class 18 -- the oceans more on the chemistry of the oceans... dissolved gases in sea water...

Class 18 -- The OceansClass 18 -- The Oceans

More on the chemistry of the Oceans...More on the chemistry of the Oceans...

DISSOLVED GASES IN SEA WATERDISSOLVED GASES IN SEA WATER• Solubility of atmospheric gasesSolubility of atmospheric gases

• COCO2 2 and Oand O22 -- Role of biological processes -- Role of biological processes

• How the oceans influence atmospheric COHow the oceans influence atmospheric CO22

(note: much of this is NOT in the text!!!)(note: much of this is NOT in the text!!!)

ATMOSPHERIC GASESATMOSPHERIC GASES•How much of each gas is present in the How much of each gas is present in the seawater that is in contact with the seawater that is in contact with the atmosphere?atmosphere?

Percent inatmosphere

Equilibrium concentrationin seawater

(ppm, or mg/kg)

N2 78% 12.5

O2 21% 7

Ar 1% 0.4

CO2 0.03% 90Why?Why?

CO O

COCO22: Carbon Dioxide: Carbon Dioxide

Oceans and Atmosphere Strongly Linked

Chemical EquilibriumChemical EquilibriumState of balance between opposing processesState of balance between opposing processesExample: OExample: O22 dissolved in water, in contact with air dissolved in water, in contact with air•Constant vibration/movementConstant vibration/movement•Some OSome O22’s in air enter the water’s in air enter the water•Some OSome O22’s in water break free => into air’s in water break free => into air

O-O

O-O O-O

O-O

O-O

O-OO-O O-O

O-O

O-O O-OO-O O-O

O-OO-O O-O

O-O

O-O

O-O O-O

O-O

O-O

O-OO-O O-O

Water

Air

At the surface, equilibrium between water and atmosphere.At the surface, equilibrium between water and atmosphere.

-Ocean water is Ocean water is saturatedsaturated with these gases with these gases

-Solubility = dissolved concentration at equilibriumSolubility = dissolved concentration at equilibrium

O-O

O-O O-O

O-O

O-O

O-OO-O O-O

O-O

O-O O-OO-O O-O

O-OO-O O-O

O-O

So then why is COSo then why is CO22 so soluble??? so soluble???

Percent inatmosphere

Equilibrium concentrationin seawater

(ppm, or mg/kg)

N2 78% 12.5

O2 21% 7

Ar 1% 0.4

CO2 0.03% 90Why?Why?

High High solubilitysolubility of CO of CO22 -- Why??? -- Why???

-CO-CO22 reacts with water to form anions. reacts with water to form anions.

Ions are held tightly in the water structure.Ions are held tightly in the water structure.

Chemical reactions:Chemical reactions:CO2(g) ↔ CO2(d)

CO2(d) + H2O ↔ H2CO3 [carboni c ac ]id

H2CO3 ↔ H+ + HCO3- [bicarbonate]

HCO3- ↔ H+ + CO32- [carbonate]

Step #1 equilibrium between water and atmosphere.Step #1 equilibrium between water and atmosphere.

O-O

O-O O-O

O-O

O-O

O-OO-O O-O

O-O

O-O O-OO-O O-O

O-OO-O O-O

O-O

CO O


O

O

OO

O O

OO

CO O


(Imagine this (Imagine this molecule molecule surrounded by surrounded by HH22O molecules)O molecules)

H

O

H

C

O

O

O

H

H

HH22COCO33: Carbonic Acid: Carbonic Acid

(Imagine this (Imagine this molecule molecule surrounded by surrounded by HH22O molecules)O molecules)

HCO3- = Bicarbonate

C

O

O

O

H

H+

H+

CO32- Carbonate

C

O

O

O

H+

Concentrations change until equilibrium is Concentrations change until equilibrium is established.established.

CO2(g) ↔ CO2(d)

CO2(d) + H2O ↔ H2CO3 [carboni c ac ]id

H2CO3 ↔ H+ + HCO3- [bicarbonate]

HCO3- ↔ H+ + CO32- [carbonate]

Dissolved inorganic carbon (DIC) Dissolved inorganic carbon (DIC) in sea water in sea water

= sum of these three species= sum of these three species

CO2(d) + H2CO3 1 %

HCO3- 93 %

CO32- 6 %

Chemical equilibrium is attained, therefore…Chemical equilibrium is attained, therefore…

The following proportions occur for present-The following proportions occur for present-day abundance of Hday abundance of H++ (or pH): (or pH):

HH++ is VERY important, is VERY important, chemically and chemically and biologicallybiologically

• [H[H++] = concentration of H] = concentration of H++ • Very important in chemistryVery important in chemistry• Varies over a huge range:Varies over a huge range:

–Coca Cola: [HCoca Cola: [H++] = 0.01, or 10] = 0.01, or 10-2-2

pH = 2pH = 2

pH = -1 x log [pH = -1 x log [aaH+H+ ]]

What is pH?What is pH?

