Composition and origin of volcanic ash zones in Late Quaternary sediments from the Reykjanes Ridge: Evidence for ash fallout and ice-rafting

Seven Quaternary volcanic ash zones including the well-known ash zones AZ I and AZ II have been investigated in five well-dated sediment cores from the Reykjanes Ridge between 59 and 60 degrees N. The zones were defined on their geochemical composition, morphological variation of the particles and stratigraphic position. The ash zones AZ I and AZ II show a mixture of colorless bubble wall shards and brown blocky glasses, whereas ash zones VZ 1 to 5 are composed of dark to light brown volcanic glasses, ranging in shape and texture from blocky, nonvesicular shards to highly vesicular, pumiceous types. The dark brown glass shards of ash zones VZ 1 to 5 represent subalkalic and low-K subalkalic basaltic series. Geochemical correlation of the ash zones with possible source areas indicates that they all derived from Iceland. The colorless shards in AZ I and AZ II are of rhyolitic composition. The Eastern Volcanic Zone on Iceland is the most important source, with abundant volcaniclastics being produced by subaerial/subglacial eruptions and secondary erosion of volcanic rocks. Explosive volcanism on Iceland is manifested by marine ash layers AZ I and AZ II. Most of the volcaniclastic zones VZ 1 to 5 coincide chronologically with peak abundances of coarse ice-rafted material (IRD) indicating that most of this volcanic glass was incorporated into the Icelandic Ice Sheet and released to the ocean by iceberg calving and subsequent melting. However, two layers in VZ 1 and VZ 5 have no correlatives in the IRD-record, suggesting that they were probably transported by local sediment gravity flows. From this we must infer that most of the ice-rafting episodes bearing volcanic particles as observed in sediments from Reykjanes Ridge, monitor glacier fluctuations on Iceland.