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Marine Geochemistry 1

Marine Geochemistry 1. Reference: Schulz and Zabel Marine Geochemistry Springer, New York 2000 453 pp. ISBN 3-540-66-453-X. The Organic Carbon Cycle. Divided into two parts : 1. Biological cycle 2. Geological cycle. Biological cycle.

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Marine Geochemistry 1

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  1. Marine Geochemistry 1 Reference: Schulz and Zabel Marine Geochemistry Springer, New York 2000 453 pp. ISBN 3-540-66-453-X

  2. The Organic Carbon Cycle Divided into two parts : 1. Biological cycle 2. Geological cycle

  3. Biological cycle Photosynthesisin surface waters of oceans or lakes • organic matter from carbon dioxide • organic matter from bicarbonate Ends with metabolic or chemical oxidation of decayed biomass to carbon dioxide

  4. Geological cycle • Incorporation of biogenic organic matter into sediments and soils • Leads to the formation of natural gas, petroleum and coal or metamorphic forms of carbon

  5. Organic matter accumulation in sediments • In the fossil record: • Dark colored sediments • periods of time favorable to organic matter accumulation • White or red colored sediments or rocks • devoid of organic matter

  6. Causes leading to deposition of massive organic-matter rocks • Good Preservation • Sluggish circulation in the deep ocean • Shallow epicontinental seas accompained by water column stratification • Good Productivity • High primary productivity in a dynamic system

  7. Primary Production Photosynthetic plankton • produce 20 to 30 billions tons/year of carbon • fixation is not evenly distributed on the oceans but display zones of: • Higher activity on continental margins • Lower activity within the central ocean gyres

  8. Export to the Ocean Bottom • Of the total biomass formed only a very small portion reaches the underlying sea floor and is ultimately buried a sediment • Most of the organic matter enters the biological food web and it is respired or used for new biomass production

  9. Sedimentation Rate vs. Organic Matter Burial • Oxic open-ocean conditions: • 2X increase in organic carbon content for every 10X increase in sedimentation rate in marine sediments • Anoxic conditions: • no change in organic carbon content over a wide range of sedimentation rates

  10. Organic Carbon Content of Marine Sediments • Mean organic carbon content : • 0.3% with a median value of 0.1% • (data from deep sea drilling) • Varies over several hundreds of magnitude

  11. Organic Carbon Content of Marine Sediments Depends on: • extend of supply of organic matter • preservation conditions • dilution by mineral matter

  12. Chemical Composition of Biomass • Chemical nature of biomass is commonly described by its elemental composition • Marine phytoplankton • Redfield et al. (1963) ratio C:N:P = 106:16:1 • Ratio changes drastically : • food chain processes • early digenetic processes

  13. Chemical Composition of Biomass • Chemical composition can also be confined to a limited number of compound classes • Their proportions will vary in the different groups of organisms (Romankevitch, 1984)

  14. Principle of Selective Preservation Organic compounds and compound classes: • differ in their potential to be preserved in sediments • differ in their potential survive early diagenesis

  15. Principle of Selective Preservation Low Preservation Potential= easily hydrolyzed • Water-soluble organic compounds • Organic macromolecules High Preservation Potential= low solubility in water • Lipids • Hydrolysis resistant molecules

  16. Biological Markers • Molecules with high degree of structural complexity provide the possibility of relating a certain product to a specific precursor • EXAMPLE: • 24-methylenecholesterol and dinoserol are preferentially biosynthesized by diatoms and dinoflagellates (Volkman et al., 1998)

  17. Marine vs. TerrigenousOrganic Matter Variations in marine and terrigenous organic matter proportions important for: • paleoclimatic studies • paleoceanographic studies

  18. Parameters used to assess the organic matter sources • Carbon / Nitrogen Ratio • 10 in marine / 20 in terrigenous • Hydrogen Indices (mg HC/g TOC) • 150 in marine / 300-800 in terrigenous • Stable Carbon Isotope Rations • d13C = -27o/oo in marine / - 7o/oo in terrigenous

  19. Molecular Paleo-SeawaterTemperature and Climate Indicators • Biosynthesis of Long-Chain Alkenones in the microalgae Class Haptophyceaedepends on the water temperature during growth Coccolithoophorids belong to this class !

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