Initial Expansion of C₄ Vegetation in Australia During the Late Pliocene

Since the late Miocene, plants using the C₄ photosynthetic pathway have increased to become major components of many tropical and subtropical ecosystems. However, the drivers for this expansion remain under debate, in part because of the varied histories of C₄ vegetation on different continents. Australia hosts the highest dominance of C₄ vegetation of all continents, but little is known about the history of C₄ vegetation there. Carbon isotope ratios of plant waxes from scientific ocean drilling sediments off north-western Australia reveal the onset of Australian C₄ expansion at ~3.5 Ma, later than in many other regions. Pollen analysis from the same sediments reveals increasingly open C₃-dominated biomes preceding the shift to open C₄-dominated biomes by several million years. We hypothesize that the development of a summer monsoon climate beginning in the late Pliocene promoted a highly seasonal precipitation regime favorable to the expansion of C₄ vegetation.