Geochronology

Geochronology is the study of how old rocks, fossils, and sediments are, using clues found within the rocks themselves. Absolute ages come from measuring radioactive isotopes, while relative ages are determined through methods like paleomagnetism and stable isotope ratios. Combining several geochronological and biostratigraphic indicators makes the resulting age estimate more precise.

This field differs from biostratigraphy, which identifies the geological period of sedimentary rocks by describing and comparing fossil plants and animals. Biostratigraphy doesn’t give a direct absolute age—it only places a rock within the time interval when that fossil set existed. Still, the two disciplines work closely together, even sharing the same naming system for rock layers and the time spans used to classify sublayers.

Geochronology is the main tool of chronostratigraphy, which aims to assign absolute dates to all fossil assemblages and reconstruct the geologic history of Earth and other celestial bodies.

**Dating Methods**

*Radiometric Dating* By measuring how much a radioactive isotope has decayed, given its known half-life, geologists can determine the absolute age of the original material. Different isotopes decay at different rates, making them useful for different geological periods—slower-decaying isotopes cover longer spans but are less precise in years. Except for radiocarbon, most techniques actually measure the increase in a radiogenic isotope (the decay product). Using two or more radiometric methods together yields more reliable results. Most are suited only for geological time, but radiocarbon and argon-argon dating can reach into early human history and recorded time.

Common techniques include: - **Radiocarbon dating**: Measures carbon-14 decay in organic material, best for samples younger than about 60,000 years. - **Uranium–lead dating**: Measures the ratio of lead-206 and lead-207 to uranium in minerals like zircon in igneous rocks. Along with argon-argon dating, it’s one of the two most common geologic methods. Monazite geochronology is a U-Pb variant used especially for dating metamorphism. It works for samples older than about 1 million years. - **Uranium–thorium dating**: Used for speleothems, corals, carbonates, and fossil bones, covering a range from a few years to about 700,000 years. - **Potassium–argon and argon–argon dating**: Date metamorphic, igneous, and volcanic rocks, as well as volcanic ash layers above or within paleoanthropological sites. The younger limit for argon-argon is a few thousand years.

*Electron Spin Resonance (ESR) Dating* *Fission-Track Dating*

*Cosmogenic Nuclide Geochronology* This set of techniques finds the age when a geomorphic surface formed (exposure dating) or when surface materials were buried (burial dating). Exposure dating uses exotic nuclides (like beryllium-10, aluminum-26, chlorine-36) created by cosmic rays hitting Earth materials to date surfaces such as alluvial fans. Burial dating relies on the different radioactive decay rates of two cosmogenic elements to determine when sediment was shielded from further cosmic rays.

*Luminescence Dating* These methods detect light emitted from materials like quartz, diamond, feldspar, and calcite. Types include optically stimulated luminescence (OSL), cathodoluminescence (CL), and thermoluminescence (TL). TL and OSL are used in archaeology to date fired objects like pottery or cooking stones, and can also track sand movement.

*Amino Acid Dating* This technique measures the ratios of amino acid isomers in fossils to estimate their age. It cannot produce numerical ages on its own—it needs site-specific calibration from other methods—but it is relatively inexpensive. The preserved protein is most common in shell-bearing organisms, so it is often applied to ocean sediments.

*Incremental Dating* These techniques build year-by-year chronologies that can be fixed (linked to the present day and calendar or sidereal time) or floating. Examples include dendrochronology, ice cores, lichenometry, and varves.

*Paleomagnetic Dating* A sequence of well-dated paleomagnetic poles (virtual geomagnetic poles) forms an apparent polar wander path (APWP) for a large continental block. APWPs from different continents serve as references for new poles from rocks of unknown age. To date a rock or sediment, its paleopole is linked to the nearest point on the APWP. Two methods are used: the angular method and the rotation method.