Carbon isotope ratios can be used to identify the presence of C3 versus C4 photosynthesis in the fossil records. Large differences in discrimination against 13C02 by the initial carboxylation reactions in C3 (RuBP carboxylase) and C4 (PEP carboxylase) photosynthesis result in significant differences in the carbon isotope ratios (8I3C) of C, and C4 plants (Farquhar et al., 1989). Modern C, plants average approximately — 27%o and C4 plants average approximately — 13%o (Fig. 3). The observed ranges of 8I3C values for both C, and C4 plants are the result of genetic differences among taxa as well as responses to variations in environmental conditions, including light and water stress (Farquhar et al., 1989; Ehleringer et al., 1993; Buchnrann et al., 1996). Differences among C4 photosynthetic subtypes (NADP-nre, NAD-me, and PCK) contribute as much as l-2%o to the range of values shown in Fig. 3 (Flattersley, 1982; 1983).
Animal tissues faithfully record the isotopic composition of their food sources (Tieszen et al, 1983; Hobson, 1999), but often are not preserved in the fossil records or are subject to alteration
diet mixed C3/C4 diet
mixed C3/C4 diet
Carbon isotope ratio [%]
FIGURE 3 Histograms of the carbon isotope ratios of modern grasses and modern tooth enamel; adopted from Cerling et al. (1997).
(diagenesis) during fossilization. However, tissues such as tooth enamel are preserved without subsequent modification, thus recording the original animal diet over periods of several million years (Lee-Thorp and van der Merwe, 1987). Tooth enamel
FIGURE 4 Relationship between the carbon isotope ratio values of estimated diet and measured tooth enamel for ungulate mammals; adopted from Cerling and Harris (1999).
(bioapatite) is enriched 14.1%o relative to a grazing mammal's diet (Fig. 4), resulting in a straightforward means of recording long-term feeding patterns by mammalian grazers (Cerling and Harris, 1999). The lower histograms in Figure 3 illustrate this offset between animals and their food sources, based on an accumulation of observations of S,3C values of apatite from a wide variety of grazing species (Cerling et al, 1997; Cerling and Harris, 1999). It is important to note that the variation in plant SI3C values is similar in magnitude to the variation in tooth-enamel SI3C values. Thus, small variations in tooth-enamel SI3C values on the order of l-2%o are just as likely to represent variations in food quality associated with changing environmental conditions as variations in the abundances of C, and C4 plants in the animal's diet or the changing carbon isotope ratio of the atmosphere.
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