The Pliocene climatic and environmental evolution of southeastern Australia: evidence from the marine and terrestrial realm

During the Pliocene the global climate fluctuated markedly with the expansion and contraction of the Northern and Southern Hemisphere ice sheets. The signals of this change are well preserved in the thick (up to 1 km) Seaspray Group cool-water carbonate sediments in the Gippsland region and associated thin terrestrial deposits in southeastern Australia. This study uses seismic, facies, foraminiferal proxy data and palaeobotanical data to chart the Pliocene climate and environmental change in the marine and terrestrial realms of southeastern Australia. Complex submarine canyoning occurred at the shelf/upper bathyal transition during the Pliocene in Gippsland. Low-energy pelagic marl (wackestone/packstone) characterise canyon and inter-canyon environments in the earliest Pliocene, depositing plankton oozes with interbedded calciturbidites. From upper Early Pliocene to Late Pliocene time high-energy limestone (grainstone) facies infilled these submarine canyons associated with progradation of the succession from outer to middle shelf palaeoenvironments. Plankton proxy data suggest cool conditions in the basal part of Early Pliocene. Relatively stable warmer marine conditions prevailed throughout most of the Early Pliocene, corresponding to a period of globally low δ18O values in the oceans associated with minor Antarctic ice sheet expansion. From middle to Late Pliocene time marked fluctuations in the abundance of cool and warmer water plankton taxa occurred, corresponding to a time of global marine δ18O fluctuations and generally heavier δ18O values associated with the expansion of the Antarctic ice and Northern Hemisphere ice sheets. Upwelling is interpreted to have occurred throughout much of the warmer Early Pliocene, caused by a more northerly (compared to today) positioned and weaker Subtropical Front. Upwelling was prevalent in the outer shelf to upper slope facies at the ‘palaeo’ Bass Canyon and the Subtropical Front migrated northwards to Gippsland during Late Pliocene glacial periods. Terrestrial palaeobotanical records indicate a shift from widespread Araucarian forests and rainforest, including ‘tropical’ taxa now extinct in the region, to a landscape by the end of the Late Pliocene similar to that of the present day with a mosaic of Eucalyptus–Acacia–Casuarinaceae sclerophyllous forests and open vegetation, with local areas of Nothofagus-dominated cool temperate rainforests. Palaeobotanical proxy data indicate that regional climate oscillated between warm–wet and cool–dry phases, with an overall cooling–drying trend through the Pliocene. Earliest Pliocene climates in southeast Australia were warm–wet with a summer rainfall peak (mean annual temperature, MAT, 2–4°C higher than present, mean annual precipitation, MAP, 50–70% higher than present), whereas terminal Late Pliocene climates were drier–cooler with a winter rainfall peak (MAT 0–2°C higher than present, MAP 0–30% higher than present).