Isotope balances

Hoefs J, Sywall M (1997) Lithium isotope compositions of Quaternary and Tertiary biogene carbonates and a global lithium isotope balance. Geochim Cosmochim Acta 61 2679-2690 Hogan JF, Blum JD (2003) Boron and lithium isotopes as groundwater tracers a study at the Fresh Kills Landfill, Staten Island, New York, USA. Applied Geochem 18 615-627 Huh Y, Chan LH, Edmond JM (2001) Lithium isotopes as a probe of weathering processes Orinoco River. Earth Planet Sci Lett 194 189-199... 

Rees C.E. (1970) The sulfur isotope balance of the ocean An improved model. Earth. Planet. Sci. Lett. 7, 366-370. 

Isotope balancing can be carried out at two levels, atom-wise or molecule-wise. In the case of isotope balancing on the level of carbon atoms, fractional enrichments are used for estimating the fluxes. Thus, the sum of labeled carbon... 

Fig. 2. Principle of metabolite (left) and isotope balances (right). Two reactions, with fluxes x and y, lead to the intermediate which is produced with a flux z, that can be measured. The enrichments of the metabolites are indicated by the boxes and the values given in percent. Using the isotope balance, x and y are determined as being of equal magnitude. Using the metabolite balance, the absolute magnitudes of x and y are determined to be 40...

As the number of equations needed for isotope balances is large, it is important to use a condensed notation to describe the carbon transitions in a reaction. In the case of isotope balancing based on fractional enrichments, the balances can be expressed using so-called atom-mapping matrices [17]. The principle of using atom-mapping matrices is illustrated by considering an example where the carbon transitions in the first reactions in the Krebs cycle are shown (Fig. 3). 

Citrate is an intermediate in this pathway, and it is therefore possible to set up metabolite and isotope balances over citrate. Since citrate is a C6-molecule,... 

Table 1. Mathematical structure of metabolite and isotope balances...

An important difference between isotope balancing and metabolite balancing is the fact that the isotope label, e.g., due to reversible reactions, may be transported in the opposite direction to the net flux. Thus, reversible reactions, or... 

Radioisotopic labels. Radioisotopes of zinc, Zn and 69mzn, have been used as labels in human absorption studies as described earlier in this symposium. Use of radioisotopes is limited because subjects are exposed to radioactivity. They should not be used in some population groups, such as Infants and pregnant women. Further, the half life of 9mzn is too short for fecal Isotope balance which requires collections over 12 to 15 days. 

Some of the Important methodological Issues that should be considered when stable Isotopes and the Isotope balance method are to be used for measurement of dietary mineral availability are ... 

Validity of in vivo Measurements. Since the stable Isotope balance approach is a novel method, its validity needs to be established, and the problems that must be Included in this examination are briefly discussed below. 

Denitrification is the largest loss term for oceanic nitrogen and plays a key role in regulating the avadabifity of combined nitrogen in the ocean on long time scales (Codispoti et al., 2001 Codispoti, 2007) (see Devol, Chapter 6, this volume). While sedimentary denitrification is unhkely to alter the of oceanic NOs" (Fig. 29.3), pelagic denitrification plays a central role in the isotopic balance of the ocean since it does not typically go to completion and therefore produces residual with an... 

Each of these hypotheses is testable using mass-balance and isotopic-balance considerations. 

The mass balance implied by hypothesis 3 is shown in Table 10. The net result of the assumptions built into this model is to decrease the amount of organic matter oxidized to carbon dioxide and to increase the amount of DIC from shell material dissolution. This, in turn, decreases the amount of shell material dissolution/calcite cement precipitation needed to achieve isotope balance. Between Olanta and MRN-77, the amount of dissolution/precipitation needed for isotope balance is 2.0 mmol CaC03 kg of H2O, and 25 mmol CaC03 from MRN-77 to HO-338. This, in turn, implies that l-13vol.% of the aquifer would be cemented by calcite, which is roughly in line with observed calcite cementation. 

---