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doi:10.2204/iodp.proc.303306.111.2006

Biostratigraphy

Core catcher samples from Holes U1312A and U1312B contain rich assemblages of calcareous nannofossils and planktonic foraminifers that are generally well preserved, with the exception of some intervals from the upper Miocene. In these sediments, dissolution of nannofossils and overgrown discoasters occur and planktonic foraminifer shells are increasingly fragmented and encrusted. In contrast, silica preservation is poor in the uppermost part of each hole and the sediments are completely barren of diatoms and radiolarians in the lower part. From top to bottom, a succession of calcareous nannofossil and planktonic foraminifer events provide a reliable biostratigraphic framework that, in the upper part, is supported by the siliceous plankton biozones (Tables T5, T6).

According to the age of biostratigraphic events recognized in this study, a sedimentary sequence encompassing the late Miocene–Holocene was recovered from Site U1312 (Fig. F12). Sediments from the late Pliocene and Pleistocene contain abundant planktonic foraminifers in relation to calcareous nannofossils, whereas planktonic foraminifer abundance from lower Pliocene and Miocene sediments is reduced relative to nannofossils. In particular, Samples 306-U1312A-18H-CC and 306-U1312B-19H-CC are poor in foraminifers, probably due to dissolution in sediments from the uppermost Miocene.

Comparison of age-depth plots from Holes U1312A and U1312B show similar trends in sedimentation rate in both holes (Figs. F13, F14). Sedimentation rates in the late Miocene–earliest Pliocene are generally low (<3 cm/k.y.), and during much of the late Miocene rates are <1.0 cm/k.y. at Site U1312. Rates increased in the early Pliocene, reaching 8.33 cm/k.y. in Hole U1312A and 7.41 cm/k.y. in Hole U1312B. Sedimentation rates decreased significantly in the late Pliocene, and late Pliocene and Pleistocene sedimentation rates are relatively low in both holes (1.53 and 1.69 cm/k.y., respectively).

Calcareous nannofossils

We examined all core catcher samples from Holes U1312A and U1312B for calcareous nannofossils. Several additional samples were taken from Cores 306-U1312A-5H and 19H to refine the biostratigraphy. All samples yielded abundant to very abundant nannofossil assemblages. Preservation is generally good to moderately good, particularly in the upper part of the section, although evidence of dissolution and overgrowth of discoasters begins downhole in Cores 306-U1312A-18H and 306-U1312B-18H. Samples below these cores contain moderate to moderately well preserved nannofossils. Pseudoemiliania lacunosa varieties dominate Pleistocene assemblages, whereas reticulofenestrids dominate Pliocene and Miocene assemblages. Some reworking is evident in both holes. Reworked specimens of Reticulofenestra pseudoumbilicus and Sphenolithus abies occur within nannofossil Zone NN16, and minor Paleogene reworking is evident within Cores 306-U1312A-25H and 306-U1312B-25H.

The sections recovered at Site U1312 yielded Pleistocene, Pliocene, and upper to uppermost middle Miocene assemblages (Tables T7, T8). Six Pleistocene nannofossil datums defined by Sato et al. (1999) are identified at Site U1312. The first occurrence (FO) of Emiliania huxleyi (0.25 Ma), which marks the base of Martini’s (1971) Zone NN21, is present in Samples 306-U1312A-1H-CC and 306-U1312B-1H-CC. The last occurrence (LO) of P. lacunosa (0.41 Ma), which defines the base of Zone NN20, is detected in Samples 306-U1312A-2H-CC and 306-U1312B-2H-CC. Additionally, the FO of Gephyrocapsa parallela (0.95 Ma) occurs in Sample 306-U1312B-2H-CC but is not found in Hole U1312A. Gephyrocapsa spp. (large) (1.21–1.45 Ma) occurs in Sample 306-U1312A-3H-CC but is not present in Hole U1312B. The LO of Helicosphaera sellii (1.27 Ma) also occurs in Sample 306-U1312A-3H-CC but is not detected in Hole U1312B until Sample 306-U1312B-4H-CC. The FO of Gephyrocapsa caribbeanica (1.73 Ma), which we use to approximate the Pliocene/Pleistocene boundary, occurs in Samples 306-U1312A-4H-CC and 306-U1312B-4H-CC.

