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 : 物性変動から予測される古環境変動

抄録

D/V CHIKYU shakedown cruises of CK05-04 Leg 2 (16 Nov.-14 Dec. 2005) and CK06-06 (6 Aug.-29 Oct. 2007) were carried out off Shimokita Peninsula of the Northeast Japan. During these two cruises, three continuous cores of 47m, 71m and 365m long were recovered from the seafloor of about 1200m water depth at Holes C9001A, C9002A/B and C9001C, respectively. Cuttings samples were collected from Hole C9001D from 522 to 647mbsf in penetration depth using riser drilling. The onboard laboratory systems integration test including primary processing and measurements of the recovered cores were performed for laboratory performance evaluation and laboratory staff training. Two coring sites (C9001 and C9002) are located in Hidaka Trough that is a forearc basin under tectonic effects of both subduction and arc-arc collision. Cored sediments at four holes (C9001A, C9002A/B, C9001C) are mainly composed of diatomaceous silty clay, and divided into four lithologic units based on the lithology at Hole C9001C. Unit A is characterized by common intercalations of tephra/sand, while Unit B is rarely found in tephra/sand intercalations. Unit C is composed of a unconsolidated sands, and the Unit D is similar to the Unit A that is common intercalations of tephra/sand layers. The integrated stratigraphy based on micropaleontology, tephrochronology and magnetostratigraphy suggests that a bottom age of Hole C9001C at 365mbsf is assigned to about 780ka, and the geological age of Hole C9001D ranges from 1.05 to 1.65Ma. The sedimentation rate is high and nearly constant of 62cm/kyr at Hole C9001C and is low of about 17cm/kyr at Hole C9001D. The core of Hole C9001C has a continuous sequence without hiatus although the disturbance interval is recognized around 130mbsf. Based on the age model, the volcanic activity around the Northwest Pacific is thought to decrease during approximately 250-640ka of Unit B of C9001C using magnetic susceptibility data and the stratigraphy of tuff layers. The physical measurements of various methods revealed that two different lithology of Clays A and B are recognized in the sediments. The Clay A contains abundant diatoms during high productivity period, while Clay B is affected by inflow of high terrigenous clastics or low diatom productivity. These clays suggest a cyclic sedimentation controlled by the Milankovitch cycles.