FALLOUT OF CHLORINE-36 TO THE EARTH SURFACE IN THE SOUTHERN-HEMISPHERE

Chlorine 36 is a radioactive isotope produced in the atmosphere by the cosmic-ray spallation of Ar-40. It has many applications as an enviro nmental tracer which require an understanding of Cl-36 in modern depos ition. Data are currently available only for high latitudes in the nor thern hemisphere and for Antarctica. In the present work, the first da ta for Cl-36 i, modern deposition for the southern hemisphere are pres ented. Excluding the tropical data, the latitude dependence of this fa llout is found to follow the form predicted by Lal and Peters, and its magnitude is within 40% of their predictions. In tropical Australia, however, a substantial excess of Cl-36 is observed in rainfall from si tes that derived most of their precipitation from the summer monsoon. If the high-latitude northern hemisphere data are representative of th e entire hemisphere, then the southern hemisphere fallout appears to b e 2-3 times less than in the northern hemisphere. It is suggested that this may be due to enhanced stratosphere-troposphere exchange north o f the equator promoted by the greater area of landmass, while the symm etric thermal structure of the southern atmosphere dampens this exchan ge.