The CO2 system

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The CO2 system
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Important Points
CO2 system buffers the Ocean TC, DIC, Alk, pH and
PCO2 all critical CaCO3 ppt impacted by CO2
system, T, ?, S
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Gases Dissolve in Water
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Why is the oceanic CO2 system important?
CO2 gas exchange Carbonate sedimentation
(CaCO3) Natural buffer of ocean pH Biologically
important Carbon cycle
1. Coral reefs 2. Foraminifera 3.
Coccolithophores 4. Mollusks
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CO2 system - biologically important.
Coccolithophores - the carbonate factory of the
surface ocean
Biology of Coccolithophores -primary producer
(autotroph) -Organism made of several individual
plates, or coccoliths, which are preserved in
sediments. These surround the cell, called a
coccosphere. -Among the fastest growing
planktonic algae most abundant in warm,
nutrient-rich areas of the ocean absent in
Arctic and Antarctic. -2-25 mm across, with
10-150 coccoliths on surface
Coccolithus pelagicus
Emiliania huxleyi
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Foraminifera -Single celled, heterotrophic -Usua
lly most abundant microfossil in
sediments -Planktonic and benthic forms -Size
range from 50-400mm -About 38,000 named species
since the Cambrian -Extremely important
geologically because of their abundance,
diversity and preservation - excellent
paleoenvironmental indicators (also, stable
isotopes)
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What about diatoms radiolarians?
Diatoms (autotrophs) and Radiolarians
(heterotrophs) are composed of silica (SiO2).
They are found abundantly in regions of high
silica, such as the Antarctic and N. Pacific.
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• Back to CO2 .
• How does CO2 enter seawater, and what happens to
  it?
• CO2 dissolves into seawater until equilibrium
  between seawater and atmosphere is reached
• This is governed by Henrys Law, so is a function
  of temperature, pressure and the composition of
  the solution.
• CO2(g) H2O H2CO3
• Where
• H2CO3 ?PCO2 (? is an equilibrium constant)

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A word on PCO2 Partial pressure of CO2 In
atmosphere Concentration of the CO2 molecules
in the air. In ocean Measurement of CO2 that is
in vacuum headspace after reaching equilibrium
(at in situ temperature).
Headspace under vacuum
Seawater sample
CO2
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CO2(g) H2O H2CO3 the oceanic
chemical pump Carbonic acid is a weak
acid a diprotic acid
(It can dissociate reasonably well)
(It will dissociate in two steps)
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This is what we are referring to when we talk
about DIC 38,000 Gt C tied up in the CO2 system
Carbonate Speciation
Carbonic acid Bicarbonate Carbonate
Seltzer water
Baking soda
Tums
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Lea Reader
Equivalence points pK1 6.2, or K1
10-6.2 pK2 9.4 or, K2 10-9.2
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Alkalinity is not
the same as Basicity!
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• Alkalinity
• The acid-neutralizing capacity (or buffering
  capacity) of a solution, or
• The concentration of negative charges in the
  solution which will react with added acid
  (donator of H) or,
• The amount of excess base relative to a specific
  end point (in seawater, the CO2 equivalence
  point).
• In seawater, (in mequiv/kg)
• Alk HCO3- 2CO32- B(OH)4- 0H--
  H
• The beauty of alkalinity
• -it is easy to measure
• -it is not dependent on state of system
  (pressure, temp)
• -it allows us to estimate HCO3- and CO32-

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Total Carbon, TC, DIC, ?CO2 ?CO2 ?H2CO3
HCO3- CO32-
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Broecker Peng (1982) developed simplifications
to look at Alk, TC and carbonate speciation.
These are - ignore acid base chemistry of
borate - assume concentration of H2CO3
relative to TC is small, so ignore, TC
HCO3- CO32- And, Alkalinity HCO3-
2CO32- CO32- Alkalinity - TC Units
mmol/kg
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CaCO3 (calcium carbonate, calcite) is the
material that is used by many organisms to
construct shells. The production
(precipitation) of calcite in surface waters is
dependant on nutrients light supersaturation
of calcite not Ca2. why? The preservation of
CaCO3 in sediments is dependent on pressure
(and therefore, topography depth) temperature
again, not Ca2
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Saturation of CaCO3 in the ocean Surface
waters all supersaturated Intermediate
waters most supersaturated Deep waters most
undersaturated Aragonite, an alternate form of
CaCO3, is undersaturated nearly everywhere, at
all depths.
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What controls the saturation of CaCO3? Pressure
(therefore, depth) increase pressure, increase
solubility Temperature decrease temp,
increase solubility Organic matter degradation
(pH) decrease pH, increase solubility
To summarize, calcite is supersaturated (and
therefore well preserved) in shallow, warm
waters, and along topographic highs in the deep
sea.
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Water Mass Changes
Broecker, W.S., Chemical Oceanography, 1974
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Calcite Saturation Horizon The depth range over
which D1 (saturation) ranges for the ocean
basins Atlantic 4.5 km Indian 3.5 km North
Pacific 3 km If the CS varied simply due to TC
content, we would expect the oldest deep waters
to have the highest CS! Remember that CS varies
due to depth, temperature, and
importantly amount of respiration (Alk/TC ratio)
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Describing the preservation of calcite in
sediments
LYSOCLINE Depth at which solution effects are
first observed in sediments CALCITE COMPENSATION
DEPTH (CCD) Rain ratedissolution rate Or, no
preservation of calcite.
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The CCD and the distribution of sediments in deep
ocean basins
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How do anthropogenic processes change carbonate
system?
What happens to the Alk/TC ratio, CO32- and
the depth of the lysocline as atmospheric CO2
increases due to the burning of fossil fuels?
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Anthropogenic CO2 where does it go?
An estimated 379 Gt of carbon have been added
anthropogenically to the atmosphere. The
atmospheric change is 160 Gt of carbon, meaning
219 GtC must have been absorbed elsewhere. Best
estimate of oceanic CO2 sink 155 GtC Missing
sink 64 GtC

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Uptake of CO2
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6.81015 g CO2
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Atmosphere
Land
Ocean
CO2 (dissolved)
H2CO3