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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 Earth’s 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 + H20Post-oxic environment : Nitrate- reduction

Mn- reduction Fe- reduction

Sulphidic environment: Sulfate-reductionMethagenic 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

Green post-oxic turbidite

Intermediate 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

FeS2 dissolves at the boundary of sulfidic/post-oxic as Fe(II)

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

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Mn distribution in the sediments

The dissolve Mn(II) forms MnS, but it dissolves easily in water.

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.

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Mn and Fe particular

More common form in the particalur form of the Mn and Fe are resp. MnCO3 and FeS

Mn can besides MnCO3 also be found as MnO at higher more oxygenated conditions.

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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

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-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

-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

Reactive fraction of these elements in the turbidite has repeatedly

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Zn, Sb, U distribution

-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.

-Sharp upper cutoff immediately below the oxidation front and a gradual decline over 12, 15 and 35cm for Zn, Sb and U, resp.(tailing)

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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