Long-range transport of Icelandic tephra to the Irminger Basin, Site 919
Résumé
The 147-m-long sediment sequence recovered from Ocean Drilling Program Leg 152 Site 919 in the Irminger Basin contains several well-preserved volcanic ash layers and ash zones that provide a record of long-range transport of tephra from Iceland toward Greenland during the Pliocene−Pleistocene. A total of eight tephra layers and ash pods as well as three ash zones recovered in two separate holes (919A and 919B) were analyzed for major and trace element chemistry and grain size. Relative ages of the tephra layers were estimated based on oxygen isotope stratigraphy and correlation with other chronostratigraphic markers present in North Atlantic sediments and ice cores in Greenland. Based on their sorting coefficient and grain size, it is inferred that discrete ash layers between 1 and 5 cm thick are the result of ash fallout from large explosive eruptions. The tephra are bimodal (colorless/rhyolitic and sideromelane-tachylite/basaltic glass) or basaltic in composition, with crystal content between less than 15% for the discrete layers to more than 50% for the ash zones. The major element composition of glasses indicates two compositional groups: basaltic and rhyolitic. All of the tephra layers have an affinity with either a tholeiitic or an alkalic source in Iceland. Two separate mixed tephra layers, occurring between 10 and 11 m below seafloor at Site 919, were found to correlate with the ice-rafted ash Zone 2, based on their rhyolitic glass chemistry. Ash Zone 2 is a chronostratigraphic marker dated at about 55−57 ka in marine sediment and at about 52 ka in a Greenland ice core. The rhyolitic mixed tephra are interpreted to have been erupted during two large explosive eruptions of Tindfjallajökull volcano in southern Iceland, at a few hundred years interval. From the current pattern of seasonal variation in the atmospheric circulation over Iceland, it is suggested that the tephra were likely transported by easterly winds occurring at about 30 km elevation in midsummer, followed by fallout in southern Greenland and in the Irminger Basin.
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