Carbon isotope changes in paleosols from Siwalik, Pakistan, and marine sediments from the Bengal Fan indicate a major C (sub 4) plant expansion in the Himalayan foreland during the late Miocene. However, the timing and mechanisms behind the C (sub 4) plant expansion remain enigmatic. Here we present high-resolution ( approximately 60 k.y.) biomarker and compound-specific isotope data spanning the past 11 m.y. from Ocean Drilling Program Site 722 in the Arabian Sea. An approximately 5 per mil-6 per mil increase in leaf wax delta (super 13) C values indicates a marked rise of C (sub 4) plants from 10 to 5.5 Ma, with accelerated expansion from 7.9 to 5.5 Ma. A concurrent approximately 50 per mil rise in leaf wax delta D values is attributed to a combined effect of changes in precipitation amount and evaporation, indicating that source regions for the plant waxes became progressively drier from 10 to 5.5 Ma. In contrast to earlier reports, our isotope records, biomarker abundances, alkenone U (super K') (sub 37) , and Globigerina bulloides abundance data do not suggest enhanced summer monsoon circulation during this time interval. Rather, our results suggest that large-scale hydrological changes drove the late Miocene expansion of C (sub 4) plants in the Himalayan foreland and Arabian Peninsula.