A BIOCLIMATIC ANALYSIS OF EARLY TO MIDDLE MIOCENE BROWN-COAL FLORAS, LATROBE VALLEY, SOUTH-EASTERN AUSTRALIA

The bioclimatic prediction system BIOCLIM is employed to provide an es timate of Early to Middle Miocene climate from the overlapping present -day climatic parameter ranges of selected taxa recorded in the Latrob e Valley coal seams. Despite taxon identification to only a coarse tax onomic level, fairly tight climatic envelopes are derived, particularl y for temperature parameters. The data suggest that mean annual temper atures may have been about 5 degrees C higher than those of today, wit h a similar seasonal temperature variation. Minimum annual precipitati on is estimated to have been at least 1500 mm and probably much higher , compared to the present average of 800 mm, with most of the addition al rainfall falling in the wet season. By comparison with previous, mo re qualitative, estimates, the degree of seasonal variation is surpris ing, but may be accounted for by the high latitudinal position of sout hern Australia during the period of coal formation. A number of factor s including the degree to which the distribution of rainforest taxa in the present day may reflect their climatic potential, the possibility of ecological change within recognisable taxa, and the influence of a swamp environment on taxon distributions, all of which may limit the accuracy or validity of these estimates, are examined. It is concluded that the estimates are realistic in broad terms, but can only apply t o the limited periods of high sea level, which allowed accumulation of the swamp precursors of the brown coals.