Floods and droughts in the Murray-Darling Basin: new insights from Lake Alexandrina.

Abstract

A study on the sediments of Lake Alexandrina, a lake located at the terminus of the River Murray in the Lower Lakes Region, South Australia, is presented in an attempt to reconstruct the flow regime of the River Murray throughout the last 4000yr BP. δ18O and Itrax data are used as environmental proxies and 210Pb and 14C techniques are used to construct age-depth models and infer accumulation rate. Recent, 210Pb-based accumulation rates of 1.5-1.57cm/yr are greater than those gathered from 14C which gave an accumulation rate of 0.1cm/yr. This change is attributed to European settlement. δ18O reveals a freshwater dominated history with a mean value of -8.24488. Itrax ratios of Al and Si show links to sediment grainsize. Thus, Itrax data is used for high resolution paleo flow reconstructions. δ18O and Itrax data agree in some stretches, but not all. A dry event between 4.0ka and 3.5ka captured by Al/Si also appears in several similar paleo limnologic reconstructions for lakes throughout south east Australia. Wavelet analysis of Al/Si does not display climatic oscillation thought to be typical of the region, such as the El Niño Southern Oscillation and the Indian Ocean Dipole. The Gleissberg cycle appears to effect the lake hydrology between ~4.0Ka and ~2.6Ka.