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Lecture 18: DISSOLVED GASES IN SEA WATER

Warning: Superscripts and subscripts do not show well on these web notes!

Solubility of atmospheric gases
CO
2 and O2 -- Role of biological processes
Importance of the dissolved "CO
2" system in sea water
... "buffers" against sudden chemical changes
... controls CaCO
3 saturation

Powerpoint Lecture Slides


ATMOSPHERIC GASES
Composition of atm.
Atmospheric gases in sea water
-- saturation = equilibrium

 Molecule  Percent in atmosphere Equilibrium concentration
in seawater
(mg per kg seawater)
 N2  78%  12.5
 O2  21%  7
 Ar  1%  0.4
 CO2  0.03%  90

Equilibrium conc. depends on T, S, & pressure
temperature -- colder sea water holds more gas
salinity -- low-S sea water holds more gas
pressure -- sea water at depth holds more gas

High solubility of CO2 -- dissolves, reacts with water, and dissociates to anions:

CO2(g) <=> CO2(d)
CO2(d) + H2O <=> H2CO3 [carbonic acid]
H2CO3 <=> H+ + HCO3- [bicarbonate]
HCO3- <=> H+ + CO3= [carbonate]

"Total Dissovled CO2" (TDC) in sea water = sum of all species

CO2(d) + H2CO3 1 %
HCO3- 93 %
CO32- 6 %

DISTRIBUTION OF CO2 AND O2
-- ROLE OF BIOLOGICAL PROCESSES
Photosynthesis (Ph) and Respiration (Re): Schematically....
Ph: CO2 + H2O => CH2O+ O2
Re: CH2O+ O2 => CO2 + H2O

Total Ph rate ~ Total Re rate for global oceans.

But, Ph rate vs. Re rate varies from surface waters to deep waters
---> TDC and [O2] vary with depth.
Ph can occur only in upper 150 m ("photic zone")
Ph > Re
[O2] is high
TDC is saturated
Re is dominant at 200-800 m (Ph = 0)
O2 is consumed, and therefore...
[O2] is low ("oxygen minimum zone")
TDC is high because CO2 is produced
Re continues at depth, slowly -- but [O2] increases!
Cold, O2-saturated water sinking at high latitudes and spreading at depth.

DISSOLVED CO2 SYSTEM -- IMPORTANT IN REGULATING SEA WATER AND ATMOSPHERE.

1. It "buffers" changes in acidity, [H+] .....pH = - log [H+]
Life processes and many chemical reactions sensitive to pH
Reactions between "CO2" species consume (or produce) H+
These reactions are fast and equilibrium is established

CO2(g) in atmosphere

CO2(d) + H2O <=> H2CO3
H2CO3 <=> H+ + HCO3-
HCO3- <=> H+ + CO3=

Example:
Catastrophic release of H+ to oceans
(e.g., super-big volcanic eruption)

Higher H+ conc. means more collisions of H+ with HCO3- and CO3=
Drives the three reactions above to the left
 
Change ratios of species (like HCO3- / CO32-) without major change in [H+]
 
2. It "buffers" ocean-atm. system against big changes in atmospheric
CO2 content.
Changes in atmospheric CO2 conc.
in 1850, CO2 conc. = 280 ppm
Fossil-fuel burning
Deforestation
in 1998, CO2 conc. = 360 ppm
Response of ocean to increased atm. CO2
Some of CO2 dissolves --> increases [H+]
CaCO3 dissolves -- consumes (neutralizes) H+
More CO2 can dissolve (~50% of CO2 produced by human activity has dissolved in oceans.

CO2(g) in atmosphere
----------------------------
CO2(d) + H2O <=> H2CO3
H2CO3 <=> H+ + HCO3-
HCO3- <=> H+ + CO3=

CaCO3 + H+ <=> Ca2+ + HCO3-

3. Respiration in deep ocean controls CaCO3 saturation (CCD)
Respiration releases CO2 --> increases [H+]
Deep waters become undersaturated in CaCO3 -- it dissolves!


