• PDF orientations in shocked quartz grains around the Chicxulub crater

      Nakano, Yoichiro; Goto, Kazuhisa; Matsui, Takafumi; Tada, Ryuji; Tajika, Eiichi (The Meteoritical Society, 2008-01-01)
      We measured 852 sets of planar deformation features (PDFs) in shocked quartz grains in impactite samples of the Yaxcopoil (YAX-1) core and from 4 Cretaceous/Tertiary (K/T) boundary deposits: the Monaca, the Cacarajcara, and the Pealver formations in Cuba, and DSDP site 536, within 800 km of the Chicxulub crater, in order to investigate variations of PDF orientations in the proximity of the crater. Orientations of PDFs show a broad distribution with peaks at omega {1013}, pi {1012}, and xi {1112}, plus r, z {1011} orientations with minor c(0001), s{1121}, t{2241} plus x{5161}, and m{1010} plus a{11-20} orientations. Planar deformation features with c(0001) orientation are relatively more abundant in the proximity of the Chicxulub crater than in distal sitessuch as North America, the Pacific Ocean, and Europe. This feature indicates that in the proximity ofthe crater, part of the shocked quartz grains in the K/T boundary deposits were derived from the lowshock pressure zones. Moreover, the orientations of PDFs with xi {1122} plus r, z {1011} are high in our studied sites, and frequencies of these orientations decrease with increasing distance from the crater. On the other hand, absence of c(0001) and the rare occurrence of PDFs with xi {1122} plus r, z {1011} orientations in the sample from the YAX-1 core that was taken at the top of the impactitelayer of the Chicxulub crater suggests that the sampling horizon that reflects a certain cratering stageis also an important factor for variations in shocked quartz.
    • Radiocarbon Marine Reservoir Ages in the Northwestern Pacific off Hokkaido Island, Japan, During the Last Deglacial Period

      Ohkushi, Ken'ichi; Uchida, Masao; Aoki, Kaori; Yoneda, Minoru; Ikehara, Ken; Minoshima, Kayo; Kawahata, Hodaka; Tada, Ryuji; Murayama, Masafumi; Shibata, Yasuyuki (Department of Geosciences, The University of Arizona, 2007-01-01)
      We measured radiocarbon ages of planktic foraminifera in 4 sediment cores from the northwestern Pacific region off northern Japan in order to estimate marine reservoir ages during the Blling-Allerd period. The ages of deglacial tephra markers from 2 Japanese source volcanoes identified in these sediment cores had been previously estimated from 14C ages of terrestrial charcoal and buried forests. By comparing the foraminiferal and tephra ages, we estimated the surface water reservoir age during the Blling-Allerd period to be ~1000 yr or more in the region off northern Japan. The deglacial reservoir ages were more than 200 yr higher than the Holocene values of ~800 yr. The older deglacial ages may have been caused by active upwelling of deep water during the last deglaciation and the consequent mixing of older deep water with younger surface waters.