وبلاگ بزرگ مقالات زمین شناسی

Organic geochemistry

The study of the abundance and composition of naturally occurring organic substances, their origins and fate, and the processes that affect their distributions on Earth and in extraterrestrial materials.

Organic geochemistry was born from a curiosity about the organic pigments extractable from petroleum and black shales. It developed with extensive investigations of the chemical characteristics of petroleum and petroleum source rocks as clues to their occurrence and formation, and now encompasses a broad scope of activities within interdisciplinary areas of earth and environmental science. This range of studies recognizes the potential of geological records of organic matter to help characterize sedimentary depositional environments and to provide evidence of ancient life and indications of evolutionary developments through the Earth's history. Organic geochemistry includes determinations of anthropogenic contaminants amid the natural background of organic molecules and the assessment of their environmental impact and fate. Marine organic geochemistry addresses and interprets aquatic processes involving carbon species. It involves investigations of the chemical character of particulate and dissolved organic matter, evaluation of oceanic primary production including the factors (light, temperature, nutrient availability) that influence the uptake of carbon dioxide (CO2), the composition of marine organisms, and the subsequent processing of organic constituents through the food web. Organic geochemistry extends to broader biogeochemical issues, such as the carbon cycle, and the effects of changing carbon dioxide levels, especially efforts to use geochemical data and proxies to help constrain global climate models. Examination of the organic chemistry of meteorites and lunar materials also falls within its compass, and as a critical part of the quest for remnants of life on Mars, such extraterrestrial studies are now regaining the prominence they held in the 1970s during lunar exploration.  See also: Cosmochemistry; Geochemistry

These activities share the common need for identification, measurement, and assessment of organic matter in its myriad forms. Hence, many advances within this area have paralleled developments in analytical methodology or instrumentation that permit new approaches to the determination of the abundance, structure, and composition of organic matter in geological materials.

Global inventories of carbon

 Carbon naturally exists as oxidized and reduced forms in carbonate carbon and organic matter. The major reservoir of both forms of carbon on Earth is the geosphere. It contains carbonate minerals deposited as sediments and organic matter accumulated from the remains of dead organisms. Estimates of the size of the geological reservoir of carbon vary within the range of 5 to 7 × 1022 g, of which 75% is carbonate carbon and 25% is organic carbon. The amounts of carbon contained in living biota (5 × 1017 g), dissolved in the ocean (4 × 1019 g), and present in atmospheric gases (7 × 1017 g) are miniscule compared to the quantity of organic carbon buried in the rock record. Most of it occurs in finely disseminated form within sedimentary rocks, especially shales. The importance of buried organic matter extends beyond its sheer magnitude; it includes the fossil fuels—coal, natural gas, and petroleum—that supply 85% of the world's energy. Global annual consumption of these three fuels represent only approximately 0.4%, 1.5%, and 2.3%, respectively, of their proven economic reserves, which in turn collectively account for only about 0.12% of the total accumulation of organic matter. These figures illustrate the immense quantities of buried organic carbon, which represent a geological legacy inherited from ancient life. They also show how the release of carbon dioxide from burning of these fossil fuels can readily increase the comparatively small atmospheric reservoir of carbon, perturbing the natural balance of the global carbon cycle.  See also: Biogeochemistry; Carbon; Carbon dioxide; Carbonate minerals; Fossil fuel; Limestone; Sedimentary rocks; Shale

