Abstract

Riverine Plain palaeochannels record periods of fluvial activity for Late Pleistocene southeastern Australia. In an attempt to develop a more detailed palaeoenvironmental record for this semi-arid region, we investigate the fine overbank sediments of the palaeochannel fill that cap and underlie the coarser-grained fluvial channel sands of the Tombullen (41-29 thousand years (ka) ago) and the Yanco (29-18 ka) phases. New single grain Optically Stimulated Luminescence (OSL) age determinations for the overlying palaeochannel fills suggest an overall slowing of sedimentation rates since the phase of fluvial activity in late Marine Isotope Stages 3 and 2. delta C-13(TOC) data for the palaeochannel fill sediments range between -28 parts per thousand and -22 parts per thousand, implying variable C4 contributions (0-40%) to a predominantly C3 vegetation mosaic. Modelled C4 abundance for the last 50 thousand years (kyr) decreased from 40% to 10% at around 40 ka with predicted representation remaining low towards the Last Glacial Maximum (LGM). Post LGM C4 grasses are seen to expand to 30-40% and sustained through the Holocene. Pollen preservation is restricted to the upper two metres of the cores, with the distribution showing shifts in dominance from Poaceae and Asteraceae to Eucalyptus up sequence. Increasing Eucalyptus/Poaceae ratios correspond with increasing C4 abundance suggesting that as woodland expanded in the Holocene the associated decreasing grass component shifted from C3 to C4. However, some evidence for the persistence of Poaceae and/or Asteraceae dominance argues that the likely shift from C3 to C4 grasses during the Holocene was in places independent of expanding woodland. Bulk sediment elemental ratios reveal wetting and drying cycles during the deposition of the overbank deposits. While these traits appear to be a product of local sedimentation changes, rather than regional climate, our model of C4 contraction and subsequent expansion across the last 50 kyr is likely reflecting changing summer monsoon intensity and its effects on regional aridity.