Organic geochemistry studies the fate of carbon-based compounds from their biological origin through burial, diagenesis, catagenesis, and metamorphism. Organic matter in sediments ranges from recognizable biomolecules to complex, insoluble kerogen. Biomarkers (molecular fossils) are specific organic compounds whose structures preserve information about their biological source, even after millions of years of burial: steranes record eukaryotic input, hopanes indicate bacterial sources, and alkenones record sea surface temperature through their degree of unsaturation. The thermal maturation of kerogen generates petroleum (oil and gas) through catagenesis, with the type and thermal history of the organic matter controlling whether oil, gas, or neither is produced. Organic carbon burial is also a primary control on atmospheric O2 through geological time.
Organic geochemistry bridges biology and geology, tracking the transformation of living matter into geological materials. The ~0.5% of photosynthetically fixed carbon that escapes remineralization and is buried in sediments drives two of the most important long-term geological processes: petroleum generation and atmospheric oxygen regulation.
Biomarkers are the most information-rich organic compounds because their molecular structures can be traced to specific biological sources. Sterols are produced exclusively by eukaryotes (their carbon skeletons -- steranes -- survive burial). Hopanoids are produced by bacteria. Specific compounds can be more diagnostic: dinosterol indicates dinoflagellates, oleanane indicates angiosperms, isorenieratane indicates green sulfur bacteria (requiring photic-zone euxinia). The presence or absence of these biomarkers in ancient rocks constrains the biological community and environmental conditions at the time of deposition.
Kerogen -- the insoluble organic fraction of sedimentary rocks -- is the most abundant form of organic carbon on Earth, vastly exceeding fossil fuels and living biomass combined. It is classified by its hydrogen/carbon and oxygen/carbon ratios (van Krevelen diagram) into types reflecting the biological source: Type I (high H/C, algal), Type II (intermediate, marine), Type III (low H/C, terrestrial plants). During burial and heating (catagenesis, 60-160 C), kerogen cracks to generate liquid hydrocarbons (oil). At higher temperatures (160-200+ C), oil is cracked to wet gas, then dry gas (methane). This oil window and gas window framework is the foundation of petroleum exploration geochemistry.
The connection between organic carbon burial and atmospheric O2 is fundamental. Photosynthesis produces O2 and organic carbon in a 1:1 stoichiometric ratio. If all organic carbon is remineralized (respired), the O2 is consumed and there is no net oxygen accumulation. Only organic carbon that escapes remineralization through burial produces a net gain of O2 to the atmosphere. The delta-13C record in marine carbonates tracks the fraction of carbon buried as organic matter (f-org), and secular trends in this record document the oxygenation history of Earth's atmosphere through the linked carbon-oxygen cycle.