A Middle Miocene Climate Optimum (MMCO) plant community in eastern Australia deconstructed – The plant microfossil record

Abstract

Isotopic (⁴⁰Ar/³⁹Ar) dates and biostratigraphic criteria demonstrate an interbedded volcaniclastic and lacustrine sedimentary sequence exposed by a landslip at c. 600 m elevation on the southern escarpment of the Bulga Plateau near the village of Elands, mid north coast of NSW, was deposited during the early stages (c. 17–16.6 Ma) of the Middle Miocene Climate Optimum (MMCO) and about 10 Ma after the collision of the Australian and Southeast Asian plates. The extinct Araucariaceae-broadleaf temperate rainforest community represented by fossil pollen and spores included Nothofagus as well as a number of thermophile taxa consistent with global warming but almost none of the subtropical taxa with Southeast Asian affinities now found in rainforest on the Bulga Plateau. We conclude that the migration of the Southeast Asian rainforest taxa did not extend as far south as latitude 31°S and upslope as high as c. 600 m elevation on this near-coastal plateau in NSW by c. 16.6 Ma. Nonetheless, MMCO warming might have prevented a number of temperate taxa from migrating further north than southeast Queensland during the Miocene e.g., Nothofagus moorei and Lophosoria. One consequence is the large number of ‘Gondwanan’ morphospecies shared with sequences in southern Australia allows biostratigraphies developed for the continental margin basins in southeast Australia to be used to date Paleogene to Early Neogene sediments in northern NSW and southern Queensland.