–Ammonia: Ammonia:

[H[H++] = 0.000000000001, or 10] = 0.000000000001, or 10-12-12

pH = 12pH = 12

•Life processes and many chemical reactions sensitive [HLife processes and many chemical reactions sensitive [H++]]

DISSOLVED INORGANIC CARBON (DIC) DISSOLVED INORGANIC CARBON (DIC) SYSTEM: SYSTEM:

1. Buffers the pH, i.e., resists changes in [H1. Buffers the pH, i.e., resists changes in [H++]]

Reactions between DIC species can consume excess HReactions between DIC species can consume excess H++

Example:

Huge release of acid (H+) to oceans

(e.g., gigantic volcanic eruption --> sulfuric acid)

CO32- + H+ <=> HCO3

-

CO32- converted to HCO3

-

Absorbs acid

Avoids major change in [H+]

•Atmosphere/Ocean equilibrium at surfaceAtmosphere/Ocean equilibrium at surface•Huge amount of DIC in OceansHuge amount of DIC in Oceans

•Ocean chemical equilibrium “buffers” atmospheric COOcean chemical equilibrium “buffers” atmospheric CO22 IF it IF it

changes slowlychanges slowly

2. 2. Influences atmosphericInfluences atmospheric CO2 contentCO2 content..

In 1850, COIn 1850, CO22 conc. was 280 ppm…but then conc. was 280 ppm…but then

Fossil-fuel burning + Deforestation added CO Fossil-fuel burning + Deforestation added CO22

2007, CO2007, CO22 conc. = 384 ppm conc. = 384 ppm

• Some of the extra COSome of the extra CO22 enters ocean, converts enters ocean, converts

to to HCOHCO33-- and CO and CO33

2-2-,, and stays dissolved and stays dissolved

• ~50% (?) of CO~50% (?) of CO22 produced by human activity produced by human activity

has dissolved into oceans).has dissolved into oceans).• But But only the upper layers respond quicklyonly the upper layers respond quickly

• AND COAND CO22 uptake may slow uptake may slow

Response of ocean to increased atm. COResponse of ocean to increased atm. CO22

3. Atmospheric CO3. Atmospheric CO22 content has gotten so high content has gotten so high

the added carbonic acid has changed pH a littlethe added carbonic acid has changed pH a little

Doubling of H+

6O2 + 6C6H12O6 => 6CO2 + 6H2O + energy

6CO2 + 6H2O + sunlight => 6C6H12O6 + 6O2

Dissolved CODissolved CO22 AND O AND O2 2

-- ROLE OF BIOLOGICAL PROCESSES-- ROLE OF BIOLOGICAL PROCESSES

Respiration Only

Photosynthesis > Respiration

•Photosynth. occurs Photosynth. occurs in upper 150 min upper 150 m ("photic zone") ("photic zone")

–Photosynth > Resp. so [OPhotosynth > Resp. so [O22] is high] is high

–[CO[CO22] is controlled by equilib. w/ atmosphere] is controlled by equilib. w/ atmosphere

•Resp. continues at Resp. continues at 200-800 m200-800 m (No Photosynth.) (No Photosynth.)

–[O[O22] is low ("oxygen minimum zone")] is low ("oxygen minimum zone")

–[CO[CO22] is high, greater than at surface] is high, greater than at surface

•Resp. continues Resp. continues weakly at >800mweakly at >800m -- but [O -- but [O22] ]

increasesincreases

–Convection currents… Cold, OConvection currents… Cold, O22-saturated water -saturated water

sinking at high latitudes and spreading at depth.sinking at high latitudes and spreading at depth.

OO22 and CO and CO22 Concentrations Depend on Concentrations Depend on

Photosynthesis vs. Respiration BalancePhotosynthesis vs. Respiration Balance

Respiration in deep ocean controls CaCORespiration in deep ocean controls CaCO33

saturation (CCD)saturation (CCD)

CO2 (respiration) => H2CO3 => HCO3+

+ H+

H+ causes dissolution of CaCO3 (calcareous particles)

CCD

class 18 -- the oceans more on the chemistry of the oceans... dissolved gases in sea water...

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