A generally continuous sequence of middle Miocene through Pleistocene sediments and sedimentary rocks was recovered from Site 1151. The 1113-m-thick section consists mainly of diatomaceous silty clay. Calcareous nannofossils are generally barren to abundant throughout, with variable preservation. Except for some ash and dolomite layers, the sequence contains few to dominant diatoms throughout. Diatoms are moderately well preserved. Because the first occurrence (FO) of the diatom Denticulopsis praelauta was not recognized, we interpret the base of the section to be younger than 16.3 Ma.
We recovered 108 RCB cores from Holes 1151A (Cores 2R-109R; no recovery for 1W), from 78.0 to 1113.46 mbsf. Calcareous nannofossils at this hole were dated primarily from core-catcher (CC) samples.The nannofossil biostratigraphy suggests the sequence represents a record from the Pleistocene to the lower Miocene (Table T6).
Cores 2R to 5R were assigned to the Pleistocene. Detailed samples were examined in each of these five cores because, except for Sample 186-1151A-2R-CC, all the core-catcher samples were either barren or contained only trace nannofossils that were poorly preserved. The last occurrence (LO) of Pseudoemiliania lacunosa was not observed. The FO of Gephyrocapsa parallela, observed in Sample 186-1151A-3R-1, 10-11 cm, and the FO of Gephyrocapsa caribbeanica, observed in Sample 186-1151A-4R-3, 10-11 cm, were used to define the bases of Subzones CN14a and CN13b, respectively.
The boundary of the Pleistocene and Pliocene lies between Samples 186-1151A-4R-3, 10-11 cm, and 6R-CC, in which the FO of Discoaster brouweri was observed. The FO of D. brouweri defines the base of Subzone CN13a. The Pliocene and Miocene assemblage was dominated by Gephyrocapsa and Coccolithus pelagicus, and the warm-water Discoaster spp. were also common. The FO of Discoaster pentaradiatus was observed in Sample 186-1151A-12R-CC. Because Samples 186-1151A-10R-CC and 11R-CC are barren, the base of Subzone CN12d was put between Samples 186-1151A-9R-CC and 12R-CC. The FO of Discoaster tamalis was observed in Sample 186-1151A-13R-CC, which indicates that both the boundary of Subzones CN12c/CN12b and CN12b/CN12a must lie in Core 13R. The LO of Reticulofenestra pseudoumbilicus was observed in Sample 186-1151A-15R-CC, which defines the base of Subzone CN12a. The LO of Amaurolithus spp., FO of Ceratolithus rugosus, and LO of Triquetrorhabdulus rugosus, which mark the CN11/CN10c, CN10c/CN10b, and CN10b/CN10a boundaries, respectively, were not observed because of the sparsity of these nannofossils. The LO of Discoaster quinqueramus, which marks the base of Subzone CN10a, was observed in Sample 186-1151A-67R-CC. From Cores 59R through 66R, all core-catcher samples are either barren or only contain trace nannofossils, so the CN10a/CN9 boundary may lie within Section 186-1150A-58R-CC to 67R-CC. There are several intervals within Zone CN10 and CN11 that are barren or contain trace nannofossils: Samples 186-1150A-16R-CC, 17R-CC, 19R-CC, 23R-CC to 27R-CC, 39R-CC, 48R-CC to 50R-CC, and 59R-CC to 66R-CC. The base of Zone CN9 is defined by the FO of Discoaster berggrenii, which was observed in Sample 186-1151A-90R-CC. In Zone CN9, the samples from Section 67R-CC to 78R-CC contain common to abundant nannofossils. From Section 79R-CC to 88R-CC, the samples are either barren or contain only trace nannofossils. Zone CN7 was not recognized at this site; its top and base are marked by the LO and FO of Discoaster hamatus, and no D. hamatus was observed at this site. Section 91R-CC to 98R-CC were assigned to Zone CN6-CN8 because the FO of Catinaster coalithus, which defines the base of Zone CN6, was observed in Sample 186-1151A-98R-CC. In this interval, Samples 186-1151A-95R-CC to 97R-CC are either barren or contain only trace nannofossils. Sample 186-1151A-99R-CC was assigned to Zone CN5. Samples 186-1151A-100R-CC and 101R-CC are barren. Sample 186-1151A-102R-CC was assigned to Subzone CN5a since it contains Cyclicargolithus floridanus but no Sphenolithus heteromorphus. Samples 186-1151A-103R-CC to 107R-CC are barren. Sample 186-1151A-108R-CC was assigned to Zone CN3 since both Discoaster deflandrei and S. heteromorphus were observed in this sample. Sample 186-1151A-109R-CC is barren, although diatom data indicate that the bottom of this core is still in Zone CN3.
