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In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism. These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture. The boundary between diagenesis and metamorphism, which occurs under conditions of higher temperature and pressure, is gradational.
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 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 tectonic history they have undergone, the nature and type of fluids that have circulated through them. From an economic standpoint, that study aids in assessing the likelihood of finding various economic minerals and hydrocarbons.
Additional recommended knowledge
The role of diagenesis in anthropology
The term diagenesis is extensively used in geology. However, this term has filtered into the field of anthropology to describe the changes and alterations that take place on skeletal (biological) material in a burial context. 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”. In order to assess the potential impact of diagenesis on archaeological 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).
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. Alteration occurs at all scales from molecular loss and substitution, through crystallite reorganization, porosity and microstructural changes, and in many cases, to disintegration of the complete unit. Three general pathways of the diagenesis of bone have been identified:
They are as follows:
The role of diagenesis in hydrocarbon generation
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Diagenesis". A list of authors is available in Wikipedia.|