Factors influencing isotopic composition

New insights on biomineralization may be revealed by measuring Ca isotope variations in shell secreting organisms (e.g. Griffith et al. 2008). Two factors influence the Ca isotope composition of shells (1) the chemistry of the solution, in which the organisms live and (2) the process by which Ca is precipitated. 

The isotope composition of biogenic and authigenic mineral precipitates from lake sediments can be used to infer changes in either temperature or the isotope composition of lake water. Knowledge of the factors that may have influenced the isotope composition of the lake water is essential for the interpretation of the precipitated phases (Leng and Marshall 2004). In many lakes the combined analysis of different types of authigenic components (precipitated calcite, ostracodes, bivalves, diatoms, etc.) may offer the possibility of obtaining seasonally specific information. 

Leaf gas exchange rates are highly dependent on local climatic factors influencing C02 diffusion and evaporation rates, especially temperature lapse rates. The dependency of gas-exchange parameters on local climatic factors and leaf anatomy may account for the wide variability in leaf stomatal responses and stable isotope composition over elevation transects found in different species and different regions. 

According to transition-state theory it is possible to consider reaction velocities in terms of a hypothetical equilibrium between reactants and transition state. It follows that the influence of the isotopic composition of the medium on reaction velocity can be considered to be the same as its influence on the concentration of transition states. The kinetic formulation of the problem can thus be replaced by one couched in equilibrium terms, and the equilibrium theory of the preceding section can be applied with a minimum of modification (Kresge, 1964). The rate constant, or catalytic coefficient, (k) for a catalysed reaction can be written as the product of three factors, viz. the equilibrium constant (K ) for the process forming the transition state from the reactants, the transmission coefficient, and the specific rate of transition state decomposition (kT/h). We recognize that the third factor is independent of the isotopic nature of the reaction and assume that there is no isotope effect on the transmission coefficient. It follows that... 

The marine strontium isotope record is the proxy record most commonly used to constrain the geologic history of chemical weathering. However, in recent years it has been widely criticized as a proxy indicator of past silicate weathering rates. The osmium isotope record is analogous to the strontium record in many respects, and can help to constrain interpretations of the marine strontium isotope record. In this section the geochemical factors that influence the osmium and strontium isotope compositions of seawater are reviewed, and the structure of these two records of Cenozoic ocean chemistry is discussed. 

Water temperature is one factor that can influence the concentration of dissolved CO2 and, thereby, the isotopic fractionation encoded during photosynthetic carbon assimilation. This has been suggested as a means through which paleolatitude could be reconstructed from the carbon-isotopic composition of petroleum hydrocarbons sourced from rocks laid down during time intervals when significant pole to equator temperature gradients prevailed (Andrusevich et al., 2001). 

In summary, the cultured B. aculeata A8 C and A8 0 values are consistent with no microhabitat 8 C effect in culture, and a carbonate ion effect on shell isotopic composition. If the carbonate ion effect is similar to the planktic foraminifera effect (Spero et al. 1997), the most consistent results for both isotopes would be obtained using an equilibrium oxygen isotopic fractionation intermediate between the predictions of McCorkle et al. (1997) and Shackleton (1974). The isotopic offsets of the field specimens are consistent with a pore water 8 C influence on shell 8 C and no carbonate ion effect, and an equilibrium oxygen isotopic composition no higher than the McCorkle et al. (1997) prediction consideration of likely trends in pore water carbonate chemistry suggests an even lower equilibrium oxygen isotopic composition, similar to the prediction of Bemis et al. (1998). Thus these culture and fleld data, considered in the context of the carbonate chemistry of the culture and fleld systems, do not yield a consistent estimate of the equilibrium fractionation factor for 8 0 in benthic foraminiferal calcite. Further culture experiments, including carbonate chemistry manipulations, may help resolve this discrepancy. 

In addition to the biological factors noted above, the isotopic composition of inorganic carbon is influenced by the exchange of carbon between surface waters and the atmosphere. Carbon isotopes are fractionated with the transfer of carbon between water and the atmosphere (Siegenthaler and Munnich 1981 Zhang et al. 1995), with equilibrium fractionation resulting in atmospheric carbon dioxide about 8%o depleted relative to the ocean. This effect is temperature dependent, with a change in fractionation of approximately -0.1%o per K (Mook 1986). Thus, at equilibrium, DIC in colder waters is enriched in C relative to warmer waters. In natural waters, the time required for isotopic equilibration is slow relative to the residence time of carbon in surface waters... 

The impact of thiosulfate disproportionation on sedimentary sulfur systematics (further discussed below) will depend on the magnitude of the fractionation in nature and whether it is influenced by isotope exchange as some of the experimental results would indicate. Of critical importance will also be the isotope difference between sulfane and sulfonate atoms of naturally formed thiosulfate in the environment, for which no information is currently available. Our understanding of the impact of thiosulfate disproportionation in the environment will, therefore, depend on our resolving what factors influence the extent of fractionation, the degree of isotope exchange, and the isotopic composition of thiosulfate in nature. 

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