Late Miocene C4 Grassland Fire Feedbacks on the Indian Subcontinent

Fire dynamics potentially account for the asynchronous timing of the expansion of C-4 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 delta C-13 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-C-4 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 delta H-2 and delta C-13 values of plant waxes at 7.4 Ma indicates increased fire coincided with the onset of C-4 expansion and hydrologic change in South Asia. The correlated C-13 enrichment in PAHs, C-13 enrichment in plant waxes, and increased abundances of PAHs suggest burning of C-4 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.

Automated summary

The study examines the impact of fire dynamics on the expansion of C-4 grasslands in South Asia using molecular proxies and carbon isotope measurements from Bengal Fan sediments. The results suggest that fires facilitated the expansion and maintenance of C-4 grassland ecosystems in the region.