PLIO-PLEISTOCENE VEGETATION AND CLIMATE CYCLICITY IN UPLAND VICTORIA, SOUTHEASTERN AUSTRALIA

Neogene climatic cooling was expressed in southeastern Australia by progressive reduction of formerly widespread mesotherm and microtherm rainforests, and their replacement by more drought- and fire-adapted, open-canopied vegetation. These long term trends of vegetation replacement accelerated with the relatively abrupt climatic shifts marking the initiation of the Quaternary. However, the response of vegetation to increasingly high amplitude orbital scale climatic cycles around the Late Pliocene-Early Pleistocene remains poorly understood. Pollen analysis of partly microlaminated, organic-rich sediments from a small volcanic paleolake in upland southeastern Australia, the Stony Creek Basin, illustrates the orbital-scale response of both “Tertiary” and “Quaternary” floristic elements to the evolving climatic cyclicity of the late Cenozoic.

9-10 consecutive cycles in the 40m lacustrine record are indicated by variations in organic matter and pollen abundance. The primary pattern of change is alternation between presence and absence of Podocarpaceae-rich conifer-angiosperm rainforest, including many taxa now extinct/very restricted in Australia, against a background of open-canopied sclerophyll forest dominated by Eucalyptus, Casuarinaceae and Cupressaceae. The repetitive changes suggest pronounced differentiation of Plio-Pleistocene “interglacial” and “glacial” climatic stages, possibly related to 40kyr variation in orbital obliquity.

A latest Pliocene-earliest Pleistocene age is supported by two independent lines of evidence. First, zircons recovered from inwashed basal sands and from a pyroclastic horizon at 29m downcore give fission track ages of 1.99 ± 0.43Ma and 1.93 ± 0.18 Ma, respectively. Second, paleomagnetic analysis indicates that the sediments in the upper 25m of the core are of reversed polarity, and must predate the Brunhes/Matuyama polarity transition at 0.78 Ma. A zone of normal polarity below 25m is tentatively attributed, consistent with the 1-sigma error of fission track ages, to the Olduvai subchron, the upper boundary of which has an age of 1.78 Ma. It is concluded that the record embraces the final one or two climate cycles of the Pliocene, and the initial eight cycles of the Pleistocene, and may therefore span marine isotope stages 67 to 47.