) imposed changes of the CO2 system:
Main reaction (buffer reaction):
k1/k2 ( S=7 )
k1/k2 ( T=10°C )

7. Characteristic (measurable) variables of the CO2 system:
1. pH
Baltic Sea:
Surface water: 8.0 – 8.6
Deep water: down to 7.3
2. CO2 partial pressure, pCO2
Baltic Sea surface water: 100 – 600 µatm (atmosphere: 380 µatm)
The difference between the atmospheric and the surface water pCO2 (disequilibrium) causes CO2 gas exchange:
FCO2 – CO2 flux
k – gas exchange transfer velocity

8. 3. Total CO2, CT Baltic Sea (highly dependent on the region):
1400 – 1700 µmol/kg (central Baltic Sea)
Relative composition of CT in clean water (CT = 23 µmol/kg) and Baltic seawater (CT = 1570 µmol/kg):
88%
96%
12%
0.0001%
1.3%
2.7%

9. 4. Alkalinity, AT 1. Dissolution of CaCO3 in river water:
2. Reaction of the CO2 system:
3. The consumption of CO2 leads to undersaturation and
CO2 uptake from the atmosphere:
4. The reaction stops when both equilibrium with the atmospheric CO2 and with
CaCO3 solubility is reached (controlled by the increase of HCO3-).

10. CONCLUSION:
The amount of CaCO3 dissolved in river water and transported into the sea controls mainly the total CO2 !
5. According to the ion balance the dissolution of CaCO3 corresponds to an
excess of bases (proton acceptors) over H3O+, which is the major contribution to
the alkalinity in seawater:
6. The excess of proton acceptors can be determined by titration with an acid
(HCl). The titration also captures also other proton acceptors present in
seawater: mainly borate and - at anoxic conditions- ammonia and hydrogen
sulphide. Thus the total alkalinity, AT, is given by:

11. Alkalinity in the Baltic Sea:
(Baltic-C data, Matti Perttilä)

12. CO2 partial pressure, pCO2 4. pH
SUMMARY:
Four variables are characterizing the marine CO2 system:
Total CO2, CT
Total alkalinity, AT
CO2 partial pressure, pCO2
4. pH
conservative, not dependend on temperature and pressure (depth)
dependend on temperature and pressure (depth)
Two variables are necessary to determine the whole CO2 system on the basis of
the equilibrium constants (co2sys programme by Pelletier, Lewis and Wallace):
ko (Weiss, 1976)
k1, k2 (Millero et al., 2006)

13. Physical and biogeochemical control on the marine CO2 system
Part 2
Physical and biogeochemical control on the marine CO2 system
temperature changes;
biomass production;
mineralization of organic matter (oxic, anoxic);

14. Effect of temperature changes on the pCO2, [CO32-] and pH:
The equilibrium of the buffer reaction is a useful tool to estimate changes of the CO2 system:
with:
With increasing temperature
both the pCO2 (about 4% per °C)
and [CO32-] must increase!

15. The increase of [CO2*] causes an increase of [H3O+] and thus a decrease of pH;
The increase of [H3O+] is partly counterbalanced by reaction with [CO32-], hence,
the pH decrease is moderate;
Excercise-1:
The previous considerations were implicitely based on the assumption that no CO2
gas exchange with the atmosphere occurred during warming.
Calculate the pH change during a temperature increase from 0°C to 25°C. Produce
a plot for pH as a function of temperature and determine the mean pH change per
°C for both cases:
a. no CO2 exchange with the atmosphere;
b. spontanous CO2 equilibration with the atmosphere;
The water mass is at equilibrium with the atmosphere and characterized by: S=7,
T=10°C, AT=1600 µmol/kg, CT=1570 µmol/kg. Use the co2sys software (Pelletier et al.)
with the constants given by Millero et al. (2006) and the total pH scale.