Zone CN9, which is defined by the total range of the two related taxa D. quinqueramus and D. berggrenii, gave younger ages than those indicated by the diatom datums. Since the ages were established mainly from the low-latitude area (Backman and Rafffi, 1997; Rio et al., 1990), they may be different in this area.
A Pleistocene nannofossil assemblage was observed from the 11 APC cores recovered from Hole 1151C and the 10 APC cores from Hole 1151D (Table T6). The assemblage was marked by Emiliania huxleyi, P. lacunosa, C. pelagicus, Calcidiscus leptoporus, and Gephyrocapsa spp.
The CN15/CN14b boundary, which is marked by the FO of E. huxleyi, lies within Samples 186-1151C-2H-CC to 3H-CC and 186-1151D-2H-CC to 3H-CC. As mentioned in "Biostratigraphy" in the "Site 1150" chapter, postcruise scanning electron microscope study will be necessary for defining this boundary. The nannofossils in Zone 15 are common to abundant and well preserved in both sites. The LO of P. lacunosa, which marks the base of the Subzone CN14b, was observed in Samples 186-1151C-5H-3, 98 cm, and 186-1151D-4H-CC. Sample 186-1151C-4H-CC is barren. The FO of G. parallela, which marks the CN14/CN13 zonal boundary, was observed in Samples 186-1151C-8H-CC and 186-1151D-7H-CC. The nannofossils in the Zone 14 are rare to common. Sample 186-1151D-9H-CC is barren. Gephyrocapsa caribbeanica was observed in both Samples 186-1151C-11H-CC and 186-1151D-10H-CC, which indicated the bottoms of these two holes are still within the Subzone CN13b.
Most of our observations come from core-catcher samples. For determination of zonal boundaries and for the sequence of lower sedimentation rates (below 923 mbsf, see "Sedimentation Rates"), additional smear slides were prepared from other core intervals. Diatoms are generally few to abundant and moderately well preserved throughout the Pleistocene through middle Miocene section recovered at Site 1151. The abundance of diatoms is common to dominant above 580 mbsf, slightly decreases to common between 930 to 580 mbsf, and then decreases to few to common occurrence below that depth. A range chart showing the distribution of the index species and other selected species is shown in Tables T7 and T8 (both also available in ASCII format). From the distribution of these species, 23 datum levels were identified (Table T9, also available in ASCII format).
The top of Hole 1151A contains Proboscia curvirostris, indicating that the age is older than 0.3 Ma. Because the other late Pleistocene diatoms such as Thalassiosira jouseae and Neodenticula seminae also are continuously present, the top of Hole 1151A apparently corresponds to the P. curvirostris Zone (North Pacific diatom [NPD] 11).
The last common occurrence (LCO) of Actinocyclus oculatus (1.01-1.46 Ma), which defines the boundary between the upper Pleistocene P. curvirostris Zone and the lower Pleistocene A. oculatus Zone (NPD 11/10), is clearly identified between Sections 186-1151A-3R-CC and 4R-2 in Hole 1151A. The FO of the Pleistocene diatom P. curvirostris lies between Sections 186-1151A-4R-CC and 5R-CC.
The base of the A. oculatus Zone (NPD 10) and the top of the underlying Neodenticula koizumii Zone (NPD 9) is defined by the LO of N. koizumii. This latest Pliocene event occurs between Sections 186-1151A-5R-CC and 6R-1. In the upper part of NPD9, Nitzschia fossilis and Thalassiosira antiqua are rare.
The LCO of Neodenticula kamtschatica (2.61-2.68 Ma) between Sections 186-1151A-11R-CC and 12R-3 marks the top of the N. koizumii-N. kamtschatica Zone (NPD 8). As at Site 1150, the FO of N. seminae is observed in the upper part of this zone (Section 186-1151A-12R-CC), together with its closed copula (see "Biostratigraphy" in the "Site 1150" chapter).
