Ocean Chemistry

1
Ocean Chemistry
2
Ocean Circulation

• The oceans are extremely well mixed
• The composition is the same anywhere in the world
• Wind stress
• Affects upper 2000 meters
• Thermohaline forcing
• Primary deep water production - occurs at two
  places
• North Atlantic Deep Water (NADW)
• forms at rate of 10 million m3/sec
• caused by evaporation in N. Atlantic (where deep
  water sinks)
• greatest volume of deepwater production at the
  surface
• Antarctic Bottom Water (AABW)
• sea ice forms around Antarctica, mainly in the
  Weddell and Ross Seas
• this produces cold, saline water that sinks
• Also in Atlantic
• Mediterranean Overflow Water
• Antarctic Intermediate Water

3
Ocean Circulation
4
Ocean Circulation
5
MOW
AIW
NADW
ABW
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Seawater Chemistry

• Sources of dissolved components
• Rivers
• Hydrothermal
• Wind/Rain
• Pore water flux
• Sinks of dissolved components
• Sediment Burial
• Hydrothermal
• Volatilization/Sea-spray

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Major Constituents of Seawater

• water (96.5)
• dissolved salts (35 or 3.5)
• Cl- (chloride) 56
• Na (sodium) 28
• SO4- (sulfate) 8
• Mg2 (magnesium) 4
• Ca2 (calcium) 1.5
• K (potassium) 1
• HCO3- (bicarbonate) 0.5
• all other ions 1
• dissolved gases
• N2, O2, CO2, He, Ar

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Seawater Chemistry
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All Elements are Found in SeawaterRange of
Concentrations
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All Elements are Found In Seawater Residence
Times of Elements
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Seawater Composition

• Sillèn (1967) proposed that the ocean represented
  a chemical equilibrium between seawater,
  atmosphere, and solids deposited on the ocean
  floor.
• Ignores biological activity
• Not supported by isotopic studies of sediments
• Good in concept, but falls apart in the details

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Seawater Composition

• Box Models ocean divided into individual boxes
  of uniform composition where input-output is
  steady state.
• Conservative element box models
• Entire well mixed ocean
• Inputs and outputs balance
• Non-conservative element box models
• Divide ocean into a number of homogeneous boxes
• Surface waters
• Deep waters
• Homogeneous water masses

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The Steady-State Approximation
rate of input rate of losses composition of
seawater remains constant with time
There is good evidence that over the last 100 M
years, the composition of seawater has remained
constant
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Seawater Chemistry
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Seawater Composition

• Continuum Models
• Ocean divided into an infinite number of boxes
• Concentrations change continuously from place to
  place
• Turbulent diffusion
• Advective transport
• Chemical processes (biochemical,
  dissolution-precipitation, etc.)

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Seawater Modification

• Biological Processes
• Synthesis of soft tissue organic matter
• Bacterial decomposition of organic matter
• Secretion of skeletal hard parts

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Seawater Modification

• Synthesis of soft tissue organic matter
• Deep sea vent production (minor)
• Dominated by photosynthesis
• Photic zone
• lt200 m depth
• Phytoplankton
• Marine ratio CNP 106161
• 106CO2 16NO3- PO42- 122H2O 18H ?light?
  C106H263O110N16P 138O2
• Limiting nutrients are N and P

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Seawater Modification

• Synthesis of soft tissue organic matter (cont.)
• Controlled by
• Coastal upwelling
• High-latitude mixing and the formation of deep
  water

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Seawater Modification

• Synthesis of soft tissue organic matter (cont.)
• Respiration
• Bacterial decay (deep water)
• Higher organisms (fish, etc.)
• C106H263O110N16P 138O2 ? 106CO2 16NO3-
  PO42- 122H2O 18H
• Nearly balance with photosynthesis is surface
  waters
• Net output of nutrients

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Nonconservative Properties - Nutrients
N ?
river input
sea surface
mixed layer
N
net photosynthesis
organics
pycnocline
sinking
? depth
N
deep water
rising water
net respiration
organics
burial
sediments
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Global Nutrient Distributions
Nitrate NO3- (?M)
Phosphate PO43- (?M)
0
10
20
30
40
0
1
2
3
4
0
0
1000
1000
2000
Depth (m)
2000
Depth (m)
3000
3000
Atlantic
Atlantic
Pacific
Pacific
4000
4000
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Seawater Modification

• Biological Processes
• Bacterial decomposition of organic matter
• In water column
• In sediments
• Sediments are anoxic
• Reactions follow typical redox sequence

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Seawater Modification

• Bacterial decomposition of organic matter
• Sulfate reduction
• 2CH2O SO42- ? H2S 2HCO3-
• Anaerobic reaction
• H2S reacts with iron minerals to form pyrite
• Dominant mechanism to remove SO42-
• Dependent on
• availability of organic matter
• Sulfate
• Iron minerals
• Fine-grained oxyhydroxides

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Seawater Modification

• Biological Processes
• Secretion of skeletal hard parts
• Removes Ca2, HCO3-, Mg2, H4SiO4
• Discussed further under Ca2 below.

