ABSENCE OF CESIUM-137 FROM RECENT SEDIMENTS IN EASTERN AUSTRALIA - INDICATIONS OF CATCHMENT PROCESSES

Sediment and soil containing the isotope caesium-137 (Cs-137) are generally understood to have been sub-aerially exposed within the last 35 to 40 years. The horizon of first appearance of Cs-137 in sedimentary deposits in Australia is equated with the mid-1950s, the time of first appearance of the isotope in Australia. Some sediments, however, which are known to have been deposited in alluvial cutoffs since the mid-1950s do not contain the isotope. This is interpreted as resulting from the high magnitude of the events which entrained and deposited the sediment. The sediment was eroded from sub-surface sites, and deposited and buried rapidly, thereby preventing the adsorption of the isotope. The data indicate that Cs-137 should be used very cautiously as a dating tool in settings where a good knowledge of the fluvial and sedimentological events accompanying the emplacement of the sediments is not available.