Chemical Oceanography

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Chemical Oceanography

• Lecture 1 Primary Production
• Lecture 2 Marine Bio-geochemistry and
  Sedimentation

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Lecture 2 Marine Bio-geochemistry and
Sedimentation

• Distribution of Marine Sediments
• Carbonate Equilibrium and the CCD
• Organic Carbon and Sediments
• Bacterial Respiration and Subsurface Redox
  Zonation
• Fe/Mn Nodule Formation

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Three Main Sediment Types

• Lithogenic Physically supplied by weathering of
  sediments from continents, e.g. ice rafted
  sediment, terrigenous sands and muds, aeolian
  dust, sediments become finer away from source,
• Biogenic Biological inputs - mineral tests and
  shells, organic carbon, form oozes
• Chemical (Authigenic) diagenetic alteration of
  sediments, precipitation and dissolution of
  different minerals. e.g. dissolution of carbonate
• In reality many sediments made up of mix of
  lithogenic, biogenic and chemical components

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Lithogenic sediments

• lithogenic particles are produced by weathering
  of rock and minerals from land
• Transported by rivers, glaciers, and wind.
  Results in thickest seds at continental margins
• Transport downslope by gravity-slumps and
  turbidity currents
• Windblown (Aeolian) and volcanic Dust)
  Components-quartz 2-10 microns-deserts. Important
  in open ocean.
• Ice-rafting- up to 2000km from Antarctica, N
  Atlantic and Arctic

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Biogenic sediments

• Made up of shells (skeletal material produced
  by biogenic activity)
• Calcium carbonate shells as corals, algae,
  molluscs, foraminifera (zooplankton /benthic),
  Coccoliths (algae), pteropods, gastrops (1cm)
• Silica (opal) as diatoms (algae), radiolarians
  (animals, zooplankton), sponges.
• Most microscopic and referred to as oozes.
  deposition rates of 1-6 cm/1000yrs
• Found in most open ocean seafloor

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Dissolution of Carbonate at Depth

• Chemical processes modify biological (biogenic)
  sediments through the dissolution of CaCO3 and
  opal silica in deep water.
• Temperature and pressure play a role in
  increasing the corrosiveness of deep waters
• The other major control on carbonate dissolution
  is due to the creation of CO2 by the oxidation of
  organic matter
• This creates bicarbonate ions at the expense of
  carbonate ions thereby driving the dissolution of
  carbonate tests.
• CO2 (aq) CO32-(s) H2O ? 2HCO3-(aq)

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Carbonate dissolution and the CCD

• Carbonate content of deep sea sediments
  decreases with increasing water depth.
• lysocline, where the proportion of
  solution-resistant tests increases abruptly
• calcite compensation depth (CCD) which is the
  boundary between carbonate-bearing and
  carbonate-free sediments

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The Depth of the CCD in Oceans

• CCD normally at 5Km depth but can vary depending
  on local conditions

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Global Distribution of Marine Sediments
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Organic Carbon Supply to Sediments

• Organic Carbon supply is very important in
  sediments food for bacterial respiration
• Most (99) organic mater is recycled in water
  column aerobic respiration
• On average only 1 stored in sediments
• Open ocean, long water column (1000s m), low
  primary production, low organic matter supply
  Oxic sediments
• Near land, short water column (100s m), high
  primary production, High organic matter supply
  Anoxic sediments

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Bacterial Respiration and Subsurface Redox
Zonation

• Where primary production is high, or mixing of
  oxygen is low (e.g. in enclosed basins, Black
  Sea), oxygen is consumed before all available
  organic matter Aerobic respiration stops
• A large number of bacterial species have evolved
  to utilise other anaerobic processes to
  extracting energy from organic mater to live,
  grow and reproduce.
• Main species utilised are Nitrate, Mn(IV)
  oxides, Fe(III) oxides, sulphate and
  methanogenesis

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Sedimentary REDOX Processes

• Process that transfer electrons, resulting in
  oxidation of organic carbon, (oxidation is loss
  of electrons)
• CH2O (reduced) e- ? CO2 (oxidised) H2O
• And reduction (reduction is gain of elections)
• X (oxidised) e- ? X (reduced)
• And energy is released
• (E.g. aerobic respiration, CH2O O2 ? CO2 H2O)

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Nitrate Reduction (Denitrification)

• Uses nitrate in place of oxygen
• Nitrate Reduced to N2 gas
• Produces CO2
• CH2O NO3- ? CO2 H2O N2

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Fe/Mn oxide reduction

• Uses solid metal oxides in place of oxygen
• Metal oxides are dissolved,
• Sediment colour changes, brown Fe(III) green
  Fe(II)
• Produces CO2
• CH2O Mn(IV)O2 (s) ? CO2 Mn2 (aq)
• CH2O Fe(III)OOH (s) ? CO2 Fe2(aq)

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Sulphate Reduction

• Uses Sulphate in place of oxygen
• Produces toxic hydrogen sulfide
• Produces HCO3- alkalinity
• HS- and Fe2 (from Fe(III) reduction) combine to
  produce FeS minerals (sediments turn black)
• Burial of FeS an important S removal process
• CH2O SO42- H ? HCO3- H2O H2S

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Methanogenesis

• methanogenesis uses CO2, H2 (from fermentation of
  organic mater) etc. and organic matter directly
  to extract energy
• Produces methane
• Some e.g.
• CH2O ? CH4 CO2
• CO2 H2 ? CH4 H2O
•    

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Energy Yield and Physical Separation of Redox
Process

• Energy yield is different for each process
• When a energetically favourable can occur, it
  will occur to the exclusive of all other
  processes
• Leads to a physical separation of processes
  vertical succession

Aerobic respiration 3190 KJ/Mole
Denitrification 3030KJ/Mole
Mn-reduction 2920-3090 KJ/Mole
Fe-reduction 1330-1410 KJ/Mole
Sulphate-reduction 380 KJ/Mole
Methanogenesis 350 KJ/Mole
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Solid and Aqueous phase distribution
Physical separation of anaerobic process, and
changes in sediment colour
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Effect of Environments on Redox Process

• Where supply of Org C is low, no anaerobic
  process may occur
• Ocean nitrate concentrations are low - Fe and
  Sulphate reduction more important
• In freshwater environments sulphate is absent -
  early methanogenesis, landfill/marsh gas,
  willow-the-wisp.

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Fe/Mn nodules

• Fe.Mn Nodules Can be litter sea bed, most common
  near MOR where supply of Fe/Mn is greatest
  (Hydrothermal)
• Slow Growth, characteristic banding 1mm/million
  years

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Biogeochemical cycling of Fe and Mn

• Diagenetic cycling

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Fe/Mn Nodule Distribution
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Fe/Mn nodules as a resource
Nodules very rich in metals, potentially a ore
deposit