MOLECULAR INSIGHTS ON FIRE ECOLOGY AND CARBON CYCLING DURING THE NEOGENE C4 EXPANSION IN AUSTRALIA (Invited Presentation)

C₄ ecosystems proliferated on the Australian continent in the late Pliocene (~3.5 Ma), relatively late compared to other regions (~8-5 Ma). The unique climate and evolutionary history of the Australian continent may be partially responsible for this delay, although the late timing was potentially influenced by distinct fire traits of Australian vegetation. We applied a biomarker approach in marine sediments off the coast of western Australia (ODP 763) to test the potential influence fire regime changes might have had on the timing of C₄ expansion. Pyrogenic PAH concentrations normalized to plant inputs do not have a strong relationship with previously published d¹³C values of n-alkanes. Increased fire occurrence did not initiate nor result from the expansion of C₄ grasslands in northwestern Australia, which indicates fire-C₄ grassland relationships were not tightly coupled in this region. Australian C₃ woody vegetation has been uniquely adapted to frequent fires since the early Cenozoic, which may explain why the transition to C₄ dominated vegetation was not accompanied by a large change in fire tendency, unlike other continents. Thus, the delay in C₄ expansion in Australia potentially reflects a diminished role for fire as an accelerant of change.

PAHs can be produced from both the burning of terrestrial vegetation as well as via thermal maturation of organic matter. Petrogenic PAHs are sourced from thermally mature organic matter in parent rocks and can be found in their derivative soils. Alkylated PAH distribution patterns between 3.0 and 3.5 Ma were largely petrogenic in nature, and revealed a 10-fold increase petrogenic-sourced PAHs’ mass accumulation rates. This jump coincided with the C₄ biome expansion, and it signals greater remobilization of recalcitrant organic matter accompanied increased dust transport of weathered material off of the continent. A corresponding 10-fold increase in clay mass accumulation rate at ODP 763 supports this interpretation. The Australian C₄ biome expansion was potentially associated with intensified seasonal extremes in rainfall and increased aeolian transport. Our PAH record links destabilized soil carbon to these climate and ecological shifts and further suggests they decreased soil fertility and diminished carbon storage in the terrestrial system.