Organic geochemistry is the study of the impacts and processes that organisms have had on the Earth. It is mainly concerned with the composition and mode of origin of organic matter in rocks and in bodies of water. The study of organic geochemistry is traced to the work of Alfred E. Treibs, "the father of organic geochemistry." Treibs first isolated metalloporphyrins from petroleum. This discovery established the biological origin of petroleum, which was previously poorly understood. Metalloporphyrins in general are highly stable organic compounds, and the detailed structures of the extracted derivatives made clear that they originated from chlorophyll.
Oceanic anoxic events or anoxic events (anoxia conditions) describe periods wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic (anoxic and sulphidic) waters. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events coincided with several mass extinctions and may have contributed to them. These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating. On the other hand, there are widespread, various black-shale beds from the mid-Cretaceous which indicate anoxic events but are not associated with mass extinctions. Many geologists believe oceanic anoxic events are strongly linked to slowing of ocean circulation, climatic warming, and elevated levels of greenhouse gases. Researchers have proposed enhanced volcanism (the release of CO2) as the "central external trigger for euxinia."
Pristane is a natural saturated terpenoid alkane obtained primarily from shark liver oil, from which its name is derived. It is also found in the stomach oil of birds in the order Procellariiformes and in mineral oil and some foods. Pristane and phytane are used in the fields of geology and environmental science as biomarkers to characterize origins and evolution of petroleum hydrocarbons and coal.
Cholestane is a saturated tetracyclic triterpene. This carbon-27 biomarker is produced by diagenesis of cholesterol and is one of the most abundant biomarkers in the rock record. Presence of cholestane in environmental samples are commonly interpreted as an indicator of animal life and/or traces of O2, as animals are known for exclusively producing cholesterol, and thus has been used to draw evolutionary relationships between ancient organisms of unknown phylogenetic origin and modern metazoan taxa. Cholestane is made in low abundance by other organisms (e.g., rhodophytes), but because these other organisms produce a variety of sterols it cannot be used as a conclusive indicator of any one taxa. It is often found in analysis of organic compounds in petroleum.
A carbon-to-nitrogen ratio is a ratio of the mass of carbon to the mass of nitrogen in a substance. It can, amongst other things, be used in analysing sediments and compost. A useful application for C/N ratios is as a proxy for paleoclimate research, having different uses whether the sediment cores are terrestrial-based or marine-based. Carbon-to-nitrogen ratios are an indicator for nitrogen limitation of plants and other organisms and can identify whether molecules found in the sediment under study come from land-based or algal plants. Further, they can distinguish between different land-based plants, depending on the type of photosynthesis they undergo. Therefore, the C/N ratio serves as a tool for understanding the sources of sedimentary organic matter, which can lead to information about the ecology, climate, and ocean circulation at different times in Earth’s history.
Phytane is the isoprenoid alkane formed when phytol, a constituent of chlorophyll, loses its hydroxyl group. When phytol loses one carbon atom, it yields pristane. Other sources of phytane and pristane have also been proposed than phytol.
Archaeol is one of the main core membrane lipids of archaea, one of the three domains of life. One of the key features that distinguishes archaea from bacteria and eukarya is their membrane lipids, where archaeol plays an important role. Because of this, archaeol is also broadly used as a biomarker for ancient archaea, especially methanogens, activity.
Dinosterol is a type of steroid produced by several genera of dinoflagellates. It is a 4α-methyl sterol (4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol), a derivative of dinosterane, rarely found in other classes of protists.
Peregrinella is an extinct Early Cretaceous Rhynchonellide genus with scattered, global representation from North America to Europe and Tibet. These brachiopods are stationary epifaunal suspension feeders, its most distinguishing feature is the size, considered to be the largest of all Mesozoic Rhynchonellides, which has long puzzled paleontologists because of its unusual morphology, stratigraphic occurrence, and distribution patterns.
