Light isotopes, characterization

Besides the components discussed in detail above, a number of lesser and/or less-well characterized components have been identified, primarily for xenon, which cannot be exhaustively discussed here for several a confirmation of their existence would be welcome. One of these components, found in sulfides of the Allende meteorite, is characterized by overabundances of " Xe in gas fractions released in the 1400°C -1500°C range, without corresponding overabundances in the other light isotopes as expected for Xe-L or spallation xenon (Lewis et al. 1979). Others are connected with achondrites other than ureilites. Among these achondrites are the brachinites and related... 

NRA is an effective technique for measuring depth profiles of light elements in solids. Its sensitivity and isotope-selective character make it ideal for isotopic tracer experiments. NRA is also capable of profiling hydrogen, which can be characterized by only a few other analytical techniques. Future prospects include further application of the technique in a wider range of fields, three-dimensional mapping with microbeams, and development of an easily accessible and comprehensive compilation of reaction cross sections. 

Variations in content of some important carbon compounds are schematically demonstrated in Fig. 2.11 The two most important carbon reservoirs on Earth, marine carbonates and the biogenic organic matter, are characterized by very different isotopic compositions the carbonates being isotopically heavy with a mean 5 C-valne around 0%c and organic matter being isotopically light with a mean 5 C-value aronnd -25%c. For these two sedimentary carbon reservoirs an isotope mass balance mnst exist such that ... 

The most important modem system of units is the SI system, which is based around seven primary units time (second, abbreviated s), length (meter, m), temperature (Kelvin, K), mass (kilogram, kg), amount of substance (mole, mol), current (Amperes, A) and luminous intensity (candela, cd). The candela is mainly important for characterizing radiation sources such as light bulbs. Physical artifacts such as the platinum-iridium bar mentioned above no longer define most of the primary units. Instead, most of the definitions rely on fundamental physical properties, which are more readily reproduced. For example, the second is defined in terms of the frequency of microwave radiation that causes atoms of the isotope cesium-133 to absorb energy. This frequency is defined to be 9,192,631,770 cycles per second (Hertz) —in other words, an instrument which counts 9,192,631,770 cycles of this wave will have measured exactly one second. Commercially available cesium clocks use this principle, and are accurate to a few parts in 1014. 

Much work on characterizing changes in the isotopic composition of DOC and DIC within aquatic systems lies ahead. Carbon isotopes promise to shed much light on the primary biogeochemical processes involved in the carbon cycle. 

The basin margins are characterized by calcite and minor dolomite cements, many of which which formed in isotopically light brackish or meteoric water at low temperature. In general, calcites did not form near the sediment-water interface, but during shallow burial. On the east side of the basin these cements are characterized by widely varying 5 Cpdb values (+20 to -30) compared with central basin cements (+5 to -10). Sr isotopic ratios in cements are lower than the marine depositional waters on the east side of the basin, but are higher than expected for depositional waters on the west side. 

---