Molecular and isotopic records of C-4 grassland expansion in the late Miocene

Siwalik paleosol and Bengal Fan sediment samples were analyzed for the abun dance and isotopic composition of n-alkanes in order to test for molecular evidence of the expansion of C-4 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 delta C-13 values (ca. -30 parts per thousand) to higher values (ca. -22 parts per thousand) prior to 6 Ma. This shift is similar in magnitude to that recorded by paleosol c arbonate and fossil teeth. and is consistent with a relatively rapid transi tion from dominantly C-3 vegetation to an ecosystem dominated by C-4 plants typical of semi-arid grasslands. The n-alkane values from our paleosol sam ples indicate that the isotopic change began as early as 9 Ma, reflecting e ither a growing contribution of C-4 plants to a dominantly C-3 biomass or a decrease in water availability to C-3 plants. Molecular and isotopic analyses of other compounds, including n-alcohols an d low-molecular weight n-alkanes indicate paleosol organic matter contains contributions from a mixture of sources, including vascular plants, algae a nd/or cyanobacteria and microorganisms. A range of inputs is likewise refle cted in the isotopic composition of the total organic carbon from these sam ples. 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 ancien t terrestrial ecosystems is better reconstructed using isotopic signatures of molecular markers, rather than bulk organic carbon. This approach provid es a means of expanding the spatial and temporal records of C-4 plant bioma ss which will help to resolve possible tectonic, climatic or biological con trols on the rise of this important component of the terrestrial biosphere. Copyright (C) 2001 Elsevier Science Ltd.