地球深部探査船「ちきゅう」の下北半島沖慣熟航海コア試料の年代モデル

抄録

Three continuous cores of 47m, 71m and 365m in thickness were recovered from the subseafloor at about 1200m water depth at Holes C9001A, C9002A/B and C9001C/D during D/V CHIKYU shakedown cruises of CK05-04 Leg 2 (16 November-14 December 2005) and CK06-06 (6 August-29 October 2007). The core sediments are composed mainly of diatomaceous silty clay, and are divided into four lithologic units from Units A to D at Hole C9001C. The Unit A is characterized by common intercalations of tephra/sand and higher magnetic susceptibility (MS), while the Unit B is rare in tephra/sand intercalations with lower MS. The Unit C is composed of unconsolidated sands, and the Unit D is similar in lithology with the Unit A. The integrated stratigraphy of micropaleontology, tephrochronology and magnetostratigraphy suggest that the bottom age of Hole C9001C core is correlated with the base of the Brunhes Chron (Chron C1n) that is assigned to about 780ka (Middle Pleistocene). The geological age of Hole C9001D ranges from 1.05 to 1.65Ma (Early Pleistocene). The oxygen isotope stages from Marine Isotope Stage (MIS) 1 to MIS 18 are recognized in Hole C9001C based on the correlation with a standard isotope curve, and these data accord well with the results of biostratigraphy and magnetostratigraphy. The Unit A ranges from MIS 1 to MIS 8 (about 300ka), and Unit B spans from MIS 9 to MIS 16 (300ka-640ka). The Units C and D are assigned to MIS 17 and MIS 18, respectively. Hole C9001C core has a continuous sequence without hiatus although a disturbance layer occurs around 150mbsf. Thus, the Shimokita cores have a strong potential to improve biochronology and revise geological time scales for the past 800kyr in the Northwest Pacific region.