Physico-chemical changes in sediments occurring after their deposition
Diagenesis
(
) is the process that describes
physical
and
chemical changes
in
sediments
first caused by water-rock interactions, microbial activity, and compaction after their
deposition
. Increased pressure and temperature only start to play a role as sediments become buried much deeper in the
Earth's crust
.
[1]
In the early stages, the transformation of poorly consolidated sediments into
sedimentary rock
(
lithification
) is simply accompanied by a reduction in porosity and water expulsion (
clay
sediments), while their main
mineralogical
assemblages remain unaltered. As the rock is carried deeper by further deposition above, its organic content is progressively transformed into
kerogens
and
bitumens
.
The process of diagenesis excludes surface alteration (
weathering
) and deep
metamorphism
. There is no sharp boundary between diagenesis and
metamorphism
, but the latter occurs at higher
temperatures
and
pressures
. Hydrothermal solutions, meteoric groundwater, rock porosity,
permeability
, dissolution/
precipitation reactions
, and time are all influential factors.
After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from
solution
. Grains of sediment,
rock
fragments and
fossils
can be replaced by other minerals (e.g.
calcite
,
siderite
,
pyrite
or
marcasite
) during diagenesis.
Porosity
usually decreases during diagenesis, except in rare cases such as
dissolution
of minerals and
dolomitization
.
The study of diagenesis in rocks is used to understand the geologic history they have undergone and the nature and type of fluids that have circulated through them. From a commercial standpoint, such studies aid in assessing the likelihood of finding various economically viable mineral and
hydrocarbon
deposits.
The process of diagenesis is also important in the decomposition of bone tissue.
[2]
Role in anthropology and paleontology
[
edit
]
The term diagenesis, literally meaning "across generation",
[3]
is extensively used in
geology
. However, this term has filtered into the field of
anthropology
,
archaeology
and
paleontology
to describe the changes and alterations that take place on skeletal (biological) material. Specifically, diagenesis "is the cumulative physical, chemical, and biological environment; these processes will modify an organic object's original chemical and/or structural properties and will govern its ultimate fate, in terms of preservation or destruction".
[4]
[5]
In order to assess the potential impact of diagenesis on archaeological or
fossil
bones
, many factors need to be assessed, beginning with elemental and mineralogical composition of bone and enveloping soil, as well as the local burial environment (geology,
climatology
,
groundwater
).
[5]
The composite nature of bone, comprising one-third organic (mainly
protein
collagen
) and two thirds mineral (
calcium phosphate
mostly in the form of
hydroxyapatite
) renders its diagenesis more complex.
[6]
Alteration occurs at all scales from molecular loss and substitution, through crystallite reorganization, porosity, and microstructural changes, and in many cases, to the disintegration of the complete unit.
[7]
Three general pathways of the diagenesis of bone have been identified:
- Chemical deterioration of the organic phase.
- Chemical deterioration of the mineral phase.
- (Micro) biological attack of the composite.
[8]
They are as follows:
- The
dissolution
of collagen depends on time, temperature, and environmental
pH
.
[8]
At high temperatures, the rate of
collagen loss
will be accelerated, and extreme pH can cause collagen swelling and accelerated
hydrolysis
.
[8]
Due to the increase in porosity of bones through collagen loss, the bone becomes susceptible to hydrolytic
infiltration
where the hydroxyapatite, with its affinity for
amino acids
, permits charged species of
endogenous
and
exogenous
origin to take up residence.
[2]
- The hydrolytic activity plays a key role in the mineral phase transformations that expose the collagen to accelerated chemical- and bio-degradation.
[8]
Chemical changes affect
crystallinity
.
[2]
[9]
Mechanisms of chemical change, such as the uptake of F
?
or
CO
2?
3
may cause
recrystallization
where hydroxyapatite is dissolved and re-
precipitated
allowing for the incorporation or substitution of exogenous material.
[2]
[9]
- Once an individual has been
interred
, microbial attack, the most common mechanism of bone deterioration, occurs rapidly.
[8]
During this phase, most bone collagen is lost and porosity is increased.
[2]
The dissolution of the mineral phase caused by low pH permits access to the collagen by extracellular microbial enzymes thus microbial attack.
[8]
Role in hydrocarbon generation
[
edit
]
When animal or plant matter is buried during sedimentation, the constituent organic
molecules
(
lipids
,
proteins
,
carbohydrates
and
lignin
-
humic
compounds) break down due to the increase in
temperature
and
pressure
. This transformation occurs in the first few hundred meters of burial and results in the creation of two primary products:
kerogens
and
bitumens
.
It is generally accepted that hydrocarbons are formed by the thermal alteration of these kerogens (the
biogenic
theory). In this way, given certain conditions (which are largely temperature-dependent) kerogens will break down to form hydrocarbons through a chemical process known as
cracking
, or
catagenesis
.
A kinetic model based on experimental data can capture most of the essential transformation in diagenesis,
[10]
and a mathematical model in a compacting porous medium to model the dissolution-precipitation mechanism.
[11]
These models have been intensively studied and applied in real geological applications.
Diagenesis has been divided, based on hydrocarbon and coal genesis into:
eodiagenesis
(early),
mesodiagenesis
(middle) and
telodiagenesis
(late). During the early or eodiagenesis stage shales lose pore water, little to no hydrocarbons are formed and
coal
varies between
lignite
and
sub-bituminous
. During mesodiagenesis, dehydration of
clay minerals
occurs, the main development of oil genesis occurs and high to low volatile
bituminous coals
are formed. During telodiagenesis, organic matter undergoes cracking and dry gas is produced; semi-
anthracite
coals develop.
[12]
Early diagenesis in newly formed aquatic sediments is mediated by microorganisms using different electron acceptors as part of their metabolism. Organic matter is mineralized, liberating gaseous
carbon dioxide
(CO
2
) in the porewater, which, depending on the conditions, can diffuse into the water column. The various processes of mineralization in this phase are
nitrification
and
denitrification
,
manganese oxide
reduction,
iron hydroxide
reduction,
sulfate reduction
, and
fermentation
.
[13]
Role in bone decomposition
[
edit
]
Diagenesis alters the proportions of organic collagen and inorganic components (hydroxyapatite, calcium, magnesium) of bone exposed to environmental conditions, especially moisture. This is accomplished by the exchange of natural bone constituents, deposition in voids or defects, adsorption onto the bone surface and leaching from the bone.
[2]
[14]
See also
[
edit
]
- Chalcedony
? Microcrystalline varieties of silica
- Chert
? Hard, fine-grained sedimentary rock composed of cryptocrystalline silica
- Flint
? Cryptocrystalline form of the mineral quartz
- Concretion
? Compact mass formed by precipitation of mineral cement between particles
- Fossil
? Preserved remains or traces of organisms from a past geological age
- Petrogenesis
? Processes that form rock
References
[
edit
]
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- ^
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