Early Diagenesis/ Metal behaviour

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Early Diagenesis/ Metal behaviour
Kamuran Yasadi
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References
Redox zonation of elements at an oxic/post-oxic
boundary in deep-sea sediments, J.Thomson et al.,
Geochimica et Cosmochimica Acta, Vol. 57, pp.
579-595 Bacterial Manganese and Iron reducton in
aquatic sediments, Bo Thamdrup, Advanced in
Microbiol Ecology, Vol.16, pp. 41-72 Authigenic
mineral formation resulting from organic matter
decomposition in modern sediments, Robert
A.Berner, Stuttgart 1981
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• Introduction
• What is Early diagenesis
• Fe and Mn in deep sea sediments
• Trace elements behaviour
• Conclusion

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Diagenesis

• Two processes are the main factor that cause
  disequilibrium on earth
• Tectonism (and volcanism)
• Physical process which derives the energy
  necessay to disturb equilibrium on earth. Uplift
  of thermodynamically unstable minerals by
  volcanic extrusion.
• On land weathering
• Under Sea Diagenesis
• Biosynthesis
• Biosynthesis derives the energy nessecary for
  perturbing equilibrium from sunlight to
  photosynthesis. This brings about the
  thermodynamically unstable substances on the
  Earths surface.

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Diagenesis
Diagenesis is the chemical, physical and
biological process that brings about the changes
in the sediments subsequent to its
deposits After organic matter deposition on the
sediments The diagenetic sequence of organic
matter degradation by redox reactions. Oxic
environment CH2O O2 CO2 H20 Post-oxic
environment Nitrate- reduction Mn- reduction
Fe- reduction Sulphidic environment
Sulfate-reduction Methagenic environment
Methane-formation
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Metals behaviour in sediments

• Metals are derived into the sea by weathering
  processen on land. These weathering products are
  settled down to the bottom of the sea.
• Metals occur as dissolved in water, complexed
  with organic matter and as minerals.
• Metal in sediments are classified in
• -High content of metals such as Mn and Fe who
  have large influence on the diagenetic processes.
• Other trace elements with low content, like As,
  Ca, Cu, I, Pb, Ni, Se, U, V, Zn.
• And elements who do not influence the diagenetic
  processes in sediment like Al, Ti.

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Diagenetic sequence
Prediction of valery changes with Eh for various
elements in seawater which should be applicable
to sediment porewater as a first approximation
Eh decrease
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Diagenic sequence of metals

• The diagenic sequence predicted for metals do not
  occur, because of
• Primary fluctuations in the compositions of the
  sediments occur over time and may mask signals of
  redox-related enrichments
• Diagenetic signals may be overprinted from one
  level to the earlier ones by the continuously
  accumulated sediments.
• Kinethic rather than thermodynamic may be
  dominant species going into more metastable
  deeper reducing conditions.
• Mn and Fe oxyhydroxides have a high sorptive
  capacity for many trace elements.
• Bioturbation especially at the upper decimetre
  of the sediments

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Place of investigation

• The behaviour of metals in the sediments are
  investigated in the deep sea sediments in the
  Atlantic Ocean.
• Slow accumulation rates of organic matter
  deposition on the sediment is necessary to get a
  system where elements have the time for
  redistribution within the sediments, depending on
  the redox potential.
• -Turbidite layer having fine grained particles
  smaller than 2 microns, which contain a lot of
  orqagnic carbon
• The other reason is the large input of organic
  matter into the Atlantic Ocean by the surrounding
  continents. The weathered water from the
  continents contains heavy metals that come into
  the sediments.
• Result is a marked colour change in the turbidite
  sediment.

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Diagenetic discription of the sediment
Schematic representation of the sediment
Progressive oxidation front
Brown clay
oxic turbidite
Intermediate turbidite
Green post-oxic turbidite
Sulphidic conditions
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Fe distribution in the sediment
Forming of pyrite in the post-oxic environment
Pyrite is formed by the reaction with
sulfide(produced by the sulfide-reducing
bacteria) With the burial of Pyrite the sulfide
is getting more exhausted. The reaction can go
the other way, where pyrite dissolves in
porewater
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Fe distribution in sediment
Oxidation at the oxic boundary of the brown layer
Dissolved Fe(II) in the porewater is mobile and
migrates to the upper boundary by upward fluxes,
where it reacts with oxygen forming
Fe(III)oxyhydroxides
FeS2 dissolves at the boundary of
sulfidic/post-oxic as Fe(II)
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Mn distribution in the sediments
Also Mn as MnO2 or Mn complexated in organic
matter is oxidize the organic matter by
sulfide-reducing bacteria. (Bacilles,
coccolipiths)
Mn(II) in porewater migrates(faster than Fe(II)
by the upward fluxes and forms (Mn,Ca)CO3-minerals
, called rhodochrosite.
The dissolve Mn(II) forms MnS, but it dissolves
easily in water.
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Mn and Fe particular
Mn can besides MnCO3 also be found as MnO at
higher more oxygenated conditions.
More common form in the particalur form of the Mn
and Fe are resp. MnCO3 and FeS
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Distribution of I

• -Iodium in turbidite sequences diffuse upward as
  I(I) in post-oxic conditions
• In oxic conditions I(I) is oxidized to I(V) as
  IO3-
• Absorbtion on solid-phase in the oxidation front
  and moves downward to the non-oxic conditions
• Iodium can have on the interactions with cations

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Cu, Pb and V distribution
Reactive fraction of these elements in the
turbidite has repeatedly

• mobilised to porewater solution by oxidation
• migrated downwards by diffusion
• been reassimilated to the phase below the
  progressing front
• A possibility is that these elements, in the
  reduced form, are diffused by forming a
  complexing molecule

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Zn, Sb, U distribution
-Sharp upper cutoff immediately below the
oxidation front and a gradual decline over 12, 15
and 35cm for Zn, Sb and U, resp.(tailing)
-Also these elements undergo the same
mobilisation and redox conditions as the earlier
elements but, -They diffuse deeper into the
sediments. -The reason for their behaviour is
that they prefer more the solid phase(organic
matter) than the porewater solution.
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Mn-oxyhydroxide scavenging of Cu, Pb, Ni and Zn
Similar peaks are located on the same depth as
the Mn Mn oxyhydroxide probably responsible for
the sorption of Cu, Zn, Pb, Ni.
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Inert elements and elements which have been lost
Inert elements Al, Ti. Metals lost from
the region of the oxic/post-oxic conditions are
As, Re. These elements prefer the more reductive
conditions deeper in the sediments.
Al- and Ti-oxides are inert in the sediment
profile They are only oxidized at the upper
brown layer.
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Conclusie

• There are more factors involved in the
  redistribition of metals in sediments(like
  diffusion-effects, solubility, complexing with
  organic matter) than only the redox-potentials
• Most of the metals are found in the
  oxic/post-oxic boundaries of the sediments.
• Fe and Mn are the main metals involved in
  diagenesis, because of their contribution to the
  degradation of organic matter and the
  redistribution of the metals