One important application has been to document the global expansion of C4 grass systems between and 5 Ma (Cerling et al. 1997; Jacobs et al. 1999). Since both biochemical and fossil evidence suggest that our common ancestor with chimps lived at about this time, this important environmental shift might well have had some direct impact on the origins of the hominin lineage. The phenomenon was first detected in pedogenic carbonate nodule isotope studies from East Africa and Pakistan (Cerling et al. 1988; Quade et al. 1989; Cerling and Quade 1993). In East Africa, Cerling et al. (1988) observed the first appearance of C4 grasses in the Turkana Basin ca. 7 Ma. This pattern of first appearance of significant amounts of C4 has been mirrored in d13C of fossil fauna in Africa, North and South America, and Pakistan (Cerling et al. 1997), although Fox and Koch (2003) have suggested earlier presence of minor amounts of C4 biomass in the North American Great Plains, based on pedogenic carbonate evidence. Kingston et al. (1994) and Morgan et al. (1994) have argued that modest proportions of C4 grasses were present earlier in the Mid-Miocene of East Africa, based respectively on continuous heterogeneity in a pedogenic carbonate sequence from the Tugen Hills and several slightly enriched faunal values from early Tugen Hills sites. The argument hinges largely on the weight of evidence, since the results for the Tugen Hills pedogenic sequence have not been replicated elsewhere in Africa. The values used to demarcate the certain presence of C4 in fauna also differ as Morgan et al. (1994) used a value of -10%o as the cut-off point, above which they deduced presence of C4, whereas Cerling et al. (1997) used a more conservative —8%o.
Most evidence, however, still suggests that C4 grasses first began their expansion in lower latitudes at ca.7-8 Ma, expanding over the next few million years to midlatitudes (Cerling et al. 1997). The timing is unclear in southern Africa. Ratite eggshell data document emergence of C4 and differentiation of the 813C records between the southern and northern Namib about 5 Ma (Segalen 2003; Segalen et al. 2006). A large undated, collapsed cone speleothem in the Makapansgat Limeworks in South Africa indicates relatively invariant C3 vegetation cover from the 813C record, suggesting that it was formed prior to C4 grasses reaching this region at ~27 S (Hopley 2004). A notable exception to the general picture of C4 expansion is the 5-Ma-year-old site of Langebaanweg in the southwestern Cape of South Africa (Franz-Odendaal et al. 2002), where faunal 813C data show that a winter-rainfall, Mediterranean-type ecosystem was already in place in this region in the Late Miocene and Early Pliocene. In the northeastern interior, faunal evidence suggests a minor presence of C4 vegetation by Ma or earlier from the Rodent Corner at Makapansgat Limeworks (Hopley et al. 2006), while the modest proportion of pure C4 grazers in the Makapansgat Member 3
faunal assemblage indicates clear presence by Ma (Sponheimer and Lee-Thorp 1999a).
The expansion of C4 grasslands across large parts of the world must have a global driver but the exact causes have remained curiously elusive. Cerling et al. (1997) and Ehleringer et al. (1997) proposed that plummeting CO2 levels in the Late Miocene, to a level below 500 ppm, favored C4 plants. The hypothesis is based on the known tolerance of C4 plants for lower levels of pCO2. But evidence from marine cores suggests that CO2 levels were already low prior to this period (Pagani et al. 1999). This, along with 813C data from fossil ratite eggshells in Namibia that closely track the marine-derived trends in pCO2 for the Miocene (Segalen et al. 2006), has lead to a reconsideration of the possible drivers. The emergence and spread of C4 may reside in a combination of tectonic and solar insolation forces that rearranged the earth's global heat budget, both spatially and seasonally. Resolution of the problem likely requires a good deal of more detailed evidence from the transition period, spread across different regions of Africa and elsewhere.
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