LAUSR

Fire dynamics potentially account for the asynchronous timing of the expansion of C4 grasslands throughout the Mio‐Pliocene world. Yet how fire, climate, and ecosystems interacted in different settings remain poorly constrained because it is difficult to quantify fires and fuel source over these timescales. Here, we apply molecular proxies for fire occurrence alongside records of vegetation change and paleohydrology in Bengal Fan sediments (ODP Leg 116) to examine fire feedbacks on the south Asian continent. We employ abundances of polycyclic aromatic hydrocarbons (PAHs) to reconstruct fire occurrence and δ13C measurements of pyrogenic PAHs to constrain fuel source and grassland burning. This combination allowed us to test whether: (1) a fire‐seasonality forcing facilitated the expansion of grassland ecosystems and (2) a fire‐C4 grass burning feedback maintained these systems. PAHs can be sourced from weathered fossil carbon (i.e., a petrogenic source) and from burned terrestrial biomass (i.e., a pyrogenic source). Alkylated and non‐alkylated structure abundance data distinguished pyrogenic from petrogenic sourced samples. A sharp increase in pyrogenic PAHs along with increases in δ2H and δ13C values of plant waxes at 7.4 Ma indicates increased fire coincided with the onset of C4 expansion and hydrologic change in South Asia. The correlated 13C enrichment in PAHs, 13C enrichment in plant waxes, and increased abundances of PAHs suggest burning of C4 grasslands likely maintained open ecosystems. Our results link fire to the initial opening of grassland ecosystems on a subcontinental‐scale and support disturbance as a critical mechanism of terrestrial biome transition.