NORTH-ATLANTIC DEEP-SEA SEDIMENTATION OF LATE QUATERNARY TEPHRA FROM THE ICELAND HOTSPOT

Piston cores recovered from the North Atlantic were used to study the sedimentation of Holocene and Pleistocene volcanic ash in the Irminger and Iceland Basins. Ash Zones 1 (approximate to 11,100 yr B.P.), 2 (a pproximate to 55,000 yr B.P.) and 3 (approximate to 305,000 yr B.P.) w ere identified from their major element glass composition. The silicic and alkalic Ash Zones 1 and 2 originate from the Southeastern Volcani c Zone of Iceland, where they correlate with the Solheimar ignimbrite from Katla volcano and the Thorsmork ignimbrite from Tindfjallajokull volcano, respectively. The low-alkali composition of silicic Ash Zone 3 indicates a source from one of the silicic centers in the active rif t system. Ash Zones 2 and 3 occur in the Irminger Basin as dispersed g lass shards over a depth interval of several tens of centimeters. Thei r compositional and granulometric characteristics reflect an initial f allout on pack-ice north of Iceland, followed by ice-rafting sedimenta tion in the Denmark Strait, prior to bioturbation. delta(18)O stratigr aphy of foraminifera in the cores indicates that the ash zones were de posited during a cold interval, at the time when seas north of Iceland were ice-covered. Sedimentary features indicate that turbidity curren ts were also involved in the dispersal of Ash Zones 1 and 2 south of I celand. The initiation of these gravity currents from the shelf can be attributed to either glacier bursts (jokulhlaups) carrying tephra, or the entrance of pyroclastic flows into the ocean.