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Chemical Oceanography

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... algae, molluscs, foraminifera (zooplankton /benthic), Coccoliths (algae) ... Silica (opal) as diatoms (algae), radiolarians (animals, zooplankton), sponges. ... – PowerPoint PPT presentation

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Title: Chemical Oceanography


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

2
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

3
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

4
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

5
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6
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

7
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)

8
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

9
The Depth of the CCD in Oceans
  • CCD normally at 5Km depth but can vary depending
    on local conditions

10
Global Distribution of Marine Sediments
11
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

12
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

13
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)

14
Nitrate Reduction (Denitrification)
  • Uses nitrate in place of oxygen
  • Nitrate Reduced to N2 gas
  • Produces CO2
  • CH2O NO3- ? CO2 H2O N2

15
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)

16
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

17
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
  •    

18
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
19
Solid and Aqueous phase distribution
Physical separation of anaerobic process, and
changes in sediment colour
20
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.

21
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

22
Biogeochemical cycling of Fe and Mn
  • Diagenetic cycling

23
Fe/Mn Nodule Distribution
24
Fe/Mn nodules as a resource
Nodules very rich in metals, potentially a ore
deposit
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