THE PLIOCENE PALEOENVIRONMENTAL AND CLIMATIC EVOLUTION OF SOUTH EASTERN AUSTRALIA: EVIDENCE FROM THE GIPPSLAND BASIN

Over 1km of Seaspray Group cool-water carbonates were deposited in the Gippsland Basin in SE Australia during the Pliocene. Sampled sections lay at the shelfal to slope paleoenvironmental transition. For most of the Pliocene the Subtropical Convergence Zone (SCZ) lay on the Gippsland Basin but today occurs south of Tasmania. Detailed palynological data for the Seaspray Group can be tied by foraminiferal plankton datums to a Pliocene chronostratigraphy. These factors make the Gippsland Basin ideal to study SE Australian Pliocene climate history.

In the earliest Pliocene low energy bathyal pelagic marl was deposited in a complex of canyon and inter-canyon environments. By the upper Early Pliocene to Late Pliocene high energy limestone (grainstone) facies infilled these canyons, associated with the seaward progradation of the Gippsland shelfal complex.

Foraminiferal biofacies data suggests upwelling in cool-temperate marine conditions during the lower Early Pliocene. Early Pliocene marine conditions were mostly warmer than present in the region with only minor cooler intervals. The SCZ in the Early Pliocene lay south of the area and the region was likely to have been influenced by eddies of the warm East Australian Current. Early Pliocene terrestrial paleoclimate estimates from spore-pollen data are also warmer and wetter than present (MAT 2-4°C > present, MAP 100-300cm). This warmer period corresponds with lighter than present d¹⁸O values and higher sea levels suggesting a reduced Antarctic ice sheet. A regional SCZ more northerly than present, however suggests that while the Early Pliocene was warm, glacial conditions persisted in Antarctica sufficient to cause Southern Ocean latitudinal gradients to steepen.

By the Late Pliocene, upwelling was prevalent in the area associated with several fluctuations in marine paleotemperatures from cool-temperate to warm. These events are related to changes in the relative position of the SCZ; when cooler condition persisted the SCZ migrated northward to the Gippsland region. Cooler drier conditions prevailed in the hinterland during this time (MAT 0-2°C > present, MAP 50-150cm). This time of climate variability in SE Australia corresponds to the transition to Late Pliocene ice house conditions with the expansion of the Antarctic and northern hemisphere ice sheets.