Sample 306-U1312B-4H-CC is problematic because it contains the co-occurrence of Gephyrocapsa spp. (>4 µm), Discoaster brouweri, and Discoaster pentaradiatus. According to Sato et al. (1999) and de Kaenel et al. (1999), the LO of D. brouweri (1.97 Ma) and the FO of G. caribbeanica (>4 µm) are separated by 240,000 y and thus should not co-occur. Okada (2000), however, reports medium-sized (>4 µm) Gephyrocapsa spp. in the uppermost Pliocene sequences from Blake Ridge, northwest Atlantic Ocean and indicates the Pleistocene medium-sized Gephyrocapsa datum (1.73 Ma) should be placed at the base of consistent occurrences of medium-sized Gephyrocapsa. Thus, we suggest Sample 306-U1312B-4H-CC is older than 2.38 Ma based on the presence of D. brouweri and D. pentaradiatus. The FO of planktonic foraminifer datum Globorotalia inflata, however, is also present in this sample and indicates an age younger than 2.08 Ma. Therefore, examination of samples from Core 306-U1312B-4H is necessary to determine if discoasters found in Sample 306-U1312B-4H-CC are reworked, which would yield a younger (Pleistocene) age for this sample.

Five Pliocene events dated by Sato et al. (1999) and one by Shackleton et al. (1995) occur within the sedimentary section at Site U1312. Three events from the upper Pliocene (LO of D. brouweri, LO of D. pentaradiatus [2.38 Ma], and LO of Discoaster surculus [2.54 Ma]) occur together in Sample 306-U1312A-5H-CC, which led us to subsample Core 5H in order to determine if a hiatus is present. Examination of the subsamples determined the LO of D. brouweri, which marks the base of Zone NN19, occurs in Sample 306-U1312A-5H-2, 130–131 cm. The LO of D. pentaradiatus, which marks the base of Zone NN18, is recorded in Sample 306-U1312A-5H-5, 125–126 cm. The LO of D. surculus (base of Zone NN17) also occurs in Sample 306-U1312A-5H-5, 125–126 cm. We interpret this to indicate there is not a hiatus present in Core 306-U1312A-5H, although sedimentation rates are very low. In addition, the LO of D. surculus co-occurs in Sample 306-U1312B-5H-CC with the LO of Discoaster tamalis (2.74 Ma). Within Hole U1312A, the LO of these two species is separated, as D. tamalis is not present until Sample 306-U1312A-6H-CC. Whereas the discoaster species used as markers for the upper Pliocene are easy to identify and generally abundant, they are susceptible to reworking. As a result, further work is necessary to resolve the differences in upper Pliocene biostratigraphy between Holes U1312A and U1312B.

The intervals from 306-U1312A-6H-CC to 11H-CC and from 306-U1312B-5H-CC to 12H-CC are characterized by the occurrence of D. tamalis and absence of R. pseudoumbilicus (>7 µm), indicating an age between 2.74 and 3.85 Ma. The LO of R. pseudoumbilicus (>7 µm) (3.85 Ma), which marks the base of Zone NN16, occurs in Samples 306-U1312A-12H-CC and 306-U1312B-13H-CC. This species is reworked in Zone NN16 at Site U1312. We identified the LO as the top of consistent, common occurrences of R. pseudoumbilicus, although the true LO is likely somewhere within Cores 306-U1312A-12H and 306-U1312B-13H.

Only one datum from the lower Pliocene is identified at Site U1312. The LO of Amaurolithus primus (4.56 Ma), which approximates the base of Zone NN15, is found in Samples 306-U1312A-13H-CC and 306-U1312B-15H-CC. The ceratoliths are very rare to absent at Site U1312, making it impossible to identify the lower Pliocene FO and LO of Ceratolithus acutus.

