Isotopic fractionation bodies

Bryant, J.D. and Froelich, P.N. 1995 A model of oxygen isotope fractionation in body water of large mammals. Geochimica et CosmochimicaActa 59 4523-4537. 

THE REPRESENTATION OF ISOTOPIC FRACTIONATION BETWEEN DIET AND BODY COMPONENT... 

Any body tissue in direct equilibrium with the major excretum should reflect the isotopic composition of the diet as a whole. This seems to be the case for bioapatite carbonate, which is thought to be in equilibrium with plasma bicarbonate, which itself is in equilibrium with respired CO2. In fact the Ambrose and Norr (1993) and Tieszen and Fagre (1993) data sets (among others) show clearly that the bioapatite carbonate differs from total diet (or respired CO ) by an amount approximating to the equilibrium isotopic fractionation in the system (Mook 1989) ... 

Figure 11.1. A flow-model scheme for treating the protein routing question. Labels refer to flow rates of carbon. The total carbon flux, into and out of the body, is 1, divided into F (for protein) and 1 - F for the remainder. The significant relevant internal fluxes are between the amino acid pool (coupled to the body protein pool), and the energy metabolism pool . The extent to which protein routing is observable in the body protein composition depends on the value ofX (See Fig. 11.2). Numbers in refer to suggested isotopic fractionations associated with a metabolic path, which are consistent with the data of the Ambrose and Norr (1993) and Tieszen and Fagre (1993) data set (see Section 4.1).

The body of research on isotopic fractionation induced by sulfate and nitrate reduction provides insight into selenate, selenite and chromate reduction. For sulfate and nitrate oxyanions, reduction is generally microbially mediated, is irreversible, and involves a fairly large but variable isotopic fractionation. As described below, Se and Cr oxyanion reduction follows suit, though abiotic reactions may have a greater role in some transformations. 

Wiesli RA, Beard BL, Taylor LA, Johnson CM (2003a) Space weathering processes on airless bodies Fe isotope fractionation in the lunar regolith. Earth Planet Sci Lett 216 457-465... 

In summary, a considerable body of enzymatic, genetic, and analytical data supports the view that the major, if not sole, pathway of denitrification involves NO as an obligatory intermediate and requires the action of nitric oxide reductase. On the other hand, the ability of nitrite to modify nitrosyl transfer ratios and the N isotope fractionation factor during its reduction, are consistent with the reductive scheme of Averill and Tiedje (1982). It was suggested (Goretski... 

Quantifying the chemical and isotopic fractionations in planetary samples and in meteorites, the closest analogues to the materials that formed planets, is a necessary first step. This involves careful measurement of chemical and isotopic compositions of the various bodies and an understanding of the composition of the material from which they formed. Once these fractionations have been identified, experiments (and theory, when the relevant experiments cannot be performed) that yield similar fractionations can point to the processes and conditions that produced them. 

Some of these processes also cause measurable isotopic effects. Evaporation into empty space can cause the residual liquid or solid to become enriched in heavy isotopes. Processes that do not necessarily produce chemical fractionations can also produce isotopic effects. Diffusion is an example of such a process. Also, if the various constituents that go into making asteroids and planets have different isotopic compositions, the formation of these bodies can result in bulk compositions that are isotopically fractionated. Oxygen isotopes... 

All of the bodies in the solar system formed from the same mixture of gas and dust inherited from the Sun s parent molecular cloud. The composition of the dust is best approximated by Cl chondrites. The current compositions of the bodies in our solar system came about because various chemical and physical processes fractionated the elements and isotopes in that initial composition. Understanding how and why elements and isotopes fractionate is a central theme of cosmochemistry. It is easy to visualize fractionations using certain kinds of diagrams that compare elements and isotopes with different chemical characteristics. 

Isotopic fractionation resulting from evaporation from standing water bodies can be described in terms of equilibrium and nonequilibrium fractionation effects. Equilibrium fractionation occurs when the isotopic composition of the evaporated water or lake evaporate is in thermodynamic equilibrium with the lake water (23). Equilibrium fractionation, however, can occur only when the water vapor in the air mass above the lake is 100% saturated. The process of equilibrium isotopic fractionation is described by Raleigh fractionation. The isotopic composition of water vapor in equilibrium with liquid water at any time is given by... 

Temperature. Six different cosmothermometers , b ed on kinetic or equilibrium isotope fractionations, or on chemical reactiom, consistently give temperatures near 360 K for Cl chondrites (Table 1). These values represent the temperatures at which isotopic or chemical equilibria were frozen in, owing to sluggish reaction rates or physical isolation of the meteoritic dust from the ambient gas (by accretion to larger bodies). These temperatures are lower than those for C2 chondrites, 380 to 400 K (Onuma et al, 1972, 1974 Lancet, 1972) or ordinary chondrites and achondrites, 450 to 500 K (Grossman and Larimer, 1974). 

A Moon-forming collision of an approximately Mars-sized body with Earth (Hartmann and Davis, 1975 Cameron and Ward, 1976) would clearly result in catastrophic loss of volatiles from the pre-existing atmosphere and may have caused substantial loss of deep-Earth noble gases as well. Ahrens (1990, 1993) argued that virtually complete expulsion might have occurred by direct ejection from the impacted hemisphere and by shock-induced outward ramming of the antipodal planetary surface. However, losses may have been incomplete, and this event could have been followed by additional, isotopically fractionating losses driven by thermal processes (see Section 4.12.5). 

Early work by Thode et al. (1949) established that there are large variations in the isotopic composition of sulfur compounds in nature. Since then sulfur isotope abundance data have been frequently used to elucidate many terrestrial processes including the genesis of sulfide ore bodies. The mechanisms of isotopic fractionation (alteration of relative isotopic abundances) can be broadly categorised under exchange processes or the kinetic isotope effects discussed on pp. 324ff. Isotopic exchange may be represented by the reaction ... 

The potential components that might have delivered volatile elements to the Earth are the PSN (the major reservoir in the Solar System) and solid matter bodies such as meteorites and comets. The composition of meteoritic volatiles is thought to have been derived from the PSN through elemental and isotopic fractionation. Contributions from sources outside the Solar System such as pre-solar grains or species affected by interstellar chemistry are attested by the discovery of pre-solar grains in primitive meteorites on one hand, and by the large variation of the D/H ratio in the Solar System on another hand, but their extent is a matter of debate. A comparison of the abundances of... 

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