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Organic Geochemistry

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Carbon Isotopes in Geochemistry Sedimentary Geochemistry→→Biogeochemistry

Core Idea

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.

Explainer

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.

Practice Questions 3 questions

Prerequisite Chain

Understanding Zero → The Number Zero → Counting to Five → Counting to 10 → Counting to 20 → Counting a Set of Objects Up to 20 → Cardinality: The Last Number Counted → Matching Numerals to Quantities → Subitizing Small Quantities → Addition Within 10 → Number Bonds to 10 → Addition Within 20 → Doubles and Near Doubles → Doubles Facts Within 10 → Near Doubles Facts Within 20 → Mental Math Strategies for Addition → Mental Math: Adding and Subtracting Tens → Addition Within 100 → Repeated Addition as Multiplication → Multiplication as Equal Groups → Multiplication: Arrays → Basic Multiplication Facts (0s, 1s, 2s, 5s, 10s) → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Multiplication and Division Fact Families → Relationship Between Multiplication and Division → Division Facts as Inverse of Multiplication → Remainders and Quotients in Division → Division Word Problems → Multi-Step Word Problems → Solving Multi-Step Word Problems → Multiplication Word Problems → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Integers and the Number Line → Comparing and Ordering Integers → Absolute Value → Adding Integers → Subtracting Integers → Multiplying Integers → Dividing Integers → Unit Rates → Proportions → Percent Concept → Converting Between Fractions, Decimals, and Percents → Operations with Rational Numbers → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Sine, Cosine, and Tangent Ratios → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Indefinite Integrals → Basic Integration Rules → Riemann Sums → Definite Integral Definition → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Work and Circulation → Line Integrals of Scalar and Vector Functions → Fundamental Theorem for Line Integrals → Conservative Vector Fields → Conservative Vector Fields and Potential Functions → Curl and Divergence of Vector Fields → Curl and Divergence → Divergence Theorem → Electric Flux and Divergence Theorem → Gauss's Law: Integral Form and Meaning → Solving Problems with Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → Postulates of Special Relativity → Time Dilation → Length Contraction → Lorentz Transformation → Relativistic Velocity Addition → Relativistic Momentum and Energy → Mass-Energy Equivalence and E=mc² → Photons as Particles with Energy and Momentum → Planck-Einstein Relation: Energy and Frequency → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Observables and Quantum Operators → Commutators and Commutation Relations → Quantum Angular Momentum → Quantum Mechanical Treatment of Hydrogen → Solving the Schrödinger Equation for Hydrogen Atom → Quantum Numbers → Electron Configuration → Periodic Trends → Covalent Bonding → Electronegativity and Bond Polarity → Ionic Bonding → Lewis Structures → VSEPR Theory and Molecular Geometry → Molecular Geometry and Electron Pair Geometry → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Statistical Mechanics: Ensembles and the Boltzmann Distribution → Partition Function: Definition and Properties → The Canonical Partition Function and Thermodynamic Derivation → Free Energy and Thermodynamic Relations from Partition Functions → Legendre Transformations and Thermodynamic Potentials → Chemical Potential and Partial Molar Properties → Phase Equilibrium and Coexistence Conditions → Clausius-Clapeyron Equation → Phase Diagrams and Clausius-Clapeyron Equation → Chemical Potential and Thermodynamic Equilibrium → Geochemical Thermodynamics → Trace Element Geochemistry → Partition Coefficients → REE Patterns in Geochemistry → Sedimentary Geochemistry → Organic Geochemistry

Longest path: 189 steps · 1077 total prerequisite topics

Prerequisites (2)

Carbon Isotopes in Geochemistryhard Sedimentary Geochemistrysoft

Leads To (1)

Biogeochemistrysoft