Ice core contaminants should provide permanent paleoclimatic records if, as it is generally assumed, they remained frozen in place and isolated from sunlight by the reflective overlaying snow layers. The excess CO levels recently detected in 1100--1600 AD Greenland ice core air bubbles relative to their Antarctic counterparts <Haan and Raynaud, 1998> amount, however, to an average production of about 5 ¿ 2 CO molecules cm-3 ice s-1 in that period. Here we show that such rates are quantitatively consistent with the in situ photodecarbonylation of the chromophoric organic matter present in Greenland, but not in the cleaner Antarctic, ice under the ¿erenkov radiation fluxes generated by penetrating muons of cosmic origin. The normal CO levels of modern (1600--1800 AD) Greenland records, and their variability earlier in the last millennium correlate significantly with the occurrence of boreal fires and the associated release of organic aerosol <Savarino and Legrand, 1998>.