Previous   |    Next

doi:10.2204/iodp.proc.314315316.206.2011

Results

Planktonic foraminiferal fossils occur throughout Site C0001, with the exception of several barren intervals in Unit II. Fossil abundance and preservation are generally excellent in Unit I. In contrast, sediments from Unit II have rare occurrences of planktonic foraminifers, with moderate to poor preservation or barren intervals. These barren samples include only thick-walled benthic foraminifers with surfaces disfigured by dissolution. Therefore, the rare occurrences and barren intervals in Unit II might be the result of some dissolution processes.

Stratigraphic distributions of selected planktonic foraminiferal species are given in Tables T1, T2, and T3. Scanning electron microphotographs of important age-diagnostic species are shown in Plates P1, P2, P3, and P4. The planktonic foraminiferal assemblage is characterized by dominant occurrences of such temperate to cosmopolitan taxa as Neogloboquadrina and Globigerina. Tropical to subtropical taxa such as Globigerinoides, Globorotalia, and Pulleniatina continuously occur in Unit I. Presently, the Kumano region lies beneath mixed waters caused by coastal upwelling between the warm Kuroshio Current and the Kii Peninsula (Fig. F1). Hence, the foraminiferal composition of Unit I is consistent with present-day conditions. The assemblage of Unit II includes more cool-water taxa, such as Neogloboquadrina pachyderma (sinistral), than Unit I. The most noteworthy feature of the Unit II assemblage is cyclic changes in the dominant coiling direction of N. pachyderma (Fig. F2; Table T4). At least 18 changes with 10 dextral and 9 sinistral intervals are observed in the studied section from ~3–5 Ma. The frequency of the coiling change generally corresponds to the perturbation of orbital eccentricity.

A total of 17 biohorizons are recognized in Holes C0001E, C0001F, and C0001H (Table T4). Among them, 15 biohorizons (1–3, 5–11, and 13–17) were reported on board using core catcher samples (see the “Expedition 315 Site C0001” chapter [Expedition 315 Scientists, 2009]) and refined by examination of section samples in the present study. Two biohorizons (4 and 12; see Table T4) were detected in the present study. Globigerinoides ruber (pink) continuously occurs between Samples 315-C0001E-1H-3, 5–10 cm, and 2H-5, 14–18 cm. The last occurrence (LO) of this morphotype (0.12 Ma) is recognized between Samples 315-C0001E-1H-2, 36–40 cm (0.40 m LSF), and 1H-3, 81–83 cm (1.44 m LSF). In Hole C0001B, this biohorizon is located above Sample 314-C0001B-1H-CC, 0–5 cm (2.11 m LSF), which is in good accordance with the results of Hole C0001E. Truncorotalia tosaensis occurs from the lower to middle part of Hole C0001E, and the LO (0.61 Ma) is recognized between Samples 315-C0001E-3H-1, 40–42 cm (13.33 m LSF), and 3H-CC, 23–28 cm (13.59 m LSF). The first occurrence (FO) of Truncorotalia crassaformis hessi is placed between Samples 315-C0001E-9H-8, 72–73 cm (78.55 m LSF), and 9H-CC, 29–34 cm (79.28 m LSF). The LO of Globoturborotalita obliquus lies between Samples 315-C0001F-4H-8, 55–60 cm (139.19 m LSF), and 4H-CC, 12.5–17.5 cm (139.60 m LSF). The coiling direction change from sinistral to dextral of Pulleniatina spp., mainly composed of Pulleniatina obliquiloculata and Pulleniatina primalis, is recorded twice: the lower (SD1: 4.08 Ma) is between Samples 315-C0001F-14H-CC, 0–5 cm (206.66 m LSF), and 315-C0001H-1R-CC, 26–31 cm (232.52 m LSF), and the upper (SD2: 1.7–1.8 Ma) lies between Samples 315-C0001F-6H-CC, 21–26 cm (157.67 m LSF), and 7H-2, 12–16 cm (159.45 m LSF). The FO of Truncorotalia truncatulinoides (1.93 Ma) is detected between Samples 315-C0001F-9H-6, 32–36 cm (180.75 m LSF), and 9H-CC, 27.5–32.5 cm (184.65 m LSF), and defines the base of Zone N.22. Neogloboquadrina asanoi occurs abundantly in Samples 315-C0001F-10H-11, 52–54 cm (189.49 m LSF), and 10H-CC, 37–42 cm (191.31 m LSF). The LO of this species (1.8 Ma) is clearly observed between Samples 315-C0001F-10H-9, 114–119 cm (188.70 m LSF), and 10H-11, 52–54 cm. The FO of T. tosaensis (3.35 Ma) is placed between Samples 315-C0001F-11H-CC, 13–18 cm (194.93 m LSF), and 12H-CC, 18–23 cm (195.96 m LSF), and determines the lower boundary of Zone N.21. Three biohorizons of different ages are found in the same interval, namely, the FO of Globoconella inflata modern form (2.3–2.5 Ma) and the LOs of Sphaeroidinellopsis seminulina s.l. (S. seminulina and Sphaeroidinellopsis subdehiscens) (3.59 Ma) and Dentoglobigerina altispira altispira (3.47 Ma), between Samples 315-C0001F-13H-CC, 20–25 cm (199.29 m LSF), and 14H-1, 125–129 cm (200.98 m LSF). This interval corresponds to the lithostratigraphic boundary between Units I and II. With respect to the foraminiferal ages, the sedimentation gap between Units I and II should be at least 1.09 m.y. The LO of Hirsutella margaritae (3.85 Ma) could be placed below Sample 315-C0001C-5R-1, 25–27 cm (267.69 m LSF). However, the precise position of the LO cannot be determined because the species occurs only sporadically. The FO of Truncorotalia crassaformis (4.31 Ma) is placed between Samples 315-C0001F-18H-CC, 0–1 cm (216.85 m LSF), and 20X-4, 65–67 cm (224.14 m LSF). The LO of Globoturborotalita nepenthes occurs between Samples 315-C0001H-2R-1, 110–112 cm (237.83 m LSF), and 2R-3, 127–129 cm (240.91 m LSF). The zonal maker species Globorotalia tumida appears sporadically, and its deepest occurrence is in Sample 315-C0001H-24R-1, 99–101 cm (439.47 m LSF). 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, namely, 315-C0001H-26H-CC, 15–20 cm (456.67 m LSF), contains P. primalis. Therefore, the sample is younger than the FO age of P. primalis (6.4 Ma: the lower boundary of Subzone N.17b).

