11:00 〜 11:15
[BPT07-08] 前期/中期中新世境界付近における東赤道太平洋の放散虫群集変遷
キーワード：放散虫、中新世、東赤道太平洋
The lower to middle Miocene sequence was drilled at IODP Site U1335 (5°18.735’N, 126°17.002’W, water depth 4328 m) in the eastern equatorial Pacific Ocean. In the modern ocean, Site U1335 is located under the North Equatorial Current, and the biogenic silica fluxes to the sea-floor sediments are comparatively high in this area. The sediments recovered from Site U1335 are predominantly nannofossil ooze with siliceous microfossils such as diatoms and radiolarians.
230 morphotypes of radiolarians were identified at this site. Because the low latitude species commonly occurred at this site, the tropical Cenozic zonation of radiolarians proposed by Sanfilippo and Nigrini (1998) was adopted. The studied sequence was divided into four zones, consisting of the RN2 (the Stichocorys delmontensis Interval Zone), RN3 (the Stichocorys wolffii Interval Zone), RN4 (the Calocycletta costata Interval Zone), and RN5 (the Dorcadospyris alata Interval Zone) at Site 1335. In the standard zonal scheme, the early/middle Miocene boundary corresponds to the top of C5Cn.1n with an age estimate of 16.268 Ma (Pälike et al., 2010; Gradstein et al., 2012). Hence, this boundary was placed at 189.6 mcd at Site 1335.
Radiolarian fauna was divided into three assemblages based on variations in the composition of dominant species: an early Miocene assemblage (20.0 to 16.8 Ma), a transitional assemblage (16.8 to 13.4 Ma) and a middle Miocene assemblage (13.4 to 12.0 Ma). The early Miocene assemblage is characterized by two dominant species of S. delmontensis, S. wolffii, and Tholospyris anthophora. The transitional assemblage consists of three dominant species of S. delmontensis, Calocycletta robusta group, and T. anthophora. The four dominant species of the middle Miocene assemblage present in this assemblage are Stylodictya sp. A, Lophocyrtis aspera, Disolenia spp. and Collosphaera spp. The most significant faunal turnover of radiolarians is marked at the boundary between the transitional/middle Miocene assemblages.
230 morphotypes of radiolarians were identified at this site. Because the low latitude species commonly occurred at this site, the tropical Cenozic zonation of radiolarians proposed by Sanfilippo and Nigrini (1998) was adopted. The studied sequence was divided into four zones, consisting of the RN2 (the Stichocorys delmontensis Interval Zone), RN3 (the Stichocorys wolffii Interval Zone), RN4 (the Calocycletta costata Interval Zone), and RN5 (the Dorcadospyris alata Interval Zone) at Site 1335. In the standard zonal scheme, the early/middle Miocene boundary corresponds to the top of C5Cn.1n with an age estimate of 16.268 Ma (Pälike et al., 2010; Gradstein et al., 2012). Hence, this boundary was placed at 189.6 mcd at Site 1335.
Radiolarian fauna was divided into three assemblages based on variations in the composition of dominant species: an early Miocene assemblage (20.0 to 16.8 Ma), a transitional assemblage (16.8 to 13.4 Ma) and a middle Miocene assemblage (13.4 to 12.0 Ma). The early Miocene assemblage is characterized by two dominant species of S. delmontensis, S. wolffii, and Tholospyris anthophora. The transitional assemblage consists of three dominant species of S. delmontensis, Calocycletta robusta group, and T. anthophora. The four dominant species of the middle Miocene assemblage present in this assemblage are Stylodictya sp. A, Lophocyrtis aspera, Disolenia spp. and Collosphaera spp. The most significant faunal turnover of radiolarians is marked at the boundary between the transitional/middle Miocene assemblages.