RECENT CLIMATE RECORDED IN LAKE SEDIMENTS FROM THE TASMANIAN HIGHLANDS

We present preliminary results on sedimentary records from three Tasmanian Highland lakes: Lake MacKenzie, Arthurs Lake and Woods Lake. These lakes fall on a 65 km NW-SE transect, along which elevations drop from 1120 to 735 m and modern precipitation decreases from 2200 to 700 mm/yr. All three lakes were enlarged by hydroelectric dams in the mid-20th Century, which raised their levels by between 2 and 8 m. CHIRP acoustic surveys were used to select coring sites with relatively thick and uniform sedimentary piles within the pre-existing lakes. The low tectonic uplift of Tasmania (~3m/Myr), as well as the lack of upwind sources of dust imply that these lakes are likely to have low sedimentation rates, on the order of tenths of a mm per year. We collected short (~1.5m) Livingstone cores from all three systems. At Woods Lake, the shallowest and easternmost of the three basins, CHIRP profiles and the presence of a sandy-silt layer at ~1.5 mblf imply relatively recent desiccation, suggesting that this has existed as a lake perhaps only since the early Holocene. Arthurs Lake retains sequences of >6m of lacustrine sediment in its central basin, but an apparent dessication horizon at ~70 cm depth in the core suggests that it too may have been dry during the LGM. Lake MacKenzie (the deepest and highest elevation lake) has a sediment pile thicker than 18m, representing a long, but complex, depositional history. Lithological core description has identified numerous authigenic and diagenetic iron mineral phases, and XRF core scanning results show systematic variability consistent with the climate gradient represented by these lakes.