Diagenesis (/ˌdaɪ.əˈdʒɛnəsɪs/) is the process ofphysical andchemical changes insediments first caused by water-rock interactions, microbial activity, and compaction after theirdeposition. Increased pressure and temperature only start to play a role as sediments become buried much deeper in theEarth's crust.[1] In the early stages, the transformation of poorly consolidated sediments intosedimentary rock (lithification) is simply accompanied by a reduction in porosity and water expulsion (clay sediments), while their mainmineralogical assemblages remain unaltered. As the rock is carried deeper by further deposition above, its organic content is progressively transformed intokerogens andbitumens.
The process of diagenesis excludes surface alteration (weathering) and deepmetamorphism. There is no sharp boundary between diagenesis andmetamorphism, but the latter occurs at highertemperatures andpressures. 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 fromsolution. Grains of sediment,rock fragments andfossils can be replaced by other minerals (e.g.calcite,siderite,pyrite ormarcasite) during diagenesis.Porosity usually decreases during diagenesis, except in rare cases such asdissolution of minerals anddolomitization.
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 andhydrocarbon deposits.
The process of diagenesis is also important in the decomposition of bone tissue.[2]
The term diagenesis, literally meaning "across generation",[3] is extensively used ingeology. However, this term has filtered into the field ofanthropology,archaeology andpaleontology 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 orfossilbones, 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 (mainlyproteincollagen) and two thirds mineral (calcium phosphate mostly in the form ofhydroxyapatite) 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:
They are as follows:
When animal or plant matter is buried during sedimentation, the constituent organicmolecules (lipids,proteins,carbohydrates andlignin-humic compounds) break down due to the increase intemperature andpressure. This transformation occurs in the first few hundred meters of burial and results in the creation of two primary products:kerogens andbitumens.
It is generally accepted that hydrocarbons are formed by the thermal alteration of these kerogens (thebiogenic theory). In this way, given certain conditions (which are largely temperature-dependent) kerogens will break down to form hydrocarbons through a chemical process known ascracking, orcatagenesis.
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) andtelodiagenesis (late). During the early or eodiagenesis stage shales lose pore water, little to no hydrocarbons are formed andcoal varies betweenlignite andsub-bituminous. During mesodiagenesis, dehydration ofclay minerals occurs, the main development of oil genesis occurs and high to low volatilebituminous 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 gaseouscarbon dioxide (CO2) in the porewater, which, depending on the conditions, can diffuse into the water column. The various processes of mineralization in this phase arenitrification anddenitrification,manganese oxide reduction,iron hydroxide reduction,sulfate reduction, andfermentation.[13]
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]
Geologic principles and processes | ||
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| Stratigraphic principles |  | |
| Petrologic principles | ||
| Geomorphologic processes | ||
| Sediment transport | ||
