NANNOFOSSIL ZONATION AND BIOSTRATIGRAPHY

Site 1150

Site 1150 (3911N, 14320E) is located on the deep-sea terrace on the landward side of the Japan Trench. A sedimentary section from the two holes drilled at Site 1150 recovered a total thickness of 1181.6 m of middle Miocene to Holocene sediment (Table T2). The major lithology of the recovered sediments predominantly consists of homogenous diatomaceous silty clay and diatomaceous clay and its lithified equivalents, which are variable admixtures of biogenic siliceous microfossils, siliciclastic grains, and volcaniclastic grains (Shipboard Scientific Party, 2000a).

Hole 1150A

Hole 1150A was drilled through Core 76X to a total depth of 722.6 meters below seafloor (mbsf) using the APC/XCB coring system in a water depth of 2692.2 m. The abundance and preservation of the nannofossils vary greatly. In Hole 1150A, 66 of 299 samples checked are barren (Table T3).

The Pleistocene assemblage is characterized by Calcidiscus leptoporus, C. pelagicus, E. huxleyi, and Pseudoemiliania lacunosa plus specimens of Gephyrocapsa, Reticulofenestra, and Helicosphaera. The youngest coccolith assemblages belong to the E. huxleyi Zone (CN15) and are present from Samples 186-1150A-1H-1, 75-76 cm, to 3H-5, 75-76 cm, where E. huxleyi is dominant. Samples 186-1150A-3H-CC to 5H-5, 75-76 cm, were assigned to Subzone CN14b because neither E. huxleyi nor P. lacunosa was found. Calcareous nannofossils are few to common and poorly preserved in this interval. Samples 186-1150A-5H-CC to 10H-CC were assigned to Subzone CN14a because they contain both P. lacunosa and Gephyrocapsa parallela. Calcareous nannofossils are generally abundant and poorly to moderately preserved in these samples.

Samples 186-1150A-11H-1, 76-77 cm, to 14X-CC were assigned to Subzone CN13b based on rare specimens of Gephyrocapsa caribbeanica in Sample 14X-CC. Nannofossils are poorly preserved in these samples; 16 of the 31 samples are barren. Samples 186-1150A-15X-3, 75-76 cm, to 16X-3, 76-77 cm, were assigned to Subzone CN13a because their well-preserved common calcareous nannofossils contain no Discoaster or G. caribbeanica. The Pliocene/Pleistocene boundary was placed between Samples 186-1150A-14X-3-CC and 16X-3, 75-76 cm.

Among other nannofossil events, the first occurrence (FO) of Reticulofenestra asanoi was found in Sample 186-1150A-11H-5, 75-76 cm, and its LO was found in Sample 9H-CC. The rare and scattered presence of Helicosphaera inversa made it impossible to delimit its FO and LO. The LOs of Calcidiscus macintyrei and Helicosphaera selli occur before the FO of the Gephyrocapsa oceanica. The FO of G. caribbeanica occurs in the same sample as the FO of G. oceanica (Sample 186-1150A-14X-CC), so only the FO of G. oceanica was used for the zonation.

In the Pliocene and upper Miocene sediments, the assemblage is dominated by small Reticulofenestra, Reticulofenestra pseudoumbilicus, Reticulofenestra gelida, Calcidiscus leptoporus, C. pelagicus, and C. macintyrei. The genus Discoaster, which prefers warm water, is consistently rare to few in the non-barren samples. This limited the resolution of the zonation that could be applied here. Quality of preservation also diminishes downhole.

Samples 186-1150A-16X-CC through 22X-5, 75-76 cm, were assigned to Subzone CN12c-CN12d, the base of which is marked by the LO of Discoaster pentaradiatus and Discoaster surculus. Samples 186-1150A-22X-CC to 27X-7, 10-11 cm, were assigned to Subzone CN12b, whose base is marked by the LO of Discoaster tamalis. Samples 186-1150A-27X-CC to 38X-3, 10-11 cm, were assigned to Subzone CN12a because no R. pseudoumbilicus was found. The LO of Amaurolithus spp. was not detected, and the rare occurrence of Ceratolithus rugosus makes assignment of the base of Subzone CN10c questionable.

Hole 1150B

Hole 1150B was drilled ~44 m east of Hole 1150A with the RCB coring system in a water depth of 2692.2 m (Table T4). The hole was washed down to 703.3 mbsf before coring began, at which point 50 RCB cores were taken. The first datum encountered in Hole 1150B was the LO of Discoaster quinqueramus, which defines the Zone CN10/CN9 boundary; this was found in Sample 186-1150B-10R-1, 10-11 cm. A single well-preserved Amaurolithus amplificus in Sample 186-1150B-19R-CC puts the age of this sample between 5.99 and 6.84 Ma. From Samples 186-1150B-1R-CC through 19R-CC, about one-half of the core catcher samples (9 out of 19) are barren or contain rare nannofossils. Samples 186-1150B-10R-1, 10-11 cm, through 38R-CC were assigned to Zone CN9 by the FO of Discoaster berggrenii. Below this, no nannofossil datum was found. In Cores 48R, 49R, and 50R, Catinaster coalitus is rare to common, which indicates it is still above the base of Zone CN6. The LO of D. surculus, the LO and FO of Amaurolithus spp., Ceratolithus acutus, Triquetrorhabdulus rugosus, and the FO of Discoaster loeblichii, however, were not found.

