Three times in the last 620,000 yrs, highly silicic ash ejected from Icelandic or Jan Mayer volcanoes has been deposited in sediments over an area of several million square kilometers in the North Atlantic. Sand-sized bubble-wall shards were initially deposited on ice cover on and near Iceland and later dropped from melting ice as far as 1,800 km to the south. Since the largest sand-sized shards cannot be that widely distributed by wind, they must be ice-rafted. The shards thus serve as localized injections into North Atlantic paleocirculation patterns, with the resulting geographic patterns of ash redistribution tracing the counterclockwise movement of the subarctic gyre. The total ash fall volumes thus carried out to sea (1 to 8 km3) approximate or exceed those of ash falls inferred from measurements on land near volcanic sources.
These geologically instantaneous ash falls are spread vertically through well-defined zones in cored sediments, providing models for vertical mixing of sand-sized microfossils. In cores with pelagic sedimentation rates varying from 2.4 to 6.7 cm/103 yrs, significant amounts of ash are mixed through thicknesses of core ranging from 17 to 66 cm, representing from 5,000 to 22,500 yrs of time. In cores with such high sedimentation rates, strongly defined climatic spikes should be little disturbed: episodes of only 10 to 100 yrs duration will still be visible after mixing, and those lasting 1,000 to 5,000 yrs should be left largely intact.