LIMESTONE EROSION MEASUREMENTS WITH COSMOGENIC CL-36 IN CALCITE - PRELIMINARY-RESULTS FROM AUSTRALIA

The rapid accumulation of cosmogenic Cl-36 in calcite forms the basis of a useful method for measuring exposure ages and erosion rates in limestone terrains. A unique aspect of this mineral-isotope pair is the combination of high production rate and high AMS detection sensitivity, allowing erosion rates to be determined from samples at depths where Cl-36 production is dominated by negative muon reactions. The concentration of muon-produced Cl-36 at depths beyond a few metres responds more slowly to erosion than Cl-36 at the surface, since the effective attenuation length for muons is much longer than for the fast nucleons which dominate production at the surface. Deep samples therefore integrate erosion over longer periods than surface samples, and combined measurements can be used to detect past changes in erosion rate. Limestone erosion rates in and around Australia range from 5 mum per year in arid regions to approximately 200 mum per year in the equatorial highlands of New Guinea. The measured erosion rates show the distribution expected under present environmental conditions, but are higher than both direct measurements of limestone ablation over the past 10-100 years, and valley incision rates in eastern Australia over the past 10(6)-10(7) years.