Isotope fractionation, living organisms

Isotope Fractionation by Living or Once Living Organisms... 

In an effort to further elucidate the nature of Ca isotope fractionation in animals, Skulan and DePaolo (1999) studied tissues from living and recently deceased (naturally) organisms. For the four animals studied, it was found that bone calcium typically has 5 Ca that is 1.3%o lower than the value in the dietary calcium (Fig. 8). The soft tissue calcium, however, is quite variable, and has values closer to that of the dietary calcium. Skulan and DePaolo (1999) concluded that the primary source of Ca isotope fractionation was in the formation of bone, and that the 5 Ca values of soft tissue were variable in time and dependent on the immediate status of the Ca balance in the organisms. 

Carbon isotope fractionation effects of individual compounds were observed in living organisms and also as a result of enzymatic isotope effects and reaction kinetics in biological systems. Such fractionation effects have to be examined by isotope ratio mass spectrometry in order to understand specific processes in life sciences or in environment.75... 

The humic constituents of SOM are usually regarded as the primary resistant compounds (Stout et al., 1981). In spite of the fact that the accumulation of C in soil is not indefinite even in natural ecosystems, it is certainly true that HS have been accumulating on the surface of the earth since the appearance of life. They now make up a considerable fraction of the soil organic C pool The amount of C stored as HS is 60 x 1017g, and it exceeds that which occurs in living organisms (Stevenson, 1994). 14C dating combined with isotope enrichment techniques have been used to... 

Sulfur isotope fractionation is important in the study of atmospheric gases, sulfur in the oceans (Section 5.4.4.1) and in living organisms. Microbes use 32S in preference to 34S during microbial sulfate reduction providing a sulfur isotope fingerprint for a... 

In Chapter 1 it was noted that the most common pathway for photosynthesis, the C3 path (Box 1.10), results in a comparatively low value for the 13C 12C ratio of the fixed carbon (conventionally represented by the 513C notation Box 1.3). This preference for light isotopes is a characteristic of life processes, and the extent of the depletion of the heavier isotope (or isotopic fractionation see Boxes 1.3 and 5.11) in sedimentary organic matter can provide clues about the source organisms, their trophic relationships and the environments in which they lived. However, simply consider-... 

After chemical pretreatment, the samples are burnt to produce carbon dioxide and nitrogen. A small amount of this gas is bled into a mass spectrometer, where the stable isotope ratios of carbon and nitrogen are measured. These ratios provide useful information on the purity of the sample and clues about the diet and climatic conditions of the living organism. The carbon isotope ratio is also used to correct for isotopic fractionation in the radiocarbon measurement. The carbon dioxide is collected in a glass ampoule or converted to graphite for radiocarbon measurement on the AMS system. 

Living organisms that take their carbon directly from the atmosphere share the same radiocarbon content, though there is usually some isotopic fractionation between the atmosphere and biosphere, which can be corrected for by measuring the stable isotope ratio... 

Radiocerium absorbed into the systemic circulation will be transported by blood proteins and be deposited predominantly in liver and bone. Deposition fractions will be about 0.45 for liver, 0.35 for bone, and 0.1 for other soft tissues with the remainder excreted in urine and feces. The retention times in liver and bone are long compared to the radioactive half-lives of the cerium isotopes. Therefore, their effective biological half-times in these organs will be approximately equal to their physical half-lives. Experimental data on internal organ distri-... 

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