We identified 13 nannofossil events dated by Raffi and Flores (1995) and Backman and Raffi (1997) in the Miocene sediments at Site U1312. The LO of Discoaster quinqueramus (5.537 Ma), which we use to approximate the Miocene/Pliocene boundary (5.332 Ma), is tentatively found in Samples 306-U1312A-18H-CC and 306-U1312B-19H-CC. Overgrowth of discoasters within the upper Miocene sediments make it very difficult to identify this event with certainty. Amaurolithus amplificus (5.999–6.84 Ma) also occurs in Sample 306-U1312A-18H-CC and indicates Subzone NN11D. The FO of A. primus (7.392 Ma), which marks the base of Subzone NN11B, occurs with D. quinqueramus in Samples 306-U1312A-18H-CC and 306-U1312B-19H-CC. We took subsamples from Core 306-U1312A-19H to determine if any amauroliths were present; however, no biostratigraphic events were identified within these subsamples and further refinement of the stratigraphy is impossible at this time.

The FO of Discoaster berggrenii (8.281 Ma, base of Subzone NN11A) occurs in Samples 306-U1312A-19H-CC and 306-U1312B-20H-CC and coincides with rare to absent R. pseudoumbilicus (>7 µm). Backman and Raffi (1997) indicate R. pseudoumbilicus (>7 µm) is absent between 7.10 and 8.79 Ma, which coincides with the LO of D. berggrenii and corresponds to the data from Site U1312. Sample resolution, however, made it impossible to determine the top of the small Reticulofenestra interval. The FO of Discoaster loeblichii (8.43 Ma) is detected in Sample 306-U1312A-20H-CC and coincides with the LO of D. berggrenii in Sample 306-U1312B-20H-CC, as well as the rare to absent R. pseudoumbilicus (>7 µm) interval in both holes.

The FO of Minylitha convallis (9.43 Ma) is observed in Samples 306-U1312A-21H-CC and 306-U1312B-21H-CC. The LO of Discoaster hamatus (9.64 Ma), which defines the base of Zone NN10, occurs in Samples 306-U1312A-22H-CC and 306-U1312B-22H-CC. Catinasters are very rare at Site U1312; however, Catinaster calyculus is found in Hole U1312B. The LO of C. calyculus (9.64 Ma) occurs in Sample 306-U1312B-23H-CC, whereas the FO of C. calyculus (10.705 Ma) occurs in Sample 306-U1312B-24H-CC. Additionally, the FO of Discoaster neohamatus (10.45 Ma) and D. hamatus (10.48 Ma, base of Zone NN9) co-occur in Samples 306-U1312A-24H-CC and 306-U1312B-24H-CC. Thus, these cores in both holes must be younger than 10.45 Ma. Catinaster coalitus (10.794 Ma), which marks the base of Zone NN8, is not observed at Site U1312.

The presence of rare Coccolithus miopelagicus in samples from the bottom of both holes (Samples 306-U1312A-25H-CC and 306-U1312B-25H-CC) indicates we reached the LO of C. miopelagicus (10.947 Ma), which correlates to the uppermost middle Miocene. These cores contain rare Paleogene material (Ellipsolithus macellus in Hole U1312A and Reticulofenestra umbilicus in Hole U1312B), suggesting the possibility the occurrences of C. miopelagicus are reworked. If that is the case, both holes only penetrated to the lowermost upper Miocene.

Planktonic foraminifers

We studied planktonic foraminifer assemblages in all core catchers from Holes U1312A and U1312B (Tables T9, T10). In addition, we examined a sample from Section 306-U1312A-1H-2 and washout from the top of Section 306-U1312B-1H-1, for which only the >150 µm fraction was available. All samples were washed with tap water. Planktonic foraminifers dominate the sand fraction in all core catchers and are primarily moderately to well preserved. Poor preservation is observed in some of the Miocene samples (306-U1312A-18H-CC and 306-U1312B-19H-CC) and also in the interval from Sample 306-U1312B-23H-CC to 25H-CC. The foraminifer assemblage in most core catcher samples consists of 11 to 15 different species, with lower numbers of species observed in some of the poorly preserved samples.

