Sedimentary Organic Matter

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Sedimentary Organic Matter

• Presented by
• Maaike de Winkel

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Outline

• Introduction
• Sediments
• Preservation
• Nitrification
• Degradation index

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Global processes

• Sediment burial of organic matter is the source
  for atmospheric oxygen, it links the cycles of C,
  S and O.
• About 0.1 of the carbon in the upper crust is
  active.
• The greatest reservoir is the seawater.
• Rivers provide the major conduit towards the
  preservation of terrigenous organic substances in
  marine sediment.

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Carbon reservoirs
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Marine sediments

• Most marine organic matter production is by
  phytoplankton.
• Most production occurs in the open ocean.
• About 90 of organic matter is preserved along
  the continental margins.
• Less than 10 of the organic matter reaching the
  ocean floor and less than 0.5 of the global
  productivity is preserved in marine sediments.

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Oxygen

• Preservation of Organic Carbon allows a
  comparable amount of photoynthetically produced
  oxygen to escape respiration and accumulate in
  the atmosphere.
• This means that there must be an outflux for
  oxygen as well.

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Patterns in OM preservation

• Degradation and preservation are opposite
  processes.
• Patterns can be found by relating various aspects
  of organic matter degradation and burial to the
  physical or dynamic characteristics of
  depositional environments.

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Accumulation rate

• The sediment accumulation rate has a great effect
  on the reactivity and preservation of the
  sediment.

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Organic input can be determined by

1. Measuring with sediment traps.
2. Estimation from local primary production using an
   empirical function.
3. Calculation from the sum of OM that is
   mineralized above and preserved below a specified
   sediment horizon.

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• Organic burial efficiency The accumulation rate
  of organic matter below the diagenetically active
  surficial sediment divided by the organic flux to
  the sea floor.
• Efficiencies range from less than 1 to almost
  80.

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Burial efficiency

• The burial efficiency is used as an indicator for
  preservation. It correlates directly with the
  sedimentation rate.
• Looking at the burial efficiencies is only useful
  if the organic matter is deposited for the first
  time.
• Recycled material will be more refractory.

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Increased organic matter preservation in less
oxygenated sediments due to

• 1.Lower free energy yields from suboxic
  respiration
• 2. The need to establish complex microbal
  consortia to stepwise degrade organic substrates.
• 3. The buildup of toxic waste products such as
  H2S.
• 4. Reduced sediment mixing, irrigation and
  bacteria cropping by benthic animals
• 5. The presence of highly insoluble oxygen-poor
  substrates which resist fermentative breakdown,
  but are aerobically degraded.

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Oxic degradation is a complex interaction of

• Local primary production.
• Organic matter composition and input to the sea
  floor.
• Sediment accumulation.
• Bioturbation rates, irrigation and reaction rates
  (both oxic and anoxic)

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Period of exposure to O2

• The period of exposure to O2, can be determined
  by
• Mean depth of O2 penetration
• Average accumulation rate

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Mechanisms affecting organic matter preservation

• Most organic matter is adsorbed to mineral
  surfaces, they are only degradable under oxic to
  suboxic conditions.
• Surface area seems to be the most important
  factor in organic matter content along the
  continental shelves and slopes.

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• Deep sea sediments show that some mechanism must
  overpower surface area protection.
• The reason for that is long term exposure to
  oxygen and other electron acceptors.

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Oxic degradation

• All sediments receive
• Totally refractory organic matter
• Oxygen sensitive organic matter
• Hydrolyzable organic matter

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Transition zone

• A transition zone from sorptive protection along
  coastal marine margins to oxic degradation in the
  deep sea should be expected.
• Grain size of the particles decreases while going
  upwards.
• Accumulation rates decrease towards the sea.

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Nitrification

• Denitrification is the primary mode of organic
  matter respiration in marine suboxic waters.
• Nitrification occurs most around 150 to 350 m
  depth.

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Degradation index
Var mole percentage of the amino acid AVG STD
mean and standard deviation Faccoef the
factor coefficient for the amino acid
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Articles that are used

• Sedimentary organic matter preservation an
  assessment and speculative synthesis. John I.
  Hedges, Richard G. Keil (1995)
• Impact of suboxia on sinking particulate organic
  carbon Enhanced carbon flux and preferential
  degradation of amino acidsvia denitrification.
  Benjamin A.S. et.al (2001)
• Linking diagenetic alteration of amino acids and
  bulk organic matter reactivity. Dauwe et.al
  (1999)