Carbon isotope modeling

Wittenberg U. and Esser G. (1997) Evaluation of the isotopic disequilibrium in the terrestrial biosphere by a global carbon isotope model. Tellus Ser. B Chem. Phys. Meteorol. 49(3), 263 -269. 

The carbon-isotope model and its variants have, it is fair to say, revolutionized the use of soils and paleosols (see Chapter 5.18) in paleobotany and climatology. Some of the major achievements and uses of the model include the following. 

Carbon isotope modeling has been used to distinguish closed and open carbonate systems... 

Once the model was complete, it was adjusted to a steady state condition and tested using historic carbon isotope data from the atmosphere, oceans and polar ice. Several important parameters were calculated and chosen at this stage. Sensitivity analysis indicated that results dispersal of the missing carbon - were significantly influenced by the size of the vegetation carbon pool, its assimilation rate, the concentration of preindustrial atmospheric carbon used, and the CO2 fertilization factor. The model was also sensitive to several factors related to fluxes between ocean reservoirs. 

Krueger, H.W. and Sullivan, C.H. 1984 Models for carbon isotope fractionation between diet and bone. In Tumland, J.R. and Johnson, P.E., eds.. Stable Isotopes in Nutrition. Washington D.C, American Chemical Society Symposium Series, No. 258 205-220. 

In carbon, an uncertainty exists which arises because we have neither sufficient empirical data nor an accepted theoretical model to explain the quantitative relationships between dietary carbon isotope signals and those... 

Figure 5.3. Model describing expected carbon isotopic shifts in direction of matrix values for browsers and grazers. Large arrows represent large shifts, small arrows the converse.

Some aspects of the biochemistry of metabolic processes affecting nutrients appear to have significant consequences for the expected behavior of stable carbon isotopes as tracers of diet. Specifically, we have seen that the simple model of a total scrambling of carbon atoms during endogenous biosynthesis is inconsistent with the expected pathways of some nutrients, whereas other isotopic records in ancient human tissues can be adequately accounted for by this model. 

Table 11.2. Model calculations of weighting functions for fractionation of carbon isotope ratios (in %o) from dietary nutrient biochemical fractions to tissue biochemical fractions.

Figure 11.4. (a) Flow-model scheme for a simple food chain with one predator-prey relationship. See text for discussion, (b) The steps involved whereby atoms from prey collagen (i.e., the diet) may be transferred to a predator s collagen (i.e., the consumer tissue). Each arrow represents a potential change in carbon isotopic composition, complicating the relationship between prey collagen 5 C and predator 5 C. 

Table All.l. Carbon isotope and protein content of experimental diets used for modeling in this paper. The values are 5 C in %o with reference to the PDB standard, except for protein content (by weight) which is in %. The type of diet corresponds to the Ambrose and Norr (1993) and Tieszen and Fagre (1993)...

W. Wiechert and A. A. de Graaf, Bidirectional reaction steps in metabolic networks. Part I Modeling and simulation of carbon isotope labelling experiments. Biotechnol. Bioeng. 55,101 117 (1997). 

Filer JM, Mojzsis SJ, Arrhenius G (1997) Carbon isotope evidence for early life discussion. Nature 386 665 Emerson D (2000) Microbial oxidation of Ee(II) and Mn(II) at circumneutral pH. In Environmental metal-microbe interactions. Lovley DR (ed) ASM Press, Washington DC, p 31-52 Ewers WE (1983) Chemical factors in the deposition and diagenesis of banded iron-formation. In Iron formations facts and problems. Trendall AF, Morris RC (eds) Elsevier, Amsterdam, p 491-512 Farley KJ, Dzombak DA, Morel FMM (1985) A surface precipitation model for the sorption of cations on metal oxides. J Colloid Interface Sci 106 226-242... 

From this simplified scheme, it follows that the diffusional process is reversible, whereas the enzymatic carbon fixation is irreversible. The two-step model of carbon fixation clearly suggests that isotope fractionation is dependent on the partial pressure of CO2, i.e. PCO2 of the system. With an unlimited amount of CO2 available to a plant, the enzymatic fractionation will determine the isotopic difference between the inorganic carbon source and the final bioproduct. Under these conditions, C fractionations may vary from -17 to —40%o (O Leary 1981). When the concentration of CO2 is the limiting factor, the diffusion of CO2 into the plant is the slow step in the reaction and carbon isotope fractionation of the plant decreases. 

Kump LR (1989) Alternative modeling approaches to the geochemical cycles of carbon, sulfur and strontium isotopes. Am J, Sd 289 390-410 Kump LR (2005) Ironing out biosphere oxidation. Science 307 1058-1059 Kump LR, Arthur MA (1999) Interpreting carbon-isotope excursions carbonates and organic matter, Chem Geol, 161 181-198... 

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