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Sedimentary Organic MatterSedimentary Organic Matter

Presented by:

Maaike de Winkel

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Outline:Outline:

IntroductionSedimentsPreservationNitrificationDegradation index

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Global processesGlobal 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:Carbon reservoirs:Reservoir type AmountSedimentary rocksInorganicCarbonates 60,000Organic 15,000Kerogen

Active (surficial) poolsInorqanicMarine DIC 38Soil carbonate 1.1Atmospheric CO2 0.66OrganicSoil humus 1.6Land plant tissue 0.95Seawater DOC 0.6Unit = 10 1̂8 gC

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Marine sediments: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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OxygenOxygen

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 preservationPatterns 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: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 Organic input can be determined by:determined by:

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: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 Increased organic matter preservation in less oxygenated sediments due to:less oxygenated sediments due to:

1.Lower free energy yields from suboxic respiration2. 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 animals5. 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 Oxic degradation is a complex interaction of: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 OPeriod of exposure to O22

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 Mechanisms affecting organic matter preservation: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:Oxic degradation:

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

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Transition zoneTransition 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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NitrificationNitrification

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:Degradation index:

∑ −=

ii

ii coeffacSTD

AVGDI **]

var

varvar[

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: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)