C isotopic composition

Figure 8. Figure (a) after Clayton et al. (1976, 1977). The scales are as in Figure 1. The O isotopic compositions of the different meteorite classes are represented ordinary chondrites (H, L, LL), enstatite chondrites (EFl, EL), differentiated meteorites (eucrites, lAB irons, SNCs) and some components of the carbonaceous chondrites. As the different areas do not overlap, a classification of the meteorites can be drawn based on O isotopes. Cr (b) and Mo (c) isotope compositions obtained by stepwise dissolution of the Cl carbonaceous chondrite Orgueil (Rotaru et al. 1992 Dauphas et al. 2002), are plotted as deviations relative to the terrestrial composition in 8 units. Isotopes are labeled according to their primary nucleosynthetic sources. ExpSi is for explosive Si burning and n-eq is for neutron-rich nuclear statistical equilibrium. The open squares represent a HNOj 4 N leachate at room temperature. The filled square correspond to the dissolution of the main silicate phase in a HCl-EIF mix. The M pattern for Mo in the silicates is similar to the s-process component found in micron-size SiC presolar grains as shown in Figure 7.

Even more complex is C-isotope fractionation in aquatic plants. Factors that control the of phytoplankton include temperature, availability of C02(aq), light intensity, nutrient availability, pH and physiological factors such as cell size and growth rate (Laws et al. 1995, 1997 Biigare et al. 1997 Popp et al. 1998 and others). In particular the relationship between C-isotope composition of phytoplankton and concentration of oceanic dissolved CO2 has been subject of considerable debate because of its potential as a palaeo-C02 barometer (see discussion). 

Nichols, R. H., Hohenberg, C. M., Hoppe, P., Amari, S. Lewis, R. S. 1992 22Ne-E(H) and 4He in single SiC grains and 22Ne-E(L) in single C grains of known C-isotopic composition. Lunar Planet. Sci. XXIII, 989-990. 

Kaufman A. J. and Knoll A. H. (1995) Neoproterozoic variations in the C-isotopic composition of seawater stratigraphic and biogeochemical implications. Precamb. Res. 73, 27 -49. 

Some three decades ago, researchers in New Zealand (Hendy and Wilson, 1968) and Europe (Galimov et al., 1965 Labeyrie et al., 1967 Duplessy et al.,1970 Fantidis and Ehhalt, 1970) called attention to the fact that changes in terrestrial climate might be recorded in the O- and C-isotope compositions of speleothems. Subsequently Hendy (1971). geochemically modeled the isotope effects which accompany different modes of speleothem deposition. These earliest studies recognized that ... 

Figure 10. Comparison of the C-isotopic evolution at 25°C during dissolution of limestone (typically with = -4 to +2%o average. = 0%o, Deines et al., 1973) by soil waters initially equilibrated with CO2 derived from a Cs-type vegetation source of = -24%o (a) and a C4-type vegetation source of = -12%o (b) under open (solid line) and closed (dashed line) system conditions (after Salomons and Mook, 1986 and Deines et al., 1974). The lines and curves in each figure represent solution paths for different conditions of initial CO2 partial pressure (PCO2) conditions until equilibrium with calcite (i.e. saturation) is reached. The tick marks on the lines represent the C-isotope composition of the solution at that point of on evolutionary trajectory. Note that carbonate dissolution under closed system conditions leads to higher 5 C values (%o PDB) at saturation than open system dissolution, the lower the initial PCO2 value, the greater the degree of C enrichment at saturation.

Stable hydrogen (50) and carbon (6 C) Isotopic compositions of methane gas bubbles formed In the sediments of several shallow aquatic environments were measured and found to range from -346 /oo to -263°/oo and from -75.0°/oo to -51.5°/oo, respectively. Evaluation of the <5D data with a previously published model Implies that acetate dissimilation accounts for about 50% to 802 of the total methane production. 6D-CH, and are... 

Mora, C.L, Driese, S.G. Seager, P.G. (1991) Carbon dioxide in Paleozoic atmosphere evidence from C-isotopic compositions of pedogenic carbonate. Geology, 19, 1017-1020. 

The history of the bio eochemical C cycle has been at least partially recorded in the C isotopic composition (S Cpdb) of carbonate (Scarb) and reduced C (5org) in ancient sedimentary and metamorphic rocks. To the extent that sedimentary rocks avoided deep burial and alteration, they have preserved information that indicates the status of the C cycle at the time of their deposition. 

If decarboxylation were the dominant mechanism then the (reprecipitated) carbonate cement in the cementation zone should be characterized by a lighter (5 C isotopic composition, inherited from the CO derived from kerogen, compared to that from a carbonate cement of sedimentary mineral origin. If dissociation were the dominant process then it should be controlled by the generation of H and the carbonate cement would be characterized by an S C isotope composition similar to that of marine carbonates. Furthermore, ions derived from the organic acids and in particular acetates should be encountered in the formation waters. 

These large differences in N content emphasize just one of the several contrasts between carbon and nitrogen isotopic studies of sedimentary OM. While in the former, it can normally be assumed that each component makes a proportional contribution to the bulk C-isotopic composition, this may not be the case with N isotopes. The N-rich fraction can determine the isotopic character of the sediment, even if it does not volumetrically dominate the OM assemblage. Conversely, the degraded, N-poor woody and grass cuticle fragments that occur in many lake sediments will not seriously distort the N-isotopic signature of the waterbody preserved in the autochthonous om (see, for example, Yoshioka, et al, 1989 Talbot Johannessen, 1992). 

From the studies available in the literature, it has become apparent that the composition of CAM plants is highly variable, and depends highly on the environment. It is thus possible to make some predictions about the environmental conditions during growth of a CAM plant knowing the C isotope composition (see Chap. 6.2.2). 

H Hanzawa, N Umemura, Y Nisida, H Kanda, M Okada, M Kobayashi. Disorder effecet of nitrogen impurities, irradiation-induced defects, and " C isotope composition on the Raman spectra in synthetic lb diamond. Phys Rev B 54 3793-3799, 1996. 

Appendix C Isotope composition of lead in Phanerozoic sediments... 

Engel, A. E. J., Patterson, C. C. Isotopic composition of lead in Leadville limestone, hydrothermal dolomite, and associated ore. Geol. Soc. Amer. Bull. 68, 1723 (1957). 

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