Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 125 2. LATE MIOCENE (CN9B SUBZONE) TO HOLOCENE CALCAREOUS NANNOFOSSILS FROM THE WESTERN AND NORTHWESTERN PACIFIC OCEAN, LEG 1251 Giuliano Ciampo2 ABSTRACT During Leg 125 of the Ocean Drilling Program, nine sites were drilled in the Mariana and Izu-Bonin areas. The sediments recovered range in age from early Pliocene to late Pleistocene in the Mariana Region and from middle Eocene to late Pleistocene in the Izu-Bonin region. This contribution concerns the biostratigraphic study of the latest Miocene (CN9b Subzone) to late Pleistocene interval. Aquantitative analysis of all calcareous nannofossil associations was conducted for the interval encompassing late Miocene to the top of the early Pliocene. Moreover, the genera Discoaster, Amaurolithus, and Ceratolithus were quantitatively investigated from the late Miocene to late Pliocene interval. Some bioevents were identified, and variations in the composition of assemblages were linked to climatic changes. INTRODUCTION SITE SUMMARIES During Leg 125 (February-April 1989), scientists drilled at nine sites Mariana Region (Fig. 2) in the Mariana and Izu-Bonin areas of the western Pacific (Fig. 1). In the In this area, Sites 778 to 781 were drilled on the top or on the flanks Mariana area, four sites (778-781) are located on the top and flanks of a of the serpentine Conical Seamount. serpentine seamount (Conical Seamount) in the forearc region. In the Izu-Bonin, area two sites (783-784) are located on the Torishima Forearc Site 778 19°29.93/N, 146° 39.93^; water depth, 3917.7 mbsl Seamount in the forearc basin and three others (782,785, and 786) along Site 779 19° 30.757^, \46° 41.15^; water depth, 3947.2 mbsl the eastern edge of the forearc basin. Site 780 19° 32.5^, 146° 39.2^; water depth, 3090 mbsl The recovered sediments range in age from middle Eocene (Sites 782 and 786) to Holocene. Uppermost Miocene (CN9b Subzone) to Only a few scattered samples, in the higher part of the seven holes Holocene calcareous nannofossils are discussed. drilled, provided usually poor and poorly preserved nannofossil as- semblages of Pleistocene age (CN14-15 Zones). METHODS No range charts have been provided for these sites. As a base for biostratigraphy the standard zonation of Okada and Site 781 19° 37.9m, 146° 32.56^; water depth, 4420.6 mbsl Bukry (1980) was used. Light-microscope techniques for studying coccoliths from smear slides also were used. Scanning electron micro- This site is located on a flank of Conical Seamount. Severe etching scope (SEM) was mainly used to determine the first occurrence of and reworking affect the nannofossil assemblages because of the great Emiliania huxleyi in the late Pleistocene sediments. depth and turbiditic sequences. Moreover, several cores and sections Quantitative analysis of the assemblages in the late Miocene-early are missing because recovery was poor. Pliocene interval (CN9b-CNllb Subzones) was performed. A mini- Samples from the bottom of the hole contain mixed early/late Pliocene mum of 500 specimens was counted for each sample. Furthermore, associations, with Ceratolithus armatus, C. cristatus, Discoaster asym- at least 300 specimens of the genus Discoaster for each sample were metricus, D. tamalis, Pseudoemiliania lacunosa, Reticulofenestra pseu- counted from the late Miocene to the extinction of the genus in the doumbilicus, and Sphenolithus spp. late Pliocene (CN12d Subzone). The number of specimens in the Samples 125-781A-26R-1,17-18 cm, to 125-781 A-15R-CC, below Ceratoliths group counted in the same time interval depends on the a massive basalt (lithologic Unit IB, 72.32-91.80 mbsf) are from early number of discoasters (n = 300). The quantitative analysis points out Pliocene (CNllb Subzone), if Reticulofenestra pseudoumbilicus and further events for a fine biostratigraphic resolution; the paleoclimatic Sphenolithus, spp. were not reworked, to late Pliocene (CN12a Subzone) implications will be commented upon briefly. in age. Nannofossils are abundant and well preserved, and sometimes In accordance with Backman and Shackleton (1983), Sphenol- discoasters are predominant in these assemblages (Table 1). ithus abies and S. neobies are considered together in the count; Samples above the massive basalt are Pleistocene in age, CN14a Discoaster variabilis is considered together with D. challenged and Subzone, based on the occurrence of Gephyrocapsa