Sedimentary organic matter

 The vast amounts of organic matter contained in geological materials represent the accumulated vestiges of organisms amassed over the expanse of geological time. Yet, survival of organic cellular constituents of biota into the rock record is the exception rather than the norm. Only a small portion of the carbon fixed by organisms during primary production, especially by photosynthesis, escapes degradation as it settles through the water column and eludes microbial alteration during subsequent incorporation and assimilation into sedimentary detritus. Most biological debris is rapidly broken down or remineralized by surficial processes within the oceans, in lakes, in wetlands, and in soils and other terrigenous environments. Atmospheric and aqueous oxidation reactions contribute to this recycling process, which is also enhanced by consumption associated with food web processes and facilitated by sediment bioturbation (movement of sediment by animals). Protective packaging of organic matter can improve its chance of survival. Rapidly sinking zooplankton fecal pellets contain materials derived from phytoplankton that have survived ingestion and digestion and descend in discrete capsules, thereby evading degradative influences. Hierarchical communities of aerobic and anaerobic microbes act as the predominant agents for degradation and alteration of organic matter in both the water column and sediments. They use it as their carbon source during a wide range of distinct metabolic processes, including heterotrophy, fermentation, sulfate reduction, methanogenesis, and methylotrophy. Microbial consortia selectively remove target substrates, and augment the surviving organic materials with secondary biomass, by adding distinctive components and isotopic signatures of their activity. The application of various microbiological procedures in examination of sedimentary materials, including incubation tests for specific metabolic processes and characterization of DNA isolates, is rapidly expanding the known distribution of microbial activity in the geosphere. These approaches demonstrate that the highly complex character of sedimentary organic matter often includes diagnostic evidence of its diverse biological sources and of the cumulative effects of alteration processes.  See also: Biodegradation; Petroleum microbiology

Two distinct but related phenomena can enhance sequestration of organic matter in sedimentary environments. First, high levels of primary production can generate a supply of organic matter that exceeds the capacity of microbial flora to effectively degrade it. Second, depositional conditions, especially anoxia, can restrict the active populations of microbial communities engaged in the degradative process. Both circumstances can lead to exceptional preservation of organic matter, although their comparative importance in the process continues to be debated. Depositional conditions that favor survival of greater quantities of carbon, in terms of its elemental abundance, also affect the molecular and isotopic compositions of the organic matter by increasing survival of labile components. For example, hypersaline environments tend to facilitate the process of sulfur incorporation, which aids preservation of less stable structural features.

 Kerogen and bitumen

 Sedimentary organic matter can be divided operationally into solvent-extractable bitumen and insoluble kerogen. Bitumens contain a myriad of structurally distinct molecules, especially hydrocarbons, which can be individually identified (such as by gas chromatography-mass spectrometry) although they may be present in only minute quantities (nanograms or picograms). The range of components includes many biomarkers that retain structural remnants inherited from their source organisms, which attest to their biological origins and subsequent geological fate.  See also: Bitumen; Kerogen

For many years, kerogen was thought to form in sediments by a series of condensation reactions that combined the building blocks of life (amino acids, carbohydrates, lipids) into larger insoluble components. Undoubtedly such reactions do occur, aided by microbial alteration processes, but it is now apparent that a significant portion of kerogen represents macromolecular remnants, such as fragments of cell walls, inherited directly from living organisms or partially modified by removal of labile structural elements by degradative processes. The biological sources of organic matter, their survival during sediment deposition, and the extent of alteration effectively establish the elemental composition of kerogen that determines its character. Kerogen is broadly classified into three main types based on the proportions of hydrogen and oxygen relative to carbon (H/C and O/C ratios). Type I kerogen possesses high H/C and low O/C ratios, and is comparatively rare. It arises from contributions of organic matter that are dominated by phytoplankton and bacteria and usually occur in lacustrine (lake) settings. Type III kerogen exhibits low H/C and high O/C ratios, which result from a predominance of terrestrial organic matter that is typical in continental margin settings. It tends to be gas-prone, generating methane (CH4). Type II kerogen displays intermediate H/C and O/C ratios, which reflects the mixed sources of organic matter that typify marine settings. Most oil source rocks contain this kerogen type, although some enriched in sulfur are labeled type II-S. The process of burial modifies all kerogen types, reducing both H/C and O/C ratios as organic matter experiences thermal alteration. Ultimately, the distinctive characteristics of the separate kerogen types and evidence of their sources are obscured by the burial processes that constitute diagenesis.

