Magnetic Susceptibility, Calcium Carbonate and the Geochemical Composition of Sediments from the Australian Continental Margin

Several gravity cores from the west (3), south (3) and east (5) Australian continental margin have been sampled and analysed for magnetic susceptibility, percent calcium carbonate, Fe203, Al203 and Si02 (% wt) and other trace element contents. Thecores sampled from the west Australian continental margin were from the Exmouth Plateau, and the Carnarvon Terrace. Sampling from the south Australian continental margin was from the west Ceduna Terrace and the Eyre Terrace and the cores sampled from the east Australian continental margin were collected offshore Evans Head.

8180 isotope data on planktonic foraminifera have been used to provide a chronological control on those cores collected from the western margin, to determine sediment accumulation rates and identify Late Quaternary isotopic stages, includingthe last glacial maximum (LGM) and the last interglacial (LIG). These same data are not available from other areas surveyed although sedimentation rates of about 1cm/kyr have been estimated from 14C data from the eastern continental margin sediments offshore Evans Head.

The down-core total magnetic susceptibility profiles generally show similar patterns within each survey area. The magnetic susceptibility profiles for those cores from thewestern margin (Exmouth Plateau and Carnarvon Terrace) show relatively high valuesin the Holocene and around the LIG, and relatively low values during the LGM. Patterns of down-core variations in magnetic susceptibility could also be recognised in gravity cores from the eastern margin offshore Evans Head. Here, magnetic susceptibilities were relatively high around the LIG (similar to the western margin), but magnetic susceptibility were low in Holocene sediments, perhaps because of diagenetic remobilisation of iron in the core-tops. In general no distinct patterns are recognisable in the down-core magnetic susceptibility profiles from the southern margin.

The down-core profiles of calcium carbonate from the west Australian continental margin show high values during the LGM, and minima in the Holocene and also during the LIG.Similarly, the down-core profile of calcium carbonate shows minimum values during the inferred LIG in eastern margin cores, although this interpretation is based on an approximate sedimentation rate. In general, total magnetic susceptibility is correlated with Al203, Si02 and Fe203, concentrations in sediments, indicating a control on magnetic susceptibility by terrigenous material contents. Calcium carbonate was found tobe a diluent for magnetic susceptibility in these continental margin sediments.