Carbon isotope changes in paleosols from Siwalik, Pakistan, and marine sediments from the Bengal Fan indicate a major C4 plant expansion in the Himalayan foreland during the late Miocene. However, the timing and mechanisms behind the C4 plant expansion remain enigmatic. Here we present high-resolution (∼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 ∼5‰–6‰ increase in leaf wax δ13 C values indicates a marked rise of C4 plants from 10 to 5.5 Ma, with accelerated expansion from 7.9 to 5.5 Ma. A concurrent ∼50‰ rise in leaf wax δD 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 UK′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 C4 plants in the Himalayan foreland and Arabian Peninsula.

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