(Detailed notes start here)

Atmospheric gases.

The major gases in the atmosphere are nitrogen and oxygen, with smaller amounts of argon and carbon dioxide. (Water vapor is important also, but we don't usually "count"it because water-vapor content depends on temperature -- the higher the temperature, the higher the water-vapor content.)

In surface sea water, atmospheric gases are close to their "saturation" concentration (or equilibrium concentration). At saturation, the concentration of a gas in sea water increases with
(a) decreasing temperature [cold sea water holds more gas]
(b) decreasing salinity [more dilute sea water holds more gas]
(c) increasing pressure [deeper sea water holds more gas]

But note that CO2 has a much higher solubility (equilibrium concentration) than the other gases. That's because CO2 not only dissolves, but also reacts with H2O to form H2CO3 (carbonic acid) and dissociates to form the anions HCO3- (bicarbonate) and CO3= (carbonate)

"Total Dissolved CO2" (TDC) in sea water is the sum of those species. This most abundant dissolved CO2 species is bicarbonate.

Role of biological processes in the distribution of CO2 and O2 in sea water.

The most important life processes in the oceans (and everywhere else) are photosynthesis and respiration.
* Photosynthesis by plants (and some other "lower" organisms) uses sunlight to convert CO2 and H2O to a carbohydrate compound; O2 is a by-product of that reaction.
* Respiration is the process that all organisms use to recover the chemical energy stored in organic compounds. O2 is used to oxidize organic matter and release chemical energy; CO2 and H2O are by-products.

On an ocean-wide basis, the rates of photosynthesis and respiration are nearly balanced. In other words, all of the organic matter produced by photosynthetic algae is respired by consumer and decomposing organisms. (There is, in fact, a slight excess in photosynthesis that is balanced by organic matter deposited and preserved in sediments.)

But photosynthesis and respiration do not balance at any given depth in the oceans. Because of that imbalance, the concentrations of TDC and O2 also vary with depth.
* Photosynthesis only occurs in the upper 150 m. In that "photic zone" Ph > Re and thus the concentration of O2 is high.
* Respiration reaches a maximum at 200-800 m, and thus O2 concentrations are low. (This depth range is called the "oxygen minimum zone.")
* Respiration continues at depth. But there is often an increase in O2 in deep and bottom water. This is due to the sinking and spreading of cold, O2-saturated water from high latitudes. These waters transport O2 faster than it is utilized by respiration.

Dissolved CO2 system in sea water.

Reactions involving dissolved CO2 species are very important in regulating the chemistry of sea water and the atmosphere -- and in making the Earth's surface a habitable environment.

1. Reactions involving dissolved CO2 species buffers oceans against sudden changes in acidity.
* Life processes and many chemical reactions in the oceans are quite sensitive to the hydrogen ion concentration [H+], or pH, in sea water.
* The dissolved CO2 system is a powerful pH buffer. Reactions between those species can consume (or produce) large amounts of hydrogen ion.

2. Reactions involving dissolved CO2 species buffers the atmosphere-ocean system against rapid changes in CO2 content.
The burning of fossil fuels and deforestation over past 150 years has increased CO2 concentration in the atmosphere from 280 ppm to 360 ppm. How does the oceans respond to an increase in atmospheric CO2?
Higher atm. CO2 --> more dissolution in sea water --> higher [H+]
Dissolution of CaCO3 neutralizes (consumes) H+
CaCO3 + H+ ---> Ca2+ + HCO3-
Neutralization allows more atm. CO2 to dissolve. About 50 % of CO2 released by human activity has dissolved in the oceans.

3. Respiration of CO2 in the deep ocean controls CaCO3 saturation and thus the distribution of calcareous sediments.
Respiration of organic matter at depth releases dissolved CO2 (d).
This CO2 (d) reacts with water, dissociates, and increases [H+].
As a result, deep waters are undersaturated in CaCO3. In other words, CaCO3 dissolves.
CaCO3 + H+ -> Ca2+ + HCO3-

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