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

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

3. Biological cycle Photosynthesis in 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. Terrigenous Organic 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 / in terrigenous
Stable Carbon Isotope Rations
d13C = -27o/oo in marine / - 7o/oo in terrigenous

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