Fingerprinting Australia's rivers with clay minerals and the application for the marine record of climate change

Abstract

Clay mineral assemblages from sediments taken from 46 rivers around Australia and in the Murray-Darling Basin were analysed and found to be characteristic for each individual river and drainage area. These clay mineral fingerprints are not necessarily associated with present climate in the drainage area, but represent an input from a local clay reservoir that was formed from the weathering of a typical combination of local bedrock over prolonged periods of time. Consequently, the clay composition of these reservoirs does not correspond to simple latitudinal patterns of clay minerals, which conventionally states that physically eroded mica/illite and chlorite occur in cold and high latitudes, and smectite and kaolinite formed from chemical weathering are found under the warm and humid climate conditions of low latitudes. Likewise, there is no latitudinal zonation of clays on the sea floor surrounding Australia. The comparison of river clays and clay mineral assemblages on the sea floor suggests that river suspensions contribute considerably to the terrigenous component in marine sediments around Australia. Although highly episodic and event-controlled, significant amounts of river clays are discharged into the ocean. They are entrained in ocean currents and can be traced for long distances. Comparing clay mineral suites in different time-slices of marine sediment cores can be a suitable tool to reconstruct palaeoclimatic processes such as varying river discharge, aridity or humidity of the source area, sea-level fluctuations and intensity and direction of ocean currents. With clay mineral fingerprints from individual drainage areas, clays in sediment cores can be traced back in many cases to the source area. Thus, the different clay mineral suites from the Last Glacial Maximum and from the present day in 14 deep-sea sediment cores from around Australia can be explained by fluctuations in climate-related processes.