Information geochronological

The abundance of a trace element is often too small to be accurately quantihed using conventional analytical methods such as ion chromatography or mass spectrometry. It is possible, however, to precisely determine very low concentrations of a constituent by measuring its radioactive decay properties. In order to understand how U-Th series radionuclides can provide such low-level tracer information, a brief review of the basic principles of radioactive decay and the application of these radionuclides as geochronological tools is useful. " The U-Th decay series together consist of 36 radionuclides that are isotopes (same atomic number, Z, different atomic mass, M) of 10 distinct elements (Figure 1). Some of these are very short-lived (tj j 1 -nd are thus not directly useful as marine tracers. It is the other radioisotopes with half-lives greater than 1 day that are most useful and are the focus of this chapter. 

Furthermore, a long term storage in flow restricted subaquatic areas (e.g. harbours or bayous) represent a metastable output of sedimentary riverine matter. This phenomena is considered in an investigation performed on a river sediment core from the Teltow Canal (chapter 5.2.3). In particular DDT-related substances are analysed in order to obtain information on the long time emission of an industrial point source. Also an undisturbed sedimentation allows the correlation of the quantitative data obtained from the core samples with a geochronological determination of the pollutants. 

The geochronological information that can be obtained from such Hf isotopic analysis is a so-called model age (Figure 9.6), representing the moment at which the zircon would have formed from a reservoir with an Hf isotopic composition similar to the depleted mantle (DM) or chondritic uniform reservoir (CHUR), the latter being a model for the whole Earth. 

The dating of terrestrial materials (geochronology) does not provide any significant information on cosmochronology, but as early as 1953 an estimate for the age of formation of the Solar System was obtained from U-Pb isotopic analyses on meteoritic lead. Subsequently meteorites were dated by the U-Pb technique to give an age of 4.551 0.004 x 10 y, and the fine-scale separation of events that took place during planetary formation can be resolved down to a few million years. 

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