LONG-RANGE TRANSPORT OF ICELANDIC TEPHRA TO THE IRMINGER BASIN , SITE 9191

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. res) yers ted 4a). ened tics t the band sednate is the ard eanoat d as ver ticand nd ). ith ion ced , or 029 INTRODUCTION A total sediment sequence of 147 m was cored in the Irminger Basin at Site 919, in three drill holes (62°40.20 ′N, 37°27.61′W; 2088 m water depth; Fig. 1). Quaternary sediments of Site 919 were foun be the only sediments recovered during Leg 152 that preserved crete ash layers. The scarcity of ash layers is likely to be a resu the great distance from Iceland, which is approximately 750 km to east-northeast. Magnetobiostratigraphic data indicate a minim sedimentation rate for Holes 919A and 919B of about 12 cm/k.y. w the Brunhes/Matuyama boundary located at about 120 m below floor (mbsf), dating the entire sequence of Pliocene−Pleistocene age. This study focuses on the Pliocene −Pleistocene tephra layers recov ered from Cores 152-919A-1H through 10H, and from Cores 1 919B-1H through 8H, with the sequence in Hole 919B drilled b tween 18.7 and 90.0 mbsf. Continuous advanced piston corer (A coring from the top to the bottom of the Hole 919A sequence w completed with a total recovery of 100%. Such excellent recovery lowed an age model to be constructed, based on the δ18O of planktonic foraminifers, in spite of limited sampling resolution (Flower, th volume). Among all the records from Deep Sea Drilling Proje (DSDP) and Ocean Drilling Program (ODP) and current piston c ing, drilling at Site 919 was by far the longest recovered in t Irminger Basin, providing new information on the marine sedimen tion of volcanic ash as well as ice-rafted debris for approximately last 800 k.y. , a period characterized by full-scale glacial-interglac oscillations. Based on the high accumulation/resolution record of S 919 and its proximity to Greenland, the recovery and study of vol nic ash layers present in this sequence are of particular interest fo 1Saunders, A.D., Larsen, H.C. , and Wise, S.W. , Jr. (Eds.), 1998. Proc. ODP, Sci. Results,152: College Station, TX (Ocean Drilling Program). 2Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, U.S.A. Lacasse: lacasse@gsosun1.gso. uri.edu 3GEOMAR, Wischhofstrasse, D-2300 Kiel 14, Federal Republic of Germany. 4Centre des Faibles Radioactivités, Gif-sur-Yvette, France. 5Centre d’Etudes Nucléaires de Saclay, Laboratoire Pierre Süe, Gif-sur-Yve France. d to dislt of the um ith sea-