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doi:10.2204/iodp.proc.314315316.123.2009

Biostratigraphy

Calcareous microfossils belonging to two major groups, nannofossils and planktonic foraminifers, were recovered from the sedimentary succession at Site C0001 in the trench slope basin, off the Kumano coast of Honshu Island, in the northwest Pacific Ocean. Combined biostratigraphic results suggest at least one time break in the sequence from the late Quaternary to the Miocene/​Pliocene boundary (Fig. F23).

Foraminifers and nannofossils are generally abundant within lithologic Subunit IA, with preservation ranging from moderate to good; the most well-preserved assemblages are found in the Pleistocene. Sedimentation rates are relatively high in Subunit IA, ranging from 140 to 180 m/m.y. (Fig. F23). In Unit II, which spans the accretionary prism sediments, sedimentation rates are difficult to pinpoint because of the lack of reliable biostratigraphic events. However, preliminary results suggest sedimentation rates between 31 and 44 m/m.y. (Fig. F23). The Pliocene/​Pleistocene boundary is placed between Sections 315-C0001F-9H-CC and 10H-CC based on planktonic foraminiferal evidence (last occurrence [LO] of Neogloboquadrina asanoi). Definite Pliocene nannofossil assemblages are present in Sample 315-C0001F-10H-CC, 37–42 cm, and correspond well to foraminifers. A marked change in both nannofossil and foraminifer assemblages occurs in Sections 315-C0001F-13H-CC through 14H-CC, spanning the interval between 207.16 and 213.27 m CSF. Here, mixed assemblages of late Pliocene to late Miocene age were encountered. In Samples 315-C0001F-19H-CC, 33–38 cm, and 315-C0001H-1R-CC, 26–31 cm, microfossil assemblages indicate an early Pliocene age.

The Miocene/​Pliocene boundary is placed below Section 315-C0001H-21R-CC based on foraminiferal evidence. Nannofossil results, however, suggest that the boundary is located between Section 315-C0001H-24R-CC and the bottom of Hole C0001H. The exact position of the Miocene/​Pliocene boundary at Site C0001 remains equivocal and warrants more detailed investigation.

Generally, the applicability of nannofossil zonation based mostly on low-latitude records benefits from the Kuroshio Current transporting warm equatorial surface waters to higher latitudes. Therefore, most samples studied here contain subtropical species within the Pliocene and Miocene intervals. However, there are indications that downhole variations in microfossil assemblage composition are partly invoked by regional paleoenvironmental changes. Early Pliocene to late Miocene planktonic foraminiferal assemblages exhibit low species diversity, and their downhole distribution is patchy, with several barren samples. A particularly poor interval in terms of nannofossil preservation and abundance stretches from Sections 315-C0001H-7R-CC to 10R-CC (296.64–319.95 m CSF). The interval is characterized by elevated numbers of the subarctic species Neogloboquadrina pachyderma sinistral form, indicating the presence of cooler water masses.

Calcareous nannofossils

All core catcher samples plus additional samples from some critical intervals in the vicinity of zonal boundaries were examined for calcareous nannofossils at Site C0001. Calcareous nannofossils are generally abundant and well to moderately preserved throughout the slope sediments (lithologic Unit I; Table T8). In contrast, within the accretionary prism sediments (Unit II) nannofossil abundance is generally low with moderately to mostly poorly preserved specimens (Tables T9, T10). Several samples from the lower Pliocene section yield only few or no nannofossils. A total of 15 nannofossil biostratigraphic events recorded at Site C0001 are listed in Table T11.

In Hole C0001B, two nannofossil events were recognized that assign its entire sedimentary succession a Pleistocene age. The first occurrence (FO) of Emiliania huxleyi, which defines the base of nannofossil Zone NN21 at 0.291 Ma, was placed between Samples 314-C0001B-1H-CC, 0–5 cm, and 2H-CC, 0–5 cm (2.11–11.86 m CSF). The LO of Pseudoemiliania lacunosa is located between Samples 314-C0001B-2H-CC, 0–5 cm, and 3H-CC, 15–17 cm (11.86–22.06 m CSF), marking the top of nannofossil Zone NN19.

