Late Glacial and Holocene Palaeoecology of the Lake St Clair Region, Tasmania

Abstract

This thesis presents a history of regional and local changes in vegetation in the Lake St Clair region, supported by modern vegetation and pollen analyses. The records span into oxygen isotope stage 3, with a focus on the last Glacial Holocene transition and fill a gap in the poorly studied region of Central Tasmania. The vegetation during the last glaciation prior to the Last Glacial Maximum was characterised by widespread Pherosphaera hookeriana dominated alpine coniferous heath growing together with a mosaic of alpine grasslands, herbfields, heathland and sedgeland. The lakes at Clarence Lagoon and Excalibur Bog are inferred to have dried up under the cold and dry climate during the Last Glacial Maximum. The legacy of Pherosphaera hookeriana dominated vegetation extends to the early deglacial sediments in Lake St Clair, declining to only trace representation through the remainder of the late Glacial and Holocene in the wider Lake St Clair region. Deglaciation of Lake St Clair was complete by c. 18.3 cal kyr BP and subsequent replacement of an early mosaic of alpine vegetation types, by subalpine Athrotaxis cupressoides and Diselma archeri dominated rainforest and/or woodland in response to rapidly rising temperatures and precipitation, is in sync with postglacial Antarctic warming and rising sea surface temperatures. A 900-year period of renewed grassland expansion is inferred to represent slightly cooler/drier conditions leading into the Antarctic Cold Reversal, abruptly ended by strong increases in rainforest mid-way through the Antarctic Cold Reversal suggesting a shift to a wetter and warmer climate leading to the establishment of Phyllocladus aspleniifolius-Nothofagus cunninghamii rainforest. The abrupt decline in Phyllocladus aspleniifolius at c. 12.4 cal kyr BP marks the expansion of Nothofagus cunninghamii-Atherosperma moschatum callidendrous rainforest growing under optimal conditions during the Early Holocene, which becomes more complex between c. 10-8 cal kyr when Phyllocladus aspleniifolius returns to high values, together with a secondary peak of Athrotaxis/Diselma under an inferred wetter and warm climate. At c. 8 cal kyr BP, the rainforest taxon Anodopetalum/Eucryphia becomes important and the remainder of the Holocene sees an overall decline in rainforest taxa and increase in sclerophyll and herbaceous taxa and fire activity, which intensifies during the late Holocene. The observed changes in the record are consistent with the onset of ENSO and a more variable climate from c. 8 cal kyr BP and an intensification and cooling temperatures from c. 5 cal kyr BP. Show more