The FOs of G. inflata and Globorotalia truncatulinoides are contemporaneous in Sample 306-U1312A-4H-CC. In Hole U1312B, however, G. inflata occurs in Sample 306-U1312B-4H-CC and G. truncatulinoides in Sample 3H-CC. The absence of G. truncatulinoides in Sample 306-U1312B-4H-CC is most probably related to hydrographic conditions because the fauna in Sample 4H-CC indicates colder, which for G. truncatulinoides signifies less favorable surface water temperatures than the fauna in Sample 306-U1312A-4H-CC. The FO events of both species occur in the North Atlantic and Mediterranean at 2.09 Ma (Weaver and Clement, 1987).

The LO of Globorotalia puncticulata is the next biostratigraphic event and occurs in Samples 306-U1312A-5H-CC and 306-U1312B-5H-CC. In Hole U1312A, it is accompanied by the LO of Neogloboquadrina atlantica. The LOs of both species are calibrated to the astronomical timescale in the Mediterranean and dated to 2.41 Ma by Lourens et al. (1996). Because of its highly convex dorsal side, the species described as Globorotalia cf. crassula by Weaver and Clement (1987) is referred to as Globorotalia hirsuta during Expedition 306. Consequently, the LO of G. cf. crassula at 3.18 Ma (Weaver and Clement, 1987) is designated as “disappearance of G. hirsuta.” This event can clearly be defined only in Hole U1312A, where it occurs in Sample 306-U1312A-6H-CC. Although based on very few specimens, we place the LO of Sphaeroidinellopsis seminulina in Sample 306-U1312B-7H-CC, which indicates an age of 3.19 Ma (Lourens et al., 1996). In Hole U1312A, this species last occurs in Sample 306-U1312A-11H-CC, which is not believed to be the real LO since this species is often rare in the studied samples (Tables T9, T10) and could therefore be present higher in the sedimentary record. As in records from the Mediterranean Sea, a well-defined gap in the occurrence of G. puncticulata (Lourens et al., 1996) is observed at Site U1312. The reappearance of this species occurs in Samples 306-U1312A-10H-CC and 306-U1312B-8H-CC and the disappearance in Samples 306-U1312A-12H-CC and 306-U1312B-12H-CC, respectively. In the Mediterranean, this gap occurs between 3.31 and 3.57 Ma (Lourens et al., 1996).

The last common occurrence (LcO) of Globorotalia margaritae defines the next biostratigraphic marker horizon and is located in Samples 306-U1312A-13H-CC and 306-U1312B-13H-CC. Weaver and Clement (1987) state this event is diachronous between the high and low latitudes. The latitude of Site U1312 is similar to that of the Mediterranean, and we therefore consider the 3.98 Ma age of the event reported by Lourens et al. (1996) to be reliable. In the lower Pliocene, the first abundant occurrence (FaO) of G. puncticulata, together with the FaO of Globorotalia crassaformis, is observed in Samples 306-U1312A-14H-CC and 306-U1312B-14H-CC. The FaO of G. puncticulata can be traced throughout the North Atlantic and Mediterranean and is calibrated to the astronomical timescale and dated to 4.52 Ma (Lourens et al., 1996).

