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Early Diagenesis

Early Diagenesis. Organic matter concentration low in deep sea sedimentsAreas of high organic matter compositionContinental marginsOffshore basinsUpwelling areas. Early Diagenesis. Effects of organic matter diagenesis can be seen over wide range of scale lengthsBox coresPiston coresODP/DSDP c

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Early Diagenesis

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    1. Early Diagenesis Deep Sea Mostly carbonate, silica, and clay alteration Continental Margins and Basins Organic matter diagenesis!

    2. Early Diagenesis Organic matter concentration low in deep sea sediments Areas of high organic matter composition Continental margins Offshore basins Upwelling areas

    3. Early Diagenesis Effects of organic matter diagenesis can be seen over wide range of scale lengths Box cores Piston cores ODP/DSDP cores Varies with: Sedimentation rate Amount of food available

    5. Early Diagenesis Organic matter is FOOD for microbes Degradation of organic matter ? energy source Tend to think of O2 as only possible oxidant Oxidant = element or molecule which is an e- acceptor

    6. Oxidation/Reduction or Redox Reactions Acid-Base reactions Proton transfer pH = -log a H+ ; determines tendency for a solution to accept or transfer protons Redox reactions Electron transfer pe = -log ae- ; determines tendency for a solution to accept or transfer electrons High pe = high tendency for oxidation

    7. Oxidation/Reduction Reactions No free electrons in aqueous solutions Reactions occur in pairs Different half reactions provide and consume electrons

    8. Oxidation/Reduction Reactions Example:

    9. Corg + oxidant ? oxidized C + reduced oxidant Many different oxidants Different metabolic reactions, different oxidants, release different amounts of free energy (Gibbs free energy) Oxidation/Reduction Reactions

    10. Oxidation/Reduction Reactions

    11. Decomposition Reactions

    12. Oxidation/Reduction Reactions Can express the electron activity in 2 ways: pe - dimensionless Eh – measured in volts using electrochemical half cells. Positive Eh = oxidizing conditions Negative Eh = reducing conditions At 25°C Eh = 0.059 pe

    13. Half Reactions

    14. Redox Reactions Leave distinct profiles of reactants and/or products in pore fluids

    15. Pore Fluid Profiles

    16. Pore Fluid Profiles Depths of variations Highly variable Can have no sulfate reduction in 100’s m of sediment or within the bottom water Depends on: Organic carbon rain rate More Corg ? more rapid depletion Sedimentation rate Slower sedimentation allows more downward diffusion of O2

    17. Organic-rich Sediments Significance of organic-rich sediments Implies anoxia, low bottom water O2 anoxia usually produced by high organic rain rate But anoxia does not necessarily lead to Corg preservation Secondary oxidants are as effective at degrading organics as oxygen

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