In Hole C0001E, the first event recognized was the crossover in abundance from E. huxleyi to Gephyrocapsa spp. (>3.5 µm) between Samples 315-C0001E-1H-2, 10 cm, and 1H-3, 38 cm (0.17–1.83 m CSF). The FO of E. huxleyi was found between Samples 315-C0001E-1H-3, 38 cm, and 1H-5, 18 cm (1.83–3.02 m CSF). The concentration of several biostratigraphic events within the first core spanning a time interval between 63 and 291 ka might be indicative of a condensation horizon in the upper part of the core. The base of Zone NN20 at 0.436 Ma is defined by the LO of P. lacunosa, located between Samples 315-C0001E-2H-6, 16.5 cm, and 2H-7, 16.5 cm (10.10–11.50 m CSF). The last consistent occurrence of Reticulofenestra asanoi (0.9 Ma) was found between Samples 315-C0001E-7H-2, 80 cm, and 7H-5, 75 cm (53.63–57.58 m CSF). Zone NN19 was further divided by biohorizons based on changes in occurrences of different sized Gephyrocapsa. The reentrance of Gephyrocapsa spp. (>4 µm) is recorded between Samples 315-C0001E-8H-CC, 48 cm, and 9H-CC, 31.5 cm (68.07–80.10 m CSF). The LO of Gephyrocapsa spp. (>5.5 µm) provides another datum level of 1.24 Ma between Samples 315-C0001F-3H-CC, 51–56 cm, and 4H-CC, 12.5–17.5 cm (131.97–145.95 m CSF).

The LO of Calcidiscus macintyrei was found between Samples 315-C0001F-6H, 23.5 cm, and 7H-CC, 71–76 cm (161.50–170.98 m CSF). Frequent reworking of discoasters in the interval above blurs the exact identification of the zonal base of NN19. However, in accordance with magnetostratigraphy the LO of Discoaster brouweri, which occurs below the Pliocene/​Pleistocene boundary, is placed between Samples 315-C0001F-9H-CC, 27.5–32.5 cm, and 10H-CC, 37–42 cm (189.24–195.79 m CSF). In the latter sample, well-preserved D. brouweri specimens were detected.

Nannofossil content is generally low, and nannofossil preservation is poor within Samples 315-C0001F-10H-CC, 37–42 cm, through 13H-CC, 20–25 cm. This sandy interval is referred to as logging Subunit IB, marking the contact between the overlying slope apron sediments and the upper accretionary prism according to seismic and log interpretation.

Samples below the sandy layers yield a mixed and rather poorly preserved nannofossil assemblage with Gephyrocapsa spp. (>3.5 µm), the occurrence of which is restricted to the upper Pliocene, and older nannofossils with LOs positioned in the upper to middle Pliocene, such as Discoaster pentaradiatus, Discoaster surculus, Discoaster variabilis, Sphenolithus spp., and Reticulofenestra pseudoumbilicus (>7 µm). In contrast, the nannofossil assemblage present in Sample 315-C0001F-18H-5, 33–38 cm, is mainly composed of abundant Reticulofenestra spp. as the dominant placoliths, D. brouweri, D. pentaradiatus, D. surculus, D. variabilis, and Sphenolithus abies. This assemblage is regarded as typical for Zones NN15 to NN12, which span the interval from 3.79 to 5.59 Ma. The LO of R. pseudoumbilicus (>7 µm) at 3.79 Ma was found between Samples 315-C0001F-17H-CC, 13–18 cm, and 18H-CC, 0–1 cm (220.31–226.18 m CSF). Therefore, these sediments are assigned at least an early Pliocene age. This age assignment implies a biostratigraphic gap of at least 1.8 m.y. between the lowermost slope sediments, which are ~2 Ma, and the accretionary prism sediments, which are early Pliocene in age.

