North Atlantic ash zones I (10.6 ka BP), II (57 .5 ka BP) and III (340 ka BP) were studied using single grain geochemical analysis (microprobe). The zones were defined only on the occurrence of rhyolitic glass which dominates each zone. The rhyolitic shards are compositionally homogeneous within each zone. We thus validate the use of these rhyolytic ash zones as marker beds representing synchronous events. Within the two younger ash zones basaltic ash of both tholeiitic and transitional alkalic types occur. All ashes apparently came from Icelandic volcanoes. Four major geochemical populations occur within ash zone I: one rhyolitic, one transitional alkalic and two tholeiitic. The rhyolitic and transitional alkalic populations are identical to the two components of the Vedde Ash in western Norway, and they certainly originale from the same eruption(s). The source volcano is suggested to be Katla if the two components were erupted from one volcano, and Hekla and Torfajøkull if they came from simultaneous
eruptions of two different volcanoes. Veidivötn and Grimsvötn volcanic systems are suggested as likely sources for each of the tholeiitic populations. Ash zone I is also correlated with ashes found in cores from the Norwegian Sea, the North Sea and in the area between Scotland and Rockall.