| Summary: | Four types of volcaniclastic deposits have been found in Cenozoic sediment of the ocean basins adjacent to the Iceland hotspot: ash fall layers derived from large explosive eruptions, ash zones derived from ice-rafting, sediment gravity flow deposits derived from glacial floods or jokulhlaups, and volcaniclastic contourites. Large explosive eruptions have been occurring for the last 6 Ma from the tholeiitic, transitional and alkalic volcanic zones in Iceland. Dispersal of the tephra from these events was controlled by the prevailing winds prior to their fallout on land, open sea or sea ice. Two of the largest silicic tephra fallouts, Ash Zone 1 (ca 11,980 yr BP) and Ash Zone 2 (ca 57,300 yrs BP) were most likely derived from major ignimbrite-forming eruptions from the transitional alkali Katla and Tindfjallajokull volcanoes of the Eastern Volcanic Zone, respectively. Owing to the changing ocean circulation characteristic of the glacial regime, the two tephra were rafted on sea ice south into the central North Atlantic, and deposited as dispersed ash zones. Sediment in the Iceland Basin also show evidence of the formation of correlative ash turbidites. These sediment gravity flows were generated either by the entrance of pyroclastic flows into the sea, or during discharge of jokulhlaups, while the volcanoes were covered by a residual or extensive ice-cap at the time of the eruption. Large glacial floods from the tholeiitic Grimsvotn-Lakagigar volcanic system, during extensive ice cover, have also triggered episodic sediment gravity flows that have introduced basaltic volcaniclastic sediment via the Myrdalsjokull Canyon. Observations of historic and postglacial Icelandic tephra falls in western Europe, Greenland, and adjacent seas agree well with model predictions of tephra transport by the present-day atmospheric circulation in the subpolar North Atlantic region. However, the present atmospheric circulation is not suitable for modelling the airborne transport of tephra during the Pliocene and Pleistocene. Significant changes in ambient atmospheric conditions, including more vigorous circulation with occasional change in wind direction, are required to explain the observed coarse grain size of the pre-Holocene tephra fall deposits.
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