Hydrogen isotope biogeochemistry is the scientific study of biological, geological, and chemical processes in the environment using the distribution and relative abundance of hydrogen isotopes. There are two stable isotopes of hydrogen, protium 1H and deuterium 2H, which vary in relative abundance on the order of hundreds of permil. The ratio between these two species can be considered the hydrogen isotopic fingerprint of a substance. Understanding isotopic fingerprints and the sources of fractionation that lead to variation between them can be applied to address a diverse array of questions ranging from ecology and hydrology to geochemistry and paleoclimate reconstructions. Since specialized techniques are required to measure natural hydrogen isotope abundance ratios, the field of hydrogen isotope biogeochemistry provides uniquely specialized tools to more traditional fields like ecology and geochemistry.
John Michael Hayes ForMemRS was a scientist emeritus at Woods Hole Oceanographic Institution in Woods Hole, Massachusetts.
Dinosterane is a steroidal alkane, also known as 4α,23,24-trimethylcholestane. It is used in geochemistry as a biomarker, interpreted as an indication of dinoflagellate presence due to its derivative dinosterol high occurrence in extant dinoflagellate species and its rarity in other taxa, although it has been shown to be produced by a single species of marine diatom as well.
Okenane, the diagenetic end product of okenone, is a biomarker for Chromatiaceae, the purple sulfur bacteria. These anoxygenic phototrophs use light for energy and sulfide as their electron donor and sulfur source. Discovery of okenane in marine sediments implies a past euxinic environment, where water columns were anoxic and sulfidic. This is potentially tremendously important for reconstructing past oceanic conditions, but so far okenane has only been identified in one Paleoproterozoic rock sample from Northern Australia.
Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly stratified, have an oxic, highly productive, thin surface layer, and have anoxic, sulfidic bottom water. The word euxinia is derived from the Greek name for the Black Sea (Εὔξεινος Πόντος (Euxeinos Pontos)) which translates to "hospitable sea". Euxinic deep water is a key component of the Canfield ocean, a model of oceans during the Proterozoic period (known as the Boring Billion) proposed by Donald Canfield, an American geologist, in 1998. There is still debate within the scientific community on both the duration and frequency of euxinic conditions in the ancient oceans. Euxinia is relatively rare in modern bodies of water, but does still happen in places like the Black Sea and certain fjords.
The sulfate-methane transition zone (SMTZ) is a zone in oceans, lakes, and rivers found below the sediment surface in which sulfate and methane coexist. The formation of a SMTZ is driven by the diffusion of sulfate down the sediment column and the diffusion of methane up the sediments. At the SMTZ, their diffusion profiles meet and sulfate and methane react with one another, which allows the SMTZ to harbor a unique microbial community whose main form of metabolism is anaerobic oxidation of methane (AOM). The presence of AOM marks the transition from dissimilatory sulfate reduction to methanogenesis as the main metabolism utilized by organisms.
24-Norcholestane, a steroid derivative, is used as a biomarker to constrain the source age of sediments and petroleum through the ratio between 24-norcholestane and 27-norcholestane, especially when used with other age diagnostic biomarkers, like oleanane. While the origins of this compound are still unknown, it is thought that they are derived from diatoms due to their identification in diatom rich sediments and environments. In addition, it was found that 24-norcholestane levels increased in correlation with diatom evolution. Another possible source of 24-norcholestane is from dinoflagellates, albeit to a much lower extent.
Chlorobactane is the diagenetic product of an aromatic carotenoid produced uniquely by green-pigmented green sulfur bacteria (GSB) in the order Chlorobiales. Observed in organic matter as far back as the Paleoproterozoic, its identity as a diagnostic biomarker has been used to interpret ancient environments.
27-Norcholestane, is a chemical compound with formula C
26H
46, that is a steroid derivative. 27-Norcholestane is used as a biomarker to constrain the source age of sediments and petroleum through the ratio between 24-norcholestane and 27-norcholestane, especially when used with other age diagnostic biomarkers, like oleanane.
21-Norcholestane, or 17β-Isoheptylandrostane is a 26-carbon (C26) sterane, found in eogene lacustrine sediments.
Ruth E. Blake is an American geophysicist and environmental scientist. She is a professor at Yale University in geology & geophysics, forestry & environmental studies, and chemical & environmental engineering. Blake's work focuses on marine biogeochemical processes, paleoclimate, astrobiology, and stable isotope geochemistry.
