Transcript Sedimentary Organic Matter

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:
Reservoir type
Sedimentary rocks
Inorganic
Carbonates
Organic
Kerogen
Active (surficial) pools
Inorqanic
Marine DIC
Soil carbonate
Atmospheric CO2
Organic
Soil humus
Land plant tissue
Seawater DOC
Unit = 10^18 gC
Amount
60,000
15,000
38
1.1
0.66
1.6
0.95
0.6
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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.
2.
3.
Measuring with sediment traps.
Estimation from local primary production
using an empirical function.
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:
vari  AVG vari
DI  [
] * fac * coefi
STD var
i
Var = mole percentage of the amino acid
AVG + STD = mean and standard deviation
Fac*coef = 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)
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