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Chemical Oceanography

Chemical Oceanography. Lecture 1: Primary Production Lecture 2: Marine Bio-geochemistry and Sedimentation. Lecture 2: Marine Bio-geochemistry and Sedimentation. Distribution of Marine Sediments Carbonate Equilibrium and the CCD Organic Carbon and Sediments

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Chemical Oceanography

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  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 suppliedby 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. 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

  6. 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)

  7. 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

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

  9. Global Distribution ofMarine Sediments

  10. 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 (1000’s m), low primary production, low organic matter supply – Oxic sediments • Near land, short water column (100’s m), high primary production, High organic matter supply – Anoxic sediments

  11. 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

  12. 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)

  13. Nitrate Reduction (Denitrification) • Uses nitrate in place of oxygen • Nitrate Reduced to N2 gas • Produces CO2 CH2O + NO3- CO2 + H2O + N2

  14. 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)

  15. Sulphate Reduction • UsesSulphatein 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

  16. 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

  17. 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

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

  19. 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.

  20. 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

  21. Biogeochemical cycling of Fe and Mn • Diagenetic cycling

  22. Fe/Mn Nodule Distribution

  23. manganese (29.40 %);  iron (6 %);  nickel (1.34 %);  copper (1.25 %);  cobalt (0.25 %);  titanium (0.6 %);  aluminum (2.9 %)  sodium, magnesium, silicium, zinc, oxygen and hydrogen (32.16 %). Fe/Mn nodules as a resource Nodules very rich in metals, potentially a ore deposit

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