Diagenetic alteration of organic matter

 Few organic constituents synthesized by living organisms remain stable in the sedimentary domain, although diagnostic features of their structures can survive for eons. To persist in sediments, organic matter must first survive the processes of recycling modification by microbial action during burial and consolidation. Subsequently, the organic matter experiences progressive transformation caused by thermal processes as its host rock undergoes compaction, dewatering, and perhaps remineralization. Modifications of organic matter proceed in a sequential manner and ultimately cause dramatic changes in its character. Hydrogen and oxygen atoms are lost incrementally, and eventually carbon-carbon bonds are broken, liberating mobile hydrocarbons, which is a crucial step in petroleum generation known as catagenesis. Continued alteration during burial at elevated temperature and pressure further depletes the hydrogen in residual organic material and ultimately produces graphite, one of the pure forms of carbon.  See also: Diagenesis; Graphite

Petroleum formation

 A number of specific geological conditions must be met for generation of recoverable petroleum, which is an inherently inefficient process. First, there is a need for the raw materials that can generate hydrocarbons, which must be provided by a source rock containing appreciable quantities (typically >0.5%) of preserved organic matter of appropriate character. The nature of the organic matter determines the potential yield of petroleum and its composition. Second, the source rock must be buried at sufficient depth that kerogen cracking liberates hydrocarbons. This process is dependent on a combination of time and temperature; petroleum generation can occur rapidly in regions where the Earth's heat flow is high, or it can proceed slowly over 108–109 years where thermal gradients are low. Third, the hydrocarbons released must migrate from the source rock to a suitable trap, often in folded or faulted rock strata, where they can accumulate in a reservoir. Primary migration occurs as generated oil moves out of the source rock aided by the thermal expansion and fracturing of kerogen. Subsequently, driven by pressure gradients, it moves through porous conduits displacing formation waters or along planes of weaknesses during the secondary migration to reach the reservoir. Finally, the integrity of the petroleum within its reservoir must be preserved for tens or hundreds of millions of years. Tectonic forces may fracture the reservoir allowing petroleum to escape, or the reservoir may be uplifted and eroded at the surface. Contact with meteoric waters percolating through the petroleum reservoir can lead to water washing, which selectively extracts the soluble petroleum constituents. It may also introduce microbial populations, which can sequentially remove components amenable to biodegradation, leaving residues that are increasingly resistant to alteration. Alternatively, petroleum may seep upward through fractures, to emanate as surface or submarine seeps, where it can evaporate or be washed or degraded by microbes. All of these processes tend to reduce the quality and value of petroleums.  See also: Petroleum; Petroleum geology; Sedimentology

The composition of petroleum reflects its source materials, thermal and migration history, and extent of biodegradation. The combined effects and disparate interactions between these influences produce oils that vary widely in their chemistry and physical properties. Thus, investigation of the compositional characteristics of petroleum enables the cumulative history of these processes, and their effects, to be interpreted. Oil generation is an essential part of a continuum of changes that affect source rocks during their progressive burial, and that lead to successive liberation of petroleum and gases, contingent on the composition of the source material. Thus, the thermal processes that transform kerogen into petroleum continue beyond oil generation, and fragment organic matter into progressively smaller molecules, which leads to production of hydrocarbon gases. The volume and composition of the gases generated is dependent on whether the source rock is oil- or gas-prone, which in turn is a function of its composition, especially its dominant kerogen type. However, as thermal maturity increases, there is a trend in composition from wet gas to dry gas, as the proportion of hydrocarbon gases that condense as liquids (for example butane and propane) decreases while methane becomes predominant.  See also: Methane; Natural gas

Methane can also be produced in shallow sediments by the microbial activity of methanogenic bacteria. Such bacterial methane forms many recoverable gas accumulations at comparatively shallow depths and represents the source of methane in gas hydrates. Hydrates occur in sediments where the combination of low temperature and high pressure stabilizes the ice cages that can entrap methane. The worldwide frequency of hydrate occurrences supports estimates that they may greatly exceed the combined resources of oil and gas (0.5% versus 0.017% of global carbon). Moreover, massive release of methane from hydrates is a direct consequence of global warming and is now recognized as the climatic perturbation that caused significant changes in Earth's flora and fauna associated with the late Paleocene thermal maximum, 55 million years ago.  See also: Hydrate; Methanogenesis (bacteria)