The FaO of G. margaritae is observed in Samples 306-U1312A-17H-CC and 306-U1312B-18H-CC and occurs at ~6.0 Ma near the base of Subchron C3A.1n (F.J. Sierro, unpubl. data). In Samples 306-U1312A-18H-CC and 306-U1312B-19H-CC, two biostratigraphic events occur: (1) the preferentially sinistral to dextral coiling direction change in Neogloboquadrina pachyderma and (2) the first common occurrence of the G. miotumida group. N. pachyderma, which was preferentially sinistral in the late Miocene, underwent a series of changes in coiling beginning at 6.3 Ma to become dominantly dextral during the Pliocene (Hilgen and Krijgsman, 1999; Sierro et al., 2001; Hodell et al., 2001). The G. miotumida group, including Globorotalia conomiozea, expanded in the North Atlantic and Mediterranean when these species replaced the Globorotalia menardii 5 (dextral forms) group (Sierro, 1985; Sierro et al., 1993); however, the latter group was not observed in these samples, probably due to low sampling resolution (one sample per core) or the possible presence of a disconformity at this site. This is a distinct event that occurs at 7.24 Ma and has been used to mark the Tortonian/Messinian boundary (Sierro et al., 1993; Hilgen et al., 1995). The G. menardii 4 group LcO appears in Samples 306-U1312A-19H-CC and 306-U1312B-20H-CC, respectively. This event is astronomically dated to 7.51 Ma (Krijgsman et al., 1995; Hilgen et al., 1995).

From Samples 306-U1312A-20H-CC and 306-U1312B-21H-CC to the base of each hole, N. pachyderma is again preferentially dextral. In the Mediterranean, this species becomes dominantly dextral at 9.5 Ma (Krijgsman et al., 1995, Hilgen et al., 1995); however, several fluctuations in coiling are recorded between 7.8 and 9.55 Ma. Although a higher resolution analysis of this species is needed to accurately date this interval, an age older than 7.8 Ma is assigned to this core. This event is preceded in Samples 306-U1312A-21H-CC and 306-U1312B-22H-CC by the LO of Globorotalia lenguaensis, which occurs in the tropical Atlantic at 8.99 Ma (Turco et al., 2002). This species, however, is always rare, and therefore the position of this event is probably less precise. The last stratigraphic marker observed in both holes is the FO of N. pachyderma as its morphotype Neogloboquadrina acostaensis, which is present in Samples 306-U1312A-24H-CC and 306-U1312B-24H-CC. This event has been astronomically dated to 10.55 Ma (Hilgen et al., 2000) in the Mediterranean and to 9.89 Ma in the tropical Atlantic (Turco et al., 2002), with the former age probably more reliable in the North Atlantic at middle latitudes.

Globigerina nepenthes, which first appears at 11.6 Ma (Spezzaferri, 1998), occurs in Samples 306-U1312A-25H-CC and 306-U1312B-25H-CC. Neogloboquadrina mayeri, with its LO at 11.2 Ma (Hilgen et al., 2000), is not observed in these samples, so the age at the bottom of both holes must be younger than 11.2 Ma. In addition, specimens of Neogloboquadrina other than the morphotype N. acostaensis, including large encrusted N. atlantica, are still present in Samples 306-U1312A-24H-CC and 306-U1312B-24H-CC, indicating that the bottom of the hole is above the FO of the neogloboquadrinids, which is dated to 11.78 Ma (Hilgen et al., 2000). According to the planktonic foraminifer stratigraphy, Holes U1312A and U1312B encompass the interval from the base of the Tortonian (late Miocene) to the late Quaternary.

IRD is observed in the two uppermost core catcher samples of both holes. Based on correlation of the lightness L* record with the benthic δ¹⁸O stack of Lisiecki and Raymo (2005) (see “Stratigraphic correlation”), these samples correlate with the transition from marine isotope Stage (MIS) 11 to MIS 10 (Samples 306-U1312A-1H-CC and 306-U1312B-1H-CC) and with MIS 22 (Samples 306-U1312A-2H-CC and 306-U1312B-2H-CC). For these stages, the planktonic foraminifer fauna suggest the presence of transitional, rather than subpolar, waters, as high abundances of G. inflata, N. pachyderma (dextral), Globigerina bulloides, and Globigerinella aequilateralis appear together with minor numbers of the subtropical species Globigerinoides ruber and G. crassaformis (Tables T9, T10). Subtropical to tropical species of the Globigerinoides group are also present in rare to common abundances throughout the entire Pliocene. Samples 306-U1312A-18H-CC and 306-U1312B-17H-CC, both from the Miocene, contain heavily encrusted, relatively large sized Neogloboquadrina species.