The FO of N. koizumii (3.53-3.95 Ma), which defines the boundary between the upper Pliocene N. koizumii-N. kamtschatica Zone and the lower Pliocene Thalassiosira oestrupii Zone (NPD 8/7B boundary), is clearly identified between Sections 14R-CC and 15R-4 in Hole 1151A. Within NPD 7B, the FO of Thalassiosira latimarginata (5.07 Ma) lies between Sections 186-1151A-29R-CC and 30R-CC.
The FO of T. oestrupii sensu lato defines the top of the upper Miocene N. kamtschatica Zone of Koizumi (1992). This datum exists between Sections 38R-1 and 38R-CC of Hole 1151A. Though the abundance is very rare, Rouxia californica appears continuously below Section 186-1151A-66R-CC, suggesting that the boundary of NPD 7Ba/7A (6.65 Ma) exists between Sections 186-1151A-66R-1 and 66R-CC. The presence of the LO of Cavitatus jouseanus (6.7-6.8 Ma) between Sections 186-1151A-69R-CC and 70R-CC supports this zonal boundary. The FO of Pyxidicula (= Thalassiosira) zabelinae falls in the bottom of this zone.
The upper Miocene R. californica Zone of Koizumi (1992) is situated on the lower part of NPD 7A. The top of this zone is defined by the FO of N. kamtschatica (7.3-7.4 Ma). In this site, however, this species has a sporadic occurrence down to the underlying Thalassionema schraderi Zone (see below). Thus this datum is documented as the first common occurrence (FCO) between Sections 186-1151A-77R-CC and 78R-1. In this zone, R. californica is continuously present, and Ikebea tenuis appears sporadically.
The LO of T. schraderi (7.6 Ma) is between Sections 186-1151A-79R-CC and 80R-1 and marks the top of the upper Miocene T. schraderi Zone (NPD 6B). As at Site 1150, Actinocyclus ingens appears continuously from the lower part of this zone. The LO of Denticulopsis katayamae occurs between Sections 186-1151A-89R-CC and 90R-CC. The distinction of N. kamtschatica from its ancestor Nitzschia rolandii is difficult because of its gradual evolutionary change (Yanagisawa and Akiba, 1998), but according to Yanagisawa and Akiba (1990), N. kamtschatica can be distinguished from N. rolandii by broader spacing of pseudosepta or costae (5-8 in 10 µm). In this sense, N. kamtschatica occurs sporadically in the upper part of this zone. Koizumi and Tanimura (1985) also reported such phenomena at DSDP Site 581; thus, the datum level of the FO of N. kamtschatica should be redefined as the FCO.
The zonal boundary between the T. schraderi Zone and the D. katayamae Zone (NPD 6A) is determined by the LCO of Denticulopsis simonsenii (8.6 Ma), as between Sections 186-1151A-91R-3 and 91R-5.
The top of the Denticulopsis dimorpha Zone (NPD 5D) is defined by the LO of D. dimorpha (9.16 Ma). This datum is recognized between Sections 186-1151A-91R-CC and 92R-2, together with the FO of T. schraderi. Below this depth, the FO of D. katayamae (9.26 Ma) is observed (between Sections 186-1151A-93R-CC and 94R-CC). The base of this zone is defined by the FO of D. dimorpha (9.9 Ma).
The FO of D. dimorpha (9.9 Ma), which defines the top of Thalassiosira yabei Zone (NPD 5C), is clearly identified between Sections 186-1151A-95R-CC and 96R-1. But its nominate species, T. yabei, is very rare. The bottom of this zone is determined by the LCO of Denticulopsis praedimorpha (11.5 Ma) between Sections 186-1151A-99R-CC and 100R-1.
The middle Miocene D. praedimorpha Zone (NPD 5B) at this site contains four important marker species: Crucidenticula nicobarica, Denticulopsis hyalina, D. simonsenii, and the nominate species D. praedimorpha. The LO of C. nicobarica (12.5 Ma) lies at the top of this zone, suggesting that there is possibly a redeposition of this species, coring gap, or a hiatus (<1.0 m.y.) between Sections 186-1151A-99R-CC and 100R-1. The bottom of this zone is defined by the FO of D. praedimorpha (12.9 Ma) between the Samples 186-1151A-101R-1, 71-73 cm, and 101R-1, 90-91 cm.
