A detailed proxy rainfall record from Lake Broadwater, Dalby, Southeast Queensland

Abstract

Lake Broadwater is located in southeast Queensland and the upper reaches of the Murray-Darling Basin and was selected to develop a detailed proxy rainfall record and palaeoenvironmental history for this region. Lake sediments from 0-171 cm were collected and returned to USQ for geological logging, grain size and geochemical analyses as well as lead (210Pb) and carbon (14C) dating. The lake sediments consist mostly of fine dark grey silts and clays with small amounts of fine sands, which increase towards the surface. Detailed particle size analysis (0.05-880μm diameter) using laser diffraction was used to determine grain size distributions. A bimodal population of very fine clays/silts and fine sands occur in most samples, with sand becoming more abundant from 38 cm towards the surface. Multi-element geochemistry using ICP/AES and CNS analyser was completed to determine Fe, Mn, Al, Na, K, Ca, Mg, Sr, Cu, Zn, Pb, C, N and S values. Organic, carbonate and silicate matter percentages were derived from LOI. Elevated C, Ca, Sr, N, Cu and Zn values were identified in the top 1 cm. Ba, Na, Al, Mg, Mn and Fe increase from the surface down the profile whilst siliciclastic % increases. Lead (210Pb) dating was attempted but was unsuccessful. Carbon (14C) dating of the humic acid fraction gave a maximum age of 9470 years (+/-100 years) at 170-171 cm, which is close to the Pleistocene-Holocene boundary. This indicates an average rate of sedimentation of ~0.22 mm per year. Lake Broadwater is a mature, eutrophic lake that has had a relatively stable recent past making it ideal for developing a palaeoenvironmental history of the region. Two significant changes in sedimentation occur at 38 cm (~3240 yr B.P) and 16 cm (~2190 yr B.P) indicating a change to drier conditions and the onset of stronger ENSO variability.