Site 1151

Site 1151 (3845N, 14320E) is 48 km south of Site 1150 (Fig. F1). The sedimentary section from 0 to 1113 mbsf was cored with the RCB and/or APC coring systems in three holes (1151A, 1151C, and 1151D). The dominant lithology is that of diatom-, glass-, and spicule-bearing silty clay (Shipboard Scientific Party, 2000b). Occurrence of minor lithologies is rare. Nannofossil biostratigraphy suggests that the sequence represents a record from the middle Miocene to Pleistocene (Table T5).

Hole 1151A

From Hole 1151A, 108 cores were recovered (Cores 2R through 109R; no recovery for Core 1W) from 78.0 to 1113.46 mbsf at a water depth of 2182.2 m. The abundance and preservation of nannofossils vary greatly (Table T6).

The Pleistocene assemblage was moderate to well preserved; 4 of 12 samples are barren. Core 2R was assigned to Zone CN14 because of G. parallela found in Sample 186-1151A-2R-CC. All three samples checked in this core yielded well-preserved abundant nannofossils. Samples 186-1151A-3R-1, 10-11 cm, through 3R-6, 10-11 cm, were assigned to Subzone 13b because of the presence of G. oceanica and G. caribbeanica. Samples 186-1151A-3R-6, 75-76 cm, to 6R-4, 75-76 cm, were assigned to Subzone CN13a because no Discoaster was found. The three samples between them are barren.

The Pliocene and Miocene assemblages are dominated by Reticulofenestra, C. pelagicus, and C. leptoporus. Discoasters are also common. The LO of Discoaster brouweri was observed in Sample 186-1151A-6R-CC, which indicates the base of Subzone CN13a. The LO of D. pentaradiatus was observed in Sample 186-1151A-10R-1, 75-76 cm, which indicates the base of Subzone CN12d. The nannofossils are generally abundant and moderately to well preserved in Subzone CN12d. Samples 186-1151A-10R-1, 75-76 cm, to 13R-3, 10-11 cm, were assigned to Subzones CN12b to CN12c, whereas the LO of D. tamalis was observed in Sample 186-1151A-13R-3, 75-76 cm, which indicates the base of Subzone CN12b. Ten of 20 samples within this interval are barren. The LO of D. surculus was not detected. The LO of R. pseudoumbilicus was observed from Sample 186-1151A-15R-CC, which indicates the base of Subzone CN12a. Nannofossils are generally abundant and moderately preserved in Subzone CN12a except for Samples 186-1151A-15R-5, 76-77 cm, and 16R-6, 10-11 cm, which are barren. The CN11/CN10c, CN10c/CN10b, and CN10b/CN10a zonal boundaries were not observed owing to the absent or sparse occurrence of the marker species.

The LO of D. quinqueramus, which marks the base of Subzone CN10a, was detected in Sample 186-1151A-59R-5, 75-76 cm. In Zones CN10-CN11, 42 of 169 samples are barren. The FO of D. berggrenii, observed in Sample 186-1151A-90R-CC, defined the base of Zone CN9. In Zone CN9, 42 of 111 samples are barren. The CN8/CN7 and CN7/CN6 zonal boundaries were not observed in this hole. The base of Zone CN6 was recognized in Sample 186-1151A-99R-2, 15-16 cm, as indicated by the FO of Catinaster coalitus. Farther downhole, poorly preserved nannofossils were found in only 8 samples; the other 27 samples examined are barren. It is difficult to determine the age of the sediments from nannofossils because of their low abundance and poor preservation. Diatom data suggest that the hole terminated in the upper part of Zone CN3.

Holes 1151C and 1151D

A Pleistocene nannofossil assemblage was observed in 11 APC cores recovered from Hole 1151C (Table T7) and 10 APC cores from Hole 1151D (Table T8). The assemblage is characterized by E. huxleyi, P. lacunosa, C. pelagicus, C. leptoporus, and Gephyrocapsa.

The Subzone CN15/CN14b boundary, which is marked by the FO of E. huxleyi, lies between Samples 186-1151C-2H-1, 25-26 cm, and 2H-7, 25-26 cm. The nannofossils in Zone CN15 are common to abundant and well preserved at both sites. The LO of P. lacunosa, which marks the base of the Subzone CN14b, was observed between Samples 186-1151C-4H-4, 100-101 cm, and 4H-5, 25-26 cm. Sample 186-1151C-4H-CC is barren. The FO of G. parallela, which marked the Zone CN14/CN13 boundary, was observed in Samples 186-1151C-8H-CC and 186-1151D-7H-CC. The nannofossils in Zone CN14 are rare to common. Sample 186-1151D-9H-CC is barren. G. caribbeanica was observed in both Samples 186-1151C-11H-CC and 186-1151D-10H-CC, which indicates that the bottom of these two holes is still within Subzone CN13b.

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