Global Expansion of C4 Ecosystems

Figure 5 shows that between 8 and 6 Ma there was a global expansion of C4 ecosystems (Cerling et al, 1997, 1998a). There is no conclusive evidence for the presence of C4 biomass in the diets of mammals before 8 Ma (Cerling et al, 1997; 1998a), although the presence of small amounts of C4 biomass in diets is not excluded because of the uncertainty in the SI3C endmember for C, plants. By 6 Ma there is abundant evidence for significant C4 biomass in Asia (Cerling et al, 1993; 1997; Morgan et al, 1994), Africa (Morgan et al, 1994; Cerling et al, 1997), North America (Cerling, et al 1993; MacFadden and Cerling, 1999; Cerling et al, 1999), and South America (MacFadden et al, 1996; Cerling et al, 1997), but not in Europe (Cerling et al, 1997). Figure 5 documents several different ecosystem type changes as recorded in mammalian tooth enamel. While each of these regions appears to have been dominated by C, ecosystems earlier in the Miocene, the C3 Pakistani ecosystem was almost completely replaced by a C4 ecosystem; African, North American, and South American ecosystems retained both C3 and C4 components; European and northern portions of North American ecosystems did not show any change in the fraction of C3 biomass, remaining at virtually 100% C, ecosystems. The mixture of C, and C4 components within a grazing ecosystem can be achieved by one of two ways: a temporal separation with C3 grasses active in winter-spring and C4 grasses active in summer or a monsoonal system with C4 grasses and C, woody vegetation. Without fine-scale analyses of the seasonal dynamics with tooth enamel, the isotopic record is silent as to which pattern existed.

The isotopic evidence is clear that the expansion of C4 ecosystems was a global phenomenon, persisting until today. It was accompanied by significant faunal changes in many parts of the world. It is unlikely that the global expansion of C4 biomass in the late Miocene was due solely to higher temperatures or to the development of arid regions. There have always been regions on earth with hot, dry climates. To explain the simultaneous global expansion of C4 plants requires a global phenomenon. Changes in

Carbon isotope ratio [%

Carbon isotope ratio [%o

Carbon isotope ratio [%

Carbon isotope ratio [%o

Carbon isotope ratio [%o

Carbon isotope ratio [%o

FIGURE 5 Histograms comparing the carbon isotope ratio values for fossil tooth enamel older than 8 Ma (lower charts) with those that are younger than 6 Ma for six regions of earth; adopted from Cerling et al. (1998a).

atmospheric C02 as predicted by the quantum-yield model are a strong possibility for this global mechanism. Supporting evidence indicates that the global expansion of C4 ecosystems appears to have originated in warmer, equatorial regions and then spread to cooler regions, consistent with the temperature-sensitivity predictions of the quantum-yield model. Cerling et al. (1997) documented that within both modern and fossil horses (equids), the distributions of isotope ratios strongly support a decrease in the importance of C4 photosynthesis in moving from warm equatorial to cooler temperate latitudes.

Stable-isotope studies of paleosols from Pakistan and East Africa are in good agreement with the paleodietary studies. The Siwalik sequence in Pakistan has excellent exposures covering the last 20 Ma. SI3C studies of paleosol carbonates show a virtually pure C, ecosystem up to about 7.5 Ma ago, a transitional period of ecosystem change lasting 1-1.5 Ma, and then C4-dominated ecosystems from 6 Ma ago to nearly the present (Quade et al., 1989; Quade and Cerling, 1995). Studies of fossil eggshell show that C, plants were present throughout the sequence, even in the last 6 Ma (Stern et al, 1994). Studies of paleosols in the Turkana Basin in Africa, covering in detail the period from about 7.5 Ma to the present, show mixed C,/C4 ecosystems throughout this period (Cerling et al, 1993; 1997).

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