Matthew Wooller¹, Gifford Miller², Nigel Spooner³, John Magee⁴, Beverly Johnson⁵, and Marilyn Fogel¹

1 Geophysical Laboratory, Carnegie Institution of Washington, 5152 Broad Branch Road, NW Washington DC, 20015-1305, USA

2 INSTAAR and Geological Sciences, University of Colorado at Boulder, Boulder, CO 80309-0450, USA.

3 Environmental Geochemistry and Geochronology, Research School of Earth Sciences, The Australian National University, Canberra 0200, Australia.

4 Department of Geology, The Faculties, The Australian National University

Canberra, ACT, 0200, Australia.

5 Department of Geology 214 Carnegie Science, Bates College, Lewiston, ME 04240, USA

The impacts, if any, of changes in atmospheric CO₂ concentrations over the last 100ka or human colonization (~60ka BP) on Australian vegetation composition are the subject of current debate. Records of vegetation changes through the Late Quaternary across semi-arid Australia are hampered by a lack of plant fossils. Alternatively carbon stable isotope measurements of organics from three sediment cores, dated using Optically Stimulated Luminescence, taken from longitudinal dunes in Western Australia, document the proportions of plants using the C₃ and C₄ photosynthetic pathways, which have diagnostic d ¹³C values. The dune sediments have a limited source of C₄-like carbon (C₄ grasses) and cover a large portion of the Late Quaternary. The d ¹³C records from geographically separate dunes show a similar magnitude of variation (~ -20 to -14 ‰) and a stratigraphic trend towards more C₄ plants in recent (Holocene) samples. This stratigraphic trend is also evident in a fluvial sequence (Pelican Meander) from the same region, implying landscape-scale environmental changes.