Fig. 1.

Intensities of Trp fluorescence (red points) and non-Trp fluorescence (blue points) versus age. (a) WAIS ice at ≈127 m depth. (b) GISP2 ice at 1,557 m. Spectra were taken at ≈300-μm intervals along each core scanned; full spectra for all points are stored in memory. (c) For comparison, the rate of arrival of bacteria (red points) and dust (blue points) blown from African desert sources to an air collector on Barbados (5) during a 2-year period (1996 and 1997).