Progress in AMS measurement of natural 32 Si for glacier ice dating

Abstract

AMS measurement of 32Si can allow for ice core dating over the last thousand years. Technique developments are reported. Necessary negative-ion yields of 20-30% can now be consistently achieved, and permit an overall efficiency from ice sample to detector of ∼1%. A 30Si-spike technique has overcome the problem of extremely low intrinsic silicon concentration, with the added benefit of allowing determination of ppb-level silicon via isotope dilution. Improvements have also been made to the ionization detector in the gas-filled magnet that separates the accelerated 32Si ions from the intense flux of 32S ions. Preliminary 32Si AMS results of snow and ice samples from Mt. Cook National Park, New Zealand, are reproducible, and with 32Si concentrations 1.2-7.2 mBq/m3 comparable to results from mid-latitude snow samples measured previously via the radiometric technique, demonstrating the feasibility of the method. With these developments, the potential of 32Si as ice core dating tool is close to being realized, and attempts to determine chronologies for both alpine and Antarctic glaciers are underway.