An Upper Pliocene lacustrine environmental record from south-Western Australia — preliminary results

A crater created by a supposed meteor impact and located about 30.5°S latitude in Western Australia contains a well-preserved lake sequence of the Upper Pliocene age. This paper reports on the palaeomagnetic record, sediment patterns and selected chemical analyses, and alludes to some of the preliminary results from pollen, charcoal and diatom analyses. The chemical analyses indicate that the lake varied in water depth and that salinity and diatom production were related to climate change rather than changes in major nutrients. The palynological analysis, for the upper 20 m of sediment, shows a woodland and heath similar to today, interspersed with some chenopod shrub land phases, and minor elements include Nothofagus, Araucaria and Phyllocladus which are known from other Tertiary records from southern Western Australia. Charcoal is abundant in the record indicating that fire was a major environmental feature, long before Aboriginal people inhabited the continent. The palaeomagnetic record shows a good match with the Gaussian chron and includes the Kaena and Mammoth subchrons; and these were used as a basis for developing an age-depth model for the sequence. The sediment was subdivided into an upper unit of four lithozones and a lower unit. The upper unit, that dates from ca. 2.5 to 3.51 Ma was interpreted to have formed under a generally wetter climate than today, and with some relatively short intervals of semi-arid climate identified around 2.56 and 2.59 Ma. They were interpreted from episodes of chenopod shrub land that were associated with assemblages of saline habitat diatoms and carbonate and gypsum in the sediments. The sediment profile also included sections of laminated sediments which indicate that around 2.82, 2.86 and 2.96 Ma the climate was more variable and possibly seasonal compared to the rest of the record. These data indicate that the last million years of the Pliocene for south-western Australia endured several episodes of climate variability, thus lending support to the idea that the Pliocene was not a period of a uniform cooling trend gradually leading into the Quaternary glacial episodes.