OPTICAL-PROPERTIES OF DEEP ICE AT THE SOUTH-POLE - ABSORPTION

We discuss recent measurements of the wavelength-dependent absorption coefficients in deep South Pole ice. The method uses transit-time dist ributions of pulses from a variable-frequency laser sent between emitt ers and receivers embedded in the ice. At depths of 800-1000 m scatter ing is dominated by residual air bubbles, whereas absorption occurs bo th in ice itself and in insoluble impurities. The absorption coefficie nt increases approximately exponentially with wavelength in the measur ed interval 410-610 nm. At the shortest wavelength our value is approx imately a factor 20 below previous values obtained for laboratory ice and lake ice; with increasing wavelength the discrepancy with previous measurements decreases. At similar to 415 to similar to 500 nm the ex perimental uncertainties are small enough for us to resolve an extrins ic contribution to absorption in ice: submicrometer dust particles con tribute by an amount that increases with depth and corresponds well wi th the expected increase seen near the Last Glacial Maximum in Vostok and Dome C ice cores. The laser pulse method allows remote mapping of gross structure in dust concentration as a function of depth in glacia l ice. (C) 1997 Optical Society of America.