Benthic foraminifers

Rare to few well preserved benthic foraminifers are present in the oozes at Site U1312 (Tables T11, T12). Three assemblages are distinguished based on their faunal composition.

Assemblage I (Oridorsalis umbonatus)

This assemblage occurs between Samples 306-U1312A-1H-CC and 14H-CC and also between Samples 306-U1312B-1H-CC and 14H-CC. O. umbonatus is the dominant species and is associated with Cibicidoides wuellerstorfi, Globocassidulina subglobosa, and Melonis barleeanus.

Assemblage II (Nuttalides umboniferus-Pullenia bulloides-Uvigerina spp.)

This assemblage occurs between Samples 306-U1312A-14H-CC and 19H-CC and also between Samples 306-U1312B-14H-CC and 20H-CC. It is characterized by deepwater foraminifers such as N. umboniferus, P. bulloides, Uvigerina peregrina, and Uvigerina proboscidea. The relative abundances of these species vary from sample to sample.

Assemblage III (Astrononion stelligerum)

This assemblage occurs below Sample 306-U1312A-19H-CC in Hole U1312A and Sample 306-U1312B-20H-CC in Hole U1312B. It is characterized by the highest relative abundance of A. stelligerum. C. wuellerstorfi and P. bulloides are frequently found with A. stelligerum in Hole U1312A.

The faunal change from Assemblage III to II occurs at ~7.246 Ma, near the Tortonian/Messinian boundary. The stratigraphic distribution of the benthic foraminifer assemblages likely reflects changes in deepwater conditions through time. Other faunal differences may possess additional paleooceanographic significance, but shipboard time constraints prohibited a more detailed investigation.

Diatoms

We investigated diatoms in smear slides from 50 core catcher samples and 125 depth intervals in core sections from Holes U1312A and U1312B (Tables T13, T14). Trace numbers of diatoms are generally present within the upper 60 m in Hole U1312A and the upper 90 m in Hole U1312B, corresponding to the Pliocene–Pleistocene interval. The next two to three cores below these levels are marked by dissolution and a decrease in diatom abundance. Below Samples 306-U1312A-11H-CC and 306-U1312B-12H-CC, the core catchers are almost completely devoid of diatoms. Silicoflagellates are generally present with the diatoms.

The generally low abundance of diatoms in the upper one-third of Holes U1312A and U1312B made placement of defined datums (Baldauf, 1987) difficult, and the almost complete lack of diatoms in the lower two-thirds of both holes made it impossible.

The base of the Fragilariopsis doliolus Zone, defined by the LO of Fragilariopsis reinholdii (0.48–0.45 Ma), occurs in Samples 306-U1312A-2H-CC and 306-U1312B-3H-CC (ignoring a single, probably reworked specimen in Sample 306-U1312B-2H-CC). Rhizosolenia curvirostris occurs in samples taken from the first sections of Core 306-U1312B-1H, suggesting an age of at least 0.3 Ma.

The F. reinholdii Zone is not present in the Hole U1312A material. In Hole U1312B, this zone spans from Samples 306-U1312B-3H-CC to 306-U1312B-6H-CC, based on the co-occurrence of F. doliolus (FO = 1.9 Ma), F. reinholdii, and Fragilariopsis fossilis (LO = ~0.5 Ma).

The upper part of the Alveus marinus Zone is present from Samples 306-U1312A-3H-CC to 5H-CC based on the occurrence of F. reinholdii and F. fossilis without F. doliolus. This zone is not readily apparent in Hole U1312B. The bottom of the A. marinus Zone is not identified, as the LO of the zonal marker Fragilariopsis jouseae (2.66–2.83 Ma) is not observed in material from Site U1312. Thalassiosira convexa last occurs in the lower part of the A. marinus Zone (2.58–2.68 Ma) and gives a secondary datum to indicate the beginning of this zone.