The uppermost core catcher sample of Hole C0001H contains basically the same assemblage as that recognized in Sample 315-C0001F-19H-5, 33–38 cm, recovered from Hole C0001F. Nannofossil abundance is generally low in sediments recovered from Hole C0001H, and their preservation is often poor. Dissolution and overgrowth affected the assemblage to a minor degree, whereas mechanical breakage was more severe, especially with respect to discoasters, which made species assignment sometimes difficult. The genera Amaurolithus and Ceratolithus provide important marker species for dividing the lower Pliocene nannofossil zones. However, these marker species appear only in low numbers in Hole C0001H, which impairs their biostratigraphic usefulness. Ceratolithus acutus was first recorded in Sample 315-C0001H-17R-CC, 0–5 cm (380.83 m CSF). This species occurs last at 5.04 Ma. One Amaurolithus primus/​Amaurolithus delicatus intergrade was first observed in Sample 315-C0001H-22R-2, 139–140 cm. The LO of A. primus at 4.5 Ma is placed between Samples 315-C0001H-21R-CC, 0–5 cm, and 22R-2, 139–140 cm (417.85–422.32 m CSF). The FO of Ceratolithus rugosus, which defines the base of Zone NN13 at 5.12 Ma, was placed between Samples 315-C0001H-22R-CC, 0–5 cm, and 23R-CC, 29–34 cm. C. acutus appears sporadically downhole to Sample 315-C0001H-25R-CC, 0–5 cm. Because of its rare occurrence, the placement of the FO of C. acutus between Samples 315-C0001H-25R-CC, 0–5 cm, and 26R-CC, 0–5 cm (449.53–456.55 m CSF), is questionable. Yet this species is confined to a relatively short stratigraphic interval between 5.04 and 5.32 Ma, providing a good biostratigraphic approximation of the Miocene/​Pliocene boundary, which is generally located slightly above the FO of C. acutus.

Planktonic foraminifers

All core catcher samples and 13 additional core samples within stratigraphically important intervals were examined. Planktonic foraminifers successively occurred in Holes C0001B, C0001E, C0001F, and C0001H, with the exception of several barren samples. Fossil abundance and preservation are generally excellent within the upper interval of the slope apron deposits (lithologic Unit I). In contrast, sediments of the lower accretionary prism (lithologic Unit II), recovered from the lower part of Hole C0001F to Hole C0001H, show relatively low numbers of fossil foraminifers with moderate to poor preservation. Some samples are even barren. These barren samples include only thick-walled benthic foraminifers such as Amphicoryna spp. with dissolved surface structures. Therefore, the discontinuous occurrence of planktonic foraminifers in the lower part of Hole C0001F could be due to dissolution of their more delicate shells, which might be related to a depositional environment below the CCD. Stratigraphic distribution of selected species is given in Tables T12, T13, and T14.

The planktonic foraminiferal assemblage significantly changes around the boundary between lithologic Units I and II: the upper assemblage of Unit I is characterized by dominant occurrences of temperate to cosmopolitan taxa such as Neogloboquadrina, Globigerina, and Globoconella. Tropical to subtropical genera such as Globogerinoides, Globorotalia, and Pulleniatina exhibit a continuous record. In contrast, the lower Unit II comprises relatively cool assemblages mainly consisting of Neogloboquadrina spp. and Globigerina bulloides. Noteworthy features of the foraminiferal record are cyclic variations in species composition within Unit II. In particular, the dominant coiling direction of N. pachyderma changes every 20–40 m in the lower sequence of Hole C0001H. These cyclic variations might result from changes in paleoclimatic conditions.

A total of 14 biohorizons are recognized at Site C0001 (Table T15). Globigerinoides ruber rosa continuously occurs between Samples 315-C0001E-1H-3, 81–83 cm, and 2H-5, 14–18 cm. The LO of this species (0.12 Ma) is recognized between Samples 315-C0001E-1H-2, 36–40 cm (0.47 m CSF), and 1H-3, 81–83 cm (2.27 m CSF). In Hole C0001B, this biohorizon is located above Sample 314-C0001B-1H-CC, 0–5 cm (2.11 m CSF), which is in good accordance with the results of Hole C0001E. Truncorotalia tosaensis continuously occurs from the lower to middle part of Hole C0001E, and its LO (0.61 Ma) is recognized between Samples 315-C0001E-2H-CC, 46–51 cm, and 3H-CC, 23–28 cm. However, the sporadic presence of reworked specimens in this interval, such as the early Pleistocene species Globoturborotalita obliquus (LO = 1.30 Ma) and Truncorotalia crassaformis viola (LO = around the base of the Pleistocene), might indicate that the actual LO of T. tosaensis should be located further downhole. Thus, its LO only serves as a maximum age estimate. Four samples from Hole C0001E contain Truncorotalia crassaformis hessi. The FO of this species (0.81 Ma) is placed below Sample 315-C0001E-6H-CC, 32.5–37.5 cm (51.60 m CSF). However, the precise position of the FO remains uncertain because of the sporadic occurrence of the species. Moreover, previous research has already mentioned that the FOs of several Truncorotalia species are generally delayed in mid-latitudes (Brunnes et al., 2002). At the present site, the FOs of Truncorotalia truncatulinoides, T. tosaensis, and T. crassiformis lag behind other foraminiferal events.

