Absolute dating radiocarbon dating. The development of accelerator mass spectrometry (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard. Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium–argon dating (K–Ar dating). Potassium-40 is a radioactive isotope of potassium that decays into argon-40. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated. Argon,
Hydrogen isotope biogeochemistry samples with more precision. Hydrogen isotope applications quickly emerged in petroleum geochemistry by measuring oil, paleoclimatology by observing lipid biomarkers, and ecology by constructing trophic dynamics. Modern advances are currently underway in the clumped isotope composition of methane after development of the carbonate thermometer. Precise measurements are also enabling focus on microbial biosynthetic pathways involving hydrogen. Ecologists studying trophic levels are especially interested in compound specific measurements for construction of past diets and tracing predator-prey relationships. Highly advanced machines are now promising position specific hydrogen isotope analysis of biomolecules and natural gases. All isotopes of a chemical element contain the