Isotopic fluxes

DERIVATION BY THE STEADY-STATE METHOD. Britton S first derived isotope flux equations under steady-state rather than equilibrium conditions. To illustrate his procedure, we shall again use Scheme 1, the B P exchange in Ordered Bi Bi mechanism, as an example, so that the results can be compared (Scheme la). 

Marty, B., Torgersen, T., Meynier, V, O Nions, R. K., de Marsily, C. (1993) Helium isotope fluxes and groundwater ages in the Dogger Aquifer, Paris Basin. Water Resources Res., 29(4), 1025-35. 

Figure 12.5 Sulfur isotopic budget based on mass isotopic fluxes (bar graphs) and their isotopic composition on a monthly basis in coastal marine sediments of cape Lookout Bight, NC (USA). Units are in mol S m-2 m-1. (Modified from Chanton and Martens, 1987b.)...

An animal at isotopic steady state must consume and release nitrogen with the same average These isotopic fluxes are not weU characterized for any animal... 

Based on the discussion in the previous section it is apparent that plant photosynthesis produces organic matter from today s atmosphere, but respiration releases carbon that has been stored for different periods in plant tissues and soil organic matter. It was also noted already that over the industrial era, the of the atmosphere has been declining at a mean rate of 0.02%cyr The carbon respired today is, therefore, enriched in compared to current photosynthate, to an extent that depends on the residence time of the carbon in the biosphere (Figure 4). For example, adopting the estimated global mean t of 27 yr mentioned above, soil respired CO2 could be 27 X 0.02 = 0.5%o more enriched in with respect to current atmospheric CO2. Under such circumstances, an isotopic flux between the land biosphere and the atmosphere will exist even if there is no net CO2 flux. Similar to the case for the ocean, this represents the land disequilibrium, Dl. Note that Dl will tend to enrich the atmosphere in... 

Bowling D. R., Tans P. P., and Monson R. K. (2001) Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2. Global Change Biol. 7(2), 127-145. 

Gautason B, Muehlenbachs K (1998) Oxygen isotope fluxes associated with high-temperature processes in the rift zones of Iceland. Chem Geol 145 275-286... 

The sum of the two fluxes of isotopes results in the change of stable isotope flux,... 

Turning to the minor isotope, the ratio of minor to major isotope fluxes in the open air (on Sq) is the same as the isotopic concentration ratio R, because there is negligible dLscrimination by fluid motion and mixing. Hence, on Sq, the advective flux for the minor isotope is pcR u — v) and the molecular or turbulent flux is J F. However, transport across plant or other solid surfaces (Sp) does discriminate between minor and major Isotopes. The flux of minor isotope across these surfaces is J pF, where Jfp is the isotopic ratio of the scalar exchanged (transported across Sp) by the flux F. Hence the molar balance for the minor isotope can be written in either of the forms... 

Gaily, H., et al. (1983). A Simple and Sensitive Procedure for Measuring Isotope Fluxes Through Ion-specific Channels in Heterogenous Populations of Membrane Vesicles, J Biol. Chem. 258 13094—13099. 

The membrane potential of many biological cells has been analyzed quantitatively by using the above equation [Eq. (126)] with known bulk ion concentrations and ion permeabilities across the membrane (Figure 27). The latter (permeabilities) were determined from isotope flux measurements across membranes together with the use of the flux equation [Eq. (124)]. However, it should be remembered that the flux equation [Eq. (124)] used to obtain the permeability was derived under the conditions of the constant field assumption. Therefore, if this membrane potential theory is not a correct expression for biological membranes, the permeability values obtained using Eqs. (124) and (126) will lose their validity. 

More recently it has been suggested that if the isotopic flux of water appears to be smaller than the flux calculated from a volume-change measurement, this is due to the existence of a poorly stirred layer of water on either side of the cellular membrane. A stagnant layer obviously does not disturb appreciably the results obtained when measuring an osmotic flow but has a profound influence on the tracer results. 

Let us imagine an experiment for which v = V2 = 0 Vs O such an experimental situation is easily devised using for instance labelled polyions and measuring isotopic fluxes in a self-diffusion process, from (7) ... 

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