中国科学院南京地质古生物研究所机构知识库

Sedimentary rocks are one of the most abundant repositories of Earth history, preserving records of climate change, tectonic events, atmospheric and hydrospheric chemistry, and biological evolution in deep time. A precise and accurate temporal framework for the sedimentary rocks is indispensable to investigating the phenomena, linkages, cause and effect relationships, and rates of processes and events in Earth history. The U-Pb radioisotopic system in U-bearing accessory minerals analyzed by the isotope dilution thermal ionization mass spectrometry (ID-TIMS) technique offers one of the most precise and accurate means of measuring absolute time in the geologic past. The magmatic mineral zircon is particularly useful for its mechanical and chemical persistence and is the most commonly used accessory mineral for U-Pb ID-TIMS geochronology. Nevertheless, the application of this technique to the stratigraphic record relies on the presence of relatively pure volcanic ash (tuffaceous) deposits mixed in - or interbedded with - sedimentary rocks of interest. Innovations in modern U-Pb ID-TIMS geochronology over the past three decades have allowed great improvements in precision and reproducibility, with analytical uncertainties approaching similar to 0.2 parts per thousand in calculated ages of analyzed samples. Simultaneously, the quest for higher precision and resolving power of geochronology has elucidated new sources of complexity in the U-Pb systematics of minerals that now result in previously undetectable dispersals of analyses. These complexities must be recognized and carefully treated by appropriate analytical and statistical methods before a measured date can be translated into a meaningful depositional age. The application of Bayesian statistics allows stratigraphic uncertainties to be objectively incorporated in age-stratigraphic models. Limitations due to paucity of datable ash beds can be overcome by integrating radioisotope geochronology with other stratigraphic time markers, such as carbon isotope excursions, geomagnetic polarity reversals and orbitally-driven sedimentary cyclicity.