Because of the rarity and poor preservation of diatoms in the lower two-thirds of both holes, the assignment of biostratigraphic zones is tentative. The absence of F. doliolus, continued presence of F. reinholdii and F. fossilis, and occasional occurrence of T. convexa indicate the oldest diatom-containing intervals of both cores belong somewhere between the A. marinus and T. convexa Zones (6.2–1.9 Ma). This interval spans from Samples 306-U1312A-6H-CC to 10H-CC and from 306-U1312B-7H-CC to 11H-CC. Below Samples 306-U1312A-10H-CC and 306-U1312B-11H-CC, the core catchers are essentially devoid of diatoms.

The diatom flora in the upper six core catchers of both holes is diverse considering the preservation. A total of 42 diatom species are identified, along with several types of silicoflagellates. The assemblage is dominated by warm-water flora, as observed by Baldauf (1987), including Coscinodiscus radiatus, Hemidiscus cuneiformis, Thalassiosira oestrupii group, A. marinus, Thalassionema nitzschioides group, F. reinholdii, F. fossilis, and species of the Thalassiosira genus with a linear areolae array. Chaetoceros resting spores are often present, indicating high productivity. Both holes contain plentiful Thalassiothrix fragments, indicating influence of colder waters. Several samples from Hole U1312B contain Actinocyclus curvatulus, also indicating a cold-water influence. In both holes, coastal diatoms such as Paralia sulcata and Raphoneis are observed in the upper three core catchers. Additionally, Fragilariopsis aff. pseudocylindrus is present in Sample 306-U1312A-3H-CC, which suggests transport of coastal diatoms to open waters.

Radiolarians

We examined radiolarians in all core catcher samples from Holes U1312A and U1312B. Only eight samples (306-1312B-1H-CC through 4H-CC, 6H-CC, and 8H-CC through 10H-CC) include identifiable radiolarian specimens (Tables T15, T16). These samples generally contain rare to few poorly preserved radiolarians, but common, well preserved specimens are present in Sample 306-U1312B-3H-CC. This sample contains numerous species, 60 of which are listed in Table T15.

According to Ciesielski and Bjørklund (1995), Cycladophora davisiana first occurs in the middle latitude North Atlantic at 2.59 Ma in Sample 94-609-19X-CC, 2–4 cm. Haslett (1994, 2004) recognized this species in the same site (Sample 94-609-18X-CC) at 2.44 Ma. The two age models applied by Haslett (1994, 2004) and Ciesielski and Bjørklund (1995) are in close agreement; therefore, we assume the FO of C. davisiana is close to 2.59 Ma.

In Hole U1312B, C. davisiana is observed in all core catcher samples from Sample 306-U1312B-1H-CC to 4H-CC, whereas 5H-CC is barren of radiolarians. In Sample 306-U1312B-6H-CC, where radiolarians are abundant, C. davisiana is absent. This indicates the FO of C. davisiana is somewhere in Core 306-U1312B-5H. The maximum age for Sample 306-U1312B-4H-CC is therefore 2.59 Ma.

Motoyama (1997) demonstrated at DSDP Site 192 that C. davisiana evolved from Cycladophora sakaii in the northwest Pacific. The oldest specimens assignable to C. davisiana are observed in Sample 19-192-18R-1, 50–52 cm (4.2 Ma), with a typical rich C. davisiana population occurring at ~2.7–2.3 Ma. This indicates C. davisiana evolved in the northwest Pacific and then spread to the rest of the world ocean. Ciesielski and Bjørklund (1995) reported the FO of C. davisiana at 2.99–3.08 Ma in the Labrador Sea in Sample 105-646B-25X-CC. They suggested that C. davisiana evolved in the Labrador Sea and migrated through the Arctic Ocean into the North Pacific (2.62–2.64 Ma; Stage 114) before migrating into the Norwegian Sea (2.63–2.53 Ma) and then the North Atlantic (2.59–2.44 Ma; Stage 109–102). According to the new results by Motoyama (1997), however, the most likely route of migration is from the northwest Pacific to the Labrador Sea via the Arctic Ocean.

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