THE EVOLUTION OF C4 GRASSLANDS IN AUSTRALIA: PHYTOLITH ASSEMBLAGE DATA FROM NEOGENE MARINE CORES

The global expansion of C₄ grasses has a spatiotemporally complex history, and phytolith analysis has been helpful in deciphering the varying controls that influenced this phenomenon. Phytoliths have the advantage that they often accurately reflect the relative abundances of grasses vs. trees and shrubs, and they provide information about different types of C₃ and C₄ grasses. Apart from Australia, phytolith assemblage analysis has been used to investigate the origin and spread of C₄-grass-dominated biomes on all continents that occupied low-mid latitudes during the Neogene. Grass phytolith abundances measured in sediment samples from the Deep Sea Drilling Program (DSDP) site 591, located east of Australia, combined with faunal evidence, suggest a later emergence of these biomes than on other continents. However, detailed phytolith assemblage analysis has yet to be conducted. Here we analyze phytolith assemblages extracted from marine sediments off the Australian coast to illuminate Australia’s vegetative history. Analyzed samples spanning the last 15Myr from DSDP Site 591 and Ocean Drilling Program (ODP) Site 763 reflect terrestrial vegetation from southern and central-northwestern Australia, respectively. Today southern Australia is covered primarily with eucalypt and acacia woodlands, eucalypt forests and chenopod shrublands, whereas the vegetation in central-northwestern Australia consists mainly of grassland and shrubland. Our preliminary studies show that samples from both cores contain well-preserved, terrestrially derived, but rare phytoliths as well as abundant marine biosilica (e.g., diatoms, radiolarians). Identified phytoliths include morphotypes typical of grasses (C₃ pooids, C₃/C₄ PACMADs), palms, and other forest indicators, suggesting that late Miocene vegetation on mainland Australia contained a mix of grasses, trees and shrubs. Further research, which will include additional phytolith work and comparison with contemporaneous compound-specific stable carbon isotopic data, will enable a more refined understanding of how grass community composition and vegetation types changed during the Neogene of Australia.