Class 18 -- The Oceans

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Class 18 -- The Oceans

• More on the chemistry of the Oceans...
• DISSOLVED GASES IN SEA WATER
• Solubility of atmospheric gases
• CO2 and O2 -- Role of biological processes
• How the oceans influence atmospheric CO2
• (note much of this is NOT in the text!!!)

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

Why?
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C
CO2 Carbon Dioxide
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Oceans and Atmosphere Strongly Linked
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Chemical Equilibrium

• State of balance between opposing processes
• Example O2 dissolved in water, in contact with
  air
• Constant vibration/movement
• Some O2s in air enter the water
• Some O2s in water break free gt into air

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Air
Water
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• At the surface, equilibrium between water and
  atmosphere.
• Ocean water is saturated with these gases
• Solubility dissolved concentration at
  equilibrium

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So then why is CO2 so soluble???
Why?
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High solubility of CO2 -- Why??? -CO2 reacts with
water to form anions. Ions are held tightly in
the water structure. Chemical reactions
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Step 1 equilibrium between water and atmosphere.
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C
CO2 Carbon Dioxide
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CO2 Carbon Dioxide
C
(Imagine this molecule surrounded by H2O
molecules)
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H2CO3 Carbonic Acid
H
C
(Imagine this molecule surrounded by H2O
molecules)
H
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HCO3- Bicarbonate
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CO32- Carbonate
C
H
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Concentrations change until equilibrium is
established.
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Chemical equilibrium is attained, therefore The
following proportions occur for present-day
abundance of H (or pH)
Dissolved inorganic carbon (DIC) in sea water
sum of these three species
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H is VERY important, chemically and
biologically
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What is pH?

• H concentration of H
• Very important in chemistry
• Varies over a huge range
• Coca Cola H 0.01, or 10-2
• pH 2
• pH -1 x log aH

• Ammonia
• H 0.000000000001, or 10-12
• pH 12

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DISSOLVED INORGANIC CARBON (DIC) SYSTEM 1.
Buffers the pH, i.e., resists changes in H

• Life processes and many chemical reactions
  sensitive H

Reactions between DIC species can consume excess
H
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Example Huge release of acid (H) to oceans
(e.g., gigantic volcanic eruption --gt sulfuric
acid) CO32- H ltgt HCO3- CO32- converted to
HCO3- Absorbs acid Avoids major change in H
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Oceans and Atmosphere Strongly Linked
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2. Influences atmospheric CO2 content.

• Atmosphere/Ocean equilibrium at surface
• Huge amount of DIC in Oceans
• Ocean chemical equilibrium buffers atmospheric
  CO2 IF it changes slowly

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Response of ocean to increased atm. CO2

• In 1850, CO2 conc. was 280 ppmbut then
  Fossil-fuel burning Deforestation added CO2
• 2007, CO2 conc. 384 ppm
• Some of the extra CO2 enters ocean, converts to
  HCO3- and CO32-, and stays dissolved
• 50 (?) of CO2 produced by human activity has
  dissolved into oceans).
• But only the upper layers respond quickly
• AND CO2 uptake may slow

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Oceans and Atmosphere Strongly Linked
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3. Atmospheric CO2 content has gotten so high the
added carbonic acid has changed pH a little
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Doubling of H
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Dissolved CO2 AND O2 -- ROLE OF BIOLOGICAL
PROCESSES
6CO2 6H2O sunlight gt 6C6H12O6 6O2
6O2 6C6H12O6 gt 6CO2 6H2O energy
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Photosynthesis gt Respiration
Respiration Only
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O2 and CO2 Concentrations Depend on
Photosynthesis vs. Respiration Balance

• Photosynth. occurs in upper 150 m ("photic zone")
• Photosynth gt Resp. so O2 is high
• CO2 is controlled by equilib. w/ atmosphere
• Resp. continues at 200-800 m (No Photosynth.)
• O2 is low ("oxygen minimum zone")
• CO2 is high, greater than at surface
• Resp. continues weakly at gt800m -- but O2
  increases
• Convection currents Cold, O2-saturated water
  sinking at high latitudes and spreading at depth.

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Respiration in deep ocean controls CaCO3
saturation (CCD)
CO2 (respiration) gt H2CO3 gt HCO3 H H
causes dissolution of CaCO3 (calcareous particles)
CCD