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 estimate of Early to Middle Miocene climate from the overlapping present-day climatic parameter ranges of selected taxa recorded in the Latrobe Valley coal seams. Despite taxon identification to only a coarse taxonomic level, fairly tight climatic envelopes are derived, particularly for temperature parameters. The data suggest that mean annual temperatures may have been about 5°C higher than those of today, with a similar seasonal temperature variation. Minimum annual precipitation 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 additional rainfall falling in the wet season. By comparison with previous, more qualitative, estimates, the degree of seasonal variation is surprising, but may be accounted for by the high latitudinal position of southern Australia during the period of coal formation. A number of factors 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 to the limited periods of high sea level, which allowed accumulation of the swamp precursors of the brown coals.