Late Quaternary Vegetation and Early Holocene Quantitative Climate Estimates from Morwell Swamp, Latrobe Valley, South-eastern Australia

Abstract

A pollen diagram from Morwell Swamp provides a record of vegetation and climate through the Holocene period while the application of a bioclimatic analysis of the aquatic species Brasenia schreberi to the occurrence of its pollen in the record allows the first quantitative reconstruction of early Holocene climate from mainland south-eastern Australia. The beginning of the Holocene, c. 10000 years before present (BP), was marked by the establishment of permanent water within the basin and an expansion of forest under conditions of increasing precipitation and probably also temperature. The early Holocene forests were dominated by Casuarinaceae, a situation typical of lowland south-eastern Australia. The presence of Brasenia schreberi Gmel., a species now restricted to lower latitudes, suggests that, by c. 9000 years BP, mean annual temperatures had risen to slighly above today’s values, while summer temperatures may have been at least 1.3˚C higher. These results are surprising considering that most previous evidence has suggested that optimal climatic conditions were achieved between about 7000 and 5000 years ago, and that radiation levels are predicted, from Milankovitch forcing, to have been lower than today at this time in the Southern Hemisphere. It is clearly necessary to be somewhat cautious about the wholescale acceptance of the quantitative values at this stage, although they are not contradicted by other palynological data. Subsequent regional increases in the wetter forest elements, Nothofagus and Pomaderris, indicate a middle Holocene peak in precipitation, although it is estimated, from a bioclimatic analysis of Nothofagus, that summer temperatures had become substantially lower than today. This lowering may have been due to a local or regional increase in cloud cover. There is evidence for minor variation in vegetation and climate within the late Holocene, which is consistent with evidence from elsewhere within the region.