oceanica. Rework- D. decorus. Rare Discoaster challenged is scattered; Discoaster ing is common, and it was impossible to recognize some extinction events decorus has its first occurrence at the base of the CN11 b Subzone and (such as Calcidiscus macintyrei or Helicosphaera sellii) within the early is always very rare. and middle Pleistocene interval (Gartner, 1977); however, the first occur- rence of Emiliania huxleyi is at the bottom of Core 125-781 A-1R; Cores 125-781A-2R and 125-781A-3R were not recovered. The late Pliocene Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992. Proc. ODP, Sci. Results, 125: to early Pliocene age indicated in Fig. 21 (p. 195 of the Initial Reports College Station, TX (Ocean Drilling Program). volume for Leg 125; Pearce, Fryer, et al., 1990) for the post-basalt interval 2 Dipartimento Scienze della Terra, Universita di Napoli, Italy. must be corrected to Pleistocene. G. CIAMPO B 40° N 33°N \ Aoga Shima ^^—J 30< 20° 20c \ Conical7?1 780 Pacman Seamount-4 19' Agπgan & Backarc \ % Basin ç^CZ^i-Pa > 18C )w r V 145°E 146° 147' 148C 149' 150c Figure 1. Location map of the sites drilled during Leg 125. 16 LATE MIOCENE TO HOLOCENE CALCAREOUS NANNOFOSSILS HOLES Magnetic Polarity Zones Epochs 778 779A 779B 78_0A 780B 78OC 78OD 781 \ 1 p 2 3 4 s 3 6 7 4 \ 5 \ 6 \ 7 8 9 10 10 11 12 12 13 13 14 15 16 17 18 19 20 21 • 22 23 24 \ 25 \ 26 Figure 2. Correlation of Leg 125 drill sites in the Mariana area to the absolute time scale (after Berggren et al., 1985) and calcareous nannofossil zones (Okada and Bukry, 1980). 17 G. CIAMPO Table 1. Distribution of calcareous nannofossils at Hole 781A.a Age Zones Samples P 6 7 10 12 13 14 19 20 21 24 25 28 29 30 31 32 33 34 35 36 37 1R-1-2 P F R R R F R F R 1R-1-65 P R R 1R-3-30 P R R CN15 1R-4-69 M R R R R R 1R-5-69 M R R R R R R 1R-CC p R R R R F F F R 4R-1-63 M R R R R R Pleistocene 4R-CC M F R C F R 5R-CC G F R R R F F 6R-1-39 M C R R R F R CN14a 6R-2-39 G C R R R F R 6R-3-39 P R R R R R R 6R-CC M R R R R R 7R-3-63 M F R R R R R R 7R-CC M R R R R F R R 15R-CC M R R R R C R R R R R F 16R-CC M C F R R C R R R R R 18R-CC M F R R R R F F R R R R R 21R-1-4 M F F R R R R R F F R R R R CN12a Pliocene 21R-CC M F F R R R R R F F R R R 22R-CC M R C R R R R R F R R R 23R-CC G R R R R R R F R R R R R 25R-CC M F F R R R R C R R R R R R R CNllb 26R-1-1 G C C C R R R F F R R R R R R d See list for identification of species. P (preservation): G = good; M = moderate; P = poor. 1. Amaurolithus amplificus 22. Discoaster icarus 36. Hayster perplexus 2. Amaurolithus bizzarrus 23. Discoaster intercalaris 37. Helicosphaera carteri 3. Amaurolithus delicatus 4. Amaurolithus primus 24. Discoaster pentaradiatus 38. Helicosphaera hyalina 5. Amaurolithus tricorniculatus 25. Discoaster quadramus 39. Helicosphaera inversa 6. Calcidiscus leptoporus I. Calcidiscus macintyrei 26. Discoaster quinqueramus 40. Helicosphaera neogranulata 8. Ceratolithus acutus 27. Discoaster stellulus 41. Helicosphaera sellii 9. Ceratolithus annatus 10. Ceratolithus cristatus 28. Discoaster surculus 42. Helicosphaera sp. 1 I1. Ceratolithus rugosus 29. Discoaster tamalis 43. Helicosphaera wallichi 12. Ceratolithus separatus 13. Ceratolithus simplex 30. Discoaster triradiatus 44. Oolithotus fragilis 14. Coccolithus pelagicus 31. Discoaster variabilis 45. Ponthosphaera japonica 15. Coccolithuspliopelagicus 16. Cricolithus jonesi 32. Emiliania huxleyi 46. Pontosphaera multipora 17. Dictyococcites aff. antarcticus 33. Geminilithella rotula Al. Ponthosphaera sp. 1 18. Discoaster asymmetricus 19. Discoaster berggrenii 34. Gephyrocapsa caribbeanica 48. Pseudoemiliania lacunosa 20. Discoaster blackstockae 35. Gephyrocapsa oceanica 49. Reticulofenestra pseudoumbilicus 21. Discoaster brouweri Izu-Bonin Region (Fig. 3) The Pliocene/Pleistocene boundary (base of the CN14a Subzone) lies below Sample 125-782A-5H-2, 72-73 cm, at 40.52 mbsf, as identified Sites 782 through 786 were drilled in the Izu-Bonin area. by the first occurrence of Gephyrocapsa oceanica. At the top of Core 125-782A-1H, the acme of Emiliania huxleyi occurs. Contrary to the Site 782 30° 51.6H 141° 18.8^; water depth, 2959 mbsl general good agreement between nannofossil biostratigraphy and mag- netostratigraphy, there is a discrepancy in the Pleistocene interval be- This site is located on the eastern margin of the Izu-Bonin forearc tween the nannofossil and magnetostratigraphic data (see also Stabell et basin.