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Seawater Modification

• Volcanic Rock - Seawater Reactions
• spreading ridges
• volcanic islands
• New minerals react with seawater
• Olivine
• Pyroxene
• Ca-Plagioclase
• At ridges, volcanic activity and heat flow set up
  seawater convection cells within the oceanic crust

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Seawater Modification

• Volcanic Rock - Seawater Reactions (Cont.)
• At spreading ridges
• High Temperature 200-400C
• Removal from seawater of Mg2 and SO42-
• Release of Ca2, H4SiO4, and K
• Release of Li, Rb, and Ba2
• Off axis basalts, ash sediments - interstitial
  waters
• K and Mg2 removed
• Li, Rb, and Ba2 removed
• Ca2, H4SiO4 released
• Smectite (Na0.5Al1.5Mg0.5Si4O10(OH)2) formed

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Seawater Modification

• Interaction with Detrital Solids
• Silicate and Clay minerals sourced by river
  water
• Reverse weathering reactions
• New, more cation-rich minerals from clays
• Example
• Na HCO3- H4SiO4 Al-silicate ?
  NaAl-silicate CO2 H2O
• Supported by cation anion chemistries of deep
  sea sediments
• Formation of new Fe-rich and Mg-rich silicates,
• Mineral surface reactions
• Adsorption - desorption
• Cation exchange
• Na released, Ca2 taken up

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Seawater chemistry

• Cloride (Cl-)
• Output Processes
• the main sink over geological time is evaporite
  deposits.
• The deposition of evaporites is controlled by
  tectonics, which controls the geometry of
  marginal seas that become evaporite basins.
• There are no significant evaporites forming today
• Seawater cycling through aerosols is also an
  important sink for Cl.
• Pore-water burial in sediments
• Input Processes
• River water addition
• Pollution
• the balance for Cl is probably not at steady
  state.

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Seawater Chemistry

• Na
• Output Processes
• the main sink over geological time is evaporite
  deposits.
• Seawater cycling through aerosols.
• Pore-water burial in sediments
• Cation exchange
• Basalt-seawater interaction - alteration
• Input Processes
• River water addition
• Pollution

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Seawater Chemistry

• Ca2
• Output Processes
• evaporite deposits in the geologic past gypsum
  and anhydtite
• Biogenic CaCO3 deposition
• Ca2 HCO3- ? CaCO3
• Benthic aragonite and magnesian calcite in
  shallow water
• Reef builders
• Molluscs, echinoderms, others
• Planktonic deep ocean
• Foraminifera (calcite)
• Coccolithophores (Calcite)
• Pteropods (aragonite)
• Input Processes
• River water addition
• Volcanic-seawater interaction due to uptake of
  Mg2
• Cation exchange

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Seawater Chemistry

• Deep water CaCO3 reactions
• Lysocline
• depth at which the rate of dissolution increases
  markedly.
• Selectively by type of biogenous sediment
• Generally, water above is saturated, water below
  is undersaturated
• Generally 500-1000 m above CCD
• Carbonate Compensation Depth (CCD)
• depth where the rate of calcite supply is matched
  by the rate of dissolution and therefore below
  which no calcareous sediments are found.
• Complete dissolution

CCD
CCD
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Seawater Chemistry

• Deep water CaCO3 reactions
• Due to
• Increase of CO2 at depth due to respiration of
  organic matter
• Increased CO2 at depth due to low temperature of
  water
• Solubility of CaCO3 increases with increasing
  pressure
• Depth of the lysocline and CCD is influenced by
• water temperature
• Pressure
• biological activity
• calcite crystal size
• dissolved CO2 concentration.

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Seawater Chemistry

• Both the lysocline and the CCD are shallower in
  the Pacific than the Atlantic
• there is more CO2 in the Pacific water.
• reflects the evolution of bottom water chemistry
• as seawater circulates from the North Atlantic
  via the Southern Ocean to the northern Pacific
  and Indian Oceans it "ages" and accumulates
  carbon dioxide.
• Dissolution of CaCO3 in deep water releases Ca2
  and HCO3-
• Carried to shallow waters by upwelling where
  utilized by organisms

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Seawater Chemistry

• HCO3- (Alkalinity)
• Output Processes
• Neutralized by H to form CO2
• H HCO3- ? H2O CO2
• CaCO3 deposition
• Ca2 HCO3- ? CaCO3
• Combined give Ca2 2HCO3- ? CaCO3 H2O CO2
• Input Processes
• River water addition
• Biogenic pyrite formation
• Due to oxidation of organic matter and
  accompanying reduction of interstitial sulfate
• 2CH2O SO42- ? H2S 2HCO3-

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Plot of log concentrations of inorganic carbon
species H and OH-, for open-system conditions
with a fixed pCO2 10-3.5 atm.
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pH

• Average pH
• 7.7 to 8.4

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pH

• Seawater too basic pH falls
• H2CO3 ? HCO3- H
• HCO3- in abundance
• Seawater too Acid pH rises
• HCO3- H ? H2CO3
• CO32- 2H ? H2CO3
• By dissolution of CaCO3

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CO2 Flux
39
CO2

• carbon dioxide is transferred to deep waters by
  two main processes
• sinking of cold water beneath the thermocline
• "biological pump"
• driven by the sinking of decaying organic matter
  below the mixing zone

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CO2

• Surface ocean processes controlling the exchange
  of CO2

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Nonconservative Properties - Dissolved O2
Biological processes
photosynthesis (light)
organic materials O2
CO2 H2O inorganic N (NO3-, NH4) inorganic P
(PO43-)
respiration
Physcial processes
O2dissolved
O2atmosphere
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Nonconservative Properties - Dissolved O2
O2
O2 ?
atmosphere
sea surface
mixed layer
O2
net photosynthesis
organics
pycnocline
oxygen minimum
sinking
? depth
O2
deep water
rising water
net respiration
organics
burial
sediments
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Dissolved Oxygen