Coal

Peat deposits contain an abundance of terrestrial organic matter that is progressively transformed by burial compaction and thermal alteration into lignite, bituminous coal, and ultimately graphite. Coals vary in type and chemical characteristics, largely dependent on the composition of their organic matter (typically type III kerogen), its extent of bacterial degradation, and its thermal history.  See also: Lignite; Peat

Coals contain many plant fragments, which are often altered but can be identified under the microscope and classified according to their origins. These integral parts of coal are called macerals, which are individually described and named according to their source, for example, sporinite (spores), resinite (resin), and vitrinite (woody). Macerals can be recognized in other sediments, where they provide evidence of contributions of organic matter from terrestrial plants and aid assessment of depositional environments. The reflectance of the maceral vitrinite changes systematically with increasing burial and thermal maturity, whether in coals or other rocks. Thus, determination of vitrinite reflectance (Ro) by microscopic examination provides a direct measure of its thermal history, and is widely used in assessing the maturity of petroleum source rocks.  See also: Coal

Biomarkers

Biomarkers are individual compounds whose chemical structures carry evidence of their origins and history. Recognition of the specificity of biomarker structures initially helped confirm that petroleum was derived from organic matter produced by biological processes. For example, the structural and stereochemical characteristics of the hydrocarbon cholestane that occurs in petroleums can be directly and unambiguously linked to an origin from teh sterol cholesterol, which is a widespread constituent of plants and animals. Of the thousands of individual petroleum components, hundreds reflect precise biological sources of organic matter, which distinguish and differentiate their disparate origins. The diagnostic suites of components may derive from individual families of organisms, but contributions at a species level can occasionally be recognized. Biomarker abundances and distributions help to elucidate sedimentary environments, providing evidence of depositional settings and conditions. They also reflect sediment maturity, attesting to the progress of the successive, sequential transformations that convert biological precursors into geologically occurring products. Thus, specific biomarker characteristics permit assessment of the thermal history of individual rocks or entire sedimentary basins.  See also: Basin

Carbon isotopes

 Carbon naturally occurs as three isotopes: carbon-12 (12C), carbon-13 (13C), and radiocarbon (14C). The proportion of 13C in living organic matter is controlled by the organism's source of carbon, fractionation during carbon fixation, and both biosynthetic and metabolic processes. Measurements of 13C are expressed as a ratio, which compares the amount of 13C in the sample to a standard (carbonate in the Peedee belemnite) and provides a value in parts per thousand (‰), defined as 13C = {[(13C/12C)sample/(13C/12C)standard] −1} × 1000. Organic carbon tends to be depleted of 13C (negative 13C values) relative to inorganic carbon (carbon dioxide and carbonate) as organisms preferentially uptake and use 12C because its reactivity is greater than that of 13C. However, the extent of 13C depletion is dependent on biosynthetic pathways. For example, photosynthetic carbon fixation of carbon dioxide by plants that employ the C3 and C4 pathways produces organic matter averaging −26‰ and −12‰, respectively. The significant difference is caused by the ability of the latter group, mainly comprising grasses, to store carbon dioxide before fixation. Methylotrophic bacteria use methane, which is highly depleted in 13C, rather than carbon dioxide as their carbon source. Consequently, they produce organic compunds that possess highly negative 13C values (−55‰ to −120‰). By contrast, the products synthesized by bacteria that use the reverse-TCA (tricarboxylic acid, or Krebs) cycle exhibit 13C values typically less than −10‰. Thus, the carbon isotopic composition of many individual components found in sediments attests to the diversity of their origins and demonstrates the variety of microbial processes active in the shallow subsurface.  See also: Isotope; Photosynthesis; Plant respiration

