Molecular and isotopic records of C₄ grassland expansion in the late miocene

Siwalik paleosol and Bengal Fan sediment samples were analyzed for the abundance and isotopic composition of n-alkanes in order to test for molecular evidence of the expansion of C₄ grasslands on the Indian subcontinent. The carbon isotopic compositions of high-molecular-weight alkanes in both the ancient soils and sediments record a shift from low δ¹³C values (ca. -30%0) to higher values (ca. -22%0) prior to 6 Ma. This shift is similar in magnitude to that recorded by paleosol carbonate and fossil teeth, and is consistent with a relatively rapid transition from dominantly C₃ vegetation to an ecosystem dominated by C₄ plants typical of semi-arid grasslands. The n-alkane values from our paleosol samples indicate that the isotopic change began as early as 9 Ma, reflecting either a growing contribution of C₄ plants to a dominantly C₃ biomass or a decrease in water availability to C₃ plants. Molecular and isotopic analyses of other compounds, including n-alcohols and low-molecular weight n-alkanes indicate paleosol organic matter contains contributions from a mixture of sources, including vascular plants, algae and/or cyanobacteria and microorganisms. A range of inputs is likewise reflected in the isotopic composition of the total organic carbon from these samples. In addition, the n-alkanes from two samples show little evidence for pedegenic inputs and we suggest the compounds were derived instead from the paleosol's parent materials. We suggest the record of vegetation in ancient terrestrial ecosystems is better reconstructed using isotopic signatures of molecular markers, rather than bulk organic carbon. This approach provides a means of expanding the spatial and temporal records of C₄ plant biomass which will help to resolve possible tectonic, climatic or biological controls on the rise of this important component of the terrestrial biosphere.