16. Effect of photosynthesis on the CO2 system:
Redfield ratio:
C/N/P = 106/16/1
pCO2 changes:
Consumption of 4 µmol-nitrate/L
corresponding to a CT loss of
about 27 µmol/kg:
controlled by the Revelle factor:

17. pH changes during organic matter production:
The Revelle factor:
Baltic Sea: 20 < R < 30
Ocean: < R < 15
pH changes during organic matter production:
(Data from SMHI)
The loss of CT by photosythesis at the start of the spring bloom in the beginning of April causes an increase of the pH.

18. Organic matter production and alkalinity:
1. The removal or addition of CO2 does not affect the alkalinity,
because:
2. The uptake of nitrate, NO3-
during photosynthesis is
accompagnied by the
uptake of H3O+ (HNO3).
Hence, AT is increasing, but
the effect on pCO2 is small.
effect of 4 µmol-NO3 uptake

19. 3. Most ocean areas are supersaturated with regard to CaCO3:
Under these conditions some plankton organisms form CaCO3 shells. The consumption of CO32- ions decreases the alkalinity and increases the pCO2:
Exercise-2: On average the CO32- used for the formation of CaCO3 amounts to
20 % of the production of organic carbon. In the North Atlantic 12 µmol-NO3/L are
consumed during the spring bloom. Calculate the change in pCO2 for the case:
that no CaCO3 is formed;
that CaCO3 is formed;
At the start of the spring bloom (T=8°C, S=35) the water has an alkalinity of
2350 µmol/kg and a pCO2 of 410 µatm.
Use again the co2sys software.

20. Organic matter decomposition (oxidation):
Reversal of the photosynthesis:
But: After consumption of the dissolved oxygen, the sulphur in the sulfate ions acts as oxidizing agent (but not for ammonia):
During sulfate reduction the alkalinity is increased. The release of 106 CO2 molecules gives an alkalinity change of:
As a consequence the pCO2 increase and pH decrease by the release of CO2 is compensated for by the alkalinity increase!

21. Excercise-3: The deep water of the Baltic Sea contains 368 µmol/kg oxygen. How much CO2 is released after the entire consumption of the oxygen by mineralization? Before the onset of the mineralization the pCO2 was 400 µatm. How big is the pCO2 after the
consumption of all oxygen? (T = 5°C, S=12, AT=1717 µmol/kg)
Again, use co2sys.

22. The Baltic Sea CO2/carbon cycle and ist anthropogenic perturbations
Part 3
The Baltic Sea CO2/carbon cycle and ist anthropogenic perturbations
Inputs and outputs of inorganic carbon: CT (AT) and
organic carbon: dissolved (DOC) and particulate (POC)
control the total carbon inventory of the Baltic Sea:
atmophere
CO2
CT, AT
CT, AT
North Sea
Rivers
DOC
POC
DOC, POC
CT, DOC
Baltic Sea
sediments

23. The internal CO2/carbon cycle in the Baltic Sea:
Relationship between mixing/stratification and CO2 seasonality in the Baltic Proper

24. Seasonality of biomass production and mineralization above the halocline, detectable by measurements of the surface water pCO2:
pCO2 data from the northern Gotland Sea,
collected with an automated measurement
system deployed on a cargo ship;
Seasonal changes in CT calculated from the pCO2 data and the mean alkalinity;

25. Accumulation of CT in the deep water of the Gotland Sea during a period of stagnation:

26. Carbon budget for the Gotland Sea deep waterduring stagnation, fluxes in mmol/m2 yr:

27. Anthropogenic perturbations: Acidification?
Increasing atmospheric pCO2 decreases the pH and the CaCO3 saturation. This effect may be enhanced by increasing acidic precipitation over the Baltic Sea.
pH decrease during at the start
of the spring bloom caused
by doubling the pre-industrial
atmospheric CO2:

28. CaCO3 saturation:
Decrease of [CO32-] according to:
Decrease of CaCO3 saturation
at the start of the spring bloom
caused by doubling the pre-
industrial atmospheric CO2:

29. But:
It is also likely that the alkalinity input into the Baltic Sea will increase because the increasing atmospheric CO2 accelerates the weathering of CaCO3 in the catchment area. This will at least dampen acidification and CaCO3 undersaturation.
The final answer on the future Baltic Sea CO2 system will be given by the BONUS Project:
Baltic-C !