Signals of the Serengeti: how do four carbon isotopic proxies for C4 grasslands compare?

Abstract:

The carbon isotopic composition of a range of carbon-bearing materials is commonly used to identify the presence of tropical C₄ grasslands in the geologic record. However, no study of modern soils has yet combined carbon isotopic compositions (δ¹³C) of bulk soil organic matter (SOM), soil carbonate (SC), plant wax n-alkanes and n-alkanoic acids to allow for comparison between all four materials. Here, we study carbonate-precipitating, grassland soils across the Serengeti ecosystem, Tanzania, with samples from 11 sites along a NW–SE transect and depth profiles at each location. δ¹³C values of soil carbonates record C₄ grassland signals, but soil carbonates are absent from two of the three sites with trees. All four materials have high δ¹³C values indicative of C₄ grassland at most sites; lower δ¹³C values are found in the organics at three sites with C₃ trees present. In particular, the δ¹³C_29alk is a sensitive detector of woody cover. Of all materials considered (plant wax homologues, SOM and soil carbonates), the plant wax C₃₃ n-alkane and C₃₂ n-alkanoic acid homologues have the broadest applicability to tropical savannas (whether grassland or woodland) extending to non-carbonate bearing soils, and to lacustrine and marine sedimentary archives.