The FOs of T. crassaformis hessi, T. truncatulinoides, and T. tosaensis are slightly discordant with other biohorizons. Previous workers have mentioned that these events are occasionally delayed in mid-latitude regions; for example, the FO of T. truncatulinoides is about 2.4 Ma in the southwestern part of the Pacific Ocean (Dowsett, 1988) and 1.1–1.2 Ma off southern Australia (Brunner et al., 2002). This implies that the discordance could be explained by ecological diachroneities of the three biohorizons.

In addition to the key species above, two important index species, namely Globoturborotalita extremus and Pulleniatina finalis, are recognized in the middle to lower part of the section. The LO of G. extremus is 1.98 Ma (Gradstein et al., 2004), and the FO of P. finalis is 2.04 Ma (Berggren et al., 1995; recalibrated to the standard timescale of Gradstein et al., 2004). However, it is hard to identify the two biohorizons at the present site because of their discontinuous occurrences.

Figure F2 represents the refined age-depth plot of Site C0001 using biohorizons of calcareous nannofossils (see the “Expedition 315 Site C0001” chapter [Expedition 315 Scientists, 2009]) and planktonic foraminifers (the present study). The plot indicates that foraminiferal biohorizons of the present study generally are consistent with the calcareous nannofossil data. Furthermore, the planktonic foraminiferal data provide detailed constraints around the boundary between Units I and II rather than the nannofossil data. Sedimentation rates of the upper slope apron deposits (Unit I) can be divided into three intervals: the uppermost part (~0–50 m LSF) is 50–60 mm/k.y., the middle part (~50–130 m LSF) is 200–250 mm/k.y., and the lower part (~130 m LSF to the base of Unit I) is 80–90 mm/k.y. No evidence of significant stratigraphic repeat is observed in Unit II, even though the unit is composed of accretionary prism deposits. The sedimentation rates of Unit II range from 200 to 220 mm/k.y.

The Pliocene/Pleistocene boundary is constrained between Samples 315-C0001F-13H-CC, 20–25 cm (199.29 m LSF), and 14H-1, 125–129 cm (200.98 m LSF), on the basis of two planktonic foraminiferal biohorizons, namely the FO of G. inflata modern form and the LO of D. altispira altispira. This boundary corresponds to the unconformity between lithologic Units I and II. The Miocene/Pliocene boundary is suggested just above the FO of G. tumida. This biohorizon lies below Sample 315-C0001H-24R-1, 99–101 cm (439.47 m LSF). However, calcareous nannofossil results indicate that the Miocene/Pliocene boundary is located between Samples 315-C0001H-24R-CC, 0–5 cm (441.82 m LSF), and 26R-CC, 15–20 cm (456.67 m CSF) (see the “Expedition 315 Site C0001” chapter [Expedition 315 Scientists, 2009]). The precise determination of the boundary requires further studies.

Top of page   |    Previous   |    Next