The underlying C. nicobarica Zone (NPD 5A) was not identified because the FCO of D. simonsenii (13.1 Ma), which defines the bottom of NPD 5A, is found at the same interval as the FO of D. praedimorpha (the top of NPD 5A). Yanagisawa and Akiba (1998) proposed that the LCO of D. hyalina can be also used as the base of NPD 5A, and this datum is also recognized between 71-73 and 90-91 cm of Section 186-1151A-101R-1. Thus there must be a short hiatus which wiped out the zone NPD 5A (>0.2 m.y.) within this interval.
The FO of D. simonsenii (14.4-14.6 Ma), defining the base of Denticulopsis hustedtii Zone (NPD 4Bb), is recognized between Sections 186-1151A-102R-1 and 102R-2. The carbonate nodule layer in this zone (Sample 186-1151A-102R-2, 62-64 cm) contains well-preserved abundant diatoms such as A. ingens, C. nicobarica, D. hyalina, Denticulopsis lauta, and T. yabei.
The underlying D. hyalina Zone (NPD 4Ba) is marked by the continuous occurrence of D. hyalina and few D. lauta. Its base is defined by the FO of D. hyalina (14.9 Ma) between Sections 186-1151A-104R-1 and 104R-CC.
Within the D. lauta Zone (NPD 4A), the abundance becomes few to common, with sporadic poor preservation intervals. Nevertheless, the marker species D. lauta occurs continuously down to Section 186-1151A-108R-4, which defines the bottom of this zone.
The D. praelauta Zone (NPD 3B) is the oldest zone of Hole 1151A because its bottom horizon defined by the FO of D. praelauta (16.3 Ma) is not identified in this hole. Extrapolation of the 43 m/m.y. sedimentation rate (see "Sedimentation Rates") between the FO of D. hyalina (14.9 Ma) and FO of D. lauta (15.9 Ma) would give a slightly younger age of 16.2 Ma for the sediments at the bottom of the hole and is, therefore, acceptable.
Diatom assemblages from all samples consist almost entirely of oceanic species. They are mainly of the subarctic North Pacific Ocean, although such warm-water taxa as Nitzschia reinholdii and Hemidiscus cuneiformis are more frequent than those of Site 1150. Neritic species such as Actinoptychus senarius, Paralia sulcata, and Cocconeis spp. vary from few to trace, but they are recognized in most samples. Freshwater species such as Aulacoseira spp. are rare, but there is continuous appearance between the upper NPD 4A and the lower NPD 4Ba. As at Site 1150, resting spores of Chaetoceros spp. are mostly few to abundant between NPD 10 to 12 and rare to few in lower zones.
The boundary between the upper Pleistocene N. seminae and P. curvirostris Zones (NPD 12/11) is indicated by the LO of P. curvirostris (0.30 Ma) between Sections 186-1151C-3H-CC and 4H-4, and 186-1151D-3H-4 and 3H-CC. The continuous occurrence of T. jouseae, the LO of which corresponds to 0.30-0.41 Ma, is observed in this depth in Hole 1151C but is slightly shallower in Hole 1151D (between Sections 2H-CC and 3H-4). The warm-water species N. reinholdii, the LO of which corresponds to 0.62 Ma in the equatorial Pacific (Shackleton et al., 1995), is found in Sections 186-1151C-5H-CC to 6H-CC and 186-1151D-4H-CC to 5H-CC.
The LCO of A. oculatus (1.01-1.46 Ma), which defines the boundary between the upper Pleistocene P. curvirostris Zone and the lower Pleistocene A. oculatus Zone (NPD 11/10), is clearly identified between Sections 186-1151C-9H-5 and 9H-CC, and between Cores 186-1151D-8H and 9H. Thalassiosira antiqua, which has its LO from 1.52 to 1.8 Ma, is scarce in the lower part of this zone (Section 186-1151D-10H-CC).
The base of the A. oculatus Zone (NPD 10) and the top of the underlying N. koizumii Zone (NPD 9) is defined by the LO of N. koizumii. This latest Pliocene event occurs between Sections 186-1151C-11H-4 and 11H-CC, and 186-1151D-9H-CC and 10H-CC.
The latter two datums are about 20 m shallower than those in Hole 1151A, which may be caused by possible coring gaps and/or sedimentation rate variations influenced by local topographical features between Hole 1151A and Holes 1151C and 1151D.