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 li mestone 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 concent ration of muon-produced Cl-36 at depths beyond a few metres responds m ore slowly to erosion than Cl-36 at the surface, since the effective a ttenuation length for muons is much longer than for the fast nucleons which dominate production at the surface. Deep samples therefore integ rate erosion over longer periods than surface samples, and combined me asurements can be used to detect past changes in erosion rate. Limesto ne erosion rates in and around Australia range from 5 mum per year in arid regions to approximately 200 mum per year in the equatorial highl ands of New Guinea. The measured erosion rates show the distribution e xpected under present environmental conditions, but are higher than bo th direct measurements of limestone ablation over the past 10-100 year s, and valley incision rates in eastern Australia over the past 10(6)- 10(7) years.