Rapid optical method for logging dust concentration versus depth in glacial ice

We describe the design and simulated response of a dust logger consisting of a downward-pointing phototube, ~2 m below side-directed light-emitting diodes (LEDs), attached to a cable that can lower the device down a 3-in. (7.5-cm) borehole filled with butyl acetate. LED photons that enter the ice are scattered or absorbed by dust grains, and those that reach the phototube provide a measure of dust or volcanic ash concentration at a given depth. An increased dust concentration associated with an ancient colder climate will usually result in an increase in collected light, but may decrease collected light if air bubbles are present. Centimeter-thick volcanic ash bands can also be detected. The concept is based on six years of experience with pulsed light sources used to measure optical properties of deep Antarctic ice.