The FO of T. truncatulinoides (1.93 Ma) is detected between Samples 315-C0001F-2H-CC, 22–27 cm (127.00 m CSF), and 3H-CC, 51–56 cm (131.97 m CSF), and defines the base of Zone N22. The change in coiling direction (SD) of Pulleniatina spp., mainly Pulleniatina obliquiloculata and Pulleniatina primalis, from sinistral to dextral is recorded twice. The lower SD1 at 4.08 Ma occurs between Samples 315-C0001F-14H-CC, 0–5 cm (213.27 m CSF), and 315-C0001H-1R-CC, 26–31 cm (234.24 m CSF). The upper event (SD2 at 1.7–1.8 Ma) lies between Samples 315-C0001F-6H-CC, 21–26 cm (161.50 m CSF), and 7H-CC, 71–76 cm (170.98 m CSF). N. asanoi occurs abundantly in Sample 315-C0001F-10H-CC, 37–42 cm (195.79 m CSF), and the LO of this species (1.8 Ma) is placed above this sample. The FO of T. tosaensis (3.35 Ma) is located between Samples 315-C0001F-12H-CC, 18–23 cm (202.34 m CSF), and 11H-CC, 13–18 cm (200.85 m CSF). This event determines the lower boundary of Zone N21.

Between Samples 315-C0001F-13H-CC, 20–25 cm (207.16 m CSF), and 14H-1, 125–129 cm (208.45 m CSF), three biohorizons of different ages are detected—namely, the FO of Globoconella inflata modern form (2.3–2.5 Ma), the LO of Sphaeroidinellopsis seminulina s.l. (Sphaeroidinellopsis subdehiscens of the present author), and the LO of Dentoglobigerina altispira (3.47 Ma). The species concept of S. seminulina in Gradstein et al. (2004) is based on Kennett and Srinivasan (1983) and includes S. seminulina and Sphaeroidinellopsis of the present author. Therefore, the LO of S. seminulina s.l. of the present study corresponds to the LO of S. seminulina of Gradstein et al. (2004) (3.59 Ma). This interval corresponds to the lithologic boundary between Units I and II. With respect to foraminifer ages, the sedimentation gap between Units I and II should be at least 1.09 m.y.

The assemblage of Sample 315-C0001F-17H-CC, 13–18 cm, which is composed of soft mudstone with heavily disturbed bedding, contains species of different ages. Well-preserved specimens of T. truncatulinoides, G. ruber rosa, and G. inflata modern form indicate a latest Pleistocene age. However, yellowish-colored specimens of Sphaeroidinella dehiscens, G. obliquus, and Globoconella puncticulata, which are stratigraphically confined to the early Pliocene, are also observed. This implies that Section 315-C0001F-17-CC is contaminated with late Pleistocene sediments.

The FO of T. crassaformis (4.31 Ma) is placed between Samples 315-C0001F-18H-CC, 0–5 cm (226.18 m CSF), and 20X-CC, 49.5–54.5 cm (239.33 m CSF). The LO of Globoturborotalita nepenthes occurs between Samples 315-C0001H-1R-CC, 26–31 cm (234.24 m CSF), and 2R-CC, 0–5 cm (246.33 m CSF). Samples obtained from lithologic Unit II generally include specimens showing different ages, especially late Miocene to early Pliocene species of Globoconella (Globoconella conoidea, Globoconella conomiozea, and Globoconella sphericomiozea), which are often recognized within Unit II. The problem remains unsolved as to whether the morphotypes are reworked or represent regional phenotypes. The zonal marker species Globorotalia tumida appears sporadically, and its bottom occurrence is in Sample 315-C0001H-21R-CC, 0–5 cm (417.85 m CSF). It therefore indicates a maximum age of 5.57 Ma for this sample. This biohorizon also defines the lower boundary of Zone N.18. In addition, the lowermost sample of Hole C0001H, Sample 315-C0001H-26R-CC, 15–20 cm (456.68 m CSF), contains Pulleniatina primalis. Therefore, the sample is younger than the FO of P. primalis that defines the base of Subzone N.17b at 6.4 Ma.

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