Temporal excursions in the 13C values of sediment sequences can reflect perturbations of the global carbon cycle. For example, episodes of enhanced burial of organic matter that occurred during widespread oceanic black shale deposition in the Cretaceous effectively depleted the global 12C reservoir, leading to an enrichment of 13C in sedimentary organic matter. Also, global shifts in carbon isotopic signals provide critical evidence that the late Paleocene thermal maximum was associated with a release of methane from hydrates. Similarly, the remarkable variations in 13C values of organic matter in the Late Proterozoic are consistent with the “snowball Earth” hypothesis that ice sheets extended to the Equator, creating long-term episodes of glaciation. These events ended catastrophically when greenhouse conditions gradually created by a build-up of atmospheric carbon dioxide became sufficient to melt the ice, leading to extraordinarily rapid weathering of continents as their ice burden dissipated, and dramatic deposition of marine carbonates as the ocean again became a sink for carbon dioxide.  See also: Black shale; Cretaceous; Marine sediments; Paleoceanography

Radiocarbon is widely employed to date archeological artifacts, but the sensitivity of its measurement also permits its use in exploration of the rates of biogeochemical cycling in the oceans. This approach permits assessment of the ages of components in sediments, demonstrating that bacterial organic matter is of greater antiquity than components derived from phytoplankton sources.  See also: Radiocarbon dating

Applications

A major impetus for the development of organic geochemistry has been its use as a tool in petroleum exploration. Determination of the quantity, nature, and degree of thermal alteration of organic matter from kerogen analysis provides evidence of the richness and maturity of source rocks and the timing of oil generation. Such information is critical in evaluation of prospects for petroleum formation and accumulation. Similarly, chemical clues provided by the molecular and isotopic composition of petroleums enable the determination of the biological sources of organic matter (marine or terrestrial) and its environment of deposition. Such information can also place constraints on the age of petroleums based on the timing of the appearance of specific biomarkers, based on evolutionary trends in their source organisms. Molecular distributions also provide valuable fingerprints for the correlation of source rocks and oils, and the 13C values of hydrocarbon gases help distinguish their origins from bacterial or thermogenic sources, especially when coupled with the determination of their hydrogen isotope compositions (deuterium values).  See also: Deuterium; Hydrogen

Molecular and isotopic tools are gaining acceptance as paleoclimate proxies. The proportion of unsaturation (number of carbon-carbon double bonds) in a group of compounds known as alkenones is biosynthetically controlled by the growth temperature of the phytoplankton that synthesize them. The survival of this temperature signal in marine sediment sequences provides a temporal record of sea surface temperatures that reflect past climates, including evidence of the timing and extent of global cooling during ice ages, and the warming associated with El Niño events.  See also: Chemostratigraphy; El Niño

Also, differences between the carbon isotopic composition of the alkenones and co-occurring carbonate can be interpreted in terms of ancient atmospheric levels of carbon dioxide, when the effect of the growth rates of the alkenone-producing phytoplankton is excluded.  See also: Phytoplankton

Bibliography

•  M. Engel and S. A. Macko, Organic Geochemistry, 1993
• Alifazeli = egeology.blogfa.com
• J. M. Hunt, Petroleum Geochemistry and Geology, 2d ed., 1996
• Alifazeli = egeology.blogfa.com
• S. Killops and V. J. Killops, An Introduction to Organic Geochemistry, 1993
• Alifazeli = egeology.blogfa.com
• L. M. Pratt, J. B. Comer, and S. C. Brassell (eds.), Geochemistry of Organic Matter in Sediments and Sedimentary Rocks, 1992
• Alifazeli = egeology.blogfa.com

Additional Readings

•  A Community of Organic Geochemistry
• Alifazeli = egeology.blogfa.com
• The Carbon Cycle, Climate, And The Long-Term Effects Of Fossil Fuel Burning
• Alifazeli = egeology.blogfa.com