Soil carbon loss and weak fire feedbacks during Pliocene C₄ grassland expansion in Australia

Abstract

C₄ grasslands proliferated later in Australia than they did on other continents (∼3.5 Ma vs. 10–5 Ma). It remains unclear whether this delay reflects differences in climate conditions or ecological feedbacks, such as fire, that promote C₄ ecosystems. Here, we evaluated these factors using terrestrial biomarkers from marine sediments off western Australia. Fire‐derived polycyclic aromatic hydrocarbons (PAH) indicate fire ecology did not substantially change during or following C₄ expansion. The presence of fire‐adapted C₃ woody vegetation likely diminished the role of fire and delayed C₄ expansion until it was prompted by climate drying between 3.5 and 3.0 Ma. At the same time, mass accumulation rates of weathered PAHs increased 100‐fold, which indicates a significant loss of soil carbon accompanied this ecosystem shift. The tight couplings between hydroclimate and carbon storage altered boundary conditions for Australian ecosystems, and similar abrupt behavior may shape environmental responses to climate change.