Environmental conditions prevailing during build-up of the Greenland Ice Sheet (GIS) are not yet established. Here we use results from gas isotopes from basal ice of the GRIP ice core to show that central Greenland was vegetated during invasion by the ice sheet. The δ 13 CCH4 and δ18O of O2 values obtained were never encountered before in ice core studies. Such values are indeed the signature of a significant biological activity. Copyright 2006 by the American Geophysical Union.