A 200 ka record of chlorine-36 fallout measured in the unsaturated zone, south-eastern Australia

The use of chlorine-36 as a radiotracer is now well established in geology, geomorphology and hydrogeology. Use of 36Cl in groundwater dating requires a precise estimate of chlorine-36 input into the system. Variation of chlorine-36 production in the atmosphere over time scales of 101 to 106 years can be linked to changes in the cosmic flux due to changes in the earth's magnetic field. This leads to variations in atmospheric spallation reactions. Investigation into these variations have been attempted, using chloride records in the ice cores and fossil pack-rat middens covering the past 100 ka and 38 ka respectively. In the present study, variations in chlorine-36 input to a hydrogeological system is investigated through records of chloride in soil water of deep unsaturated zones of low recharge in the Murray Basin of south-eastern Australia