Early Diagenesis/ Metal behaviour
Kamuran Yasadi
1
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
2
-Introduction
-What is Early diagenesis
-Fe and Mn in deep sea sediments
-Trace elements behaviour
-Conclusion
3
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.
4
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
5
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.
6
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
7
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
8
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.
9
Diagenetic discription of the sediment
Schematic representation of the sediment
Brown clay
oxic turbidite
Progressive
oxidation
front
Intermediate turbidite
Green post-oxic turbidite
Sulphidic conditions
10
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
11
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)
12
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)CO3minerals, called
rhodochrosite.
The dissolve Mn(II)
forms MnS, but it
dissolves easily in
water.
13
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
14
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
15
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
16
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.
17
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.
18
Inert elements and elements which have been lost
Inert elements Al, Ti.
Al- and Ti-oxides are inert in the
sediment profile
They are only oxidized at the
upper brown layer.
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.
19
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
20