MOLECULAR EVIDENCE SUGGESTS AN ACTIVE FIRE-FEEDBACK TRIGGERED LATE MIOCENE C4 GRASSLAND EXPANSION ON THE INDIAN SUBCONTINENT

The Late Miocene C₄ grassland expansion had a profound impact on the terrestrial biosphere and global carbon cycle, yet regional environmental drivers for this transition are still poorly understood. On the Indian Subcontinent, the timing of the expansion is well documented, and both stratigraphic and paleontological evidence indicated it was accompanied by increased seasonality in rainfall. In modern environments, greater seasonal extremes in moisture can amplify the occurrence of fire, which disturbs forest ecosystems and creates niche space for grasses to exploit. Distributions of marine charcoal suggest a fire feedback could have accelerated Miocene grassland expansion generally, but the evidence is not definitive for ecosystems on the Indian Subcontinent. Here, we test the role of fire directly by examining the abundance and distribution patterns of fire-derived polycyclic aromatic hydrocarbons (PAHs), and terrestrial vegetation signatures from n-alkanes, and compound specific carbon isotopes. PAH, biomarker and isotope data from paleosol samples of the Siwalik Group (Pakistan) enable an integrated and synoptic terrestrial record of fire and vegetation change.

PAHs are of primarily pyrogenic origin as evidenced by the pattern of alkylated and non-alkylated forms. Fire-derived PAH concentrations increased fivefold just following loss of conifer vegetation at 10 Ma, as indicated by a decrease in the conifer-derived retene compound. We propose that the initial increase in molecular fire signatures was prompted by a transition to more fire-prone vegetation, such as a C₃ grassland state or dry deciduous woodland. A second rise in the amount of fire-derived PAHs was coincident with ¹³C enrichment in n-alkanes associated with C₄ grassland expansion and with δ¹⁸O_carb evidence for increased moisture seasonality at 6 Ma. Carbon isotopic values of n-alkanes and fire-derived PAH concentrations both have considerable scatter, which is realistic, as this record likely also reflects the inherit heterogeneity of fire processes on a non-uniform landscape.

PAH evidence clearly indicates increased fire occurrence accompanied vegetation changes and landscape opening, consistent with a significant role for a pronounced fire-grassland feedback in operation during the Late Miocene grassland expansion.