DISPERSED ORGANIC MATTER IN TWO LAKE BROWN CORES, WESTERN AUSTRALIA: DO THEY REFLECT CHANGES IN LITHOLOGY?

Lake Brown is an ephemeral acidic saline lake located 43 kilometers north of Merredin, Western Australia. Like most lakes in the region, it accumulates a thick crust of salt, mainly halite, during evapoconcentration and desiccation stages. Cored sediments from LB1-09 and LB2-09, which are 3.8 kilometers apart, have been analyzed for dispersed organic matter in order to understand the evolution of this lake. Relative abundances of the eight categories of organic components identified were statistically evaluated using cluster analysis, resulting in the identification of three palynofacies assemblages each for LB1-09 and LB2-09. Structured phytoclasts primarily defined the assemblages in LB1-09, whereas comminuted and degraded phytoclasts mainly defined those in LB2-09. Amorphous organic matter, opaques, pollen and spores, fungal remains, and algae played minor roles. Variations in the abundances of the organic matter types in each core do not apparently show any particular trend in the sedimentation pattern or correlation with the lithology. There is also no direct correlation between the two cores. These observations may be related to the effects of the highly dynamic sedimentation regimen of ephemeral lakes that can change from one stage to another in a very short period of time, such as desiccation to flooding. Such changes may even occur only within a small area in the same lake, and they can modify the organic matter contents and probably mask patterns recorded in the sediments. Further evidence of the unpredictable nature of sedimentation dynamics in the lake is reflected in the uncalibrated AMS ¹⁴C dates for sediments in the upper part of the cores. The dates obtained for LB2-09 at 31.5cm and 42.5cm were 6,060 ± 55 yrs BP and 23,300 ± 370 yrs BP, respectively. This huge age difference within 11 centimeters of deposition is probably indicative of either reworking or differences in sedimentation rates.