1

Marine Chemistry II

Environmental Geochemistry DM Sherman, University of Bristol

Carbonate Equilibria and the pH of Seawater

2

Atmosphere (748 as CO2)

Lithosphere (50 x 106 as CaCO3; 3870 as Oil/Gas/Coal)

Biosphere

(1600)

Oceans

(37000 as HCO3-)

Volc

anic

Em

issi

on

(0.0

48-0

.18)

CO2 Dissolution/Degassing (100) Respiration (60) Photosynthesis (120)

The Global Carbon Cycle (in 1015 g)

Soil (1200-1600) 0.2

Res

pira

tion

(60)

Rivers (0.6)

Speciation of H2CO3

3

Open Carbonate System (fixed PCO2= 10-3.5 bar)

CO2(g) + H2O = H2CO3 pK = 1.46

p H2CO3 = pK1+ pPCO2 (If pPCO2 = 3.5 then, pH2CO3 = 4.94)

H2CO3 +H2O = H3O+ + HCO3- pK = 6.35

pHCO3- = pK2 + pH2CO3 - pH =11.29 - pH

HCO3- +H2O = H3O+ + (CO3)2- pK = 10.33

p(CO3)2- = pK3 + pHCO3- - pH = 21.62 - 2pH

Resulting pH of Rain...The charge-balance constraint [H+] = [HCO3-] +2[CO3-2] gives pH pHCO3- = 5.6

4

Effect of CaCO3 (Calcite/Aragonite)

If the system is saturated in calcite, we need the additional equilibrium:

CaCO3(s) = Ca2+ + (CO3)2- pK = 8.48

pCa2+ = pK4- p(CO3)2- + 2pH

= -13.14 + 2pH

Charge-balance requires that

2[Ca] = 2[CO3-2] + [HCO3-] [HCO3-]

or pCa = pHCO3- + 0.301

pH of Seawater (Surface, pPCO2 = 3.5)When pCa = pHCO3 + 0.301, pH = 8.3

5

Actually, its a bit more complicated..When we include all of the other acid-base and ion-pairing equilibria, we find that surface seawater should have a pH near 7.8.

The observed surface pH = 8.3 is a consequence of supersaturation of CaCO3 (SI = 1.2).

Presumably, supersaturation is favored by calcifying organisms as it is easier to grow shells under such conditions.

Predicted Changes in Oceanic pH (Caldeira & Wickett 2003)

6

Biological Controls on Dissolved Carbonate

CO2 + 2H2O CH2O + O2

i) CO2 consumption by photosynthesis:

ii)CO2 release (mineralization) by respiration CH2O + O2 CO2 + 2H2O

(Note, the arrows indicate irreversible reactions!)

iii) Biomineralization of CO3-2 Ca+2 + CO3-2 = CaCO3(calcite)

CaCO3-Shelled ZooplanktonForaminifera

Pteropods

7

CoccolithophoridsSingle-celled phytoplankton with cell walls made of CaCO3 (calcite). Large blooms dominate the ocean surface.

Map of near-surface marine calcite (from NASA)

pH of Seawater vs. Depth

Respiration decreases pH: CH2O + O2 CO2 + H2O CO2 + H2O = H+ + HCO3-

Dissolution of CaCO3: CaCO3 + H+ = Ca2+ + HCO3-

Photosynthesis increases pH: H+ + HCO3- = CO2 + H2O CO2 + H2O CH2O + O2

8

Alkalinity and Total Dissolved Carbon

[Alk] = [HCO3-] + 2[CO3-2] + [OH-] - [H+]

Alkalinity is the capacity of a solution to neutralize added acid.

Carbonate Alkalinity is

[HCO3-] + [CO3-2] + [H2CO3]

Total Dissolved Inorganic Carbon (CO2) is

Effect of Photosynthesis

CO2 + H2O CH2O + O2

Photosynthesis will not affect alkalinity but will increase the pH:

H2CO3 = CO2 + H2O

H+ + HCO3- = H2CO3

H+ + HCO3- CH2O + O2

The pH increase will favor pptn of CaCO3:

Ca+2 + HCO3- = CaCO3 + H+

9

Effect of Respiration

CH2O + O2 CO2 + H2O

Respiration will not affect alkalinity but will decrease pH:

CO2 + H2O = H2CO3

H2CO3 = HCO3- + H+

CH2O + O2 H+ + HCO3-

The pH decrease will favor dissolution of CaCO3:

CaCO3 + H+ = Ca+2 + HCO3-

Effect of CaCO3 precipitation

Ca+2 + CO3- CaCO3

Ca+2 + 2HCO3- CaCO3 + CO2 + H2O

HCO3- = H+ + CO3-

H+ + HCO3- = H2CO3

H2CO3 = CO2 + H2O

Hence, pptn of CaCO3 actually releases CO2, decreases the alkalinity, and decreases pH.

10

Shallow vs. Deep Ocean

Shallow vs. Deep Ocean

Surface Ocean

Deep Ocean

Downwelling

Upwelling Sinking Particles

Sinking calcite shells and organic matter provides a one-way flux of C to the deep ocean.

Oxidation of organic C and dissolution of CaCO3 will increase CO2 in Deep Ocean.

11

Effect of Biological Processes

CaCO3 pptn

Photosynthesis

Distribution of Carbonate Sediments

12

Carbonate Depth Profile (Mid Atlantic)

Surface waters are super- saturated with CaCO3.

Deep waters are under- saturated with CaCO3.

Calcite Compensation Depth:

Dissolution Rate > Rain Rate

CaCO3 saturation Depth (Feely et al., 2004)

13

Change in Alkalinity/TDC in Oceans

14

Absorption of Atmospheric CO2 by the Oceans

CO2 + H2O + CO3-2 = 2HCO3-

H+ + HCO3- CH2O + O2

Ca+2 + 2HCO3- = CaCO3 + CO2 + H2O

CH2O + O2 H+ + HCO3-

CaCO3 + H+ = Ca+2 + HCO3-

Atmospheric CO2

Sinking of CH2O + CaCO3

Gas Absorption

Upwelling

Shallow Ocean: carbon fixing by photosynthesis.

Deep Ocean: mineralization by respiration.

SummaryEquilibrium with atmospheric PCO2 = 10-3.5 bar and saturation in CaCO3 buffers the pH of seawater to 8.3.

CO2 is sequestered in the deep ocean by the biological pump.

Surface waters are supersaturated in calcite; deep waters are undersaturated. The carbonate compensation depth reflects the competition between the kinetics of dissolution and the downward flux of particulate CaCO3.

15

Box Model for OceansSurface Ocean

Atlantic

Southern

Indian Pacific

Primary Productivity